Activated Carbon from Biomass Sustainable Sources

Journal of C Carbon Research

Review Activated Carbon from Biomass Sustainable Sources

Yong X. Gan

Department of Mechanical Engineering, California State Polytechnic University Pomona, 3801 W Temple Avenue, Pomona, CA 91768, USA; [email protected]; Tel.: +1-909-869-2388

Abstract: Biomass wastes are abundant around us. They are renewable and inexpensive. Product manufacturing from renewable resources has caught increasing interest recently. Activated carbon preparation from biomass resources, including various trees, leaves, plant roots, fruit peels, and grasses, is a good example. In this paper, an overview of activated carbon production from biomass resources will be given. The first part will be on the processing technologies. The second part will focus on the carbon activation methods. The third part will introduce the biomass resources. The fourth part will be on surface modification of activated carbon through the addition of various components. Finally, the development of product applications will be discussed with an emphasis on adsorption, filtration, water purification, energy conversions, and energy storage.

Keywords: activated carbon; porous carbon; biomass resources; sustainable resources; processing technology; surface modiﬁcation; adsorption; water puriﬁcation; energy storage and conversions

1. Introduction One of the most important forms of carbon, called activated carbon, has a high surface area and a large volume of micropores. The specific surface area of activated 2 Citation: Gan, Y.X. Activated Carbon carbon can reach as high as 3000 m /g, which makes it very effective in the removal of from Biomass Sustainable Sources. C inorganic pollutants such as heavy metals from water [1]. Activated carbon has also been 2021, 7, 39. https://doi.org/10.3390/ studied for mercury removal from water [2,3]. Activated carbon is sometimes called active c7020039 carbon because it can participate in chemical reactions or it can be used as the support for catalysis. Recent applications of activated carbons are in the field of energy storage and Academic Editors: Jorge Bedia and conversions [4]. What are the differences between porous carbon and activated carbon? Martin Oschatz Generally speaking, porous carbon is characterized by its physicochemical properties, such as large surface area, large pore size range, relatively low density, etc. Activated carbon Received: 8 March 2021 refers to carbon materials experienced with the activation of their surfaces or modification Accepted: 26 April 2021 on the structures via functionalization, metal or oxide deposition, etc., for well-defined Published: 27 April 2021 applications. All activated carbons are porous carbons. However, not all porous carbons are activated carbons. The porosity of porous carbons spans a very wide range of pore Publisher’s Note: MDPI stays neutral sizes, while the activated carbons are, in essence, microporous materials. Although this with regard to jurisdictional claims in fundamental difference should not be overlooked, sometimes the boundary between the published maps and institutional affil- activated carbons and porous carbons may not be so distinct, especially from processing iations. and application perspectives. As will be discussed in next section, the pore generation and activation of carbon materials happen in the same process. Surface functionalization and activation result in pore generation simultaneously. Traditionally, activated carbon is made from coal or charcoal. However, making Copyright: © 2021 by the author. activated carbon from renewable resources is more intriguing because it is sustainable. The Licensee MDPI, Basel, Switzerland. carbonizing of naturally grown grass and tree leaves has been studied for various potential This article is an open access article industrial applications [5]. Date palm-tree branches (DPB) generated from the regular distributed under the terms and trimming of palm-trees were carbonized to generate an activated carbon product for toluene conditions of the Creative Commons adsorption [6]. Although some biomass may be directly used for the adsorption of cationic Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ dyes with high concentration at lower cost [7], activated sorbents after carbonization 4.0/). showed higher efficiency in dye removal [8]. Usually, curry tree (Murraya koenigii) stem

C 2021, 7, 39. https://doi.org/10.3390/c7020039 https://www.mdpi.com/journal/carbon C 2021, 7, 39 2 of 33

is considered as an agricultural waste [9]. It is present in various vegetable markets. To convert it to a value-added product, carbonization of curry tree bark was conducted to generate activated carbon. The activated carbon was used to effectively remove the crystal violet dye from wastewater [9]. The abundance and diversity of bioresources are other reasons for the preparation of new activated carbon from woods, tree barks and leaves, grasses, and roots. In the following sections, recent development in various techniques for generating high performance and low cost activated carbon from representative renewable sources will be dealt with. The physical and chemical activation methods will be discussed. In the last part of the paper, typical applications of the activated carbons for gas adsorption, water puriﬁcation, and energy storage will be presented.

2. Processing Techniques Activated carbon materials are made through three required processes. The ﬁrst process is the pretreatment of biomass raw materials. The second process is carbonization. The third process is activation. During the pretreatment process, most of the nutrients and solvable impurities are removed. The carbonization process allows organic lignin and cellulose to be converted into carbonaceous materials. Carbonization also reduces the amount of water, nutrients, oxygen, hydrogen, sulfur, and other elements. Carbon loss may happen in the carbonization process due to heating to temperatures above 400 ◦C. With the increase of temperature, the grains in raw materials are dehydrated. The oxygen in the raw materials is released in forms of H2O, CO, CO2, etc. Such reaction products facilitate subsequent activation reactions [10]. Typically, the activation process follows carbonization. However, they may be conducted at the same time. The carbonaceous materials (biochars) can be activated by two different approaches: physical activation (PA), and chemical activation (CA). In the physical activation process, a raw material is activated in the temperature range from 750 to 1000 ◦C in a vacuum or inert gas atmosphere. In the chemical activation process, chemical agents are incorporated into raw materials. They are heated up together in an inert gas, and carbonization and activation occur simultaneously. Recently, chemical activation has been studied for processing high-performance activation carbons. There are some challenges associated with the chemical activation of carbon as well. A washing step is always required to remove byproducts following chemical activation. This washing, followed by drying, is typically time consuming, which is why physical activation is claimed as a more mature process used for producing most commercially available activated carbons. Considering the increased research interest in the chemical activation process, in the following subsections, the activated carbon processing techniques based mainly on the chemical activation approach will be discussed.

2.1. Pretreatment and Carbonization Activated carbon is made through the general procedures including pre-carbonization, carbonization, and activation. Sometimes a pre-treatment procedure is needed. As shown in [4], tamarisk-tree branches were collected as the starting material, and the pre-treatment was performed by soaking the tamarisk-tree branch in distilled water for 12 h. Then the sample was air dried at 60 ◦C for a sufﬁciently long period of time. The whole process for making activated carbon from the tree branch is schematically shown in Figure1a [ 4]. The abbreviation of FLC@BC in Figure1a stands for the “few-layer carbon@bulk carbon”, a unique structure due to the activation treatment of the tamarisk tree sample with the KOH solution. The pre-carbonization was carried out at 320 ◦C for 5 h. The carbonization and activation of the sample are shown in more detail in [4]. Brieﬂy, a typical pre-carbonized sample with a weight of 1 g was soaked in 20 mL KOH solution for a day. For multiple experiments, the mass ratios of the pre-carbonized product to KOH were kept as 1:1, 2:1, 3:1, 5:1, and 7:1, respectively. The samples were dried and then carbonized at 700 ◦C for 2 h in N2. Then, the samples were washed by 1 wt% HCl solution and distilled water until the pH value reached 7. For comparative studies, the product carbonized at 700 ◦C for C 2021, 7, x FOR PEER REVIEW 3 of 33

C 2021, 7, 39 3 of 33 kept as 1:1, 2:1, 3:1, 5:1, and 7:1, respectively. The samples were dried and then carbonized at 700 °C for 2 h in N2. Then, the samples were washed by 1 wt% HCl solution and distilled water until the pH value reached 7. For comparative studies, the product carbonized at 2700 h without °C for 2 soakingh without in soaking KOH solution in KOH wassolution made. was Figure made.1b Figure shows 1b optical shows images optical ofimages the tamariskof the tamarisk tree and tree branches. and branches. Under scanningUnder scan electronning electron microscope, microscope, the tamarisk the tamarisk tree branch tree samplebranch demonstratedsample demonstrated a porous a microstructure, porous microstr anducture, the ligaments and the ligaments were relatively were rough,relatively as shownrough, byas theshown SEM by image the SEM in Figure image1c in [4 Figure]. 1c [4].

FigureFigure 1. 1. SchematicSchematic andand images showing ( (aa)) the the tamarisk tamarisk tree tree branches branches under under pre-carbonization, pre-carbonization, carbonization,carbonization, andand activationactivation treatment procedures; ( b) tamarisk tamarisk tree tree and and branches branches under under visible visible light; (c) the tamarisk tree branch under scanning electron microscope (SEM); (d–f) carbonized light; (c) the tamarisk tree branch under scanning electron microscope (SEM); (d–f) carbonized sample without KOH activation under SEM; (g) carbonized sample without KOH activation under sample without KOH activation under SEM; (g) carbonized sample without KOH activation under transmission electron microscope (TEM); (h) carbonized sample with KOH activation under SEM; transmission(i,j) carbonized electron sample microscope with KOH (TEM); activation (h) carbonizedunder TEM. sample Reproduced with KOH with activationpermission under from SEM;[4], (i©2020,j) carbonized Elsevier sampleLtd. with KOH activation under TEM. Reproduced with permission from [4], ©2020 Elsevier Ltd. Carbonization of the tamarisk tree branch sample caused the reduction of the surface roughness,Carbonization which ofcan the be tamarisk seen from tree the branch three sample SEM causedimages the as reductionshown in of Figure the surface 1d–f. roughness, which can be seen from the three SEM images as shown in Figure1d–f. Through Through the comparison, it is evident that the ligament surface as shown in Figure 1c is the comparison, it is evident that the ligament surface as shown in Figure1c is much rough much rough than that of those observed in Figure 1d–f. Under transmission electron mi- than that of those observed in Figure1d–f. Under transmission electron microscope (TEM), croscope (TEM), the carbonized tamarisk tree branch sample surface is relatively smooth, the carbonized tamarisk tree branch sample surface is relatively smooth, as revealed by as revealed by Figure 1g. The KOH pretreatment changed the morphology of the activated Figure1g. The KOH pretreatment changed the morphology of the activated carbon ﬁber carbon fiber significantly. The KOH treated sample has a structure of few-layer car- signiﬁcantly. The KOH treated sample has a structure of few-layer carbon@bulk carbon bon@bulk carbon (FLC@BC). Such a unique structure was synthesized by controlling the (FLC@BC). Such a unique structure was synthesized by controlling the diffusion depth diffusion depth from the aqueous KOH liquid phase to the tamarisk tree branch carbon from the aqueous KOH liquid phase to the tamarisk tree branch carbon precursor solid precursor solid phase [4]. After the KOH treatment, the carbonized and activated tamarisk phase [4]. After the KOH treatment, the carbonized and activated tamarisk tree branch tree branch sample, named as few-layer carbon@bulk carbon, demonstrated both macro- sample, named as few-layer carbon@bulk carbon, demonstrated both macro-pores and numerous protrusions at the pore surface, as can be seen from the SEM image in Figure1h. Under the TEM, it was observed that the few-layer carbon (loose carbon) attached to the surface of the dense, bulk carbon. Figure1i,j reveal said microstructure. Through KOH C 2021, 7, 39 4 of 33

activation and carbonization, the few-layer carbon or loose carbon was formed in the range of the diffusion depth. The activation process also generated micropores at the surface of the bulky carbon. Consequently, high speciﬁc surface area was obtained, which enhanced the speciﬁc capacitance when the activated carbon was used for energy storage [4]. The role of KOH or NaOH in the activation of carbon was reported [11]. The reaction mechanisms were proposed as described below. After the contact with the KOH, the grains of the raw materials were activated at a high temperature. The carbon between the crystallites was burned out by a series of chemical reactions, resulting in the formation of micropores. The activation time and temperature should be well controlled. Generally, the pore size increases with the increase of activation time and processing temperature, while the carbon yield decreases due to the loss in the reactions. If the activation time is too long and the temperature is too high, the micropores will be destroyed. In the activation process, the main reaction is [10]

4 KOH + C → K2CO3 + K2O + 2 H2 (1)

Carbon was etched by KOH, and the reaction released hydrogen. Consequently, micropores or voids formed. At temperatures lower than 500 ◦C, dehydration of KOH happened following the reaction shown by Equation (2).

2 KOH → K2O + 2 H2O (2)

The generated water vapor sustained the reforming reactions of Equations (3) and (4):

H2O + C → CO + 2 H2 (3)

CO + H2O → CO2 + 2 H2 (4) The carbon dioxide and potassium oxide further reacted with each other to produce potassium carbonate according to Equation (5):

K2O + CO2 → K2CO3 (5)

Therefore, the activation of carbon by KOH generated hydrogen and potassium carbonate as the main products. Carbon monoxide and carbon dioxide formed as the by-products [10].

2.2. Simultaneously Carbonization and Activation To reduce the energy consumption and save the processing time, carbonization and activation can be executed at the same time. Sun et al. [12] reported the processing and adsorbent application of activated carbon from oak tree and apple tree wood waste, as shown in Figure2. In the lower part of Figure2, the multiple reactions of the activated carbon to the surrounding organic groups are shown. During processing, small pieces of wood waste were crushed into particles with a size less than 125 µm. The particles were ◦ pyrolyzed in a tube furnace at 900 C for 2 h in N2 or CO2 gas flow. The obtained activated carbon products were named as OAC (oak tree-derived activated carbon) and ABC (apple tree-derived activated carbon). In addition, a suffix was added on each of the names, indicating the carbonization temperature and the gas used. For example, the obtained oak and apple tree activated carbon were denoted as OBC-900N, OBC-900C, ABC-900N, and ABC-900C, respectively. The suffix “N” stands for N2 purging gas, while suffix “C” refers to the CO2 gas. It was found that the pyrolysis of oak tree wood waste in CO2 flow produced activated carbon with increased aromaticity. CO2 allowed the formation of a hierarchical porous structure, as revealed by the SEM images in Figure3[ 12]. Enhanced surface hydrophilicity was found for such structures. It was also possible to change the polarity and acidity of the carbon surface, which provided higher densities of functional C 2021, 7, 39 5 of 33 C 2021, 7, x FOR PEER REVIEW 5 of 33

C 2021, 7, x FOR PEER REVIEW 5 of 33

groupsgroups near near the the surface surface of of the the activated activated carbon. carbon. The The two two SEM SEM images images in in Figure Figure3g,h 3g,h show show groups nearthe thethe hierarchical surfacehierarchical of the porous po activatedrous structure. structure. carbon. The two SEM images in Figure 3g,h show the hierarchical porous structure.

FigureFigure 2. 2. Schematic and and images images showing showing the the conversion conversion of oak of oak tree tree wood wood waste waste to activated to activated car- Figure 2. Schematicbon via and pyrolysis images atshowing 900 °C◦ thein CO conversion2, adsorption of oak of 2,4-dichlorophentree wood waste oxyto activated acetic acid car- (2,4-D) by the carbon via pyrolysis at 900 C in CO2, adsorption of 2,4-dichlorophenoxy acetic acid (2,4-D) by the bon via pyrolysisactivated at 900 carbon°C in CO under2, adsorption microwave of 2,4-dichlorophen irradiation, and theoxy development acetic acid (2,4-D) of th eby microporous the structure activated carbon under microwave irradiation, and the development of the microporous structure in activated carbonin graphitizedunder microwave carbon. irradiation, Reproduced an dwith the pedevelopmentrmission from of th[12],e microporous ©2020 Elsevier structure B.V. graphitized carbon. Reproduced with permission from [12], ©2020 Elsevier B.V. in graphitized carbon. Reproduced with permission from [12], ©2020 Elsevier B.V.

Figure 3. SEM images of activated carbon derived from oak tree and apple tree wood waste ((a–d): 5000× magnification; (e–h): 20,000× magnification). Reproduced with permission from [12], ©2020 Figure 3. SEMFigure images 3. of SEM activated images carbon of activated derived carbon from oakderived tree andfrom apple oak tree tree and wood apple waste tree (( wooda–d): 5000waste× ((magniﬁcation;a–d): 5000× magnification;Elsevier (e B.V.–h): 20,000× magnification). Reproduced with permission from [12], ©2020 (e–h): 20,000× magniﬁcation). Reproduced with permission from [12], ©2020 Elsevier B.V. Elsevier B.V. 2.3. Activation

2.3. Activation2.3. ActivationTypical activation agents include KOH, K2CO3, ZnCl2, and CaCl2. In the work per- Typical formedactivationTypical by agentsBai activation et al.include [13], agents 25KOH, wt% include K ammonia2CO3 KOH,, ZnCl wa2 K, 2andsCO used 3CaCl, ZnClto 2hydrothermally. In2, the and work CaCl per-2. treat In the wood work for formed by Baiperformedremoving et al. [13], the by 25 Bailignin. wt% et al. Theammonia [13 temperature], 25 wt%was used ammonia for hydrothermalto hydrothermally was used processing to hydrothermally treat wood was carried for treat out wood at 190 removing thefor°C lignin. removingfor 6 Theto 8 temperature theh before lignin. carbonization The for temperature hydrothermal in vacuum. for processing hydrothermal After wasthe ammonia processingcarried out treatment, wasat 190 carried the out wood at °C for 6 to 8 wash before heat carbonizationtreated at 900 in°C vacuum. for 8 to 10After h in the vacuum. ammonia The treatment, carbon was the soaked wood in saturated was heat treated at 900 °C for 8 to 10 h in vacuum. The carbon was soaked in saturated C 2021, 7, 39 6 of 33

C 2021, 7, x FOR PEER REVIEW 6 of 33

190 ◦C for 6 to 8 h before carbonization in vacuum. After the ammonia treatment, the wood was heat treated at 900 ◦C for 8 to 10 h in vacuum. The carbon was soaked in saturated KOH solution for two to three days. Finally, the carbon was heat treated at 750 °C ◦for 8 to 10KOH h. After solution rinsing for twoby HCl to three solution days. and Finally, cleaning the carbonby water, was the heat activated treated carbon at 750 withC for a 8 three-dimensionalto 10 h. After rinsing (3D) by structure HCl solution was obtain and cleaninged. The by ordered water, alignment the activated of carbonchannels with and a multi-poresthree-dimensional was observed (3D) structure using scanning was obtained. electron The microscopy ordered alignment(SEM). The of SEM channels images and in multi-pores was observed using scanning electron microscopy (SEM). The SEM images in Figure 4 show the morphology of the activated carbon material. The multiple sized pores Figure4 show the morphology of the activated carbon material. The multiple sized pores can be found in Figure 4A, the top view of the sample. The cross section view in Figure can be found in Figure4A, the top view of the sample. The cross section view in Figure4B 4B reveals the through thickness channels within the carbon material. The morphology of reveals the through thickness channels within the carbon material. The morphology of the channel wall is shown in Figure 4C. These channels are aligned regularly along the the channel wall is shown in Figure4C. These channels are aligned regularly along the longitudinal direction of the tree trunk. In addition, there are many micropores on the longitudinal direction of the tree trunk. In addition, there are many micropores on the inner inner wall of the carbon material activated by KOH. Under higher magnification, the mi- wall of the carbon material activated by KOH. Under higher magnification, the micropores cropores can be seen more clearly. In Figure 4D,E, the multi-level pores are demonstrated. can be seen more clearly. In Figure4D,E, the multi-level pores are demonstrated. Such a Such a unique structure allows the activated carbon material to have both high pore vol- unique structure allows the activated carbon material to have both high pore volume and ume and large specific surface area. A lithium–sulfur (Li/S) battery was built using the large specific surface area. A lithium–sulfur (Li/S) battery was built using the activated activated carbon as the cathode material. The electrochemical testing results showed that carbon as the cathode material. The electrochemical testing results showed that the first thedischarge first discharge capacity capacity of the activated of the activated carbon electrode carbon electrode reached 1595reached mA 1595·h/g. mA This‧h/g. value This is valuevery closedis very to closed the theoretical to the theoretical specific capacityspecific capacity of the sulfur of the electrode sulfur electrode material, material, which is which1670 mA is 1670·h/g mA [13].‧h/g [13].

FigureFigure 4. 4. SEMSEM images images of of activated activated carbon carbon derived derived from from wood wood showing showing (A (A) )the the top top surface, surface, (B (B) the) the sideside surface, surface, ( (CC)) the the wall wall of of pores, pores, ( (DD)) the the magnified magniﬁed region region in in ( (CC),), ( (EE)) the the further further magnified magniﬁed re- region gion in (D). Reproduced from [13]. in (D). Reproduced from [13].

K2CO3 is not a deleterious compound because it can be used for food. Hayashi et al. K2CO3 is not a deleterious compound because it can be used for food. Hayashi et al. [14] [14] reported that K2CO3 can be used as an effective activation reagent for activated carbon reported that K2CO3 can be used as an effective activation reagent for activated carbon generation.generation. At At the the selected temperatures forfor activation, activation, the the following following reaction reaction occurs: occurs:

K2CO3 + 2 C → 2K +3 CO (6) K2CO3 + 2 C → 2K +3 CO (6) Carbon in the chars was removed to form pores by the CO gas. Nutshells including almondCarbon shell in(AM), the charscoconut was shell removed (CN), tooil form palm pores shell by(OP), the pistachio CO gas. Nutshellsshell (PT) includingand wal- nutalmond shell shell(WN) (AM), were coconut used as shell the raw (CN), materials oil palm for shell preparing (OP), pistachio high specific shell (PT) area and activated walnut carbons.shell (WN) The were nutshell used asparticles, the raw materialsK2CO3, and for preparingwater were high mixed speciﬁc together area activated and dried. carbons. The nutshellsThe nutshell impregnated particles, with K2CO K32CO, and3 were water loaded were on mixed a ceramic together boat and and dried. placed The into nutshells a stainless-steelimpregnated reactor. with The K2 COactivation3 were loadedtemperature on a ceramic range was boat set and from placed 773 to into 1173 a stainless-steel K. Nitrogen wasreactor. used The as the activation protective temperature gas. When range prepared was at set 1073 from K, 773 all the to 1173activated K. Nitrogen carbons was reached used theas themaximum protective specific gas. area. When prepared at 1073 K, all the activated carbons reached the maximumZinc chloride, speciﬁc as area. the activating agent, can lower the carbonization temperature. The selectedZinc activation chloride, temperature as the activating for the agent, production can lower of activated the carbonization carbon using temperature. walnut shell The asselected the raw activation material temperature is approximately for the 375 production °C, as shown of activated in the carbonwork performed using walnut by Kim shell et as C 2021, 7, 39 7 of 33 C 2021, 7, x FOR PEER REVIEW 7 of 33

the raw material is approximately 375 ◦C, as shown in the work performed by Kim et al. [15]. al. [15]. Comparative studies of using ZnCl2 and CaCl2 to activate the carbon material de- Comparative studies of using ZnCl2 and CaCl2 to activate the carbon material derived rived from walnut shells were also made. The degree of activation by ZnCl2 was compared from walnut shells were also made. The degree of activation by ZnCl2 was compared with with that by CaCl2. It was found that a higher degree of activation was achieved by using that by CaCl2. It was found that a higher degree of activation was achieved by using the 2 2 2 ZnClthe ZnCl2. However,. However, CaCl CaCl2 is less is expensiveless expensive than than ZnCl ZnCl2. Both. Both the efficiency the efficiency of activation of activation and theand cost the shouldcost should be considered be considered in approaches in approaches to a practical to a practical activation activation process. process. During During the experimentthe experiment of ZnCl of 2ZnClactivation,2 activation, black walnutblack walnut shells wereshells crushed were crushed and ground and intoground sizes into in thesizes range in the from range 250 from to 1410 250 µtom. 1410 The µm. prepared The prepared walnut walnut shells wereshells dried were anddried mixed and mixed with thewith activation the activation agent ZnClagent2 .ZnCl The2 weight. The weight ratios ofratios walnut of walnut shell raw shell material raw material to the ZnCl to the2 solutionZnCl2 solution were 1:1, were 1:3, 1:1, 1:5, 1:3, and 1:5, 1:7. and The 1:7. apparatus The apparatus consisted consisted of three of three parts, parts, namely namely the gasthe inlet,gas inlet, tube tube furnace, furnace, and and gas gas condenser, condenser, as schematicallyas schematically shown shown in in Figure Figure5a. 5a. High High puritypurity NN22 gasgas flewflew intointothe the furnace furnace at at a a flow flow rate rate of of 150 150 mL/min mL/minduring during the the activation activation process.process. A porcelain boat boat was was used used to to hold hold specimens. specimens. A Pt/Pt-Rh A Pt/Pt-Rh thermocouple thermocouple was wasused usedto measure to measure the temperature. the temperature. To locate To locate the boat the boatat the at center the center of tube, of tube, a molybdenum a molybdenum (Mo) (Mo)wire wirewas used was usedas the as pusher. the pusher. Figure Figure 5b illus5b illustratestrates the sample the sample holder holder [15].[ The15]. flow The flowchart chartfor processing for processing the porous the porous carbon carbon using using ZnCl2 ZnCl as the2 as activating the activating agent agentis presented is presented in Figure in Figure6 [15].6 [The15] general. The general procedures procedures for the for activa the activationtion process process can can be bedescribed described as asfollows: follows: A Amixture mixture of of walnut walnut shell shell and and ZnCl ZnCl22 solutionsolution with with the the representative concentrationconcentration ofof 1.0,1.0, 3.0,3.0, 5.0,5.0, oror 7.07.0 MM was was loaded loaded intointo thethe sample sample holder,holder, the the boat boat in in Figure Figure5 b,5b, and and heated heated in in ◦ thethe furnacefurnace toto a a controlled controlled temperature temperature between between 250 250 to to 550 550 C.°C. The The activation activation time time was was controlledcontrolled asas 0.5,0.5, 1, 2, 3, 3, and and 4 4 h, h, respecti respectively.vely. The The heat-treated heat-treated product product was was boiled boiled in 5% in 5%HCl HCl solution solution to leach to leach out outthe theZn-rich Zn-rich activating activating agent. agent. Then Then the product the product was wasrinsed rinsed with withhot distilled hot distilled water water several several times. times. Then, Then, it wa its wasneutralized neutralized in a indilute a dilute NaOH NaOH solution. solution. The Thefinal final product product was was stored stored in a indesiccator a desiccator filled filled with with N2 gas N2 togas prevent to prevent oxidation. oxidation. For com- For comparison,parison, an activation an activation experiment experiment using using CaCl CaCl2 as2 theas theactivating activating agent agent was was conducted. conducted.

FigureFigure 5. 5.Schematic Schematic of of the the experimental experimental apparatus apparatus for for activation activation (a), ( anda), and sample sample boat boat (b). ( Reproducedb). Repro- withduced permission with permission from [15 from], ©2001 [15], Elsevier ©2001 Elsevier Science B.V.Science B.V. C 2021, 7, 39 8 of 33 C 2021, 7, x FOR PEER REVIEW 8 of 33 C 2021, 7, x FOR PEER REVIEW 8 of 33

Figure 6. Flow chart for carbon activation process using ZnCl2. Reproduced with permission from Figure 6. FlowFigure chart for 6. Flowcarbon chart activation for carbon process activation using ZnCl process2. Reproduced using ZnCl with2. permission Reproduced from with permission [15], ©2001 Elsevier Science B.V. [15], ©2001 Elsevierfrom [15 Science], ©2001 B.V. Elsevier Science B.V.

ZnCl2 is considered as a strong dehydrator. It can subtract hydrogen and oxygen ZnCl2 is consideredZnCl2 is as considered a strong asdehydrator. a strong dehydrator. It can subtract It can hydrogen subtract and hydrogen oxygen and oxygen from the rawfromfrom material thethe rawraw in inert materialmaterial atmosphere in inert inert duriatmosphere atmosphereng the activation duri duringng the theprocess. activation activation As a process. result, process. the As Asa result, a result, the carbonaceousthecarbonaceous microporous carbonaceous microporous structure microporous as shownstructure structure in Figureas shown as 7 shown can in Figuredevelop in Figure 7 can[15].7 develop canThe developconcen- [15]. The [ 15 ].concen- The tration of ZnCl2 strongly affects the pore formation and determines the specific surface tration of ZnClconcentration2 strongly affects of ZnCl the2 strongly pore formatio affectsn theand pore determines formation the andspecific determines surface the speciﬁc area (SSA). Itsurfacearea was (SSA). found area It (SSA).that was the found It porous was that found microstructures the that porous the porousmicrostructures were microstructures better developed were better were with developed better 3.0 developed with 3.0 M ZnCl2 than with 1.0 M ZnCl2, as revealed by the two SEM images in Figure 7. With even M ZnCl2 thanwith with 3.0 1.0 M M ZnCl ZnCl2 than2, as revealed with 1.0 by M ZnClthe two2, as SEM revealed images by in the Figure two 7. SEM With images even in Figure7 . higher concentrations,Withhigher even concentrations, higherfor example, concentrations, for7.0 example,M, the for specific 7.0 example, M, areathe 7.0specificof M,the theactivated area speciﬁc of the carbon area activated ofde- the carbon activated decreases due tocarboncreases the collapse decreases due to of the the due collapse pore to the walls of collapse the caused pore of wallsby the the porecaused overetching walls by the caused inoveretching the by concentrated the overetchingin the concentrated in the concentratedZnCl2. ZnCl . ZnCl2. 2

Figure 7. SEM images of activated carbon using 1.0 M and 3.0 M ZnCl2. Reproduced with permis- Figure 7. SEM images of activated carbon using 1.0 M and 3.0 M ZnCl2. Reproduced with permis- Figure 7. SEM images of activatedsion from carbon [15], using©2001 1.0 Elsevier M and Science 3.0 M ZnClB.V. 2. Reproduced with permission from [15], ©2001 Elsevier Sciencesion B.V. from [15], ©2001 Elsevier Science B.V. In addition to the above-mentioned activated carbon processing techniques, other In addition Into additionthe above-mentioned to the above-mentioned activated carbon activated processing carbon techniques, processing techniques,other other methods using different chemical activating reagents such as H3PO4 and H2SO4 have been methods usingmethods different using chemical different activating chemical reagents activating such reagents as H3PO such4 and as HH2POSO4 haveandH beenSO have been studied and used for a considerably long period of time [16]. Conceptually,3 4 2 chemicals4 with studied and studiedused for and a considerably used for a considerably long period of long time period [16]. ofConceptually, time [16]. Conceptually, chemicals with chemicals with dehydration and oxidation characteristics are suitable for making activated carbon [15]. dehydrationdehydration and oxidation and characteristics oxidation characteristics are suitable arefor suitablemaking activated for making carbon activated [15]. carbon [15]. In the following section, the activated carbon generating raw materials from biomass re- In the followingIn the section, following the activated section, thecarbon activated generating carbon raw generating materials rawfrom materials biomass re- from biomass sources will be discussed. sources willresources be discussed. will be discussed.

3. Biomass Resources3. Biomass for Resources Activated for Carbon Activated Carbon Various raw materialsVarious raw including materials nutshells, including fruit nutshells, pits, paper fruit mill pits, waste paper (lignin), mill waste wood, (lignin), wood, charcoal, browncharcoal, and bituminous brown and coals, bituminous lignite, coals, bone, lignite, and peat bone, are andthe some peat areof the the starting some of the starting C 2021, 7, 39 9 of 33

C 2021, 7, x FOR PEER REVIEW 9 of 33 3. Biomass Resources for Activated Carbon Various raw materials including nutshells, fruit pits, paper mill waste (lignin), wood, charcoal, brown and bituminous coals, lignite, bone, and peat are the some of the starting materials for the production of activated carbon. It is not the intent to list every category materials for the production of activated carbon. It is not the intent to list every category of materialof material here. here. Instead, Instead, the discussionthe discussion will will be on be certain on certain representative representative sustainable sustainable biomass bio- sources.mass sources. Nut shells, Nut treeshells, leaves, tree leaves, tree woods, tree w willowoods, catkins,willow catkins, and vegetable and vegetable wastes will wastes be discussedwill be discussed in the following in the following subsections. subsections.

3.1.3.1. WalnutWalnut ShellShell First,First, walnutwalnut shellshell forfor carboncarbon generationgeneration isis presented presented because because it it contains contains a a relatively relatively highhigh percentage percentage of of carbon. carbon. The The weightweight percent percent of of carbon carbon in in walnutwalnut shellshell isis aboutabout 47%47% [[15].15]. AsAs is is known, known, walnut walnut is is a a generally generally accepted accepted nutrient nutrient food, food, but but its its shell shell as as one one of of the the waste waste productsproducts from from the the meat meat processing, processing, as as shown shown in in FigureFigure8 [8 [17],17], hashas littlelittle commercialcommercial value.value. FromFrom Figure Figure8 ,8, it it can can be be seen seen that that the the walnut walnut fruit fruit contains contains four four main main parts: parts: the the kernel, kernel, the the skin,skin, the the shell, shell, and and the the husk. husk. The The importance importance of of walnuts walnuts is is mostly mostly related related to to the the valuable valuable kernels.kernels. TheThe walnutwalnut fruitfruit processingprocessing industryindustry generatesgenerates aaconsiderable considerablelarge large amount amount of of shellsshells as as an an agricultural agricultural by-product by-product and and are are discarded discarded or or burned burned as as fuel. fuel. WalnutWalnut shell shell has has caughtcaught muchmuch interestinterest asas a a naturally naturally inert inert plant-based plant-based biosorbent biosorbent [ 17[17].]. Recently,Recently, turning turning thethe produced produced walnut walnut shell shell biomass biomass into into another another valuable valuable product, product, activated activated carbon, carbon, has has beenbeen considered considered [ 15[15].]. The The preparation preparation of of activated activated carbon carbon from from walnut walnut shell shell through through both both physicalphysical activationactivation andand chemicalchemical activationactivation werewere describeddescribed inin [[15]15] andand reviewedreviewed inin [[17].17]. ForFor example,example, usingusing potassium potassium carbonate carbonate (K (K2CO2CO33)) asas aa chemicalchemical activator,activator, walnut walnut shells shells ◦ werewere converted converted into into activated activated carbon carbon at at 800 800 °C.C. The Thewalnut walnut shell-derived shell-derived activated activated carbon carbon asas anan effectiveeffective adsorbent adsorbent for for the the removal removal of of various various hazardous hazardous materials materials including including heavy heavy metalsmetals (HMs),(HMs), syntheticsynthetic industrialindustrial dyes,dyes, andand harmfulharmful chemicalschemicals waswas discusseddiscussed inin more more detaildetail in in [ 17[17].].

FigureFigure 8. 8.Different Different parts parts of of walnut walnut fruit fruit and and the the corresponding corresponding by-products. by-products. The The walnut walnut fruit fruit mainly consistsmainly ofconsists the husk, of the shell, husk, skin, she andll, skin, kernel. and The kernel. husk, The shell husk, and she skinll and arethe skin main are the agricultural main agricul- waste tural waste products of walnut fruit. Reprinted from [17]. products of walnut fruit. Reprinted from [17].

3.2.3.2. Citrus-LimonCitrus-Limon TreeTree Leaves Leaves Nemati,Nemati, Jafari,Jafari, and Esmaeili Esmaeili [18] [18 ]converted converted citrus-limon citrus-limon tree tree leaves leaves into into activated activated car- carbonbon with with the the purpose purpose of ofremoving removing lead lead and and arsenic arsenic ions ions from from aqueous aqueous solutions. solutions. The The tree treeleaves leaves were were collected collected and anddedusted dedusted by washin by washing.g. Then, Then, the tree the leaves tree leaves were were dried dried com- completelypletely at 100 at °C 100 for◦C 2 forh in 2 an h oven. in an The oven. dried The leaves dried were leaves placed were in placed a furnace in a at furnace 700 °C atfor 7004 h ◦withC for an 4 inert h with gas an protection. inert gas Adsorption protection. tests Adsorption showed teststhat showedthe highest that adsorption the highest ef- adsorptionficiencies were efﬁciencies 98% and were 96% 98% for and Pb (II) 96% and for As Pb (III), (II) and respectively. As (III), respectively. Additionally, Additionally, the adsor- thebent adsorbent was successfully was successfully regenerated regenerated four times four and times therefore and therefore it was able it was to ableperform to perform the ad- thesorption adsorption and anddesorption desorption processes processes well. well. Moreover, Moreover, the the results results of of adsorptionadsorption kineticskinetics showed that the pseudo second-order kinetic model was more effective for the description C 2021, 7, 39 10 of 33 C 2021, 7, x FOR PEER REVIEW 10 of 33

showed that the pseudo second-order kinetic model was more effective for the description ofof adsorptionadsorption mechanism of of both both metallic metallic ions. ions. Furthermore, Furthermore, the the equilibrium equilibrium studies studies in- indicateddicated that that Langmuir Langmuir and and Freundlich Freundlich isotherm isotherm models models were were desirable desirable for lead for leadand arse- and arsenicnic ions, ions, respectively. respectively.

3.3.3.3. GuavaGuava TreeTree WoodWood GuavaGuava treetree woodwood growngrown inin EgyptEgypt waswas processedprocessed intointo activatedactivated carboncarbon (AC)(AC) [[19].19]. TheThe removalremoval ofof brilliantbrilliant greengreen (BG)(BG) dyedye fromfrom aqueousaqueous solutionssolutions withwith differentdifferent concentrationsconcentrations usingusing thethe processedprocessed activatedactivated carboncarbon waswas performed.performed. FigureFigure9 a9a is is an an SEM SEM image image showing showing thethe morphologymorphology of of the the activated activated carbon carbon adsorbed adsorbed with with the the green green dye. dye. Figure Figure 9b9 showsb shows the thetime time dependent dependent adsorption adsorption behavior behavior of the of thecarbon carbon in solutions in solutions with with different different initial initial dye dyeconcentrations. concentrations. From From Figure Figure 9b, it9 wasb, it found was found that the that dye the adsorption dye adsorption reached reached the equilib- the equilibriumrium within within 15 min. 15 The min. effects The effects of other of otherfactors factors such suchas contact as contact time, time, pH, adsorbent pH, adsorbent dos- dosage,age, and and temperature temperature on onthe theadsorption adsorption of BG of BGonto onto the theAC ACwere were investigated. investigated. The Theiso- isothermtherm results results were were analyzed analyzed using using different different isothermal isothermal kinetic kinetic models models proposed proposed by Lang- by Langmuir,muir, Freundlich, Freundlich, Temkin, Temkin, and andDubinin-Radushkevich. Dubinin-Radushkevich. Linear Linear regression regression shows shows that that the theequilibrium equilibrium data data can can be bebest best represented represented by by the the Freundlich Freundlich isotherm. isotherm. Each Each gram ofof ACAC cancan removeremove 9090 mgmg BGBG dye.dye.

(a) (b)

FigureFigure 9.9. SEMSEM imageimage andand adsorptionadsorption behaviorbehavior ofof activatedactivated carboncarbon derivedderived fromfrom guavaguava treetree wood:wood: ((aa)) SEMSEM image,image, ((bb)) adsorption of brilliant green dye solution with different concentrations (-■- 25 ppm; -●- 50 ppm; -▲- 75 ppm). Reprinted adsorption of brilliant green dye solution with different concentrations (-- 25 ppm; -•- 50 ppm; -N- 75 ppm). Reprinted from [19], ©2020 The Authors. from [19], ©2020 The Authors.

3.4. Willow Catkin 3.4. WillowZhang Catkin et al. [20] showed willow catkin as a raw material to produce activated carbon. The Zhangactivated et al.carbon [20] showedwas used willow to make catkin a hydrophilic as a raw material composite to produce membrane activated for efficient carbon. Thesolar activated steam generation, carbon was as shown used to in make Figure a hydrophilic10. The activated composite carbon membrane membrane for was efficient placed solarin a tree-like steam generation, evaporation as shownconfiguration in Figure to 10 si.mulate The activated a natural carbon transpiration membrane process. was placed They intested a tree-like the light evaporation harvesting configuration performance toof the simulate solar steam a natural generator transpiration and found process. that a They very testedstrong the light light absorption harvesting (up performance to 96%) was of ac thehieved solar steam[20]. The generator solar energy and found was that converted a very stronginto thermal light absorption energy to (upheat to the 96%) surrounding was achieved water [20 flowing]. The solar in a energyporous was water converted channel into un- thermalder the capillary energy to action. heat the surrounding water flowing in a porous water channel under the capillary action. C 2021, 7, 39 11 of 33 C 2021, 7, x FOR PEER REVIEW 11 of 33

FigureFigure 10. 10.Photo Photo and and drawing drawing showing showing the the willow willow catkin, catkin, activated activated carbon carbon membrane, membrane, and and the the solar solar steam generator. Reprinted from [20], ©2020 The Authors. steam generator. Reprinted from [20], ©2020 The Authors.

TheThe preparation preparation of of activated activated carbon carbon membrane membrane includes includes several several key key steps, steps, as schemat- as sche- icallymatically shown shown in Figure in Figure 11 and 11 an describedd described in morein more detail detail in in [20 [20].]. First, First, the the catkins catkins were were collected.collected. TheThe seedsseeds andand dustdust in in the the catkins catkins were were removed. removed. Then Then the the raw raw materials materials were were air-driedair-dried at at 80 80◦ °CC for for 6 6 h. h. TheThe cleanedcleaned catkinscatkins were were immersed immersed into into 2 2 M M KOH KOH solution solution and and thenthen dried dried at at 80 80◦ °CCfor for12 12 h.h. AfterAfter that,that, carbonizationcarbonizationwas was performed performed by by heating heating the the catkins catkins (held(held in in a a nickel nickel crucible) crucible) to 750to 750◦C °C at aat rate a rate of 5 of◦C/min 5 °C/min under under nitrogen nitrogen gas protection. gas protection. The timeThe fortime carbonization for carbonization process process was 1 h.was The 1 carbonh. The productcarbon product was washed was withwashed 1 M with HCl and1 M deionizedHCl and deionized water until water the pH until value the pH of the value ﬁltrate of the reached filtrate about reached 7. The about generated 7. The generated product C 2021, 7, x FOR PEER REVIEW product was porous and polyhedral in shape. Therefore, the samples were named12 as of po- 33 was porous and polyhedral in shape. Therefore, the samples were named as porous carbon polyhedralsrous carbon (PCPs). polyhedrals (PCPs). The PCPs were further processed using a mixed acid consisting of H2SO4 and HNO3 with a molar ratio of 3:1 for 1 h. Deionized (DI) water was used to wash the acid treated PCPs until the pH value of the filtrate reached about 7. Surface modification on the acidi- fied PCPs, as shown in Figure 11, was conducted by adding them into dopamine tris- hydrochloride buffer solution (pH = 8.5, 50 mM) and under vigorous stirring for 8 h at 25 °C. More details about this surface modification process are described in references [21] and [22]. The modified samples were washed by deionized water several times, and they were called modified PCPs [20]. The as-prepared product was filtered in a cellulose filter with a thickness of 200 µm under a pressure of 0.1 MPa and dried at 60 °C for 4 h. A hydrophilic light-absorber layer remarkably increased the attachment sites of water molecules, thereby maximizing the use of thermal energy. At the same time, hierarchically porous structure and large specific surface area (as high as 1380 m2/g) supplied more available channels for rapid water vapor diffusion. The as-prepared composite membrane with a low-cost advantage realized a high evaporation rate (1.65 kg/(m2·h) only under 1 sun illumination (1 kW/m2), which was improved by roughly 27% in comparison with the unmodified hydrophobic composite membrane. The tree-like evaporation configuration with excellent heat localization resulted in the evaporator achieving a high solar-to-vapor conversion efficiency of 90.5%. Moreover, the composite membrane could remove 99.9% sodium ions from seawater and

99.5% heavy metal ions from simulated wastewater, and the long-term stable evaporation Figureperformance 11. Photo proved and schematic its potential showingshowing in thethe actual preparationpreparation solar desalination. ofof willow willow catkin-derived catkin-derived This work active active not carbononly carbon fabri- and andthecated compositethe an composite efficient membrane. membrane. evaporator Reprinted Reprinte but fromalsod from [provi20], [20], ©2020ded ©2020 a The strategy Authors.The Authors. for reusing willow catkin natural wastes for water purification [20]. The PCPsmorphology were further and structure processed of usingthe samples a mixed were acid observed consisting under of H2 aSO scanning4 and HNO elec-3 withtron microscope. a molar ratio The of 3:1porous for 1 structure h. Deionized and inner (DI) water morphologies was used of to the wash catkin-derived the acid treated car- bonPCPs and until composite the pH membrane value of the are filtrate shown reached in Figure about 12. In 7. Figure Surface 12a, modification the porous oncarbon the polyhedralsacidified PCPs, (PCPs) as shownwere revealed. in Figure Such 11, wasa microstructural conducted by feature adding provides them into the dopaminetranspira- tiontris-hydrochloride channels for water buffer vapor. solution The (pH photo = 8.5, as 50an mM)inset andin Figure under 12b vigorous shows stirring that the for compo- 8 h at site25 ◦ C.membrane More details had abouta black this color surface due to modification biochar formation process during are described carbonization. in references The SEM [21] images in Figure 12b,c show the surface and enlarged morphological features of the surface modified PCPs, respectively, illustrating the interconnected microporous structure of the biochar layer. In comparison with the PCPs, many polydopamine (PDA) particles were deposited on the surface of the modified PCPs, and their porous structures were maintained after the PDA modification. The TEM image in Figure 12d shows that even a thin slice of the activated carbon consisted of a large number of mesopores [20].

C 2021, 7, x FOR PEER REVIEW 12 of 33

C 2021, 7, 39 12 of 33

and [22]. The modified samples were washed by deionized water several times, and they were called modified PCPs [20]. The as-prepared product was filtered in a cellulose filter with a thickness of 200 µm under a pressure of 0.1 MPa and dried at 60 ◦C for 4 h. A hydrophilic light-absorber layer remarkably increased the attachment sites of water molecules, thereby maximizing the use of thermal energy. At the same time, hierarchically porous structure and large specific surface area (as high as 1380 m2/g) supplied more available channels for rapid water vapor diffusion. The as-prepared composite membrane with a low-cost advantage realized a high evaporation rate (1.65 kg/(m2·h) only under 1 sun illumination (1 kW/m2), which was improved by roughly 27% in comparison with the unmodified hydrophobic composite membrane. The tree-like evaporation configuration with excellent heat localization resulted in the evaporator achieving a high solar-to-vapor conversion efficiency of 90.5%. Moreover, the composite membrane could remove 99.9% sodium ions from seawater and 99.5% heavy metal ions from simulated wastewater, and the long-term stable evaporation performance provedFigure 11. its Photo potential and inschematic actual solarshowing desalination. the preparation This of work willow not catkin-derived only fabricated active an carbon efficient evaporatorand the composite but also membrane. provided Reprinte a strategyd from for reusing[20], ©2020 willow The Authors. catkin natural wastes for water purification [20]. TheThe morphology morphology and and structure structure of of the the samples samples were were observed observed under under a scanning a scanning electron elec- microscope.tron microscope. The porous The porous structure structure and inner and morphologies inner morphologies of the catkin-derived of the catkin-derived carbon and car- compositebon and composite membrane membrane are shown are in Figure shown 12 in. In Figure Figure 12. 12 a,In theFigure porous 12a, carbon the porous polyhedrals carbon (PCPs)polyhedrals were revealed.(PCPs) were Such revealed. a microstructural Such a microstructural feature provides feature the transpirationprovides the transpira- channels fortion water channels vapor. for The water photo vapor. as an The inset photo in Figure as an inset12b shows in Figure that 12b the shows composite that membranethe compo- hadsite amembrane black color had due a black to biochar color due formation to biochar during formation carbonization. during carbonization. The SEM images The SEM in Figureimages 12 inb,c Figure show 12b,c the surface show andthe surface enlarged and morphological enlarged morphological features of the features surface of modified the sur- PCPs,face modified respectively, PCPs, illustrating respectively, the illustrating interconnected the interconnected microporous microporous structure of the structure biochar of layer.the biochar In comparison layer. In with comparison the PCPs, with many the polydopamine PCPs, many (PDA)polydopamine particles were(PDA) deposited particles onwere the deposited surface of on the the modified surface PCPs, of the and modifi theired porous PCPs, structuresand their porous were maintained structures afterwere themaintained PDA modification. after the PDA The modification. TEM image inThe Figure TEM 12 imaged shows in Figure that even 12d ashows thin slicethat ofeven the a activatedthin slice carbonof the activated consisted carbon of a large consis numberted of of a mesoporeslarge number [20 of]. mesopores [20].

Figure 12. Microstructure of the porous carbon polyhedrals (PCPs): (a) SEM image of the porous carbon polyhedrals (PCPs), (b,c) SEM images of the modiﬁed PCPs, and (d) TEM image of the modiﬁed PCPs. The inset in (b) is the photo of the composite membrane with a diameter of 2 cm. Reprinted from [20], ©2020 The Authors. C 2021, 7, x FOR PEER REVIEW 13 of 33

Figure 12. Microstructure of the porous carbon polyhedrals (PCPs): (a) SEM image of the porous carbon polyhedrals (PCPs), (b,c) SEM images of the modified PCPs, and (d) TEM image of the modified PCPs. The inset in (b) is the photo of the composite membrane with a diameter of 2 cm. C 2021, 7, 39 13 of 33 Reprinted from [20], ©2020 The Authors.

3.5. Onion Peel 3.5. OnionOther Peel biomass resources for activated carbon production include but are not limited to vegetableOther biomass wastes. resources For example, for activated onion peel carbon has been production considered include for butthis are purpose not limited in the toresearch vegetable reported wastes. by For Musyoka, example, Mutuma, onion peel and has Manyala been considered [23]. Briefly, for red-onion this purpose skin in waste the research(peel) was reported collected, by Musyoka,washed, an Mutuma,d air-dried and at Manyala 60 °C for [ 2312]. h. Briefly, The cleaned red-onion onion skin peel waste was (peel)put into was an collected, autoclave washed, filled with and deioniz air-drieded at water 60 ◦C and for converted 12 h. The cleanedinto hydrochar onion peel through was putthe intohydrothermal an autoclave carbonization filled with deionizedtreatment waterat 200 and°C for converted 4 h. The intoresulting hydrochar hydrochar through was thegathered hydrothermal by filtration, carbonization water-cleaned, treatment and air-dried at 200 ◦ Cat for90 °C 4 h. for The 12 h. resulting Then, high hydrochar specific wassurface gathered area activated by filtration, carbon water-cleaned, was prepared and from air-dried the hydrochar at 90 ◦C in for two 12 steps. h. Then, First, high pre- specificcarbonization surface of area the activated onion hydrochar carbon was was prepared carried out from at 500 the hydrochar°C for 1 h under in two argon steps. atmos- First, pre-carbonizationphere with a ramp of rate the of onion 5 °C/min. hydrochar The pr wase-carbonized carried out hydrochar at 500 ◦C was for 1then h under mixed argon with ◦ atmosphereKOH. The mass with ratio a ramp of ratehydrochar of 5 C/min. to KOH The was pre-carbonized 1 to 4. A few hydrochardrops of water was thenwere mixedadded withinto KOH.the mixture The mass to form ratio a of slurry. hydrochar After tobein KOHg dried was 1at to 70 4. °C A for few 12 drops h, the of mixture water were was ◦ addedheated into up theto a mixture pre-activation to form temperature a slurry. After of being 400 °C dried for at30 70 minC and for 12 then h, the to an mixture activation was ◦ heatedtemperature up to aof pre-activation 600 °C for 1 h. temperatureThe product ofwas 400 immersedC for 30 in min 10 wt% and thenHCl for to an24 activationh followed ◦ temperatureby washing ofwith 600 deionizedC for 1 h. water The product to a neutral was immersedpH = 7. This in 10 onion wt% peel-derived HCl for 24 h followedactivated bycarbon washing (OP withAC) deionizedwas named water as OP to AC-600 a neutral °C. pH Activation = 7. This onionat higher peel-derived temperatures activated of 700 ◦ ◦ carbon°C and (OP 800 AC)°C was was also named performed. as OP AC-600 The generaC. Activationted products at higher were named temperatures as OP AC-700 of 700 C°C ◦ ◦ andand 800OP AC-800C was also °C, performed.respectively The [23]. generated As shown products in Figure were 13, named the resulting as OP AC-700ACs hadC a ◦ andunique OP thin AC-800 graphitic-likeC, respectively nanosheet [23]. morpho As shownlogy. in The Figure small 13 ,amount the resulting of N remaining ACs had ain uniquethe activated thin graphitic-like carbon was nanosheet believed to morphology. be from the The inherent small amountnitrogen of groups N remaining in the inonion the activatedpeel. carbon was believed to be from the inherent nitrogen groups in the onion peel.

FigureFigure 13. 13.Images Images of of onion onion peel-derived peel-derived activated activated carbon: carbon: (a ()a SEM) SEM image image of of OP OP AC-600 AC-600◦C, °C, (b ()b SEM) imageSEM image of OP AC-700of OP AC-700◦C, (c) °C, SEM (c) image SEM image of OP AC-800of OP AC-800◦C, (d) °C, TEM (d) image TEM ofimage OP AC-600of OP AC-600◦C, (e) TEM°C, (e) TEM image of OP AC-700 °C, and (f) TEM image of OP AC-800 °C. The insets in (a–c) are the image of OP AC-700 ◦C, and (f) TEM image of OP AC-800 ◦C. The insets in (a–c) are the SEM images SEM images taken at higher magnifications. Reprinted from [23], ©2020 The Authors. taken at higher magniﬁcations. Reprinted from [23], ©2020 The Authors.

Figure 14 shows the N2 sorption isotherms of the three carbon materials activated at Figure 14 shows the N2 sorption isotherms of the three carbon materials activated atthe the three three temperature temperature conditions. conditions. The The sample sample generated generated at 600 at 600 °C ◦exhibitedC exhibited a type a type I iso- I isotherm,therm, whereas whereas the the other other two two products products obtained obtained from from the activation atat 700700◦ °CC andand 800 800◦ C°C displayed both type I and IV isotherms. A type I isotherm is indicative of a microporous material having pores that are less than 2 nm, while a type IV isotherm depicts the presence of both micropores and mesopores [23,24]. As shown in Figure 14a, the sharp gas uptake in all the samples at low partial pressure region was a conﬁrmation that most of the exposed surface resides mostly inside the micropores. This observation was further conﬁrmed by the C 2021, 7, x FOR PEER REVIEW 14 of 33

displayed both type I and IV isotherms. A type I isotherm is indicative of a microporous C 2021, 7, 39 material having pores that are less than 2 nm, while a type IV isotherm depicts the pres- 14 of 33 ence of both micropores and mesopores [23,24]. As shown in Figure 14a, the sharp gas uptake in all the samples at low partial pressure region was a confirmation that most of the exposed surface resides mostly inside the micropores. This observation was further confirmed bypore the sizepore distribution size distribution curves curves presented presented in Figure in Figure 14b (as 14b an (as inset). an inset). In this In case, this the samples ◦ ◦ case, the samplesproduced produced at 600 atC 600 and °C 700 andC 700 portrayed °C portrayed a bimodal a bimodal pore sizepore distribution size distribu- with the main tion with thepores main having pores having dimensions dimensions less than less 2nm than (i.e., 2 nm 1.05 (i.e., and 1.05 1.08 and nm, 1.08 respectively). nm, respec- However, the ◦ tively). However,OP AC the 800 OPC AC sample 800 °C exhibited sample a exhibited trimodal porea trimodal size distribution pore size havingdistribution pores positioned having poresat positioned 1.08, 1.48, andat 1.08, 2.31 1.48, nm. and The 2.31 presence nm. The of pores presence above of 2 pores nm is above an indication 2 nm is ofan the presence indication ofof the small presence pores, of which small is pores, further wh corroboratedich is further by corroborated the occurrence by ofthe the occurrence slight hysteresis loop of the slight inhysteresis the isotherm, loop in as the shown isotherm, in Figure as shown 14a [23 in]. Figure 14a [23].

Figure 14. Nitrogen isothermal sorption results of the onion peel-derived activated carbons: (a) Figure 14. Nitrogen isothermal sorption results of the onion peel-derived activated carbons: (a) quantity of N2 sorption, and (b) pore sizequantity distribution. of N2 sorption, Reprinted and from (b) pore [23], ©2020size distribution. The Authors. Reprinted from [23], ©2020 The Authors.

The specific Thesurface specific area, surface pore volume, area, pore and volume, hydrogen and hydrogenuptake of uptake the onion of the peel-de- onion peel-derived rived activatedactivated carbons carbons produced produced at 600, at700, 600, and 700, 800 and °C 800are ◦listedC are in listed Table in 1 Table [23]. 1The[ 23 one]. The one with with the bestthe performance best performance is the OP is the AC-800 OP AC-800 °C sample.◦C sample. The specific The specific area achieved area achieved 3150 3150 m2/g. m2/g. The poreThe volume pore volume is as high is as as high cm as3/g. cm It 3can/g. take It can hydrogen take hydrogen up to 3.67 up to wt% 3.67 at wt% 77 K at 77 K and 1 and 1 bar. Thebar. specific The specific capacitance capacitance of this of sa thismple sample reached reached 169 F/g. 169 Its F/g. specific Its specific current current was was found found to be 0.5to be A/g. 0.5 A A/g. symmetric A symmetric supercapacitor supercapacitor based basedon the on OPAC-800 the OPAC-800 °C sample◦C sample was was made. made. It showedIt showed a capacitance a capacitance of 158 of F/g 158 at F/g 0.5 at A/g. 0.5 A/g.Such Suchresults results suggested suggested that the that high the high surface surface area areagraphene-like graphene-like carbon carbon nanostructures nanostructures are potential are potential candidate candidate materials materials for hy- for hydrogen drogen storagestorage and andsupercapacitor supercapacitor applications applications [23]. [23].

Table 1. TexturalTable properties 1. Textural of propertiesthe onion peel-derived of the onion activated peel-derived carb activatedons produced carbons at 600, produced 700, and at 600, 700, and 800 °C. Adopted800 with◦C. Adopted permission with from permission [23], ©2020 from The [23 Authors.], ©2020 The Authors.

BET Surface Micropore Total Pore vol. Micropore H2 Uptake Sample NameSample BET Surface Micropore Total Pore Micropore H2 Uptake NameArea (mArea2/g) (mArea2/g) (mArea2/g) (m2/g)(cm3/g)vol. (cmvol.3/g) (cm3vol./g) (cm(wt%)3/g) (wt%) OP AC-600 °C OP 2241 2220 0.94 0.89 3.08 2241 2220 0.94 0.89 3.08 OP AC-700 °CAC-600 ◦C 2706 2651 1.28 1.10 3.29 OP AC-800 °C OP 3150 3080 1.64 1.39 3.67 2706 2651 1.28 1.10 3.29 AC-700 ◦C OP 3150 3080 1.64 1.39 3.67 AC-800 ◦C C 2021, 7, x FOR PEER REVIEW 15 of 33 , , 39 15 of 33

4. Functionalization of Activated Carbon 4. Functionalization of Activated Carbon In this section, the functionalization of activated carbon by adding some elements or In this section, the functionalization of activated carbon by adding some elements or compoundscompounds will will be be discussed. discussed. These These added added elem elementsents or or molecules molecules have have special special properties properties toto enhance enhance the the functions functions of of porous porous carbons. carbons. As As an an example, example, incorporating incorporating iron iron into into acti- acti- vatedvated carbon waswas studiedstudied to to promote promote the the adsorption adsorption of Asof As (V) (V) from from aqueous aqueous solutions solutions [25]. [25].As is As well is well known, known, arsenic arsenic compounds compounds in aqueous in aqueous environment environment arehighly are highly poisonous poisonous and andthey they are are harmful harmful to humanto human health. health. More More details details on on the the iron iron decorateddecorated carboncarbon will be discusseddiscussed below. below. In In addition, addition, titanium titanium ox oxideide and manganese oxide modified modiﬁed activated carboncarbon will will also also be be discussed discussed because because they they take take significant signiﬁcant roles roles in photocatalysis in photocatalysis and andde- salination.desalination.

4.1.4.1. Iron Iron Decorated Decorated Activated Activated Carbon Carbon SawoodSawood and and Gupta Gupta [25] [25] made made two two functional functional activated activated carbons carbons for for removing removing As As (V) (V) fromfrom water. water. One One was was an an iron-impregnated iron-impregnated acti activatedvated carbon synthesized from the powder ofof Azadirachta indicaindicatree tree bark bark named named as as Fe-AIB, Fe-AIB, and and the the other other was was iron iron containing containing activated acti- vatedcarbon carbon derived derived from Azadirachtafrom Azadirachta indica indicatree leaf tree named leaf named as Fe-AIL. as Fe-AIL. TheThe activated carboncarbon derived derived from from the the bark bark and and leaf leaf of Azadirachta of Azadirachta indica indicatree was tree made was madefollowing following the physical the physical activation activation method. method. The pre-treated The pre-treated bark and bark leaf wereand groundedleaf were ◦ groundedinto powders. into powders. The powders The powders were carbonized were carbonized at 750 C at under 750 °C nitrogen under nitrogen protection. protec- The ◦ tion.generated The generated carbon was carbon immersed was immersed in 0.5 M FeCl in 0.52 solution M FeCl and2 solution stirred and at 70 stirredC for at 24 70 h. °C Then for 24NaOH h. Then was NaOH used towas adjust used the to adjust pH of the pH suspension of the suspension to 8. This to can 8. increaseThis can theincrease negative the negativecharge sites charge in the sites activated in the carbon.activated The carbon. filtered The product filtered was product washed was several washed times several to get timesrid of to the get colloidal rid of the precipitates. colloidal precipitates. Sufficient iron Sufficient salt attached iron salt to attached the surface to the of surface Fe-AIB orof Fe-AIBFe-AIL or [26 Fe-AIL]. It was [26]. also It found was also that found the iron that impregnation the iron impregnation led to the rearrangementled to the rearrange- of the mentpore structureof the pore of structure activated of carbon. activated Consequently, carbon. Consequently, better affinity better towards affinity adsorbate towards wasad- sorbateachieved was as achieved reported inas [reported27]. in [27]. InIn terms terms of of the surface morphology of the two iron-impregnated activated carbon materials,materials, Fe-AIB Fe-AIB and and Fe-AIL Fe-AIL were were observed observed by by SEM. SEM. The The images images revealed revealed porous porous micro- mi- structures.crostructures. As Asshown shown in inFigure Figure 15, 15 micropores, micropores and and grooves grooves could could be be clearly clearly seen. seen. Such Such featuresfeatures contribute contribute to to the the large large specific specific surf surfaceace area area of of the the activated activated carbon. carbon. The The impreg- impreg- natednated iron iron particles particles are are observed observed as as marked marked by by the the yellow yellow arrows. arrows. The The micrographs micrographs of Fe- of AILFe-AIL in Figure in Figure 16 16illustrate illustrate fewer fewer pores pores and and less less uniformity uniformity as as compared compared with with the the images images ofof Fe-AIB. Fe-AIB. In In some some of the pores, the aggregates of Fe can be seen as well.

(a) (b)

FigureFigure 15. 15. SEMSEM images images of of Fe-AIB Fe-AIB showing ( a)) iron particles in pores, andand ((b)) uniformuniform microgroovesmicro- grooveson the pore on the wall. pore Reprinted wall. Reprinte from [25d from], ©2020 [25], The ©2020 Authors. The Authors.

C 2021, 7, 39 16 of 33 CC 2021 2021, 7, ,7 x, xFOR FOR PEER PEER REVIEW REVIEW 1616 of of 33 33

(a(a) ) (b(b) )

FigureFigureFigure 16. 16.16. SEM SEMSEM images imagesimages of ofof Fe-AIL Fe-AILFe-AIL showing showingshowing ( a ((aa) ))Fe FeFe particles particlesparticles in inin pores, pores,pores, and andand ( b ((bb) ))irregular irregularirregular micro- microgrooves.microgrooves. Reprinted from [25], ©2020 The Authors. grooves.Reprinted Reprinted from [25 ],from ©2020 [25], The ©2020 Authors. The Authors.

TheTheThe experiments experimentsexperiments on onon removal removalremoval of ofof As AsAs (V) (V)(V) by byby Fe-AIB Fe-AIBFe-AIB and andand Fe-AIL Fe-AILFe-AIL from fromfrom aqueous aqueousaqueous solutions solutionssolutions werewerewere carried carriedcarried out outout under underunder different differentdifferent conditions. conditions.conditions. As AsAs shown shownshown in inin Figure FigureFigure 17,17 17,, the thethe Fe-AIB Fe-AIBFe-AIB adsorbent adsorbentadsorbent showedshowedshowed up up toto to 96%96% 96% As As As (V) (V) (V) removal. removal. removal. The The The Fe-AIL Fe-AIL Fe-AIL adsorbent adsorbent adsorbent demonstrated demonstrated demonstrated up toup up 90% to to removal.90% 90% re- re- moval.Themoval. kinetic The The kinetic datakinetic fit data data best fit fit in best thebest in pseudo-second-order in the the pseu pseudo-second-orderdo-second-order model. model. model. Intraparticle Intraparticle Intraparticle diffusion, diffusion, diffusion, pore porediffusionpore diffusion diffusion and filmand and diffusionfilm film diffusion diffusion also contributedalso also contri contributedbuted to the to to overall the the overall overall adsorption adsorption adsorption rate. rate. Mechanisticrate. Mecha- Mecha- nisticframeworksnistic frameworks frameworks related related related to the to to the adsorptionthe adsorption adsorption process process process were were were analyzed analyzed analyzed using using using variousvarious various isotherm isotherm models.models.models. Both Both Both Langmuir Langmuir Langmuir and and and Freundlich Freundlich Freundlich models models models can can can explain explain explain the the theAs As (V) As (V) (V)adsorption adsorption adsorption by by Fe- Fe- by AIBFe-AIBAIB and and andFe-AIL. Fe-AIL. Fe-AIL. Thermodynamic Thermodynamic Thermodynamic analysis analysis analysis reve reveals revealsals the the spontaneous thespontaneous spontaneous adsorption adsorption adsorption on on Fe-AIB. Fe-AIB. on Fe- TheAIB.The endothermic Theendothermic endothermic nature nature nature of of the the of theadsorption adsorption adsorption pr process processocess for for forboth both both the the the adsorbents adsorbents adsorbents was was was found. found. ConsistentlyConsistentlyConsistently greater greatergreater than thanthan 90% 90%90% As AsAs (V) (V)(V) removal removalremoval up upup to toto few fewfew cycles cyclescycles for forfor both bothboth the thethe adsorbents adsorbents waswaswas observed observedobserved in inin regeneration regenerationregeneration studies. studies.studies. Both BothBoth adsorbents adsorbentsadsorbents showed showedshowed good goodgood reusability. reusability.reusability. It ItIt was waswas concludedconcludedconcluded that thatthat the thethe two twotwo functionalized functionalizedfunctionalized carbon carboncarbon have havehave the thethe potential potentialpotential to toto be bebe used usedused as asas efficient efficientefficient adsorbentsadsorbentsadsorbents for forfor practical practicalpractical As(V) As(V)As(V) removal removalremoval application. application.application.

Figure 17. FigureFigure 17. 17. Effect EffectEffect of of ofadsorbent adsorbent adsorbent dose dose dose on on adsorption on adsorption adsorption of of As As of (V) As(V) by (V)by Fe-AIB Fe-AIB by Fe-AIB and and Fe-AIL. Fe-AIL. and Fe-AIL. Reprinted Reprinted Reprinted from from [25],from[25], ©2020 ©2020 [25], ©2020 The The Authors. Authors. The Authors.

4.2. TiO2 Decorated Activated Carbon 4.2.4.2. TiO TiO2 2Decorated Decorated Activated Activated Carbon Carbon Titanium dioxide decorated activated carbons have been studied for their enhanced Titanium dioxide decorated activated carbons have been studied for their enhanced photocatalyticTitanium dioxide performance decorated [28, 29activated]. Such carbon functionalizeds have been carbon studied materials for their have enhanced found photocatalytic performance [28,29]. Such functionalized carbon materials have found ap- photocatalyticapplications in performance degradation [28,29]. of certain Such pharmaceutical functionalized products carbon inmaterials the environment, have found for ap- ex- plications in degradation of certain pharmaceutical products in the environment, for ex- plicationsample, the in safe degradation removal of of diclofenac certain pharmaceutical (DCF) and its by-products products in from the environment, water [30]. Activated for example, the safe removal of diclofenac (DCF) and its by-products from water [30]. Acti- ample,carbon the was safe obtained removal from of Arganiadiclofenac spinosa (DCFtree) and nutshells. its by-products Functionalization from water was [30]. achieved Acti- vated carbon was obtained from Argania spinosa tree nutshells. Functionalization was vatedby incorporating carbon was Degussaobtained P25 from titanium Argania dioxide spinosa into tree the nutshells. porous carbon.Functionalization Three different was achieved by incorporating Degussa P25 titanium dioxide into the porous carbon. Three achievedtechniques by were incorporating described byDegussa El-Sheikh P25 ettitani al. [31um] fordioxide deposition into the of titaniumporous carbon. dioxide Three on ac- different techniques were described by El-Sheikh et al. [31] for deposition of titanium di- differenttivated carbon. techniques These were methods describ areed chemical by El-Sheikh vapor et deposition al. [31] for (CVD), deposition direct of air-hydrolysis titanium dioxide on activated carbon. These methods are chemical vapor deposition (CVD), direct oxide(DAH), on and activated high-temperature carbon. These impregnation methods are (HTI) chemical techniques. vapor Indeposition [29], immobilizing (CVD), direct TiO air-hydrolysis (DAH), and high-temperature impregnation (HTI) techniques. In [29], im-2 air-hydrolysisin the pores of (DAH), activated and carbon high-temperature was performed impregnation using the high (HTI) temperature techniques. impregnation In [29], immobilizing TiO2 in the pores of activated carbon was performed using the high tempera- mobilizingmethod. A TiO slurry2 inwas the madepores byof activated mixing TiO carbon2 nanoparticles, was performed activated using carbon, the high and tempera- distilled ture impregnation method. A◦ slurry was made by mixing TiO2 nanoparticles, activated turewater impregnation at a temperature method. of 70 A C.slurry The carbonwas made to TiO by 2mixingmass ratio TiO2 was nanoparticles, 1:2. The mixture activated was C 2021, 7, 39 17 of 33

continuously stirred for 2 h until the mixture changed color into gray. The mixture’s color change indicated that the interaction between the activated carbon and the TiO2 certainly occurred. The mixture was settled for 15 min, and then the supernatant was decanted and the precipitate dried in the oven at a temperature of 200 ◦C for 12 h [29]. The scanning electron microscopic (SEM) images of activated carbon and the surface modiﬁed composite of TiO2/AC are shown in Figure 18a,b, respectively. From Figure 18a, it can be seen that the activated carbon particulates are in irregular shapes with sharp edges. The activated carbon’s rough surface and heterogeneous pores provide sites for trapping the TiO2 nanoparticles. In the SEM image of TiO2/AC shown in Figure 18b, the TiO2 particulates are found uniformly dispersed on the surface of the activated carbon. Not only the surface of activated carbon can be covered, but also the mesopores and macrospores of activated carbon can be covered by TiO2, as reported earlier in [32]. The efﬁciency of photocatalysis is dependent highly on the light absorption capability. Since the deposition of TiO2 on the external surface of activated carbon can provide more sites to trap light energy, the photocatalytic degradation property should be enhanced due to the existence of the titanium dioxide nanoparticles. The higher the content of TiO2 on the external surface C 2021, 7, x FOR PEER REVIEW of activated carbon, the more active the photocatalyst in degradation of pollutants18 such of 33 as trichloroethylene [33].

(a) (b)

(c)

Figure 18. Morphology of TiO2 and TiO2/AC, and the pharmaceutical degradation property of TiO2/AC: (a) SEM image of Figure 18. Morphology of TiO2 and TiO2/AC, and the pharmaceutical degradation property of TiO2/AC: (a) SEM image TiO2, (b) SEM image of TiO2/AC, and (c) time-dependent concentration of pharmaceutical products in the photocatalytic of TiO2,(b) SEM image of TiO2/AC, and (c) time-dependent concentration of pharmaceutical products in the photocatalytic degradation process by the TiO2/AC catalyst. The initial concentration of all pharmaceuticals (co) was 50 mg/L. The degradation process by the TiO2/AC catalyst. The initial concentration of all pharmaceuticals (co) was 50 mg/L. The TiO2/AC TiO2/AC dosage was 1.2 g/L. Reprinted with permission from [29], ©2016 Elsevier Ltd. dosage was 1.2 g/L. Reprinted with permission from [29], ©2016 Elsevier Ltd.

4.3. MnO2 Decorated Activated Carbon Removing ions from saline water using 3D hierarchical carbon architectures via capacitive deionization has caught increasing attention [39]. Specifically, manganese oxide has been used as an active functional component to modify activated carbons for making the capacitive deionization anode [40]. MnO2 showed the pseudocapacitive behavior that involves a reversible redox-mediated intercalation process [41]. As an anode intercalating material in an asymmetric or hybrid capacitive deionization cell, MnO2 in nanosheet form led to the selective removal of H+ and Na+ following the reactions shown in Equations (7) and (8) [42]:

MnO2 + H+ + e− ↔ MnOOH (7)

MnO2 + Na+ + e− ↔ MnOONa (8)

Therefore, the decoration of the activated carbon surface with MnO2 nanostructures could significantly improve the ion-storage behavior of the cell by catalyzing the selective and reversible H+ and Na+ insertion [42]. C 2021, 7, 39 18 of 33

The performance on degradation of pharmaceuticals by TiO2/AC is shown in Figure 18c [29]. The elimination of pharmaceuticals in the dark process by TiO2/AC was faster than pure TiO2. The removal efficiencies of 17% for amoxicillin, 9% for ampicillin, 10% for diclofenac, and 11% for paracetamol were achieved for the TiO2/AC. This is because the adsorption capacity of TiO2/AC is substantially higher than that of the pure TiO2 [34]. During the first 90 min of irradiation, high degradation rates of pharmaceuticals were observed [29]. This phenomenon is due to the large free surface of activated carbon at the early stage, which led to higher adsorption capability of the catalyst. In addition, in the early stage of the photocatalytic degradation process, there are plenty of hydroxyl radicals near the surface of the catalyst. Nevertheless, the produced hydroxyl radicals are completely consumed at the late stages of the reaction, which reduced the photocatalytic degradation rates, as shown in Figure 18c [29]. The complete degradation of amoxicillin by TiO2/AC was achieved after 120 min of irradiation. The TiO2/AC is better than the pristine TiO2, as only 89% of amoxicillin was removed after a longer time (150 min) of illumination. This indicates that the TiO2/AC has not only a higher tendency to degrade the amoxicillin but also generates faster photocatalytic reaction. Similar trends were found for the removal of ampicillin, diclofenac, and paracetamol [29]. It was observed that the deposition of titanium oxide on carbon enhanced the reaction between the produced hydroxyl radicals and the amoxicillin molecules at the surface of the TiO2/AC. The reason is the increase in the attraction between the pharmaceutical molecule and the catalyst, leading to the formation of more active sites for photocatalytic reaction. Therefore, the change in degradation efficiency of pharmaceuticals comes from the difference in the number of active sites and the affinity to the binding sites on the catalyst. The enhancement of photocatalytic degradation of pharmaceuticals by using TiO2/AC agrees with the findings of other investigators who studied the behavior of the immobilized TiO2 on activated carbon for removing various organic compounds such as phenol [35], 2-propanol VOC pollutant [36], 4-chlorophenol [37], and methyl orange dye [38].

4.3. MnO2 Decorated Activated Carbon Removing ions from saline water using 3D hierarchical carbon architectures via capacitive deionization has caught increasing attention [39]. Speciﬁcally, manganese oxide has been used as an active functional component to modify activated carbons for making the capacitive deionization anode [40]. MnO2 showed the pseudocapacitive behavior that involves a reversible redox-mediated intercalation process [41]. As an anode intercalating material in an asymmetric or hybrid capacitive deionization cell, MnO2 in nanosheet form led to the selective removal of H+ and Na+ following the reactions shown in Equations (7) and (8) [42]: + − MnO2 + H + e ↔ MnOOH (7) + − MnO2 + Na + e ↔ MnOONa (8)

Therefore, the decoration of the activated carbon surface with MnO2 nanostructures could signiﬁcantly improve the ion-storage behavior of the cell by catalyzing the selective and reversible H+ and Na+ insertion [42]. Govindan et al. [43] used a biomass source, the Phoenix dactylifera (palm tree) leaf, for making activated carbon. The activated carbon via classical thermo-chemical conversion and activation was functionalized by incorporating MnO2. The following part will brieﬂy introduce the processes and the functionalization procedures of MnO2-loaded activated carbon, as shown in Figure 19[ 43]. Porous activated carbon prepared from the leaf base wastes of the date palm tree was functionalized by α-MnO2 to form a composite. The composite was named as MnO2/f-AC. The hydrothermal synthesis method [44] was used to deposit the α-MnO2 nanoparticles on the activated carbon. The conditions include temperature at 180 ◦C and time period for 12 h [43]. C 2021, 7, x FOR PEER REVIEW 19 of 33

Govindan et al. [43] used a biomass source, the Phoenix dactylifera (palm tree) leaf, for making activated carbon. The activated carbon via classical thermo-chemical conversion and activation was functionalized by incorporating MnO2. The following part will briefly introduce the processes and the functionalization procedures of MnO2-loaded activated carbon, as shown in Figure 19 [43]. Porous activated carbon prepared from the leaf base wastes of the date palm tree was functionalized by α-MnO2 to form a composite. The com- C 2021, 7, 39 posite was named as MnO2/f-AC. The hydrothermal synthesis method [44] was used19 of 33to deposit the α-MnO2 nanoparticles on the activated carbon. The conditions include temperature at 180 °C and time period for 12 h [43].

2 FigureFigure 19. 19.Procedures Procedures for for immobilizing immobilizing MnO MnO2 nanostructure nanostructure on on sustainable sustainable source-derived source-derived activated acti- carbon.vated carbon. Reprinted Reprinted from [43 from], ©2020 [43], The©2020 Authors. The Authors.

TheThe microstructuresmicrostructures of the the activated activated carbons carbons with with and and without without the the α-MnOα-MnO2 function-2 func- tionalizationalization were were examined examined using using field field emission emission scanning scanning electron electron microscopy microscopy (FESEM). (FESEM). The TheX-ray X-ray diffraction diffraction energy energy dispersive dispersive spectrum spectrum (EDS) (EDS)for the forfunctionalized the functionalized carbon was carbon cap- wastured captured by FESEM by FESEMas well. asFigure well. 20 Figure presents 20 presentsboth the bothFESEM the images FESEM and images the elemental and the elementalanalysis result. analysis The result. well-developed The well-developed tube-like tube-likepores can pores be seen can from be seen the fromimages the in images Figure in20a,bFigure for 20thea,b as-preparedfor the as-prepared AC samples. AC It samples. is believed It isthat believed such tubular that such pores tubular increase pores the increasespecific thesurface specific area, surface thus providing area, thus an providing abundanc ane of abundance active sites of for active salt sitesions forintercalation salt ions intercalationand de-intercalation and de-intercalation during the capacitive during the deionization capacitive deionization(CDI) process (CDI) [43]. processFigure 20c–e [43]. Figureshows 20thec–e FESEM shows images the FESEM of the images α-MnO of2/f-AC the α -MnOcomposite2/f-AC specimen composite at different specimen magnifi- at dif- ferentcations. magnifications. From the three From images, the threewe can images, see that we the can octahedral see that theα-MnO octahedral2 nanostructuresα-MnO2 nanostructureswere dispersed were uniformly dispersed on uniformlythe surface on of the the surface f-AC. ofThe the EDS f-AC. in TheFigure EDS 20f in reveals Figure 20 thef revealssignals the from signals C, Mn, from and C, O, Mn, the and three O, major the three elements major elementsexisting in existing the MnO in2 the/f-AC MnO specimen.2/f-AC specimen.The characterization studies as presented in [43] include the selectivity and capaci- tanceThe performance characterization as well studies as the as kinetics presented of ion-storage in [43] include and the removal selectivity of the and prepared capacitance elec- performancetrodes installed as well in a ashybrid the kinetics CDI cell. of ion-storageThe cell consists and removalof an f-AC of thecathode prepared and electrodesα-MnO2/f- installedAC anode. in aFigure hybrid 21a CDI shows cell. the The relation cell consists of solution of an conductivity f-AC cathode vs. and time.α-MnO2/f-AC A steep drop anode.in the conductivity Figure 21a shows can be the seen relation in the ofα-MnO solution2/f-AC//f-AC-based conductivity vs. electr time.odes A steep at 1.2 drop V. This in theindicates conductivity that the can salt be ions seen are in quickly the α-MnO captured2/f-AC//f-AC-based by the charged electrodes. electrodes The at 1.2conductiv- V. This indicatesity of the that salt thesolution salt ions decreased are quickly significantly captured byin the the first charged 10 min electrodes. and declined The conductivity slowly until ofreaching the salt equilibrium solution decreased at 40 min. significantly As shown in in the the first inset 10 of min Figure and declined21a, the concentration slowly until reachingdropped equilibriumrapidly within at 4010 min min. and As gradually shown in ente thered inset the of flat Figure zone 21 arounda, the concentration40 min as well. droppedIn Figure rapidly 21b within is shown 10 min the and electrosorption gradually entered rate theand flat electrosorption zone around 40capacity min as of well. the preparedIn Figure electrodes. 21b is shownThe desalination the electrosorption capacity wa rates found and electrosorptionto be 17.8 mg/g in capacity NaCl solution of the preparedwith a concentration electrodes. The of desalination600 mg/L. The capacity electros wasorption found rate to be is 17.8 typically mg/g inevaluated NaCl solution by the withsalt removal a concentration capacity. of It 600 can mg/L. be seen The that electrosorption the maximum rate electrochemical is typically evaluated adsorption by therate salt removal capacity. It can be seen that the maximum electrochemical adsorption rate was about 1.7 mg NaCl per electrode mass per second. Figure 21c reveals the current re- was about 1.7 mg NaCl per electrode mass per second. Figure 21c reveals the current sponse of the prepared electrodes during the desalination process. The results indicate response of the prepared electrodes during the desalination process. The results indicate that the electrode has low charge consumption. The α-MnO2/f-AC electrode exhibited that the electrode has low charge consumption. The α-MnO2/f-AC electrode exhibited ideal electrical double layer and pseudocapacitive behavior with low charge transfer resistance. Therefore, the CDI is a relatively low energy consuming process, and the ion adsorption is fast. The Figure 21d schematic illustrates the CDI hybrid system with the α-MnO2/f-AC and f-AC two electrodes. Both electrodes are porous carbon based with high surface areas. C 2021, 7, x FOR PEER REVIEW 20 of 33

ideal electrical double layer and pseudocapacitive behavior with low charge transfer resistance. Therefore, the CDI is a relatively low energy consuming process, and the ion adsorption is fast. The Figure 21d schematic illustrates the CDI hybrid system with the α- MnO2/f-AC and f-AC two electrodes. Both electrodes are porous carbon based with high surface areas. Table 2 lists eleven different CDI systems with the specific capacitance and electrosorption capacity data taken from references [43,45–54]. It can be seen that the CDI performance of the MnO2/f-AC//f-AC-based electrode takes the lead among various carbon- C 2021, 7, 39 based and metal–oxide–carbon electrodes. Since this functionalized activated carbon-20 of 33 based CDI is derived from a palm tree biomass sustainable resource, it should be more cost-effective than other CDI systems.

Figure 20. FESEM images of AC samples (a,b) and α-MnO2/f-AC nanocomposites prepared Figure 20. FESEM images of AC samples (a,b) and α-MnO2/f-AC nanocomposites prepared through hydrothermalthrough hydrothermal at 180 ◦C at for 180 12 °C h ( cfor–e )12 along h (c– withe) along the EDSwith resultthe EDS (f). result Reprinted (f). Reprinted from [43], from©2020 [43], The ©2020 The Authors. Authors.

Table2 lists eleven different CDI systems with the speciﬁc capacitance and electrosorption capacity data taken from references [43,45–54]. It can be seen that the CDI performance of the MnO2/f-AC//f-AC-based electrode takes the lead among various carbon-based and metal–oxide–carbon electrodes. Since this functionalized activated carbon-based CDI is derived from a palm tree biomass sustainable resource, it should be more cost-effective than other CDI systems. CC2021 2021, 7, ,7 39, x FOR PEER REVIEW 2121 of of 33 33

FigureFigure 21.21.( (aa)) CapacitiveCapacitive deionizationdeionization (CDI)(CDI) proﬁleprofile forfor solutionsolution conductivityconductivity vs.vs. time (inset Figure concentrationconcentration (ppm) (ppm) vs. vs. time time (min)), (min)), (b ()b electrosorption) electrosorption rate rate and and electrosorption electrosorption capacity capacity of preparedof prepared electrodes, (c) current response of the prepared electrodes during desalination perfor- electrodes, (c) current response of the prepared electrodes during desalination performances, and (d) mances, and (d) schematic illustration of the CDI hybrid system. Reprinted from [43], ©2020 The schematic illustration of the CDI hybrid system. Reprinted from [43], ©2020 The Authors. Authors.

TableTable 2. 2.Comparison Comparison ofof thethe speciﬁcspecific capacitancecapacitance andand electrosorptionelectrosorption capacity of different CDI electrodes. Adopted with permission from [43], ©2020 The Authors. trodes. Adopted with permission from [43], ©2020 The Authors. Desalination Electrodes and Specific CapacitanceSpeciﬁc Applied NaClNaCl Concentration Con- Desalination Electrodes and Applied Capacity Source Materials Capacitance(F/g) Voltage (V) centration(ppm) Capacity Source Materials Voltage (V) (F/g) (ppm) (mg/g)(mg/g) Anode: α- Anode: MnO2/f-AC 388 1.2 600 17.8 [43] α-MnO /f-AC 388 1.2 600 17.8 [43] Cathode: f-AC2 Cathode: f-AC ZnO/activated ZnO/activated 66 1.2 500 9.4 [45] carbon 66 1.2 500 9.4 [45] carbon N-doped cluster- N-doped cluster-like 199 1.2 100 11.98 [46] like porous C 199 1.2 100 11.98 [46] Graphene–chi-porous C 190 1.6 300 12.7 [47] Graphene–chitosan–tosan–Mn3O4 190 1.6 300 12.7 [47] MnOMn2-nano-3O4 292 1.2 --- 5.01 [48] rods@grapheneMnO - 2 292 1.2 — 5.01 [48] nanorods@grapheneSulfonated car- 238 1.2 500 10.0 [49] bon/TiOSulfonated2 N-doped porous 238 1.2 500 10.0 [49] carbon/TiO2 292 1.4 500 16.63 [50] C N-doped porous C 292 1.4 500 16.63 [50] rGO–SnO2 nano- rGO–SnO 142 1.2 400 17.62 [51] composite 2 142 1.2 400 17.62 [51] nanocomposite TiO2-nanotube array with car- 238 1.2 500 13.11 [52] bon embedded electrode Nanoporous 3D 200 1.6 500 17.1 [53] Graphene C 2021, 7, 39 22 of 33

Table 2. Cont.

Speciﬁc NaCl Con- Desalination Electrodes and Applied Capacitance centration Capacity Source Materials Voltage (V) (F/g) (ppm) (mg/g)

TiO2-nanotube array with carbon 238 1.2 500 13.11 [52] C 2021, 7, x FOR PEER REVIEW embedded electrode 22 of 33

Nanoporous 3D 200 1.6 500 17.1 [53] Graphene

GO/ZrO210% 452 1.2 50 4.76 [54] GO/ZrO210% 452 1.2 50 4.76 [54]

4.4.4.4. NobleNoble MetalMetal Particles-DecoratedParticles-Decorated ActivatedActivated CarbonCarbon NobleNoble metal-decorated metal-decorated activated activated carbons carbons have have found found applications applications in catalysis,in catalysis, electro- elec- chemicaltrochemical assay, assay, and and sensing. sensing. In [ 55In ],[55], a biomass a biomass porous porous carbon carbon (BPC) (BPC) was was prepared prepared using us- freshing fresh oyster oyster mushroom mushroom as the as the raw raw material. material. Au–Pt Au–Pt nanoparticles nanoparticles were were deposited deposited onto onto thethe mushroom-derivedmushroom-derived activatedactivated carboncarbon forfor baicalinbaicalin detection. detection. TheThe goldgold andand platinum platinum bimetalbimetal decorateddecorated biomassbiomass porousporous carboncarbon (Au–Pt@BPC)(Au–Pt@BPC) compositecomposite waswas synthesizedsynthesized byby carbonization of oyster mushroom at 700 ◦C in N followed by solvothermal deposition of carbonization of oyster mushroom at 700 °C in N22 followed by solvothermal deposition of Au–Pt.Au–Pt. The The composite composite was was made made into into an electrodean electrode for electrochemicalfor electrochemical detection detection of baicalein. of bai- Figurecalein. 22 Figure shows 22 the shows procedure the procedure for synthesis for synthesis and baicalein and baicalein detection detection [55]. [55].

FigureFigure 22.22. PhotoPhoto andand schematicschematic illustrations showing the the synthesis synthesis of of the the Au–Pt@BPC Au–Pt@BPC composite composite andand thethe preparationpreparation ofof carboncarbon ionicionic liquidliquid electrodeelectrode (CILE)(CILE) forfor detectingdetecting baicalein.baicalein. Reprinted with permissionpermission from from [ 55[55],], ©2020 ©2020 Elsevier Elsevier B.V. B.V.

DuringDuring Au–PtAu–Pt particleparticle deposition,deposition, 5.05.0 mLmL 0.02430.0243 MM HAuClHAuCl44 solutionsolution andand 5.05.0 mLmL 0.01930.0193 M M HPtCl HPtCl4 solution4 solution were were mixed mixed with with 10 mL10 mL ethylene ethylene glycol glycol solution. solution. Then, Then, 1.0 g BPC1.0 g − − − − wasBPC added was added in 20 in mL 20 mixture mL mixture solution solution containing containing 6.0 mM 6.0 mM [AuCl [AuCl4] and4] and 4.8 4.8 mM mM [PtCl [PtCl4] 4] withwith 50% 50% ethylene ethylene glycol. glycol. TheThe mixturemixture waswas transferredtransferred intointo aa 5050 mLmL TeflonTeflon lined lined stainless stainless autoclave.autoclave. TheThe solvothermalsolvothermal synthesissynthesis waswas performedperformed atat 120120 ◦°CC forfor 11 hh toto generategenerate thethe Au–Pt@BPCAu–Pt@BPC composite.composite. TheThe finalfinal productproduct was was washed washed with with water water and and dried dried at at 120 120◦ °CC for for 66 hh [[55].55]. TheThe structuresstructures ofof BPCBPC andand Au–Pt@BPCAu–Pt@BPC werewere analyzedanalyzed byby fieldfield emission emission scanning scanning electronelectron microscopymicroscopy andand transmissiontransmission electronelectron microscopy.microscopy. From From the the SEM SEM images images shown shown in Figure 23A,B, a multi-layered 3D porous structure of BPC can be seen with pore sizes ranging from 0.5 to 1.0 µm. Such a porous structure provided the space for the adsorption of molecules. Figure 23C,D reveals the Au–Pt microparticles on the surface of the BPC. The average size of the Au–Pt microparticles was found to be 4 µm. The TEM images of the BPC, as shown in Figure 23E,F, show frameworks consisting of thin carbon sheets and pores. These pores were formed due to the etching effect of the KOH activating agent on the carbon substrate, as shown by Song et al. in [56]. In Figure 23G,H, the TEM images show that the entrapped smaller Au–Pt nanoparticles are uniformly distributed within the pores of the BPC [55]. The reactions are expressed by Equations (9) and (10) as presented in [57,58]: C 2021, 7, 39 23 of 33

in Figure 23A,B, a multi-layered 3D porous structure of BPC can be seen with pore sizes ranging from 0.5 to 1.0 µm. Such a porous structure provided the space for the adsorption C 2021, 7, x FOR PEER REVIEW of molecules. Figure 23C,D reveals the Au–Pt microparticles on the surface23 of 33 of the BPC. The average size of the Au–Pt microparticles was found to be 4 µm. The TEM images of the BPC, as shown in Figure 23E,F, show frameworks consisting of thin carbon sheets and pores. These pores were formed due to the etching effect of the KOH activating agent on

the carbon substrate,CH2OH as shown− CH2OH by → Song CH et3CHO al. in + [ 56H2].O In Figure 23G,H, the(9) TEM images show that the entrapped smaller Au–Pt nanoparticles are uniformly distributed within the C 2021, 7, x FOR PEER REVIEW 23 of 33 pores6CH of3CHO the BPC + 2[AuCl [55]. The4]− → reactions 2Au0 + are3CH expressed3CO − COCH by Equations3 + 6H+ + 8Cl (9)− and (10)(10) as presented The Au–Pt@BPCin [57,58]: modified carbon ionic liquid electrode (CILE) was prepared as a − → working electrode for the detectionCH of 2baicalOH ein,CH and2OH the redoxCH3CHO reaction + H2 mechanismO was (9) − 0 → + − proposed and shown6CH in Figure3CHO 24 + 2[AuCl[55].CH The42]OH mo→ −dified 2AuCH2OH electrode+ 3CH 3CHCO was3CHO− COCHsuccessfully + H23O+ 6H used+ 8Cl for (10)(9) detecting baicalein in drug and human urine samples. 6CH3CHO + 2[AuCl4]− → 2Au0 + 3CH3CO − COCH3 + 6H+ + 8Cl− (10) The Au–Pt@BPC modified carbon ionic liquid electrode (CILE) was prepared as a working electrode for the detection of baicalein, and the redox reaction mechanism was proposed and shown in Figure 24 [55]. The modified electrode was successfully used for detecting baicalein in drug and human urine samples.

Figure 23. SEMFigure images 23. of BPCSEM (imagesA,B) and of Au–Pt@BPCBPC (A,B) and (C ,Au–Pt@BPCD), TEM images (C,D of), TEM BPC (imagesE,F) and of Au–Pt@BPC BPC (E,F) and (G Au–,H) at various magniﬁcations.Pt@BPC Reprinted (G with,H) at permission various magnifications. from [55], ©2020 Reprinted Elsevier with B.V. permission from [55], ©2020 Elsevier B.V.

The Au–Pt@BPC modiﬁed carbon ionic liquid electrode (CILE) was prepared as a working electrode for the detection of baicalein, and the redox reaction mechanism was proposedFigure 23. andSEM shownimages inof BPC Figure (A ,B24) [and55]. Au–Pt@BPC The modiﬁed (C,D electrode), TEM images was of successfully BPC (E,F) and used Au– for detectingPt@BPC (G baicalein,H) at various in drug magnifications. and human Reprinted urine samples. with permission from [55], ©2020 Elsevier B.V.

Silver nanoparticles have the function of disinfection and cleaning. Chang et al. [59] studied the adsorption of formaldehyde on silver nanoparticle modified activated carbon. The activatedFigureFigure carbon 24.24. Thewas redox redoxwashed mechanism mechanism with deionized of of baicalein baicalein water at at Au–P followed Au–Pt@BPC/CILE.t@BPC/CILE. by pre-oxidization Reprinted Reprinted with treat- with permission permission ment in 1.0from fromM nitric [ 55[55],], ©2020 acid©2020 solution. Elsevier Elsevier B.V. InB.V. the functionalization process, aqueous silver nitrate solutions with three different concentrations of 0.1 M, 0.01 M, and 0.001 M were used for silver deposition.SilverSilver In each nanoparticlesnanoparticles case, the activated havehave thethe carbon functionfunction and of of 5 disinfection dig chitosansinfection were and and cleaning. addedcleaning. intoChang Chang one et et al. al. [ 59[59]] of the silverstudiedstudied nitrate the thesolutions. adsorptionadsorption The ofofmixture formaldehydeformaldehyde was put on oninto silversi lveran autoclave nanoparticlenanoparticle and modiﬁedmodified kept at 120 activatedactivated °C carbon.carbon. for 1 h. Then,TheThe the activatedactivated hydrothermally carbon carbon was wastreated washed washed samp withle with was deionized deionized cleaned water and water dried followed followed in an by oven pre-oxidization by at pre-oxidization 150 treat- °C for 12 h.treatmentment The modifiedin 1.0 in M 1.0 nitric activated M nitric acid acidcarbonsolution. solution. was In thetested In functionalization the as functionalizationthe filter media process, for process, indoor aqueous aqueousfor- silver nitrate silver maldehydenitrate solutionsremoval solutions [59].with three with different three different concentrations concentrations of 0.1 M, of 0.01 0.1 M,M, 0.01and M,0.001 and M 0.001were Mused were for The functionusedsilver for deposition. of silver Ru functionalized deposition. In each case, In activated each the case, activated carbon the activated carbon as a catalyst and carbon 5 gfor chitosan and sorbitol 5 g chitosan were produc- added were into added one tion from biomassintoof the one silver was of the demonstratednitrate silver solutions. nitrate [60]. solutions. The The mixture activated The mixture wa scarbon put was into was put an oxidized intoautoclave an autoclave in andHNO kept3 and, at kept 120 at°C ◦ chlorinated120 forin thionyl1C h. for Then, 1 chloride, h. the Then, hydrothermally theand hydrothermally doped with treated nitrogen treated samp atle sample washigh cleaned temperatures was cleaned and driedand of 700 driedin an°C ovenin an at oven 150 or 900 °C in°C N2 foratmosphere. 12 h. The Tomodified deposit activated Ru nanoparticles, carbon was 250 testedmg activated as the filter carbon media and 150for indoor for- mg of urea maldehydewere added removal into 100 [59]. mL water. In order to obtain 3 wt% Ru loading on the support, 18.4 mgThe of RuCl function3 was of then Ru added.functionalized After mixing, activated the reactioncarbon as temperature a catalyst for was sorbitol set produc- to 120 °C. Ittion is noted from thatbiomass urea wasshould demonstrated start decomposing [60]. The at activated 90 °C. After carbon cooled was down oxidized to in HNO3, room temperature,chlorinated 2 g ofin sodiumthionyl formatechloride, we andre dopedadded. withThen, nitrogen the solution at high was temperatures heated up of 700 °C to 130 °C toor degrade 900 °C in the N 2formate. atmosphere. The Tosoluti depositon was Ru leftnanoparticles, at room temperature 250 mg activated overnight carbon and 150 mg of urea were added into 100 mL water. In order to obtain 3 wt% Ru loading on the support, 18.4 mg of RuCl3 was then added. After mixing, the reaction temperature was set to 120 °C. It is noted that urea should start decomposing at 90 °C. After cooled down to room temperature, 2 g of sodium formate were added. Then, the solution was heated up to 130 °C to degrade the formate. The solution was left at room temperature overnight C 2021, 7, 39 24 of 33

at 150 ◦C for 12 h. The modiﬁed activated carbon was tested as the ﬁlter media for indoor formaldehyde removal [59]. The function of Ru functionalized activated carbon as a catalyst for sorbitol production from biomass was demonstrated [60]. The activated carbon was oxidized in HNO3, chlorinated in thionyl chloride, and doped with nitrogen at high temperatures of 700 ◦C ◦ or 900 C in N2 atmosphere. To deposit Ru nanoparticles, 250 mg activated carbon and 150 mg of urea were added into 100 mL water. In order to obtain 3 wt% Ru loading on the

C 2021, 7, x FOR PEER REVIEW support, 18.4 mg of RuCl3 was then added. After mixing, the reaction temperature was24 of set 33 to 120 ◦C. It is noted that urea should start decomposing at 90 ◦C. After cooled down to room temperature, 2 g of sodium formate were added. Then, the solution was heated up to 130 ◦C to degrade the formate. The solution was left at room temperature overnight under agitation.under agitation. After that, After the that, product the product was filtrated was out,filtrated washed, out, andwashed, oven and dried oven overnight. dried over- The ◦ ◦ solidnight. was The heat-treated solid was heat-treated at 400 C or 600at 400C for°C or two 600 hours °C for in atwo reductive hours atmospherein a reductive (5% atmos- H2 + 95%phere Ar) (5% to obtainH2 + 95% the desiredAr) to obtain particles thesize desired [60]. particles Azar, Angeles size [60]. Lillo-Rodenas, Azar, Angeles and Lillo-Ro- Carmen Roman-Martinezdenas, and Carmen [61] Roman-Martinez prepared a catalyst [61] consisting prepared of a Ru catalyst nanoparticles consisting (1 wt%),of Ru supportednanoparti- oncles mesoporous (1 wt%), supported activated on carbons mesoporous (ACs). activa The catalystted carbons was used(ACs). in The the catalyst one-pot was hydrolytic used in hydrogenationthe one-pot hydrolytic of cellulose hydrogenation to obtain sorbitol. of cellul Theose electronic to obtain statesorbitol. and The particle electronic size of state Ru wereand particle determined. size of It Ru was were found determined. that the It amount was found and that type the of amount surface functionaland type of groups surface infunctional the carbon groups materials in the were carbon modified materials as a were result modified of the Ru as incorporation. a result of the TheRu incorpora- activated carbontion. The kept activated a high mesoporecarbon kept volume a high after meso functionalizationpore volume after and functionalization Ru incorporation. and The Ru catalystincorporation. was found The tocatalyst be very was active, found resulting to be very in aactive, high celluloseresulting conversionin a high cellulose rate of 50%con- andversion selectivity rate of to50% sorbitol and selectivity above 75% to [ 61sorbitol]. above 75% [61]. InIn [[62],62], 22 wt.%wt.% Pd–PtPd–Pt catalystcatalyst was was loaded loaded onto onto activated activated carbon carbon by by the the impregnating impregnating approach.approach. The metal metal acetylacetonates acetylacetonates or or metal metal chlorides chlorides were were used used as as the the precursors precursors to togenerate generate metallic metallic nanoparticles. nanoparticles. The The Pd–Pt Pd–Pt nanoparticles nanoparticles with with sizes sizes ranging ranging from from 2 to 23 tonm 3 nmwere were well-dispersed well-dispersed on the on carbon. the carbon. The ca Thetalyst catalyst was tested was testedfor the for gas the phase gas hydro- phase hydrodechlorinationdechlorination of chlorodifluoromethane of chlorodifluoromethane (HCF (HCFC-22).C-22). Zhang Zhang et al. et [63] al. [studied63] studied the thePd- Pd-functionalizedfunctionalized activated activated carbon carbon for for selective selective phenol phenol hydrogenation. hydrogenation. Direct Direct calcination calcination of ofactivated activated carbon carbon (AC) (AC) at high at high temperature temperature in argon in argon was wasperformed performed to modify to modify the struc- the structureture and andsurface surface conditions. conditions. The The AC ACwithout without high high temperature temperature treatment treatment was was marked marked as ◦ asC-raw. C-raw. The The calcination calcination temperature temperature of 600 of °C 600 resultedC resulted in an AC in ancalled AC C600. called The C600. modified The modifiedAC materials AC materialswere decorated were decoratedwith Pd nanoparticles with Pd nanoparticles (NPs) to make (NPs) the to Pd/C make catalyst. the Pd/C The catalyst.Pd/C600 catalyst The Pd/C600 exhibited catalyst higher exhibited catalytic acti highervity catalyticthan the Pd/C-raw. activity than This the is because Pd/C-raw. the ThisPd/C600 is because catalyst the possesses Pd/C600 higher catalyst hydrophobici possesses higherty and hydrophobicity contains more and structural contains defects more structuralthan the Pd/C-raw, defects than as shown the Pd/C-raw, in Figure as25. shown The hydrophobicity in Figure 25. Theand the hydrophobicity surface defect and im- theproved surface the defect dispersibility improved in thethe dispersibilityphenol-cyclo inhexane the phenol-cyclohexane reaction solution. The reaction better solution. Pd dis- Thepersion better and Pd smaller dispersion Pd size and for smaller the Pd/C600 Pd size promoted for the Pd/C600 the catalytic promoted performance. the catalytic The performance.Pd/C600 catalyst The also Pd/C600 shows catalyst better recyclability also shows better than recyclabilitythe unmodified than Pd/C-raw the unmodified catalyst Pd/C-raw[63]. catalyst [63].

FigureFigure 25.25. Temperature effect on on the the hydrophobicity hydrophobicity of of carbon carbon and and the the disper dispersionsion of the of the Pd Pdon on carbon. Reprinted with permission from [63], © 2020 The Chemical Industry and Engineering Soci- carbon. Reprinted with permission from [63], © 2020 The Chemical Industry and Engineering Society ety of China, and Chemical Industry Press Co., Ltd. of China, and Chemical Industry Press Co., Ltd. Rare-earth elements such as lanthanum (La) and cerium (Ce) have been reported to functionalize activated carbons (ACs) and chars derived from a lignocellulosic-based precursor [64]. The La- and Ce-based ACs were used for controlling the water pollution caused by fluoride and arsenic (V) ions. It was found that the incorporation of these rare- earth elements on the activated carbon or char adsorbents increased the adsorption capacities 25 times, especially for arsenic (V). Chars and activated carbons functionalized with lanthanum and cerium demonstrated the adsorption capacities of 1.3–9.2 mg/g and 0.1– 9.2 mg/g for fluoride and arsenic (V) ions, respectively. A ligand exchange of [OH]− from the adsorbent surface and the presence of electrostatic forces could be the reasons for enhanced adsorption of fluoride and arsenic (V) ions [64]. C 2021, 7, 39 25 of 33

Rare-earth elements such as lanthanum (La) and cerium (Ce) have been reported to functionalize activated carbons (ACs) and chars derived from a lignocellulosic-based precursor [64]. The La- and Ce-based ACs were used for controlling the water pollution caused by fluoride and arsenic (V) ions. It was found that the incorporation of these rare-earth elements on the activated carbon or char adsorbents increased the adsorption capacities 25 times, especially for arsenic (V). Chars and activated carbons functionalized with lanthanum and cerium demonstrated the adsorption capacities of 1.3–9.2 mg/g and 0.1–9.2 mg/g for fluoride and arsenic (V) ions, respectively. A ligand exchange of [OH]− from the adsorbent surface and the presence of electrostatic forces could be the reasons for enhanced adsorption of fluoride and arsenic (V) ions [64].

5. Applications Activated carbons have found wide applications for catalysis [65,66], adsorption [67–79], templating [80], desalination [81], and electromagnetic interference shielding [82]. In addition, they have been extensively used for making supercapacitors [83–87], battery electrodes [88,89], and solar photothermal energy converters [90]. Activated carbons are accepted adsorbents for the purification of gaseous and aqueous solution systems at a large scale. In addition to purification application, energy conversion and energy storage are some of the most important applications. Some of these applications will be briefly discussed in the following section. Although in the previous section, the catalysis applications were presented, it is worth mentioning that biochars and activated carbons derived from different woods are efficient catalysts for toluene conversion [65]. In [66], new bio-composite materials consisting of TiO2 (Degussa P25) and activated carbon (AC) of Argania spinosa tree nutshells by calcination and H3PO4 activation were made. The composites were used as photocatalysts for the elimination of pharmaceuticals, including diclofenac (DCF), carbamazepine (CBZ), and sulfamethoxazole (SMX), from aqueous solution. The TiO2 was attached to the AC to form the composite materials by high temperature impregnation. The drug elimination efficiency was evaluated. The adsorption applications of biomass-derived activated carbons may be divided into several sub-categories. One of the categories is on heavy metal adsorption [67–72]. There is also a report on vitamin B adsorption [73]. Another category is about dye adsorption and decomposition [74–77]. Recently, carbon dioxide adsorption using activated carbon has become an increasingly important branch of research [78,79]. In [78], the porous carbon materials prepared from camellia leaves at the hydrothermal carbonization (HTC) temperature of 240 ◦C followed by KOH activation showed the microporous structure. From the HTC, the tree leaves were converted to hydrochars or biochars in solid form. The biochars were used as the raw materials for activated carbon preparation. The specific 2 surface area was as high as 1824 m /g. A maximum CO2 adsorption capacity of 8.30 mmol/g at 25 ◦C under 0.4 MPa was achieved. Xu et al. [79] prepared nitrogen doped carbons from camphor tree leaves. The tree leaves were carbonized at 500 ◦C for 2 h to generate chars. The chars were then activated with KOH at 600 ◦C in nitrogen gas flow. The nitrogen contained in the tree leaves also served as the nitrogen source for doping. The 2 carbon showed a relatively high surface area of 1736 m /g. A fairly high CO2 uptake of 5.86 mmol/g at 1 bar and 273 K was attained. Porous carbon can be used as the template for fabricating nanostructures with different compositions. Activated carbon from biomass through physical activation in an inert atmosphere was chemically treated using tetraethyl orthosilicate (TEOS) [80]. Porous carbons were obtained from carbonization of the Platanus orientalis L. plane tree fruit (PTF) precursor and activated at 850 ◦C. The activated carbon as a template allowed the creation of highly porous and spatially ordered bio-SiC ceramics. The SiC nanostructures were generated at several processing temperatures. The carbothermal reduction occurred at 1400 ◦C. The increase in the temperature and the duration of processing promoted the generation of the SiC particles inside the porous structure. β-SiC with the cubic structure was the major portion, and the remainder was α-SiC with a hexagonal structure [80]. C 2021, 7, 39 26 of 33

Activated carbon plays an important role in capacitive deionization and helps in the making of biologically-inspired desalination systems. As described in [81], the growth of mangrove trees in brackish swamps represents an amazing biologic adaptation to saltwater. Through water desalination, the mangrove maintains a near freshwater flow from roots to leaves to maintain growth. One-step carbonization of a plant with developed aerenchyma tissue to enable highly-permeable, freestanding flow-through capacitive deionization electrodes was performed [81]. The resistance to water flow through the electrode made by carbonized aerenchyma from red mangrove roots was more than 60 times lower than that through the electrode from carbonized common woody biomass. The practical use of the intact carbonized red mangrove roots as electrodes in a flow-through capacitive deionization system was illustrated [81]. Farhan, Wang, and Li [82] made a green carbon foam from the fibrous fruits of Platanus orientalis L. (plane) along with the tar oil as binder via the powder molding route. The porous carbon derived from biomaterials showed a considerably high strength. Various physical, thermal, and electromagnetic shielding properties were investigated. The application for electromagnetic interference shielding was proposed because the carbon foam exhibited shielding effectiveness of more than 20 dB over the X-band frequency. A fast carbonization approach was performed at 1000 ◦C under the cover of the pyrolyzed tree seeds without using extra protective gas. In some samples, 5 wt.% iron chloride was added during the molding process. Iron chloride is a graphitization catalyst and activating agent, which helped increasing the specific surface area from 88 to 294 m2/g, but the flexural strength of the carbon foam was decreased by 25%. Thermal stability was improved due to the incorporation of more graphitic phases in the sample. The thermal conductivity was increased slightly from 0.22 to 0.67 W/(m·K) due to the graphitization catalyzed by the iron chloride. In an electromagnetic (EM) field, the EM wave absorption by the carbon foam was dominant with only 8–10% reflection. This indicates that the EM wave absorption is the dominant shielding mechanism. The new carbon foam material preserved the light weight and was highly porous with interconnected pore morphology from the original biomaterial. It is suggested for high temperature thermal insulation as well [82]. Activated carbons have long been studied for energy storage and conversions [83–90]. A lot of researchers investigated the supercapacitors made from activated carbons [83–87]. In [83], a symmetric ionic liquid-based supercapacitor was fabricated with porous carbon derived from capsicum (bell pepper) seeds. The porous carbon with the nickname of “peppered”-activated carbon (ppAC) was obtained through the carbonization at 850 ◦C using KHCO3 as the activating agent. The ppAC-based supercapacitor operated at a maximum cell voltage of 3.20 V and was filled with an ionic liquid electrolyte, 1-ethyl-3- methylimidazolium bistrifluorosulfonylimide (EMIM-TFSI). The highest specific energy was 37 Wh/kg with a power density of 0.6 kW/kg at 0.5 A/g. A specific energy of 26 Wh/kg was obtained when the applied current was increased to 1.0 A/g. After being tested for 25,000 cycles, the capacitor was proven to have a high cyclic stability. The coulombic efficiency was kept at 99% after the cycling. He, Huang, and Wang [84] introduced porous nitrogen and oxygen co-doped carbon microtubes (PCMTs) generated from the carbonization and activation of plane tree fruit fluffs (PTFFs). The PCMTs were proposed as high-performance supercapacitor electrode materials. The pore structures, surface chemistry, and degree of graphitization of the porous carbon tubes can be tailored by varying the activation temperature in a range from 650 to 900 ◦C. The PCMT obtained from the 850 ◦C activation, named as PCMT-850, showed a specific surface area of 1533 m2/g), with the highest mesopore ratio of 9.13%. It contains 2.2 at% nitrogen, which is the highest N content achieved among all the PCMTs. It also has the highest degree of graphitization, leading to excellent electrical conductivity. In 6 M KOH, the PCMT-850 electrode attained the lowest internal resistance and highest charge storage capacity. The specific capacitance was 257.6 F/g at a current of 1A/g. Kumar et al. [85] used a new activating agent (NaCl: KCl = 1: 1) for making a nanoporous carbon from Java Kapok tree shell. The nanoporous carbon showed a spe- C 2021, 7, 39 27 of 33

cific surface area of 1260 m2/g, pore volume of 0.439 cm3/g, pore size of 1.241 nm, and microspore volume of 0.314 cm3/g. The capacitor electrode using the nanoporous carbon demonstrated a specific capacitance of 169 F/g with 97% capacity retention after 10,000 cycles at 1 A/g. Barzegar et al. [86] prepared low-cost carbons from expanded graphite (EG) and pinecone (PC) biomass using KOH as the activation agent. The final carbonization was carried out in argon and hydrogen atmosphere. A specific surface area of 808 and 457 m2/g were obtained for the activated pinecone carbon (APC) and the activated expanded graphite (AEG), respectively. The activated carbon was used to make the electrode for asymmetric supercapacitors. A specific capacitance of 69 F/g was reported. Nitrogen-doped porous carbon nanosheets prepared from eucalyptus tree leaves by simply mixing the leaf powders with KHCO3 and subsequent carbonization were used for electrodes in supercapacitors and lithium batteries [87]. The specific surface area of the porous carbon nanosheets was as high as 2133 m2/g. For supercapacitor application, the porous carbon nanosheet electrode exhibited a supercapacitance of 372 F/g at a current density of 500 mA/g in 1 M H2SO4 aqueous electrolyte and excellent cycling stability over 15,000 cycles. In an organic electrolyte, the nanosheet electrode demonstrated stable cycling behavior with a specific capacitance of 71 F/g at a current density of 2A/g. When applied as the anode material for lithium ion batteries, the carbon nanosheets showed good rate capability and stable cycling performance with a high specific capacity of 819 mAh/g at a current density of 100 mA/g [87]. Another area of energy storage research is in utilizing activated carbon for battery electrodes, because the biomass-derived carbon electrodes have low cost [87–89]. There are various carbon-based electrodes for lithium–sulfur batteries [88,89]. Zhang et al. [88] carbonized and activated palm tree fibers with KOH to obtain novel highly ordered carbon tube (OCT) arrays. The OCT was taken as the host in lithium–sulfur batteries. The electrode made from OCT was found effective on sulfur storage. The large specific area and pore volume were also found. The S@OCT composite with 65% (w/w) sulfur exhibited satisfactory electrochemical performance. It delivered an initial discharge capacity of 1255.2 mAh/g or 1.8 mAh/cm2 and retained 756.9 mAh/g after 100 cycles with a high coulomb efficiency [88]. Selva et al. [89] also showed that biomass-derived porous carbon could be a promising sulfur host material for lithium sulfur batteries because it is highly conductive and has large porosity. Two different carbons were prepared from oak tree fruit shells by carbonization with and without KOH activation. It was found that the KOH activated carbon (AC) revealed a much higher surface area of 796 m2/g than the pyrolyzed carbon (PC) (334 m2/g) without KOH activation. The activated-carbon contains more single-layer sheets with a lower degree of graphitization. The biomass-derived porous carbon was coated onto a separator, which led to an improved electrochemical performance in Li–S cells. The Li–S cell assembled with porous carbon modified separator demonstrated an initial capacity of 1324 mAh/g. This value for the cell with the uncoated separator was 875 mAh/g. The charge transfer resistance measurement confirmed the high ionic conductivity nature of porous carbon modified separator. The biomass-derived activated carbon can be considered as an alternative material for the polysulfide inhibition in Li–S batteries [89]. Activated carbons have been studied for energy converters, for example, solar thermal convertors or solar steam generators [20,90]. In [90], a photothermal generator inspired from banyan tree using the synthetic material, polyester, was prepared. However, sustainable resources, for example, willow catkin-derived porous carbon membrane demonstrated the potential for efficient solar steam generation [20]. Activated carbon possesses hydrophilic properties, allowing solar energy to be converted into thermal energy to heat the surrounding water flowing in a porous water channel under capillary action.

6. Perspectives and Conclusions Activated carbon from sustainable resources have the advantages of low cost and eco- friendliness. This is meaningful because the prices for high end AC products have increased C 2021, 7, 39 28 of 33

over time. There are two types of activation methods: physical activation and chemical activation. The physical activation process is much more time consuming than the chemical process. Moreover, the pore size and porosity are very difficult to control in the physical activation process. Therefore, chemical activation becomes the prevailing technique for making activated carbons. Various activating agents have been investigated. KOH, KHCO3, K2CO3,H2SO4,H3PO4, ZnCl2, NaCl/KCl, and CO2 are some of the commonly used agents. In view of the kinds of biomass for carbonization, tree woods, shell, nuts, leaves, and roots are extensively used. There are various other carbon sources under exploration [91–100]. Beech tree wood [91], plant barks [92], eucalyptus wastes [93], spruce tree sawdust [94], olive tree pruning residues [95], fruit of Brazil nut tree [96], apple tree small branches [97], wood of cherry tree [98], coconut shells of tall and dwarf tree varieties [99], and tree bark waste [100] are some of the good examples of newly used biomass for activated carbon formation. The applications of the biomass-derived carbon span different fields. Traditionally, the porous carbon from coal gasification was used for nitrate removal [101]. However, biomass-derived activated carbon has found extensive application in various fields for adsorption, energy storage, and conversion. Inspired by the design of the electrochemical flow reactor [102], various activated carbons simulating the flow reactors were built for water purification and desalination. Characterization of the activated carbon materials derived from renewable resources is not just limited to morphology observation and porosity measurement. The electrochemical properties for capacitors and batteries have been emphasized in recent studies. Moreover, some uncommon properties such as ice nucleation behavior on the surface of activated carbon [103], and thermal tension responses [104] are also dealt with. In addition to the processing technologies, several questions related to the activated carbon remain to be answered. The first question is about the CO2 footprint of producing activated carbon. Gu et al. [105] conducted a life cycle assessment of woody source-derived activated carbon. It was found that the greenhouse gas emissions for activated carbon production from biochar are less than half of those for AC production from coal. Another question is about the energy demand for activated carbon production. Studies show that using woody biomass for both feedstock and processing can significantly lower the energy consumption [105]. High performance activated carbon processing and new application are still under exploration. Dependence of the microporosity of activated carbons on the lignocellulosic composition of the precursor such as almond tree pruning and walnut shell was investigated [106]. The lignocellulosic and porosity properties of the raw materials can affect the activation processes. Olive oil waste-derived activated carbon for absorbing various organic contaminants including triclosan (TCS), ibuprofen (IBP), and diclofenac (DCF) was also developed [107]. Such fundamental research could guide us to further improve the quality of activated carbons. It is also essential to explore new methods for activated carbon production using nonconventional and low-cost processing and manufacturing technologies.

Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Conﬂicts of Interest: The author declares no conﬂict of interest.

References 1. Bolisetty, S.; Peydayesh, M.; Mezzenga, R. Sustainable technologies for water puriﬁcation from heavy metals: Review and analysis. Chem. Soc. Rev. 2019, 48, 463–487. [CrossRef] 2. Liakos, E.V.; Rekos, K.; Giannakoudakis, D.A.; Mitropoulos, A.C.; Fu, J.; Kyzas, G.Z. Activated porous carbon derived from tea and plane tree leaves biomass for the removal of pharmaceutical compounds from wastewaters. Antibiotics 2021, 10, 65. [CrossRef] C 2021, 7, 39 29 of 33

3. Egirani, D.; Latif, M.T.; Wessey, N.; Poyi, N.R.; Shehata, N. Preparation and characterization of powdered and granular activated carbon from Palmae biomass for mercury removal. Appl. Water Sci. 2021, 11.[CrossRef] 4. Tan, Y.T.; Li, Y.; Wang, W.C.; Ran, F. High performance electrode of few-layer-carbon@bulk-carbon synthesized via controlling diffusion depth from liquid phase to solid phase for supercapacitors. J. Energy Storage 2020, 32, 101672. [CrossRef] 5. Khorasgani, N.B.; Sengul, A.B.; Asmatulu, E. Briquetting grass and tree leaf biomass for sustainable production of future fuels. Biomass Conv. Bioref. 2020, 10, 915–924. [CrossRef] 6. Vohra, M.; Al-Suwaiyan, M.; Hussaini, M. Gas phase toluene adsorption using date palm-tree branches based activated carbon. Int. J. Env. Res. Public Health 2020, 17, 9287. [CrossRef][PubMed] 7. Peng, L.C.; Gao, J.; Yao, S.; Lan, X.Q.; Li, H.P.; Song, H. Modified ginkgo leaves for adsorption of methyl violet and malachite green dyes in their aqueous system. Desal. Water Treat. 2020, 206, 358–370. [CrossRef] 8. Yargic, A.S. Evaluation of poplar tree-based sorbents in dye uptake via 2(5) full factorial experimental design and statistical analysis of %removal efficiency. J. Polytech. Politek. Derg. 2020, 23, 941–954. 9. Saniya, A.; Sathya, K.; Nagarajan, K.; Yogesh, M.; Jayalakshmi, H.; Praveena, P.; Bharathi, S. Modelling of the removal of crystal violet dye from textile effluent using Murraya koenigii stem biochar. Desal. Water Treat. 2020, 203, 356–365. [CrossRef] 10. Yang, H.M.; Zhang, D.H.; Chen, Y.; Ran, M.J.; Gu, J.C. Study on the application of KOH to produce activated carbon to realize the utilization of distiller’s grains. In Proceedings of the 3rd International Conference on Advances in Energy, Environment and Chemical Engineering, Chengdu, China, 26–28 May 2017; p. 012051. [CrossRef] 11. Yamashita, Y.; Ouchi, K. Influence of alkali on the carbonization process-I: Carbonization of 3,5-dimethylphenol-formaldehyde resin with NaOH. Carbon 1982, 20, 41–45. [CrossRef] 12. Sun, Y.Q.; Yu, I.K.M.; Tsang, D.C.W.; Fan, J.J.; Clark, J.H.; Luo, G.; Zhang, S.C.; Khan, E.; Graham, N.J.D. Tailored design of graphitic biochar for high-efficiency and chemical-free microwave-assisted removal of refractory organic contaminants. Chem. Eng. J. 2020, 398, 125505. [CrossRef] 13. Bai, Y.Q.; Wang, C.G.; Li, X.; Fan, W.Q.; Song, P.H.; Gu, Y.C.; Liu, F.Q.; Liu, G.Y. Preparation and electrochemical properties of S@C composite material with high capacity and ordered alignment of channels. Chem. J. Chin. Univ. 2020, 41, 1306–1312. [CrossRef] 14. Hayashi, J.; Horikawa, T.; Takeda, I.; Muroyama, K.; Nasir Ani, F. Preparing activated carbon from various nutshells by chemical activation with K2CO3. Carbon 2002, 40, 2381–2386. [CrossRef] 15. Kim, J.-W.; Sohn, M.-H.; Kim, D.-S.; Sohn, S.-M.; Kwon, Y.-S. Production of granular activated carbon from waste walnut shell and its adsorption characteristics for Cu2+ ion. J. Hazard. Mater. 2001, 85, 301–315. [CrossRef] 16. Molina-Sabio, M.; Rodriguez-Reinoso, F.; Caturla, F.; Selles, M.J. Porosity in granular carbons activated with phosphoric acid. Carbon 1995, 33, 1105–1113. [CrossRef] 17. Jahanban-Esfahlan, A.; Jahanban-Esfahlan, R.; Tabibiazar, M.; Roufegarinejad, L.; Amarowicz, R. Recent advances in the use of walnut (Juglans regia L.) shell as a valuable plant-based bio-sorbent for the removal of hazardous materials. RSC Adv. 2020, 10, 7026–7047. [CrossRef] 18. Nemati, F.; Jafari, D.; Esmaeili, H. Highly efficient removal of toxic ions by the activated carbon derived from citrus limon tree leaves. Carbon Lett. 2020.[CrossRef] 19. Mansour, R.A.; El Shahawy, A.; Attia, A.; Beheary, M.S. Brilliant green dye biosorption using activated carbon derived from Guava tree wood. Int. J. Chem. Eng. 2020, 2020, 8053828. [CrossRef] 20. Zhang, S.C.; Zang, L.L.; Dou, T.W.; Zou, J.L.; Zhang, Y.H.; Sun, L.G. Willow catkins-derived porous carbon membrane with hydrophilic property for efficient solar steam generation. ACS Omega 2020, 5, 2878–2885. [CrossRef][PubMed] 21. Xing, R.; Wang, W.; Jiao, T.; Ma, K.; Zhang, Q.; Hong, W.; Qiu, H.; Zhou, J.; Zhang, L.; Peng, Q. Bioinspired polydopamine sheathed nanofibers containing carboxylate graphene oxide nanosheet for high-efficient dyes scavenger. ACS Sustain. Chem. Eng. 2017, 5, 4948–4956. [CrossRef] 22. Zhang, C.; Ma, M.Q.; Chen, T.T.; Zhang, H.; Hu, D.F.; Wu, B.H.; Ji, J.; Xu, Z.K. Dopamine-triggered one-step polymerization and codeposition of acrylate monomers for functional coatings. ACS Appl. Mater. Interfaces 2017, 9, 34356–34366. [CrossRef] 23. Musyoka, N.M.; Mutuma, B.K.; Manyala, N. Onion-derived activated carbons with enhanced surface area for improved hydrogen storage and electrochemical energy application. RSC Adv. 2020, 10, 26928–26936. [CrossRef] 24. Sing, K.S.W.; Everett, D.H.; Haul, R.A.W.; Moscou, L.; Pierotti, R.A.; Rouquerol, J.; Siemieniewska, T. Reporting physisoption data for gas/solid systems with special reference to the determination of surface area and porosity. Pure Appl. Chem. 1985, 57, 603–619. [CrossRef] 25. Sawood, G.M.; Gupta, S.K. Kinetic equilibrium and thermodynamic analyses of As (V) removal from aqueous solution using iron-impregnated Azadirachta indica carbon. Appl Water Sci. 2020, 10, 131. [CrossRef] 26. Kalaruban, M.; Loganathan, P.; Nguyen, T.V.; Nur, T.; Johir, M.A.; Nguyen, T.H.; Trinh, M.V.; Vigneswaran, S. Iron-impregnated granular activated carbon for arsenic removal: Application to practical column filters. J. Environ. Manag. 2019, 239, 235–243. [CrossRef][PubMed] 27. Shah, I.; Adnan, R.; Ngah, W.S.W.; Mohamed, N. Iron impregnated activated carbon as an efficient adsorbent for the removal of methylene blue: Regeneration and kinetics studies. PLoS ONE 2015, 10, e0122603. [CrossRef] 28. Kakavandi, B.; Bahari, N.; Kalantary, R.R.; Fard, E.D. Enhanced sono-photocatalysis of tetracycline antibiotic using TiO2 decorated on magnetic activated carbon (MAC@T) coupled with US and UV: A new hybrid system. Ultrason. Sonochem. 2019, 55, 75–85. [CrossRef][PubMed] C 2021, 7, 39 30 of 33

29. Alalm, G.M.; Tawfik, A.; Ookawara, S. Enhancement of photocatalytic activity of TiO2 by immobilization on activated carbon for degradation of pharmaceuticals. J. Environ. Chem. Eng. 2016, 4, 1929–1937. [CrossRef] 30. Daou, C.; Hamade, A.; El Mouchtari, E.; Rafqah, S.; Piram, A.; Wong-Wah-Chung, P.; Najjar, F. Zebrafish toxicity assessment of the photocatalysis-biodegradation of diclofenac using composites of TiO2 and activated carbon from Argania spinosa tree nutshells and Pseudomonas aeruginosa. Environ. Sci. Pollut. Res. 2020, 27, 17258–17267. [CrossRef][PubMed] 31. El-Sheikh, A.H.; Newman, A.P.; Al-Daffaee, H.; Phull, S.; Cresswell, N.; York, S. Deposition of anatase on the surface of activated carbon. Surf. Coat. Technol. 2004, 187, 284–292. [CrossRef] 32. Wang, X.; Liu, Y.; Hu, Z.; Chen, Y.; Liu, W.; Zhao, G. Degradation of methyl orange by composite photocatalysts nano-TiO2 immobilized on activated carbons of different porosities. J. Hazard. Mater. 2009, 169, 1061–1067. [CrossRef] 33. Salih, H.H.; Sorial, G.A.; Patterson, C.L.; Sinha, R.; Krishnan, E.R. Removal of trichloroethylene by activated carbon in the presence and absence of TiO2 nanoparticles. Water Air Soil Pollut. 2012, 223, 2837–2847. [CrossRef] 34. Liu, Q.-S.; Zheng, T.; Wang, P.; Jiang, J.P.; Li, N. Adsorption isotherm, kinetic and mechanism studies of some substituted phenols on activated carbon fibers. Chem. Eng. J. 2010, 157, 348–356. [CrossRef] 35. Lam, S.-M.; Sin, J.-C.; Mohamed, A.R. Parameter effect on photocatalytic degradation of phenol using TiO2-P25/activated carbon (AC). Korean J. Chem. Eng. 2010, 27, 1109–1116. [CrossRef] 36. Horikoshi, S.; Sakamoto, S.; Serpone, N. Formation and efficacy of TiO2/AC composites prepared under microwave irradiation in the photoinduced transformation of the 2-propanol VOC pollutant in air. Appl. Catal. B Environ. 2013, 140–141, 646–651. [CrossRef] 37. Matos, J.; Garcia, A.; Cordero, T.; Chovelon, J.-M.; Ferronato, C. Eco-friendly TiO2-AC photocatalyst for the selective photooxida- tion of 4-chlorophenol. Catal. Lett. 2009, 130, 568–574. [CrossRef] 38. Jamil, T.S.; Ghaly, M.Y.; Fathy, N.A.; Abd el-Halim, T.A.; Österlund, L. Enhancement of TiO2 behavior on photocatalytic oxidation of MO dye using TiO2/AC under visible irradiation and sunlight radiation. Sep. Purif. Technol. 2012, 98, 270–279. [CrossRef] 39. Han, J.; Shi, L.; Yan, T.; Zhang, J.; Zhang, D. Removal of ions from saline water using N, P co-doped 3D hierarchical carbon architectures via capacitive deionization. Environ. Sci. Nano 2018, 5, 2337–2345. [CrossRef] 40. Wu, T.; Wang, G.; Wang, S.; Zhan, F.; Fu, Y.; Qiao, H.; Qiu, J. Highly stable hybrid capacitive deionization with a MnO2 anode and a positively charged cathode. Environ. Sci. Technol. Lett. 2018, 5, 98–102. [CrossRef] 41. Umeshbabu, E.; Justin, P.; Rao, G.R. Tuning the surface morphology and pseudocapacitance of MnO2 by a facile green method employing organic reducing sugars. ACS Appl. Energy Mater. 2018, 1, 3654–3664. [CrossRef] 42. Qi, H.; Bo, Z.; Yang, S.; Duan, L.; Yang, H.; Yan, J.; Cen, K.; Ostrikov, K. Hierarchical nanocarbon-MnO2 electrodes for enhanced electrochemical capacitor performance. Energy Storage Mater. 2019, 16, 607–618. [CrossRef] 43. Govindan, B.; Alhseinat, E.; Darawsheh, I.F.F.; Ismail, I.; Polychronopoulou, K.; Jaoude, M.A.; Arangadi, A.F.; Banat, F. Activated carbon derived from phoenix dactylifera (palm tree) and decorated with MnO2 nanoparticles for enhanced hybrid capacitive deionization electrodes. ChemistrySelect 2020, 5, 3248–3256. [CrossRef] 44. Gan, Y.X.; Jayatissa, A.H.; Yu, Z.; Chen, X.; Li, M.H. Hydrothermal synthesis of nanomaterials. J. Nanomater. 2020, 2020, 8917013. [CrossRef] 45. Liu, J.; Lu, M.; Yang, J.; Cheng, J.; Cai, W. Capacitive desalination of ZnO/activated carbon asymmetric capacitor and mechanism analysis. Electrochim. Acta 2015, 151, 312–318. [CrossRef] 46. Li, Y.; Liu, Y.; Shen, J.; Qi, J.; Li, J.; Sun, X.; Shen, J.; Han, W.; Wang, L. Design of nitrogen-doped cluster-like porous carbons with hierarchical hollow nanoarchitecture and their enhanced performance in capacitive deionization. Desalination 2018, 430, 45–55. [CrossRef] 47. Gu, X.; Yang, Y.; Hu, Y.; Hu, M.; Wang, C. Fabrication of graphene-based xerogels for removal of heavy metal ions and capacitive deionization. ACS Sustain. Chem. Eng. 2015, 3, 1056–1065. [CrossRef] 48. El-Deen, A.G.; Barakat, N.A.M.; Kim, H.Y. Graphene wrapped MnO2-nanostructures as effective and stable electrode materials for capacitive deionization desalination technology. Desalination 2014, 344, 289–298. [CrossRef] 49. Min, B.H.; Choi, J.-H.; Jung, K.Y. Improved capacitive deionization of sulfonated carbon/titania hybrid electrode. Electrochim. Acta 2018, 270, 543–551. [CrossRef] 50. Wang, Z.; Yan, T.; Fang, J.; Shi, L.; Zhang, D. Nitrogen-doped porous carbon derived from a bimetallic metal-organic framework as highly efficient electrodes for flow-through deionization capacitors. J. Mater. Chem. A 2016, 4, 10858–10868. [CrossRef] 51. Sami, S.K.; Seo, J.Y.; Hyeon, S.-E.; Shershah, M.S.A.; Yoo, P.-J.; Chung, C.-H. Enhanced capacitive deionization performance by an rGO-SnO2 nanocomposite modified carbon felt electrode. RSC Adv. 2018, 8, 4182–4190. [CrossRef] 52. Wei, K.J.; Zhang, Y.H.; Han, W.Q.; Li, J.S.; Sun, X.Y.; Shen, J.Y.; Wang, L.J. A novel capacitive electrode based on TiO2-NTs array with carbon embedded for water deionization: Fabrication, characterization and application study. Desalination 2017, 420, 70–78. [CrossRef] 53. Shi, W.H.; Li, H.B.; Cao, X.H.; Leong, Z.Y.; Zhang, J.; Chen, T.P.; Zhang, H.; Yang, H.Y. Ultrahigh performance of novel capacitive deionization electrodes based on a three-dimensional graphene architecture with nanopores. Sci. Rep. 2016, 6, 18966. [CrossRef] [PubMed] 54. Yasin, A.S.; Mohamed, H.O.; Mohamed, I.M.A.; Mousa, H.M.; Barakat, N.A.M. Enhanced desalination performance of capacitive deionization using zirconium oxide nanoparticles-doped graphene oxide as a novel and effective electrode. Sep. Purif. Technol. 2016, 171, 34–43. [CrossRef] C 2021, 7, 39 31 of 33

55. Cheng, H.; Weng, W.J.; Xie, H.; Liu, J.; Luo, G.L.; Huang, S.M.; Sun, W.; Li, G.J. Au-Pt@Biomass porous carbon composite modified electrode for sensitive electrochemical detection of baicalein. Microchem. J. 2020, 154, 104602. [CrossRef] 56. Cheng, S.; Zhang, L.; Xia, H.; Zhang, S.; Peng, J.; Wang, S. Crofton weed derived activated carbon by microwave-induced KOH activation and application to wastewater treatment. J. Porous Mat. 2016, 23, 1597–1607. [CrossRef] 57. Xu, J.; Li, S.; Weng, J.; Wang, X.; Zhou, Z.; Yang, K. Hydrothermal syntheses of gold nanocrystals: From icosahedral to its truncated form. Adv. Funct. Mater. 2008, 18, 277–284. [CrossRef] 58. Fievet, F.; Lagier, J.P.; Figlarz, M. Preparing monodisperse metal powders in micrometer and submicrometer sizes by the polyol process. MRS Bull. 1989, 14, 29–34. [CrossRef] 59. Chang, S.M.; Hu, S.C.; Shiue, A.; Lee, P.Y.; Leggett, G. Adsorption of silver nano-particles modified activated carbon filter media for indoor formaldehyde removal. Chem. Phys. Lett. 2020, 757, 137864. [CrossRef] 60. Carlier, S.; Gripekoven, J.; Philippo, M.; Hermans, S. Ru on N-doped carbon supports for the direct hydrogenation of cellobiose into sorbitol. Appl. Catal. B Environ. 2021, 282, 119515. [CrossRef] 61. Azar, F.Z.; Lillo-Rodenas, M.A.; Roman-Martinez, M.C. Mesoporous activated carbon supported Ru catalysts to efficiently convert cellulose into sorbitol by hydrolytic hydrogenation. Energies 2020, 13, 4394. [CrossRef] 62. Radlik, M.; Juszczyk, W.; Matus, K.; Rarog-Pilecka, W.; Karpinski, Z. Hydrodechlorination of CHClF2 (HCFC-22) over Pd-Pt catalysts supported on thermally modified activated carbon. Catalysts 2020, 10, 1291. [CrossRef] 63. Zhang, C.H.; Yang, G.X.; Jiang, H.; Liu, Y.F.; Chen, R.Z.; Xing, W.H. Phenol hydrogenation to cyclohexanone over palladium nanoparticles loaded on charming activated carbon adjusted by facile heat treatment. Chin. J. Chem. Eng. 2020, 28, 2600–2606. [CrossRef] 64. Merodio-Morales, E.E.; Reynel-Avila, H.E.; Mendoza-Castillo, D.I.; Duran-Valle, C.J.; Bonilla-Petriciolet, A. Lanthanum- and cerium-based functionalization of chars and activated carbons for the adsorption of fluoride and arsenic ions. Int. J. Environ. Sci. Technol. 2020, 17, 115–128. [CrossRef] 65. Korus, A.; Samson, A.; Szlek, A. Catalytic conversion of toluene over a biochar bed under an inert atmosphere—The comparison of chars from different types of wood and the role of selected metals. Fuel 2020, 279, 118468. [CrossRef] 66. El Mouchtari, E.; Daou, C.; Rafqah, S.; Najjar, F.; Anane, H.; Piram, A.; Hamade, A.; Briche, S.; Wong-Wah-Chung, P. TiO2 and activated carbon of Argania Spinosa tree nutshells composites for the adsorption photocatalysis removal of pharmaceuticals from aqueous solution. J. Photochem. Photobiol. A Chem. 2020, 388, 112183. [CrossRef] 67. Hasanpour, M.; Hatami, M. Application of three-dimensional porous aerogels as adsorbent for removal of heavy metal ions from water/wastewater: A review study. Adv. Colloid Interf. Sci. 2020, 284, 102247. [CrossRef][PubMed] 68. Ullah, M.; Nazir, R.; Khan, M.; Khan, W.; Shah, M.; Afridi, S.G.; Zada, A. The effective removal of heavy metals from water by activated carbon adsorbents of Albizia lebbeck and Melia azedarach seed shells. Soil Water Res. 2020, 15, 30–37. [CrossRef] 69. Liang, S.; Shi, S.Q.; Zhang, H.H.; Qiu, J.J.; Yu, W.H.; Li, M.Y.; Gan, Q.; Yu, W.B.; Xiao, K.K.; Liu, B.C.; et al. One-pot solvothermal synthesis of magnetic biochar from waste biomass: Formation mechanism and efficient adsorption of Cr(VI) in an aqueous solution. Sci. Total Environ. 2019, 695, 133886. [CrossRef][PubMed] 70. Vo, A.T.; Nguyen, V.P.; Ouakouak, A.; Nieva, A.; Doma, B.T.; Tran, H.N.; Chao, H.P. Efficient Removal of Cr(VI) from water by biochar and activated carbon prepared through hydrothermal carbonization and pyrolysis: Adsorption-coupled reduction mechanism. Water 2019, 11, 1164. [CrossRef] 71. Shi, S.Q.; Yang, J.K.; Liang, S.; Li, M.Y.; Gan, Q.; Xiao, K.K.; Hu, J.P. Enhanced Cr(VI) removal from acidic solutions using biochar modified by Fe3O4@SiO2-NH2 particles. Sci. Total Environ. 2018, 628–629, 499–508. [CrossRef][PubMed] 72. Egirani, D.E.; Poyi, N.R.; Shehata, N. Preparation and characterization of powdered and granular activated carbon from Palmae biomass for cadmium removal. Int. J. Environ. Sci. Technol. 2020, 17, 2443–2454. [CrossRef] 73. Lupascu, T.; Petuhov, O.; Timbaliuc, N.; Cibotaru, S.; Rotaru, A. Adsorption capacity of vitamin B(12) and creatinine on highly-mesoporous activated carbons obtained from lignocellulosic raw materials. Molecules 2020, 25, 3095. [CrossRef] 74. Chong, M.Y.; Tam, Y.J. Bioremediation of dyes using coconut parts via adsorption: A review. SN Appl. Sci. 2020, 2, 187. [CrossRef] 75. Tolosa, N.C.; Mendoza, K.D.; Dumayas, D.L.P.; De Silva, J.M.D.F. Preparation and characterization of activated carbon derived from antidesma bunius L. in methylene blue removal from wastewater. J. Environ. Sci. Manag. 2020, 2020, 18–28. 76. Ghaedi, A.M.; Baneshi, M.M.; Vafaei, A.; Nejad, A.R.S.; Tyagi, I.; Kumar, N.; Galunin, E.; Tkachev, A.G.; Agarwal, S.; Gupta, V.K. Comparison of multiple linear regression and group method of data handling models for predicting sunset yellow dye removal onto activated carbon from oak tree wood. Environ. Technol. Innov. 2018, 11, 262–275. [CrossRef] 77. Khafri, H.Z.; Ghaedi, M.; Asfaram, A.; Safarpoor, M. Synthesis and characterization of ZnS:Ni-NPs loaded on AC derived from apple tree wood and their applicability for the ultrasound assisted comparative adsorption of cationic dyes based on the experimental design. Ultrason. Sonochem. 2017, 38, 371–380. [CrossRef][PubMed] 78. Yang, G.Z.; Song, S.; Li, J.; Tang, Z.H.; Ye, J.Y.; Yang, J.H. Preparation and CO2 adsorption properties of porous carbon by hydrothermal carbonization of tree leaves. J. Mater. Sci. Technol. 2019, 35, 875–884. [CrossRef] 79. Xu, J.G.; Shi, J.S.; Cui, H.M.; Yan, N.F.; Liu, Y.W. Preparation of nitrogen doped carbon from tree leaves as efficient CO2 adsorbent. Chem. Phys. Lett. 2018, 711, 107–112. [CrossRef] 80. Dodevski, V.; Pagnacco, M.C.; Radovic, I.; Rosic, M.; Jankovic, B.; Stojmenovic, M.; Mitic, V.V. Characterization of silicon carbide ceramics obtained from porous carbon structure achieved by plant carbonization. Mater. Chem. Phys. 2020, 245, 122768. [CrossRef] C 2021, 7, 39 32 of 33

81. Wood, A.R.; Garg, R.; Justus, K.; Cohen-Karni, T.; LeDuc, P.; Russell, A.J. Intact mangrove root electrodes for desalination. RSC Adv. 2019, 9, 4735–4743. [CrossRef] 82. Farhan, S.; Wang, R.M.; Li, K.Z. Physical and electromagnetic shielding properties of green carbon foam prepared from biomaterials. Trans. Nonferr. Met. Soc. China 2018, 28, 103–113. [CrossRef] 83. Momodu, D.; Sylla, N.F.; Mutuma, B.; Bello, A.; Masikhwa, T.; Lindberg, S.; Matic, A.; Manyala, N. Stable ionic-liquid-based symmetric supercapacitors from Capsicum seed-porous carbons. J. Electroanalyt. Chem. 2019, 838, 119–128. [CrossRef] 84. He, D.; Huang, Z.H.; Wang, M.X. Porous nitrogen and oxygen co-doped carbon microtubes derived from plane tree fruit fluff for high-performance supercapacitors. J. Mater. Sci. Mater. Electron. 2019, 30, 1468–1479. [CrossRef] 85. Kumar, K.T.; Sundari, G.S.; Kumar, E.S.; Ashwini, A.; Ramya, M.; Varsha, P.; Kalaivani, R.; Andikkadu, M.S.; Kumaran, V.; Gnanamuthu, R.; et al. Synthesis of nanoporous carbon with new activating agent for high-performance supercapacitor. Mater. Lett. 2018, 218, 181–184. [CrossRef] 86. Barzegar, F.; Bello, A.; Dangbegnon, J.K.; Manyala, N.; Xia, X.H. Asymmetric supercapacitor based on activated expanded graphite and pinecone tree activated carbon with excellent stability. Appl. Energy 2017, 207, 417–426. [CrossRef] 87. Mondal, A.K.; Kretschmer, K.; Zhao, Y.F.; Liu, H.; Wang, C.Y.; Sun, B.; Wang, G.X. Nitrogen-doped porous carbon nanosheets from eco-friendly eucalyptus leaves as high performance electrode materials for supercapacitors and lithium ion batteries. Chem. A Euro. J. 2017, 23, 3683–3690. [CrossRef] 88. Zhang, M.Y.; You, X.L.; Liu, L.J.; Walle, M.D.; Li, Y.J.; Liu, Y.N. Biomass derived highly-ordered carbon tube as athode material for high performance lithium-sulfur batteries. Chin. J. Inorg. Chem. 2019, 35, 1493–1499. [CrossRef] 89. Selva, R.K.; Zhu, P.; Yan, C.I.; Zhu, J.D.; Dirican, M.; Shanmugavani, A.; Lee, Y.S.; Zhang, X.W. Biomass-derived porous carbon modified glass fiber separator as polysulfide reservoir for Li-S batteries. J. Colloid Interf. Sci. 2018, 513, 231–239. [CrossRef] 90. Zhang, Q.; Hu, R.; Chen, Y.L.; Xiao, X.F.; Zhao, G.M.; Yang, H.J.; Li, J.H.; Xu, W.L.; Wang, X.B. Banyan-inspired hierarchical evaporators for efficient solar photothermal conversion. Appl. Energy 2020, 276, 115545. [CrossRef] 91. Narowska, B.E.; Kulaznski, M.; Lukaszewicz, M. Application of activated carbon to obtain biodiesel from vegetable oils. Catalysis 2020, 10, 1049. [CrossRef] 92. Ighalo, J.O.; Adeniyi, A.G. Adsorption of pollutants by plant bark derived adsorbents: An empirical review. J. Water Proc. Eng. 2020, 35, 101228. [CrossRef] 93. Heidari, A.; Khaki, E.; Younesi, H.; Lu, H.Y.R. Evaluation of fast and slow pyrolysis methods for bio-oil and activated carbon production from eucalyptus wastes using a life cycle assessment approach. J. Clean. Prod. 2020, 241, 118394. [CrossRef] 94. Ozbay, N.; Yargic, A.S. Carbon foam production from bio-based polyols of liquefied spruce tree sawdust: Effects of biomass/solvent mass ratio and pyrolytic oil addition. J. Appl. Polym. Sci. 2019, 136, 47185. [CrossRef] 95. Mamani, A.; Sardella, M.F.; Gimenez, M.; Deiana, C. Highly microporous carbons from olive tree pruning: Optimization of chemical activation conditions. J. Environ. Chem. Eng. 2019, 7, 102830. [CrossRef] 96. Rimoli, M.F.D.; Nogueira, R.M.; Ferrarini, S.R.; de Castro, P.M.; Pires, E.M. Preparation and characterization of carbon from the fruit of Brazil nut tree activated by physical process. Rev. Arvore 2019, 43, E430206. [CrossRef] 97. Marques, S.C.R.; Mestre, A.S.; Machuqueiro, M.; Gotvajn, A.Z.; Marinsek, M.; Carvalho, A.P. Apple tree branches derived activated carbons for the removal of beta-blocker atenolol. Chem. Eng. J. 2018, 345, 669–678. [CrossRef] 98. Ardekani, P.S.; Karimi, H.; Ghaedi, M.; Asfaram, A.; Purkait, M.K. Ultrasonic assisted removal of methylene blue on ultrasoni- cally synthesized zinc hydroxide nanoparticles on activated carbon prepared from wood of cherry tree: Experimental design methodology and artificial neural network. J. Mol. Liq. 2017, 229, 114–124. [CrossRef] 99. Sanni, E.S.; Emetere, M.E.; Odigure, J.O.; Efeovbokhan, V.E.; Agboola, O.; Sadiku, E.R. Determination of optimum conditions for the production of activated carbon derived from separate varieties of coconut shells. Int. J. Chem. Eng. 2017, 2017, 2801359. [CrossRef] 100. Momodu, D.; Bello, A.; Oyedotun, K.; Ochai-Ejeh, F.; Dangbegnon, J.; Madito, M.; Manyala, N. Enhanced electrochemical response of activated carbon nanostructures from tree-bark biomass waste in polymer-gel active electrolytes. RSC Adv. 2017, 7, 37286–37295. [CrossRef] 101. Zhu, D.D.; Zuo, J.; Jiang, Y.S.; Zhang, J.Y.; Zhang, J.P.; Wei, C.D. Carbon-silica mesoporous composite in situ prepared from coal gasification fine slag by acid leaching method and its application in nitrate removing. Sci. Total Environ. 2020, 707, 136102. [CrossRef][PubMed] 102. Walsh, F.C.; de Leon, C.P. Progress in electrochemical flow reactors for laboratory and pilot scale processing. Electrochim. Acta 2018, 280, 121–148. [CrossRef] 103. Korhonen, K.; Kristensen, T.B.; Falk, J.; Lindgren, R.; Andersen, C.; Carvalho, R.L.; Malmborg, V.; Eriksson, A.; Boman, C.; Pagels, J.; et al. Ice-nucleating ability of particulate emissions from solid-biomass-fired cookstoves: An experimental study. Atmos. Chem. Phys. 2020, 20, 4951–4968. [CrossRef] 104. Kharrazi, S.M.; Mirghaffari, N.; Dastgerdi, M.M.; Soleimani, M. A novel post-modification of powdered activated carbon prepared from lignocellulosic waste through thermal tension treatment to enhance the porosity and heavy metals adsorption. Powder Technol. 2020, 366, 358–368. [CrossRef] 105. Gu, H.M.; Bergman, R.; Anderson, N.; Alanya-Rosenbaum, S. Life cycle assessment of activated carbon from woody biomass. Wood Fiber Sci. 2018, 50, 229–243. [CrossRef] C 2021, 7, 39 33 of 33

106. Roman, S.; Ledesma, B.; Alvarez-Murillo, A.; Al-Kassir, A.; Yusaf, T. Dependence of the microporosity of activated carbons on the lignocellulosic composition of the precursors. Energies 2017, 10, 542. [CrossRef] 107. Delgado-Moreno, L.; Bazhari, S.; Gasco, G.; Mendez, A.; El Azzouzi, M.; Romero, E. New insights into the efﬁcient removal of emerging contaminants by biochars and hydrochars derived from olive oil wastes. Sci. Total Environ. 2021, 752, 141838. [CrossRef]