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,