Journal of C Research

Article Physicochemical Properties of Activated Carbon: Their Effect on the of Pharmaceutical Compounds and Adsorbate–Adsorbent Interactions

Valentina Bernal 1 , Liliana Giraldo 1 and Juan Carlos Moreno-Piraján 2,* 1 Departamento de Química, Universidad Nacional de Colombia, 11001000 Bogotá, Colombia; [email protected] (V.B.); [email protected] (L.G.) 2 Departamento de Química, Universidad de los Andes, 11001000 Bogotá, Colombia * Correspondence: [email protected]; Tel.: +57-1-3394-949 (ext. 3465, 3478 and 4753)

 Received: 3 October 2018; Accepted: 30 October 2018; Published: 19 November 2018 

Abstract: The adsorption of salicylic , acetaminophen, and methylparaben (pharmaceutical products derived from phenol) on activated with different surface chemistries was carried out. We evaluated the effect of the physicochemical properties of the adsorbent and adsorbates on the adsorption capacity. A study of the adsorbate–adsorbent interactions via immersion calorimetry in the analytes at different concentrations was included, in addition to the equilibrium data analysis. The results show that the pharmaceutical compounds (2.28–0.71 mmol g−1) have lower adsorption capacities in the activated carbon with the highest content of oxygenated groups (), while the activated carbons with amphoteric characteristics increase the capacities of adsorption (2.60–1.38 mmol g−1). This behavior may be associated with the increased affinity between the adsorbent and solvent due to the presence of polar groups, which was corroborated by the high −1 immersion enthalpy value in water (∆HimmH2O = −66.6 J g ). The equilibrium data, adjusted to the Freundlich adsorption model, indicated that the heterogeneous adsorption processes involve immersion enthalpy values between −9.42 and −24.3 J g−1.

Keywords: acetaminophen; activated carbon; adsorption; immersion enthalpy; methylparaben; phenol and salicylic acid; salicylic acid

1. Introduction The most common use of activated carbons is in adsoption processes, because the adsorbent presents the necessary physicochemical characteristics that allow it to capture substances that are desired to be removed from systems in gaseous or liquid phases [1,2]. Due to anthropogenic activities, water sources are contaminated with substances of different natures: heavy , , products of oil refineries, and chemical agents. Pharmaceutical products, which are considered to be emerging contaminants due to their presence in , surface water, and , are included within this last category [3–5]. It is a priority to find effective methods for removing compounds with biological activity, due to their toxicity, from water sources—not only for plants and animals but also for humans who are exposed to it. Active pharmaceutical ingredients such as salicylic acid induce oxidative stress in plants [6]; acetaminophen generates hepatotoxicity and neurotoxicity in aquatic vertebrates [7]; and methylparaben is associated with the proliferation of tumor cells and reproductive abnormalities in humans [8]. Adsorption is a method widely used for the purpose of removing such compounds, due to its low cost of implementation, high efficiency, and easy operational design [9]. Recent research has shown that this process, when carried out in activated carbon, allows the carbon to adsorb at least 50% of the initial concentration of contaminants present in the water [10–12].

C 2018, 4, 62; doi:10.3390/c4040062 www.mdpi.com/journal/carbon C 2018, 4, 62 2 of 20

The efficiency of the adsorption process depends on the physicochemical characteristics of the adsorbate and adsorbent. For the latter, factors such as the pore size, area, and surface chemistry are relevant if the process is carried out in activated carbon. One of the advantages of this adsorbent is that it allows modifications to the chemical surface through the addition of heteroatoms, such as oxygen, , or sulfur, which favor interactions with molecules that are desired to be adsorbed. However, this also increases the affinity with water, because it increases the surface polarity [13,14]. Organic compounds, including pharmaceutical compounds, are apolar. Thus, adsorption can be favored on adsorbents with low surface polarity, so an option to increase the adsorption capacity is to modify the surface chemistry through the reduction of chemical groups present on the activated carbon. This process can be carried out by increasing the adsorbent above the thermal stability of the surface chemical groups on the activated carbon. In this way, the oxygenated groups are converted to CO2 and CO. According to De la Puente et al. [15], the thermal stability of the main oxygen groups range between 200 K and 1500 K. Table1 shows the temperature range of the thermal stability of the main oxygenate groups present on activated carbon.

Table 1. Thermal stability temperature of functional groups on activated carbon.

Functional Group Thermal Stability Temperature (K) Carboxylic acid 523–673 Lactone 673–923 Phenol 623–923 Carbonyl 973–1173 Quinone 1073–1173 Pyrone 1273–1473

Sigmund et al. indicated that aromatic compounds such as phenols increase their adsorption capacity if π–π interactions between the adsorbate and adsorbent occur [16]. Activated carbon, due to its aromatic nature, possesses this type of electron. However, according to Long et al., if the adsorbents are subjected to high , they increase the amount of these electrons, which causes the sites to become highly basic. Therefore, adsorbates with acidic characteristics can be selectively adsorbed [17]. In this work, the adsorption of phenol, salicylic acid, acetaminophen, and methylparaben on five activated carbons with different surface chemistries—a granular activated carbon made from shell (CAG) subjected to an HNO3 oxidation or heat treatment at different temperatures (1073 K, 1173 K, and 1273 K)—was carried out. The effect of the physicochemical changes of the adsorbent in the adsorption capacity of the compounds was determined from the equilibrium data (adsorption isotherms), and the adsorbate–adsorbent affinity was evaluated by immersion enthalpy in the studied compound solutions.

2. Materials and Methods

2.1. Activated Carbons Table2 discloses preparation methods of the activated carbons analyzed in this work.

Table 2. Modifications made to carbon activated granular (CAG).

Activated Carbon Modification Method Granular activated carbon (CAG) (CARBOCHEM CARBOCHEM GS50 INC., Philadelphia, CAG PA, USA) immersed in concentrated HCl, rinsed with distilled water until reaching a constant pH value, and dried at 373 K.

Oxidized activated carbon (CAO). Activated carbon CAG immersed in HNO3 (6 M) for 6 CAO hours until it reached its boiling temperature, washed with distilled water until reaching a constant pH value, and dried at 373 K. CAR1073, CAR1173, Reduced activate (CAR). Activated carbon subjected to heat treatment in a THERMOLYNE and CAR1273 oven for 2 hours with a heating ramp of 2 K min−1 at 1073, 1173, or 1273 K, respectively. C 2018, 4, 62 3 of 20

2.2. Physical Characterization Nitrogen adsorption at 77 K was performed in a commercial semi-automatic Autorsorb IQ2 (Quantachrome Instruments, Boynton Beach, FL, USA after degassing the activated carbons for 24 h at 473 K. The Brunauer, Emmett, and Teller (BET); Dubinin–Astakhov; and density functional theory (DFT) models were applied to the isotherms to obtain models for the determination of the apparent surface area, micropore volume, and distribution of pore sizes, respectively.

2.3. Chemical Characterization

2.3.1. The Determination of Surface Chemistry (Oxygenated Functional Groups) of the Activated Carbon Boehm’s method allows the determination of the oxygenated chemical groups (phenols, lactones, and carboxylic acids). It corresponds to the acid– titration of the functional groups present on the activated carbon surface with bases that have different strengths, which allows the neutralization of acids with different pKa values [18]. This method was carried out on an Ohaus Pioneer PA 114 analytical balance (Ohaus Corporation, Parsippany, NJ, USA), with an accuracy of 0.001 g. First, 0.5 g of activated carbon CAG was weighted out in airtight glass containers. Next, 0.05 L of a NaOH 0.1 M was added, and maintained at room temperature (293 ± 1 K) for 5 days with constant agitation at 100 rpm. Subsequently, an aliquot of the supernatant was taken and titrated with HCl using a potentiometer CG840 Schott (Schott AG, Maguncia, Germany). The above procedure was repeated using carbonate (Na2CO3) and sodium bicarbonate (NaHCO3) as the immersion liquids. For the quantification of basic groups, the previous procedure was carried out using 0.05 L of HCl 0.1 M as the immersion liquid. This was maintained for 5 days, an aliquot of supernatant was taken and it was titrated with 0.1 M NaOH. The solutions of HCl and NaOH were previously standardized with boric acid and potassium biftalate.

2.3.2. pH Determination of the Zero Charge Point Some properties associated with the adsorption on solid–liquid systems are influenced by the appearance of surface electric charges in the adsorbent and the adsorbate. When activated carbon is placed in contact with an electrolytic solution, the surface ionizes depending on the pKa of the functional groups present in the surface, and the charged particle is surrounded by of the opposite charge since it is apparently electrically neutral. The pH value required for the net surface charge of activated carbon is zero and is known as the point of zero charge (pHpzc). The mass titration method was proposed by Noah and Schwartz for the determination of the net charge of alumina oxides in solution. This technique consists in measuring a pH value that becomes constant with the increase in mass of the solid in a volume of water. Under these conditions, it has been observed that the pH of the oxide aqueous suspension depends on the amount of this material in a given volume of water [19]. On an Ohaus Pioneer PA 114 balance (Ohaus Corporation, Parsippany, NJ, USA), with an accuracy of 0.001 g, between 4 g and 4.5 g of activated carbon was weighed in amber glass containers with lid, to which was added 0.01 L of a solution of NaCl 0.1 M. The containers are stored for 5 days at ambient temperature (293 ± 1 K) and constant agitation (100 rpm); past the equilibrium time, 0.005 L of the supernatant solution of each container was extracted and the pH was measured with a pH meter CG 840 Schott (Schott AG, Maguncia, Germany). C 2018, 4, 62 4 of 20

2.4. Adsorption Test

2.4.1. Preparation of Pharmaceutical Compound Solutions Adsorption tests were carried out using solutions of phenol, salicylic acid, acetaminophen, and methylparaben in concentrations between 0.07 and 10.6 mmol L−1, since at these concentrations there is a sufficient amount of adsorbate molecules to evaluate the adsorbate-adsorbent interactions without the interference of the activated carbon–solvent interactions. Stock solutions were prepared with analytical grade reagents: phenol (Merck Millipore, MA, USA) with a purity of 99%, salicylic acid (Panreac chemistry SLU, Castellar del Vallès, Barcelona, Spain) with a purity of 99%, acetaminophen (Alpha Aesar, MA, USA) with a purity of 98%, and methylparaben (Panreac chemistry SLU, Castellar del Vallès, Barcelona, Spain) with a purity of 99%. The solutions were prepared by dissolving 2 g of the reagent in 1 L of distilled water, stirring at 150 rpm and 293 ± 1 K. The dilutions were made by taking stock solution aliquots and adjusting the volume with distilled water.

2.4.2. Batch Experiments First, 0.1 g of activated carbon CAG was weighed in amber glass containers, to which 0.025 L of the analyte solution (phenol, salicylic acid, acetaminophen, and methylparaben) was added with concentrations between 0.07 mmol L−1 and 10.6 mmol L−1. The containers were kept at room temperature (293 ± 1 K) and with sporadic agitation for 7 days (150 rpm). Once the equilibrium time was reached, the solutions were filtered and the equilibrium concentration was determined by UV-Vis spectrophotometry (Thermo Fisher Scientific, Madison, WI, USA) at the maximum wavelength of the analyte. Table3 shows the maximum wavelengths used for each analyte.

Table 3. Maximum wavelengths of the analytes.

Compound λmax H2O (nm) Phenol 268 Salicylic acid 296 Acetaminophen 242 Methylparaben 254

The concentration equilibrium was calculated the from the calibration curve. Equation (1) is used to determine the adsorbed amount of analyte:

(C − C ) ∗ V Q = o e , (1) m where Q represents the adsorbed amount, Co the initial concentration, Ce the equilibrium concentration, V the total volume of solution, and m the mass of activated carbon employed.

2.5. Calorimetric Assays The immersion enthalpy on the activated carbons CAG, CAO, CAR1073, CAR 1173, and 1273 CAR were determined in aqueous solutions of phenol, salicylic acid, acetaminophen, and methylparaben with concentrations between 0.07 and 10.6 mmol L−1. For this purpose, a Tian-Calvet calorimeter of local construction was employed, in which 0.01 L cell of immersion liquid was deposite. Activated carbon (0.1 g) was previously weighed out in a glass ampoule with a fragile peak in the bulb that facilitates its later break at the immersion time. The glass ampoule was placed on the top of the calorimeter. The electric potencial was recorded until reaching a baseline, and then the immersion of the activated carbon was carried out. The increase in potential is associated with the immersion, and once the value returned to the baseline, and electrical calibration was performed. C 2018, 4, 62 5 of 20

The immersion enthalpy is obtained using Equations (2) and (3):

Qimm = Kcal * area under the immersion curve (2)

−Q ∆H = imm (3) imm m where Qimm represents the heat of immersion, Kcal is the calibration constant of the calorimeter determined from the electrical calibration, and the area under the curve is the integral of the potential’s peak generated during the adsorption.

3. Results and Discussion

3.1. Physicochemical Characterization of Activated Carbons

3.1.1. Textural Characterization Table4 shows the values of surface area and pore size of the activated carbons CAG, CAO, CAR1073, CAR 1173, and CAR 1273, determined by BET and Dubinin–Astakhov models, respectively.

Table 4. Textural characteristics of activated carbons CAO, CAG, CAR1073, CAR1173, and CAR1273.

Textural Characteristic CAO CAG CAR1073 CAR1173 CAR1273 2 −1 Surface area (SBET) (m g ) 469 864 1127 814 711 Micropore volume (cm3 g−1) 0.18 0.34 0.42 0.29 0.23 Total volume 1 (cm3 g−1) 0.21 0.35 0.48 0.34 0.30 1 Calculated with the DFT (quenched solid density functional theory) method, assuming slit cylindrical pores.

According to the data in the Table4, it is evident that the oxidation treatment with nitric acid decreases the surface area, micropore volume, and total volume of pores compared to the results of the activated carbon CAG. Gokce et al. [20] indicated that this behavior is due to the conversion of micropores into mesoporous by the effect of strong oxidation. Moreover, the decrease in the surface area is associated with the pores blocking due to the formation of acid groups that prevent nitrogen access through the activated carbon structure. According to Shim et al. [21], the formation of acid groups tends to occur to in the edges pores because it is the weakest part of the carbonaceous structure. Furthermore, the graphenic layers are very stable, so the oxidation there is very slow. With respect to the activated carbon CAG, it was determined that the increase in the thermal treatment temperature above 1073 K decreases its textural characteristics, while up to 1073 K the parameters increase. For activated carbons CAR1173 and CAR1273, it was determined that the decrease in the textural properties is associated with pore blockage with gases (CO, CO2) originating during the thermal treatment [22]. In addition, this behavior can be caused by the collapse of carbonaceous structures due to temperature; this is observed in the decrease of the total volume and micropore volume values (see Table4). Hu et al. indicated that low temperatures of burn-off favored the formation of microporosity, as is the case of activated carbon CAR1073. Likewise, the increase of this type of pores favors the adsorption of nitrogen, which generates an increase in the textural parameters for this sample [23]. Figure1 shows the pore size distribution of the activated carbons. It can be observed that these samples have pores between 10 and 14 Å—pores suitable for the adsorption of molecules such as phenols, which have areas smaller than this value. C 2018, 4, x FOR PEER REVIEW 6 of 21 C 2018, 4, 62 6 of 20

0.018

CAO 0.016 CAG CAR1073 0.014 CAR1173 CAR1173 ) 0.012

-1

g

-1 0.010

Å

3

0.008

dV(r) (cm dV(r) 0.006

0.004

0.002

0.000 10 20 30 40 50

Half width pore (Å) FigureFigure 1.1. PorePore size size distributions distributions of of the the activated activated carbons carbons CAO, CAO, CAG, CAG, CAR1 CAR1073,073, CAR1173 CAR1173,, and andCAR1 CAR1273.273.

3.1.2.3.1.2. Chemical Chemical Characterization TableTable5 shows 5 shows the the results results of of the the chemical chemical characterization characterization of of activated activated carbons carbons CAO, CAO, CAG, CAG, CAR1073,CAR1073, CAR1173, CAR1173 and, and CAR1273. CAR1273.

Table 5. Chemical characteristics of the activated carbons CAO, CAG, CAR1073, CAR1173, Table 5. Chemical characteristics of the activated carbons CAO, CAG, CAR1073, CAR1173, and and CAR1273. CAR1273. Chemical Characteristic CAO CAG CAR1073 CAR1173 CAR1273 Chemical Characteristic CAO CAG CAR1073 CAR1173 CAR1273 −1 Total acidityTotal ( µaciditymol g ) 656 90.5 93.6 93.0 94.1 Total basicity (µmol g−1) 735656 90.5 742 93.6 1210 93.0 2037 94.1 2290 (µmol g−1) Carboxylic acid (µmol g−1) 106 22.2 66.1 65.5 64.7 Total basicit−1y Lactones (µmol g ) 490735 21.8742 1210 21.2 2037 23.8 2290 46.8 Phenols(µ (µmolmol g g−1−)1 ) 53.9 46.6 6.36 3.71 17.5 CarboxylicpHpzc acid 3.40 5.40 11.1 8.90 9.96 106 22.2 66.1 65.5 64.7 (µmol g−1) Lactones Table5 presents the results of the chemical490 characterization21.8 21.2 of the activated23.8 carbons46.8 CAO, CAG, −1 CAR1073, CAR1173,(µ andmol CAR1273. g ) It can be observed that activated carbon CAO has the highest Phenols amount of acid groups (lactones > carboxylic53.9 acid46.6 > phenols)6.36 due to the3.71 oxidation 17.5 with nitric acid, −1 which gives it a total(µ aciditymol g higher) than the rest of the studied activated carbons. Boehm indicated that strong oxidations generatepHpzc functional3.40 groups 5.40 such as carboxylic11.1 acids8.90 and phenols9.96 [18], and that groups such as the lactones are the product of the condensation of phenols and vicinal carboxylic acid groupsTable 5 during presents thermal the results treatment. of the In chemical activated characterization carbon CAG, it ofis evident the activated that phenol carbons groups CAO, areCAG, prevalent CAR1073, because CAR1173 coconut, and shell CAR1273. is constituted It can by be lignin, observed cellulose, that activated and hemicellulose, carbon CAO which has are the polymershighest amount with abundant of acid groups phenol ( contents.lactones > carboxylic acid > phenols) due to the oxidation with nitric acid,The which CAR1073, gives CAR1173, it a total acidity and CAR1273 higher activated than the carbons rest of the have studied a total activated acidity equivalent carbons. to Boehm that ofindicate the startingd that activated strong oxidations carbon. However, generate atfunctional the temperature groups such of thermal as carboxylic treatment acids their and reduction phenols was[18] expected,, and thattherefore groups such the formationas the lactones of acid are groups the product after the of thermalthe condensation treatment of was phenols attributed and tovicinal the reactioncarboxylic between acid groups free radicals during generated thermal treatment. by the breakdown In activated of carbon the carbonaceous CAG, it is evident chains inthat the phenol heat treatmentgroups are and prevalent environmental because oxygen-forming coconut shell groups is constituted such as by carboxylic lignin, cellulose acids [24,]. and hemicellulose, which are with abundant phenol contents. C 2018, 4, x FOR PEER REVIEW 7 of 21

The CAR1073, CAR1173, and CAR1273 activated carbons have a total acidity equivalent to that of the starting activated carbon. However, at the temperature of thermal treatment their reduction Cwas2018 expected,, 4, 62 therefore the formation of acid groups after the thermal treatment was attributed7 of to 20 the reaction between free radicals generated by the breakdown of the carbonaceous chains in the heat treatment and environmental oxygen-forming groups such as carboxylic acids [24]. The formation of acid groups results in greater surface polarity.polarity. Therefore, contacting activated carbons with water will generate an interaction with this solvent,solvent, depending on the affinity affinity that is established between between them. them To. T establisho establ thisish relation this relation for the for studied the activatedstudied activated carbons, water carbons, immersion water calorimetryimmersion calorimetry was performed, was performed, and the results and arethe shown results in are Table shown6. in Table 6.

Table 6. ImmersionImmersion enthalpy enthalpy in inwater water for for activated activated carbons carbons CAO, CAO, CAG, CAG, CAR1073, CAR1073, CAR1173 CAR1173,, and andCAR1273. CAR1273.

∆ΔHHimmimmCAOCAO Δ∆HHimimmmCAGCAG Δ∆HHimmimmCAR1073CAR1073 ΔH∆HimmimmCAR1173CAR1173 ΔH∆HimmimmCAR1273CAR1273 (J g−1) (J g−1) (J g−1) (J g−1) (J g−1) (J g−1) (J g−1) (J g−1) (J g−1) (J g−1) −−66.666.6± ± 1.331.33 −49.749.7 ±± 0.990.99 −−27.327.3 ±± 0.550.55 −−32.432.4 ± ±0.650.65 −31.5−31.5 ± ±0.630.63

According to to the the results results of of Table Table6, the 6, activatedthe activated carbon carbon with greaterwith greater immersion immersion enthalpy enthalpy values invalues water in was water the was activated the activated carbon modified carbon modified with nitric with acid, nitric CAO; acid therefore,, CAO; therefore, the greater the the greater number the of acidnumber groups, of acid the groups higher, thethe affinityhigher the with affinity the solvent with the due solvent the greater due surfacethe greater polarity. surface polarity. In the case of of activated activated carbon CAG, the immersionimmersion enthalpy in water was greatergreater than the activated carbons with heat treatment.treatment. W Whenhen associating the data of the chemicalchemical characterizationcharacterization with thesethese results,results, itit waswas foundfoundthat that this this activated activated carbon carbon had had a a greater greater amount amount of of phenol phenol groups, groups, so so it wasit was determined determined that that the adsorbent-solvent the adsorbent-solven interactiont interaction was associated was associated with the with formation the formation of of bondshydrogen interactions bonds interactions between phenol between and water. phenol Figure and 2 water shows. Figure that for 2 the shows studied that activated for the carbons, studied thereactivated was a carbons directly, there proportional was a relationshipdirectly proportional between the relationship amount of surfacebetween phenols the amount and the of enthalpy surface ofphenols immersion and the in water.enthalpy of immersion in water.

Figure 2.2. Relationship between the amount of phenol groups and the enthalpy of immersion in water of the activated carbons CAO, CAG, CAR1073, CAR1173,CAR1173, andand CAR1273.CAR1273.

The basicity in the activated carbons was attributed to thethe formationformation ofof oxygenatedoxygenated functional groups, suchsuch asas pyrone,pyrone, chromene, chromene, diketone, diketone and, and quinone, quinone as, as well well as as the the delocalized delocalized electrons electronsπ of π the of graphenicthe graphenic layers. layers. Mont Montésés-Mor-Moránán et al. et [25al.] [2 indicated5] indicate thatd that the basicitythe basicity of groups of groups like pyroneslike pyrones is low; is however,low; however, if the if group the g isroup included is included in cycles in ofcycles more of than more one than ring, one the ring pKa, the increases pKa increases above 12 above due to 12 a 2 stabilizationdue to a stabilization of the protonated of the form protonated through form the conjugation through the of conjugationπ electrons with of π sp electronsstructures. with sp2 + structuresOn the. other hand, the basicity of the π electrons was evaluated from the adsorption of H ions in the layers, a process that was carried out by electrostatic interactions. According to Montés-Morán, delocalized π electrons are the main contributors to the basicity of activated carbon, as they are found in a greater proportion compared to the number of basic functional groups. C 2018,C4 ,2018C 62 C2018 2018, 4,, x4, ,FOR4 x, xFOR FOR PEER PEER PEER REVIEW REVIEW REVIEW 8 of8 218of8 of 21 of 21 20

OnOn Onthe the theother other other hand, hand, hand, the the thebasicity basicity basicity of ofthe of the theπ electronsπ π electrons electrons was was was evaluated evaluated evaluated from from from the the theadsorption adsorption adsorption of ofH of +H ionsH+ ions+ ions Thein in thein pH the the graphene of graphene thegraphene point layers, layers, of layers, zero a process a acharge process process thatis that one that was was ofwas carried the carried carried physicochemical out out outby by electrostaticby electrostatic electrostatic parameters interactions. interactions. interactions. with greaterAccording According According relevance to to to in theMontés adsorptionMontésMontés-Morán,-Morán,-Morán, processes delocalized delocalized delocalized carried π electronsπ π electrons out electrons in are aqueous are arethe the themain main solutions, main contributors contributors contributors because to tothe to the it thebasicity indicates basicity basicity of ofactivated theof activated activated net charge carbon, carbon, carbon, from the adsorbentas astheyas they they are whenare arefound found found it in comes ina in greatera agreater ingreater contact pr proportion proportionoportion with compared water. compared compared The to to pHthe to the thenumber of number thenumber point of ofbasic of basic of basic zerofunctional functional functional charge groups. isgroups. groups. dependent on the surfaceThe chemistryTheThe pH pH pH of of the of of the the point point point adsorbents. of of zero of zero zero charge charge As charge such, is is one is activatedone one of of the of the the physicochemical carbons physicochemical physicochemical with a highparameters parameters parameters content with of with with acid greater greater greater groups presentrelevancerelevance arelevance pH ofin theinthe in the the pointadsorption adsorption adsorption zero processes charge processes processes below carried carried carried 7 out (such out outin inaqueous in as aqueous aqueous CAO), solution solution whilesolutions, activateds,becauses, because because it indicatesit carbons it indicates indicates the with the thenet net basic net chargechargecharge from from from the the theadsorbent adsorbent adsorbent when when when it comesit it comes comes in incontact in contact contact with with with water. water. water. The The The pH pH pHof ofthe of the thepoint point point of ofzero of zero zero charge charge charge is is is structures present values above this pH (CAR). In terms of electric charge, Noah et al. [19] indicated dependentdependentdependent on on onthe the the surface surface surface chemistry chemistry chemistry of of the of the the adsorbents adsorbents adsorbents. As. . As As such, such, such, activated activated activated carbons carbons carbons with with with a higha ahigh high that at the pH of the point of zero charge the surface of the activated carbon is neutral, while below contentcontentcontent of ofacid of acid acid groups groups groups present present present a pHa apH pHof ofthe of the thepoint point point zero zero zero charge charge charge below below below 7 (7such 7(such (such as asCAO) as CAO) CAO), while, while, while activated activated activated this valuecarbonscarbonscarbons the with surfaceswith with basic basic basic structures have structures structures a positive present present present values charge values values above due above above tothis this the this pH pH protonation pH(CAR) (CAR) (CAR). In. In.terms In terms of terms functionalof ofelectric of electric electric charge, groups charge, charge, Noah suchNoah Noah as amines.et etal. Atet al. [al.19 pH [ 19][ 19indicate] values ]indicate indicated above thatdd that that at this atthe at the the givenpH pH pHof ofthevalue, of the thepoint point point the of electricofzero of zero zero charge charge chargecharge the the ofthesurface surface the surface adsorbent of ofthe of the theactivated activated activated is negative carbon c arboncarbon dueis is is to the deprotonationneutral,neutral,neutral, while while while ofbelow functionalbelow below this this this value groups value value the the the such surfaces surfaces surfaces as phenols have have have a apositive and a positive positive carboxylic charge charge charge due acids. due due to to Thus, the to the the protonation protonationat protonation the pH of of of the of solutionsfunctionalfunctionalfunctional (around groups groups 6),groups the such suchactivated such as as aminesas amines amines carbon. A. t.A pHAt carbontpH pHvalues values values CAO above above above had this thisa this given negative given given value, value, value, surface the the theelectric chargeelectric electric charge whilecharge charge of the of theof the other the activatedadsorbentadsorbentadsorbent carbons is negativeis hadis negative negative a positive due due due to tothe charge.to the thedeprotonation deprotonation deprotonation of offunctional of functional functional groups groups groups such such such as asphenols as phenols phenols and and and carboxylic carboxylic carboxylic Itacids canacidsacids. Thus, be. Thus,. observedThus, at atthe at the thepH frompH pHof ofthe of thethe thesolution solution values solutions (arounds shown s(around (around 6) in ,6) the Table6), the, theactivated activated5 activated that carbon the carbon carbon pH carbon ofcarbon carbon the CAO pointCAO CAO ha had of ha ad zerod negativa anegativ negativ chargee e e increasessurfacesurfacesurface in proportioncharge charge charge while while while tothe the theother other basic other activated activated groupsactivated carbons presentcarbons carbons ha had on ha ad thedpositivea apositive positive activated charge. charge. charge. carbons of study. In addition, a It ItcanIt can canbe be observedbe observed observed from from from the the thevalues values values shown shown shown in inTable in Table Table 5 that5 5that that the the thepH pH pHof of thof eth thpointe epoint point of of zeroof zero zero charge charge charge decrease in the pH of the point of zero charge of the activated carbons CAR1173 and CAR1273 was increasesincreasesincreases in inproportion in proportion proportion to tothe to the thebasic basic basic groups groups groups present present present on o theno nthe theactivated activated activated carbons carbons carbons of ofstudy. of study. study. In Inaddition, In addition, addition, a a a shown; this behavior was attributed to the increase of the basicity by π electrons and not by functional decreasedecreasedecrease in inthe in the thepH pH pHof ofthe of the thepoint point point of ofzero of zero zero charge charge charge of ofthe of the theactivated activated activated carbons carbons carbons CAR1173 CAR1173 CAR1173 and and and CAR1273 CAR1273 CAR1273 was was was groupsshown;shown; thatshown; canthis this donatethis behavior behavior behavior or was accept was was attributed attributed protonsattributed to with to theto the the increase increase increasemedium; of of the of thetherefore, the basicity basicity basicity by part by byπ πof electrons π electrons the electrons basicity and and and notcannot not notby by by be quantifiedfunctionalfunctionalfunctional through groups groups groups the that pH. that that can can candonate donate donate or oraccept or accept accept protons protons protons with with with the the themedium medium medium; therefore,; therefore,; therefore, part part part of ofthe of the thebasicity basicity basicity cannotcannotcannot be be quantified be quantified quantified through through through the the thepH. pH. pH. 3.2. Adsorption Test 3.23.2. Adsorption. Adsorption Test Test The3.2. 3.2Adsorption adsorption. Adsorption Test processesTest not only depend on the physicochemical characteristics of the adsorbents;TheThe propertiesThe adsorption adsorption adsorption such processes processes asprocesses , not not not only molecular only only depe depe dependnd size,nd on on onthe molecular the the physicochemical physicochemical physicochemical weight, and characteristics characteristics pKacharacteristics are also of of relevant the of the the to theadsorbents processesadsorbentsadsorbents; carriedproperties; ;properties properties out such in such solution.such as as solubility, as solubility, solubility, Table 7 molecular shows molecular molecular the size, physicochemicalsize, size, molecular molecular molecular weight weight weight characteristics, and, ,and and pKa pKa pKa are of are arealso phenol, also also salicylicrelevantrelevant acid,relevant to acetaminophen, tothe to the theprocesses processes processes carried carried and carried methylparaben.out out outin insolution. in solution. solution. Table Table Table 7 shows 7 7shows shows the the thephysicochemical physicochemical physicochemical characteristics characteristics characteristics of of of phenol,phenol,phenol, salicylic s alicylicsalicylic acid, acid, acid, acetaminophen acetaminophen acetaminophen, and, and, and methylparaben. methylparaben. methylparaben. Table 7. Physicochemical characteristics of phenol, salicylic acid, acetaminophen, and methylparaben. TableTableTable 7. Physicochemical7 .7 Physicochemical. Physicochemical characteristics characteristics characteristics of ofphenol, of phenol, phenol, salicylic salicylic salicylic acid, acid, acid, acetaminophen, acetaminophen, acetaminophen, and and andmethylparaben methylparaben methylparaben. . . Physicochemical PhysicochemicalPhysicochemicalPhysicochemical Phenol Salicylic Acid Acetaminophen Methylparaben Characteristic PhenolPhenolPhenol SalicylicSalicylicSalicylic A cidA Acid cid AcetaminophenAcetaminophenAcetaminophen MethylparabenMethylparabenMethylparaben CharacteristicCharacteristicCharacteristic

StructureStructureStructureStructure

MolecularMoleMoleMolecular sizecularcular (Åsize size2 )size 5.76 × 4.17 6.96 × 5.87 8.50 × 5.70 8.69 × 5.02 5.75.76 6× ×4 .174.17 6.966.96 × ×5.87 5.87 8.508.50 × ×5.70 5.70 8.698.69 × ×5.02 5.02 2 2 5.765.7 × 46.17 × 4 .17 6.966.96 × 5.87 × 5.87 8.508.50 × 5.70 × 5.70 8.698.69 × 5.02 × 5.02 Solubility(Å(Å2 H)(Å 2)O2 ) −1 0.882 0.017 0.093 0.016 298So KSolubility (molSolubilitylubility L H 2H)O H2 O 2O pKa298298 298K K K 0 9.99.8820.0882. 882 0.0170 2.97.0017. 017 0.0930.0 9.34093. 093 0.0160 8.20.0016. 016 Red spheres(mol(mol(mol L represent−1 L) L−1)−1 ) the oxygen atoms, blue spheres the nitrogen atoms, black spheres the carbon atoms, and white spherespKappKa Ka the hydrogen atoms.9.999.999.99 2.972.972.97 9.349.349.34 8.208.208.20 RedRed Redspheres spheres spheres represent represent represent the the othexygen o xygenoxygen atoms, atoms, atoms, blue blue blue spheres spheres spheres th eth ntheitrogen en itrogennitrogen atoms, atoms, atoms, black black black spheres spheres spheres the the carbonthe carbon carbon It isatoms shownatomsatoms, an, dan, in anwhited dwhite Table white spheres spheres 7spheres that the the hthe theydrogen h ydrogenhydrogen adsorbates atoms. atoms. atoms. present differences in their physicochemical characteristics despite having in common the group structure phenol. Properties such as solubility It It isIt isshown is shown shown in in Tablein Table Table 7 7 that 7that that the the theadsorbates adsorbates adsorbates present present present differences differences differences in in their in their their physicochemical physicochemical physicochemical and pKa vary depending on the substituent present in the aromatic ring; for example, the presence characteristicscharacteristicscharacteristics despite despite despite having having having in incommon in common common the the thegroup group group structure structure structure phenol p phenolhenol. P.roperties P. Propertiesroperties such such such as assolubility as solubility solubility of groups such as carboxylic acid (salicylic acid) and ester (methylparaben) decrease the solubility of andandand pKa pKa pKa vary vary vary depending depending depending on on theon the thesub sub substituentstituentstituent present present present in inthe in the thearomatic aromatic aromatic ring ring ring; for; for; example,for example, example, the the thepresence presence presence the compounds,of ofgroupsof groups groups such whichsuch such as ascarboxylic as iscarboxylic carboxylic an indication acid acid acid (salicylic (salicylic (salicylic that theacid) acid) acid) adsorbate–solvent and and and ester ester ester (methylparaben (methylparaben (methylparaben interactions) decrease) decrease) decrease are the disfavored, the thesolubility solubility solubility of a of step of that isthe requiredthe thecompounds compounds compounds for the, which, which, adsorption which is anis is an indication an indication toindication take that place. that that the the theadsorbate Except adsorbate adsorbate for–solvent– salicylicsolvent–solvent interactions interactions acid, interactions the are other are aredisfa disfa compoundsdisfavored,vored,vored, a stepa astep step have valuesthat ofthatthat pKais requiredis is requiredabove required for pH for thefor 8,the theadsorption which adsorption adsorption indicates to totake to take take thatplace. place. place. above Except Except Except this for for pH,salicylicfor salicylic salicylic 50% acid, of acid, acid, the the the initial theother other other compounds concentration compounds compounds have have ofhave the adsorbate is ionized. C 2018, 4, x FOR PEER REVIEW 9 of 21

values of pKa above pH 8, which indicates that above this pH, 50% of the initial concentration of the C 2018, 4, 62 9 of 20 adsorbate is ionized.

Adsorption3.2.1. Adsorpti Isothermson Isotherms TheThe datadata ofof thethe adsorptionadsorption equilibriumequilibrium allowsallows thethe determinationdetermination ofof thethe amountamount ofof compoundcompound thatthat can be adsorbed adsorbed in in each each activated activated carbon carbon once once the the equilibrium equilibrium time time has has been been reached reached.. In Ingeneral, general, once once the the adsorption adsorption isotherms isotherms are are determined, determined, it is itpossible is possible to adjust to adjust the data the data to a tomathematical a mathematical model model that that describes describes the the experimental experimental behavior behavior from from chemical, chemical, kinetickinetic,, oror thermodynamicthermodynamic assumptions assumptions.. Figures Figures 33–7– 7show show the the adsorption adsorption isotherms isotherms of phenol, of phenol, salicylic salicylic acid, acid,acetaminophen, acetaminophen, and methylparaben and methylparaben in the in activated the activated carbons carbons CAO, CAO, CAG, CAG, CAR1073, CAR1073, CAR1173 CAR1173,, and andCAR1273. CAR1273. TheThe isothermsisotherms werewere fittedfitted toto thethe Langmuir,Langmuir, Freundlich,Freundlich, Redlich-Peterson,Redlich-Peterson, andand SipsSips models,models, andand thethe parametersparameters ofof eacheach modelmodel cancan bebe foundfound inin TablesTables7 –712–1.2. TheThe selection selection of of the the adsorption adsorption models models used used allows allows the descri the descriptionption of different of different types of typessystems. of systems.The Langmuir The Langmuirmodel is used model for is adsorption used for adsorptionprocesses where processes the adsorbate where the-adsorbent adsorbate-adsorbent interactions interactionsare specific, areand specific, all active and sites all activeare energetically sites are energetically equivalent; equivalent;therefore, the therefore, process theare processconsidered are consideredhomogeneous. homogeneous. TheThe Freundlich,Freundlich, Sips,Sips, andand Redlich-PetersonRedlich-Peterson modelsmodels describedescribe systemssystems thatthat presentpresent highhigh energeticenergetic heterogeneityheterogeneity associated associated with with the the formation formation of of nonspecific nonspecific interactions interactions that that include include lateral lateral and and associativeassociative interactions. interactions. In In the Freundlich model, it is assumed that the the adsorbate adsorbate interacts interacts with with differentdifferent active sites that that differ differ in in the the energy energy they they possess; possess; therefore, therefore, the the energy energy of the of theprocess process is not is notconstant. constant. TheThe SipsSips andand Redlich-PetersonRedlich-Peterson modelsmodels are derivedderived from thethe FreundlichFreundlich andand LangmuirLangmuir models, allowingallowing thethe descriptiondescription ofof systemssystems thatthat exhibitexhibit intermediateintermediate behaviorbehavior [[2626––2828].].

FigureFigure 3.3. AdsorptionAdsorption isothermisotherm of phenol, phenol, acetaminophen, acetaminophen, salicylic acid, acid, andand methylparabenmethylparaben onon activatedactivated carboncarbon CAOCAO atat 293293 K.K. Table 8. Parameters of Langmuir, Freundlich, Redlich-Peterson, and Sips models for phenol, acetaminophen,Table 8. Parameters salicylic of acid, Langmuir, and methylparaben Freundlich, adsorption Redlich-Peterson on activated, and carbon Sips models CAO. for phenol, acetaminophen, salicylic acid, and methylparaben adsorption on activated carbon CAO. a −1 −1 2 Compound Model K1 K2 (L mmol) Qm (mmol g ) n R b K2 Qm Phenol Freundlich 2.10 a NA NA 0.56 0.982 Compound Model K1 n R Salicylic acid Redlich-Peterson 24.5 23.8(L−1 mmol) (mmol NA g−1) 0.69 0.99 AcetaminophenPhenol LangmuirFreundlich 9.14 2.10 NANA b 0.71NA 0.56 NA 0.98 0.96 Methylparaben Freundlich 0.87 NA NA 0.31 0.97 Salicylic acid Redlich-Peterson 24.5 23.8 NA 0.69 0.99 a Corresponds to the equilibrium constant and its units depend on the model of adjustment KL (Langmuir constant) Acetaminophen Langmuir− 1 9.14 NA 0.711−1/n 1/nNA g−1 0.96 and KR-P (Redlich-Peterson constant) = L g , KF (Freundlich constant) = (mmol L ) and KS (Sips constant) = (L mmol−1). b NA: Not applicable. C 2018, 4, x FOR PEER REVIEW 10 of 21

C 2018, 4, x MethylparabenFOR PEER REVIEW Freundlich 0.87 NA NA 0.31 0.97 10 of 21 a Corresponds to the equilibrium constant and its units depend on the model of adjustment KL (LangmuirMethylparaben constant) and KRFreundlich-P (Redlich -Peterson0.87 constant)NA = L g−1, KFNA (Freundlich 0.31 constant)0.97 = (mmola Corresponds1−1/n L1/n g− 1 to) and the K equilibriumS (Sips constant) constant = (L mmol and its−1) . unitsb NA: dependNot applicable on the model of adjustment KL (Langmuir constant) and KR-P (Redlich-Peterson constant) = L g−1, KF (Freundlich constant) = C 2018, 4, 62 10 of 20 (mmol1−1/n L1/n g−1) and KS (Sips constant) = (L mmol−1). b NA: Not applicable

Figure 4. Adsorption isotherm of phenol, acetaminophen, salicylic acid, and methylparaben on activated carbon CAG at 293 K. Figure 4.4. Adsorption isotherm of phenol, acetaminophen, salicylic acid, and methylparaben on Tableactivated 9. Parameters carbon CAGCA ofG Freundlich at 293 K.K. and Redlich-Peterson models for phenol, acetaminophen, salicylic acid, and methylparaben adsorption on activated carbon CAG. Table 9. Parameters of Freundlich and Redlich-Peterson models for phenol, acetaminophen, salicylic Table 9. Parameters of Freundlich and Redlich-Peterson models for phenol, acetaminophen, salicylic acid, and methylparaben adsorption on activated carbon CAG.K2 Qm acid, andCompound methylparaben adsorptionModel on activatedK 1carbon CAG. n R2 −1 −1 −(L1 mmol) (mmol g−1) 2 Compound Model K1 K2 (L mmol) Qm (mmol g ) n R Phenol Freundlich 2.25 NAK2 NAQm 0.58 0.98 CompoundPhenol FreundlichModel 2.25K1 NA NA 0.58n R 0.982 −1 −1 SalicylicSalicylic acid acid FreundlichFreundlich 1.341.34 NA(L NA mmol) (mmol NANA g ) 0.77 0.77 0.99 0.99 AcetaminophenAcetaminophenPhenol Redlich-Peterson RedlichFreundlich-Peterson11.1 11.12.25 9.569.56NA NANANA 0.860.58 0.86 0.980.98 0.98 MethylparabenMethylparabenSalicylic acid FreundlichFreundlichFreundlich 1.57 1.571.34 NANANA NANANA 0.450.77 0.45 0.970.99 0.97 Acetaminophen Redlich-Peterson 11.1 9.56 NA 0.86 0.98 Methylparaben Freundlich 1.57 NA NA 0.45 0.97

FigureFigure 5 5.. AdsorptionAdsorption isotherm isotherm of of phenol, phenol, acetaminophen, acetaminophen, salicylic salicylic acid, acid, and and methylparaben methylparaben on on activatedactivated carbon carbon CAR1073 CAR1073 at at 293 293 K K.. Figure 5. Adsorption isotherm of phenol, acetaminophen, salicylic acid, and methylparaben on activated carbon CAR1073 at 293 K. C 2018, 4, 62 11 of 20 C 2018, 4, x FOR PEER REVIEW 11 of 21

TableTable 10.10. Parameters of of Freundlich Freundlich model model for for phenol, phenol, acetaminophen, acetaminophen, salicylic salicylic acid, acid, and andmethylparaben methylparaben adsorption on on activated activated carbon carbon CAR1073. CAR1073.

K2 −1 2 2 CompoundCompound Model Model K K1 K (L mmol) n nR R 1 2(L−1 mmol) PhenolPhenol Freundlich Freundlich 3.44 3.44 NA 0.73 0.730.96 0.96 Salicylic acidSalicylic a Freundlichcid Freundlich 1.44 1.44 NA 0.61 0.610.94 0.94 AcetaminophenAcetaminophenFreundlich Freundlich 1.36 1.36 NA 0.42 0.420.97 0.97 Methylparaben Freundlich 2.08 NA 0.45 0.98 Methylparaben Freundlich 2.08 NA 0.45 0.98

FigureFigure 6. Adsorption 6. Adsorption isotherm isotherm of of phenol, phenol, acetaminophen, acetaminophen, salicylic salicylic aci acid,d, and and methylparaben methylparaben on on activatedactivated carbon carbon CAR1173 CAR1173 at at 293 293 K. K.

TableTable 11. Parameters 11. Parameters of Freundlichof Freundlich and and Sips Sips modelsmodels for phenol, phenol, acetaminophen, acetaminophen, salicylic salicylic acid,acid, and and methylparabenmethylparaben adsorption adsorption on on activated activated carbon carbon CAR1173.CAR1173.

Compound Model K Qm −1 n 2 Compound Model 1 K1 Qm (mmol g )n R2 R (mmol g−1) Phenol Freundlich 3.11 NA 0.70 0.96 Phenol Freundlich 3.11 NA 0.70 0.96 Salicylic acid Sips 4 1.35 11.4 0.99 C 2018, 4,Acetaminophen x FOR PEER REVIEWSalicylic Freundlich acid Sips 1.274 1.35 NA 11.4 0.430.99 0.97 12 of 21 MethylparabenAcetaminophen Freundlich Freundlich 1.53 1.27 NA NA 0.43 0.450.97 0.96 Methylparaben Freundlich 1.53 NA 0.45 0.96

4 Fenol Ácido salicílico Acetaminofén Metilparabeno 3 Modelo de Freundlich Modelo de Freundlich

) Modelo de Langmuir -1 Modelo de Langmuir 2

Q ( mmol g mmol ( Q

1

0 0.0 0.5 1.0 1.5 2.0

C (mmol L-1) e FigureFigure 7. Adsorption 7. Adsorption isotherm isotherm of of phenol, phenol, acetaminophen, acetaminophen, salisalicyliccylic acid, acid, and and methylparaben methylparaben on on activatedactivated carbon carbon CAR1273 CAR1273 at 293 at 293 K. K.

Table 12. Parameters of Langmuir and Freundlich models for phenol, acetaminophen, salicylic acid, and methylparaben adsorption on activated carbon CAR1273.

K2 Qm Compound Model K1 n R2 (L−1 mmol) (mmol g−1) Phenol Freundlich 2.21 NA NA 0.54 0.97 Salicylic acid Freundlich 1.09 NA NA 0.62 0.96 Acetaminophen Langmuir 25.3 NA 1.02 NA 0.93 Methylparaben Langmuir 44.2 NA 1.20 NA 0.94

The models of Freundlich and Redlich–Peterson do not include in their parameters the adsorption capacity; however, by replacing in the mathematical equations the calculated parameters of each model and taking Ce as the higher equilibrium concentration, it is possible to calculate a theoretical capacity of adsorption. The results are shown in Table 13.

Table 13. Adsorption capacities calculated with Freundlich, Langmuir, Redlich-Peterson, and Sips models for the adsorption of phenol, acetaminophen, salicylic acid, and methylparaben on the activated carbons CAO, CAG, CAR1073, CAR1173, and CAR1273.

Q Adsorbate Activated Carbon Model (Mmol g−1) CAO Freundlich 2.28 CAG Freundlich 2.58 Phenol CAR1073 Freundlich 2.60 CAR1173 Freundlich 2.50 CAR1273 Freundlich 2.40 CAO Redlich-Peterson 1.30 CAG Freundlich 1.37 Salicylic acid CAR1073 Freundlich 1.51 CAR1173 Sips 1.35 CAR1273 Freundlich 1.42 CAO Langmuir 0.71 Acetaminophen CAG Redlich–Peterson 1.21 C 2018, 4, 62 12 of 20

Table 12. Parameters of Langmuir and Freundlich models for phenol, acetaminophen, salicylic acid, and methylparaben adsorption on activated carbon CAR1273.

−1 −1 2 Compound Model K1 K2 (L mmol) Qm (mmol g ) n R Phenol Freundlich 2.21 NA NA 0.54 0.97 Salicylic acid Freundlich 1.09 NA NA 0.62 0.96 Acetaminophen Langmuir 25.3 NA 1.02 NA 0.93 Methylparaben Langmuir 44.2 NA 1.20 NA 0.94

The models of Freundlich and Redlich–Peterson do not include in their parameters the adsorption capacity; however, by replacing in the mathematical equations the calculated parameters of each model and taking Ce as the higher equilibrium concentration, it is possible to calculate a theoretical capacity of adsorption. The results are shown in Table 13.

Table 13. Adsorption capacities calculated with Freundlich, Langmuir, Redlich-Peterson, and Sips models for the adsorption of phenol, acetaminophen, salicylic acid, and methylparaben on the activated carbons CAO, CAG, CAR1073, CAR1173, and CAR1273.

Adsorbate Activated Carbon Model Q (Mmol g−1) CAO Freundlich 2.28 CAG Freundlich 2.58 Phenol CAR1073 Freundlich 2.60 CAR1173 Freundlich 2.50 CAR1273 Freundlich 2.40 CAO Redlich-Peterson 1.30 CAG Freundlich 1.37 Salicylic acid CAR1073 Freundlich 1.51 CAR1173 Sips 1.35 CAR1273 Freundlich 1.42 CAO Langmuir 0.71 CAG Redlich–Peterson 1.21 Acetaminophen CAR1073 Freundlich 1.38 CAR1173 Freundlich 1.36 CAR1273 Langmuir 1.02

Table 13. Cont.

Adsorbate Activated Carbon Model Q (Mmol g−1) CAO Freundlich 2.20 CAG Freundlich 1.43 Methylparaben CAR1073 Freundlich 1.53 CAR1173 Freundlich 1.44 CAR1273 Langmuir 1.20

It can be observed that the adsorption isotherms presented in Figures3–7 are mainly fitted to the adsorption models that contemplate within their parameters the energetic heterogeneity derived from the physicochemical characteristics of the system. The Freundlich model fits the adsorption data of most systems; this indicates that the process is carried out by the formation of multilayers with heterogeneous distributions of the adsorption enthalpy. According to the model, the energy decreases exponentially and remains constant once the process has reached equilibrium [29]. From the point of view of statistical thermodynamics, the value of the heterogeneity factor (n) depends on the coordination number of the adsorbate, adsorbate-adsorbate interactions, the Boltzmann constant, and Avogadro’s number; therefore, the heterogeneity of the systems depends on the binding energy of the active sites and the formation of lateral interactions. Values of n that tend to 0 indicate C 2018, 4, 62 13 of 20 physisorption, while values close to 1 indicate or cooperative adsorption [30]. In the case of Redlich-Peterson and Sips models, when n tends to 0 the equation takes the form of the Freundlich model and when it tends to 1 the Langmuir model is adopted. Additionally, the energetic heterogeneity of the systems is attributed to the competition between the solvent and the adsorbate for the active sites, which means that the enthalpy of adsorption is not constant throughout the process. Likewise, the change in the adjustment models shows that the adsorption depends on the type of adsorbate–adsorbent interactions, which not only depends on the chemical groups present in the adsorbent. It was found that phenol adsorption can be described in all systems with the Freundlich models, indicating that the process is highly heterogeneous, which in turn is indicative of the formation of adsorbent-adsorbate, adsorbate–solvent interactions and interactions between molecules that are already adsorbed, which in this case may be hydrogen bonds. This same trend describes the behaviors of salicylic acid, methylparaben, and acetaminophen. However, the adsorption of the latter compound on the activated carbons CAO and CAR1273 are described with the Langmuir models, indicating specific adsorbate–adsorbent interactions, so that the adsorption capacity decreases. To determine the effect of the textural characteristics on the adsorption capacity, the adsorption capacityC 2018, 4, x was FOR associated PEER REVIEW with the micropore volume. Figure8 shows the results obtained. 14 of 21

FigureFigure 8.8. RelationRelation between between the the adsorption adsorption capacity capacity of of phenol, phenol, acetaminophen,acetaminophen, salicylic salicylic acid, acid, andand methylparabenmethylparaben andand thethe microporemicropore volumevolume ofof activatedactivated carbonscarbons CAO,CAO, CAG,CAG, CAR1073,CAR1073, CAR1173,CAR1173, andand CAR1273.CAR1273. DashedDashed lineslines representrepresent aa tendencytendency ofof data.data.

FigureFigure8 8shows shows a a directly directly proportional proportional behavior behavior between between the the adsorption adsorption capacity capacity and and the the microporemicropore volume.volume.This This indicates indicates that that the the process process of of adsorption adsorption of of the the study study compounds compounds is relatedis related to theto the filling filling of micropores of micropores according according to the to energy the energy they possess. they possess. In the caseIn the of case acetaminophen of acetaminophen and phenol, and adsorptionphenol, adsorption on CAR1173 on CAR1173was not found was tonot present found thisto present tendency, this since tendency, the capacity since of the adsorption capacity in of thisadsorption case was in greater this case than was that greater of activated than that carbon of activated CAG, even carbon though CAG the, even latter though has a greaterthe latter surface has a areagreater and surface micropore area volume and micropore (Table4). A volume similar behavior(Table 4) was. A presented similar behavior by methylparaben, was present whoseed by adsorptionmethylparaben, capacity whose was greateradsorption in the capacity activated was carbon greater CAO, in which the activated presented carbon the lowest CAO, values which of surfacepresent areaed the and lowest micropore values volume. of surface These deviationsarea and micropore from the general volume. behavior These were deviations attributed from to the the effectgeneral of surfacebehavior chemistry were attributed on the adsorptionto the effect capacity. of surface chemistry on the adsorption capacity. AccordingAccording toto thethe datadata inin TableTable4 ,4, the the activated activated carbons carbons studied studied are are microporous, microporous, which which could could limitlimit thethe process of of adsorption adsorption of of large large molecules, molecules, although although this this is not is not the thecase case of phenol, of phenol, salicylic salicylic acid, acetaminophen and methylparaben, which have small dimensions. The presence of the aromatic substituents could generate steric hindrance, so the effect of the substituent on the adsorption capacity of the adsorbates on activated carbons CAO, CAG, CAR1073, CAR1173, and CAR1273 was evaluated. In Figure 9, it is evident that the behavior between the adsorption capacity and the weight of the substituent is inversely proportional in all activated carbons, as indicated by Lundelius’ rule. For the correlations presented, the slope represents the rate of change in the adsorption capacity for each gram of additional substituent from the phenolic ring. The intercept corresponds to the contribution of the phenolic ring to the adsorption capacity. It is evident that the effect of the weight of the substituent in the adsorption capacity (slope) does not vary between the activated carbons CAG, CAR1073, CAR1173, and CAR1273; therefore, a linear function can be employed to describe the general behavior in the four activated carbons. CAO presents a slope different from that of the other activated carbons; as such, its behavior was described with a different equation. C 2018, 4, 62 14 of 20 acid, acetaminophen and methylparaben, which have small dimensions. The presence of the aromatic substituents could generate steric hindrance, so the effect of the substituent on the adsorption capacity of the adsorbates on activated carbons CAO, CAG, CAR1073, CAR1173, and CAR1273 was evaluated. In Figure9, it is evident that the behavior between the adsorption capacity and the weight of the substituent is inversely proportional in all activated carbons, as indicated by Lundelius’ rule. For the correlations presented, the slope represents the rate of change in the adsorption capacity for each gram of additional substituent from the phenolic ring. The intercept corresponds to the contribution of the phenolic ring to the adsorption capacity. It is evident that the effect of the weight of the substituent in the adsorption capacity (slope) does not vary between the activated carbons CAG, CAR1073, CAR1173, and CAR1273; therefore, a linear function can be employed to describe the general behavior in the four activated carbons. CAO presents a slope different from that of the other activated carbons; as such, its Cbehavior 2018, 4, x FOR was PEER described REVIEW with a different equation. 15 of 21

FigurFiguree 9 9.. Relation between the molecular weight of the substituent and the adsorption capacity of the aadsorbatesdsorbates on the activated carbons CAO, CAG, CAR1073, CAR1173,CAR1173, andand CAR1273.CAR1273.

In Figure 99 it it cancan bebe seenseen thatthat thethe adsorptionadsorption capacitycapacity (Q)(Q) decreasesdecreases withwith thethe molecularmolecular weightweight of the substituentsubstituent (Q(Q phenolphenol >> QQ salicylicsalicylic acidacid == carboxyliccarboxylic acidacid >> QQ acetaminophenacetaminophen == amideamide ≥≥ Q mmethylparabenethylparaben == ester).ester). Also,Also, accordingaccording to to the the slope slope of of the the equations, equations the, the adsorption adsorption capacity capacity varies varies by −1 by0.02 0.02 mmol mmol g gfor−1 for each each gram gram from from the substituentthe substituent in the in phenolic the phenolic ring. ring. In the In activated the activated carbon carbon CAO, CAO,the adsorption the adsorption capacity capacity decreased decreased by 25% withby 25% the increasewith the inincrease the molecular in the molecular weight of thewe substituentight of the −1 (bsubstituent = 0.025 mmol (b = 0 g.025). mmol This may g−1). beThis related may tobe therelated decrease to the in decrease the volume in the of volume micropores, of micro but alsopores, to but the presencealso to the of presence oxygenated of groupsoxygenated in the groups entries in of the this entries type of of , this type which of porosity, generates which steric hindrancegenerates withsteric the hindrance substituents with inthe the substituents adsorbates. in the adsorbates. The process of adsorption of organic compounds from aqueous solutions can involveinvolve two driving forces: a a derivative derivative of of the the hydrophobicity of of the solute that induces adsorption by low adsorbateadsorbate-solvent-solvent affinity affinity and and the thespecific specific affinity affinity of the adsorbate of the adsorbate for the surface, for the which surface, contemplates which contemplatesthe adsorption the by ionic adsorption interactions, by ionic by van interactions, der Waals forces,by van and der by chemicalWaals forces reactions., and Therefore, by chemical the reactionsadsorbate–adsorbent. Therefore, affinitythe adsorbate depends–adsorbent on the surface affinity chemistry depends of the on activated the surface carbon. chemistry of the activatedTo analyze carbon. the correlation between the adsorbed amount and the functional groups present in the activatedTo analyze carbons, the correlation it was determined between the that adsorbed the adsorbed amount amount and the of functional phenol, salicylicgroups present acid, and in theacetaminophen activated carbons, increases it inwas a manner determined that is that inversely the adsorbed proportional amount to the of total phenol, acidity salicylic of the acid activated, and acetaminophen increases in a manner that is inversely proportional to the total acidity of the activated carbons. Figure 10 shows the relationship between the total basicity and the adsorbed amount of phenol, salicylic acid, and acetaminophen. C 2018, 4, 62 15 of 20 carbons. Figure 10 shows the relationship between the total basicity and the adsorbed amount of Cphenol, 2018, 4, x salicylic FOR PEER acid, REVIEW and acetaminophen. 16 of 21

FigureFigure 110.0. RelationshipRelationship between between the the adsorption adsorption capacity capacity and and the the number number of of basic basic groups groups in in tthehe adsorptionadsorption process process of of phenol, phenol, salicylic salicylic acid acid,, and and acetaminophen acetaminophen o onn the the activated activated carbons carbons CAO, CAO, CAG, CAG, CAR1073,CAR1073, CAR1173 CAR1173,, and CAR1273.

TheThe behavior behavior described described in in Figure Figure 1100 cancan be be explained explained by by the the formation formation of of acid acid-base-base interactions interactions andand the surface charge charge of t thehe adsorbent adsorbent.. S Surfaceurface groups groups such such as as amines amines have have a a pair pair of of free free electrons electrons notnot compromised inin thethe aromaticityaromaticity ofof thethe aromatic aromatic rings rings of of the the graphene graphene layers, layers, which which allows allows them them to tointeract interact with with Lewis Lewis acids acids such such as phenol as phenol that could that could accept accept a pair ofa pair electrons. of electrons In the case. In the of oxidized case of oxidizedactivated activated carbon, the carbon, adsorbed the adsorbed amount of amount the compound of the compound decreases becausedecreases this because adsorbent this inadsorbent solution + inyields solution H ions yields that H decrease+ ions that the decrease pH, which the changespH, which the changes surface chargethe surface of the charge adsorbent. of the This adsorbent implies. Thisthat whenimplies the that basic when groups the basic are protonated,groups are p therotonated, electronic the pair electronic that was pair free that binds was free to the binds hydrogen to the hydrogenand therefore and decreases therefore decreasesthe amount the of amount groups of available groups toavailable interact to with interact the adsorbate. with the adsorbate Likewise,. + Ltheikewise, amount the of amount H ions of in H the+ ions medium in the interacts medium with interacts the π withelectrons the π of electrons the graphene of the layers, graphene preventing layers, preventingthe formation the offormation adsorbate-adsorbent of adsorbate- interactionsadsorbent interactions by π electrons. by π Onelectrons. the other On hand,the other if the hand, pH ofif thethe pH medium of the ismedium above theis above pH at the the pH point at the of point zero charge, of zero thecharge, adsorbent the adsorbent will have wil al negativehave a negative charge chargeassociated associated with the with deprotonation the deprotonation of the acid of groups, the acid implying groups, a greater implying polarity a greater at the polarity surface, atwhich the + surface,favors the which interaction favors withthe interaction water. Therefore, with water. as the Therefore, activated as carbons the activated give less carbons H ions give to theless medium, H+ ions tothe the adsorption medium, capacity the adsorption of the pharmaceutical capacity of compoundsthe pharmaceutical is increased compounds until it remains is increased constant until in the it remainsreduced constant activated in carbons, the reduced which activated have the carbons, same values which in have the total the same acidity. values in the total acidity. ItIt was was found found that that the the adsorption adsorption capacity capacity decreases decreases with with the the molecular molecular weight weight of of the the adsorbate, adsorbate, soso mo moleculeslecules with with higher higher molecular molecular size size may may have problems of dissemination and and therefore the adsorptionadsorption depends onon thethe surfacesurface chemistry. chemistry. In In the the case case of of acetaminophen, acetaminophen, it wasit was envisaged envisaged that that the thesurface surface chemistry chemistry of activated of activated charcoal CAO CAO disfavors disfavors the the process process altogether, altogether, in part in part because because of the of thenegative negative charge charge of carboxylic of carboxylic acids acids and and the the increase increase in the in the polarity polarity of the of the surface, surface which, which favors favors the theinteractions interactions with with the the solvent. solvent. MMethylparabenethylparaben andand acetaminophenacetaminophen have have similar similar molecular molecular weights; weights; therefore, therefore, similar similar to the to latter the latteradsorbate, adsorbate, adsorption adsorption depends depends the interactions the interactions between between the adsorbate the adsorbate and surface and groups. surface However, groups. However,contrary to contrary phenol, salicylic to phenol, acid, salicylic and acetaminophen, acid, and acetaminophen, the adsorption the capacity adsorption of methylparaben capacity of methylparabenincreases with the increases number with of oxygenated the number groups. of oxygenated To analyze groups. the different To an correlationsalyze the different between correlationsthe acid groups between and thethe amountacid groups adsorbed, and the it wasamount determined adsorbed that, it was there determined is a directly that proportional there is a directlycorrelation proportional between the correlation amount of between acid groups the amount and the of adsorbed acid groups amount, and which the adsorbed can be observedamount, whichin Figure can 11 be. observed in Figure 11. C 2018, 4, 62 16 of 20 C 2018, 4, x FOR PEER REVIEW 17 of 21

Figure 11. 11. RelationRelation between between the the adsorption adsorption capacity capacity and and total total acidity oonn the adsorption of methylparaben onon the activated carbons CAO,CAO, CAG,CAG, CAR1073,CAR1073, CAR1173,CAR1173, andand CAR1273.CAR1273.

This behavior was attributed to the hydrolysis of the lactone groups in an acidicacidic medium, whichwhich form a a carboxylic carboxylic acid acid and and an an alcohol after after the reaction. the reaction This. favorsThis favors the formation the formation of a greater of a amount greater ofamount hydrogen of hydrogen bonds with bonds the with phenol the in phenol the adsorbate in the adsorbate structure; structure likewise,; likewise, the formation the formation of phenols of preventsphenols theprevents solvent the from solvent competing from forcompet the adsorptioning for the sites adsorptio created,n since sites specific created, interactions since specific are formedinteractions any are carboxylic formed acid-methylparabenany carboxylic acid-methylparaben and solvent–phenol and solvent interactions.–phenol As interactions the pH increases,. As the thepH hydrolysis increases, ofthe the hydrolysis lactones decreases of the lactones and therefore decreases the adsorption and therefore capacity the decreases adsorption as well. capacity On thedecreases other hand,as well. the On increase the other of h pHand, in the the increase medium of favors pH in the the formation medium favors of the the dissociated formation species, of the generatingdissociated repulsionsspecies, generating with carboxylic repulsions acids with on thecarboxylic surface. acids on the surface. 3.3. Calorimetric Data 3.3. Calorimetric Data In an adsorption system that consists of a solvent, an adsorbate, and an adsorbent, the immersion In an adsorption system that consists of a solvent, an adsorbate, and an adsorbent, the enthalpy represents the energy exchanged during the interaction of the adsorbate and the solvent immersion enthalpy represents the energy exchanged during the interaction of the adsorbate and with the activated carbon surface. Table 14 shows the immersion enthalpy of activated carbons CAO, the solvent with the activated carbon surface. Table 14 shows the immersion enthalpy of activated CAG, CAR1073, CAR1173, and CAR1273 in the solutions of phenol, salicylic acid, acetaminophen, carbons CAO, CAG, CAR1073, CAR1173, and CAR1273 in the solutions of phenol, salicylic acid, and methylparaben. acetaminophen, and methylparaben. Table 14. Immersion enthalpy for the adsorption of phenol, salicylic acid, acetaminophen, and Table 14. Immersion enthalpy for the adsorption of phenol, salicylic acid, acetaminophen, and methylparaben in aqueous solution on the activated carbons CAO, CAG, CAR1073, CAR1173, methylparaben in aqueous solution on the activated carbons CAO, CAG, CAR1073, CAR1173, and and CAR1273. CAR1273. Average Average Average Average ∆Himm ∆Himm Activated Carbon Adsorbate ΔH−imm1 ΔH−imm1 Activated Carbon Adsorbate (J g ) (J g ) 0.07–0.66(J mmolg−1) L−1 1.32–10.2(J mmolg−1) L−1 CAO 0.07−–24.50.66± mmol1.23 L−1 1.32−–10.243.9 ±mmol0.87 L−1 CAGCAO −−9.5724.5 ±± 0.481.23 −434.33.9 ± 0.870.69 CAR1073CAG Phenol −−15.69.57 ±± 0.780.48 −336.14.3 ± 0.690.72 CAR1173 −10.8 ± 0.54 −35.7 ± 0.71 CAR1073 Phenol −15.6 ± 0.78 −36.1 ± 0.72 CAR1273 −29.2 ± 1.46 −23.3 ± 0.47 CAR1173 −10.8 ± 0.54 −35.7 ± 0.71 CAO −27.4 ± 1.37 −35.1 ± 0.70 CAR1273CAG −−11.329.2 ±± 0.571.46 − 38.723.3± ± 0.770.47 CAR1073CAO Salicylic acid −−31.627.4 ±± 1.581.37 −336.45.1 ± 0.700.73 CAR1173CAG −−11.711.3 ±± 0.590.57 −38.737.1 ±± 0.770.75 CAR1273CAR1073 Salicylic acid −−21.531.6 ±± 1.081.58 −329.66.4 ± 0.730.59 CAR1173 −11.7 ± 0.59 −37.1 ± 0.75 CAR1273 −21.5 ± 1.08 −29.6 ± 0.59 CAO Acetaminophen −33.0 ± 1.65 −56.0 ± 1.12 C 2018, 4, 62 17 of 20

Table 14. Cont.

Average Average ∆H ∆H Activated Carbon Adsorbate imm imm (J g−1) (J g−1) 0.07–0.66 mmol L−1 1.32–10.2 mmol L−1 CAO −33.0 ± 1.65 −56.0 ± 1.12 CAG −9.83 ± 0.49 −31.9 ± 0.64 CAR1073 Acetaminophen −39.0 ± 1.95 39.6 ± 0.79 CAR1173 −9.42 ± 0.47 −34.6 ± 0.69 CAR1273 −23.4 ± 1.17 −24.3 ± 0.49 CAO −18.3 ± 0.92 −43.5 ± 0.87 CAG −11.9 ± 0.56 −51.0 ± 1.02 CAR1073 Methylparaben −41.2 ± 2.06 −42.7 ± 0.85 CAR1173 11.6 ± 0.58 −37.1 ± 0.85 CAR1273 −26.5 ± 1.33 −25.2 ± 0.50

The values in Table 14 indicate that the immersion enthalpy of phenol at low concentrations is highest for activated carbon CAR1273 (although it does not correspond to the greater adsorption capacity). This indicates that, although this surface is hydrophobic, interactions with water can be present through the formation of hydrophobic interactions, which are formed by the creation of water clusters around the carbonaceous surface through hydrogen bonds between them. These strong interactions prevent the interaction of the adsorbate with the adsorbent, even when increasing the concentration. This behavior is repeated for all adsorbates; additionally, it can be seen in Table 14 that the immersion enthalpy values for activated carbon CAR1273 decrease with the increase in adsorbate concentration, indicating that the displacement of solvent from the activated carbon surface requires a very high concentration of adsorbate molecules. In the case of activated carbon CAO, it was observed that at low concentrations the immersion enthalpy has high values due to the interactions of the adsorbent with the solvent (due to the high polarity of both). The values in the immersion enthalpy increase with the concentration of the adsorbate, indicating that when modifying the concentration of the adsorbate, there is a new equilibrium in which the adsorbate–adsorbent interactions are formed by Le Chatelier’s principle. However, the immersion enthalpy values are associated with solvent–adsorbent interactions, except in methylparaben where they can be associated with adsorbate-adsorbent interactions. The activated carbons CAG and CAR1173 at low concentrations of adsorbate presented low values of immersion enthalpy due to the competence of the solvent to adsorb the adsorbate through active sites. The increase in the adsorbate concentration increased the immersion enthalpy associated with adsorbate–adsorbent interactions.

4. Conclusions • An activated carbon prepared from coconut shell was modified by chemical treatment (impregnation with HNO3) or physical treatment (heating at 1073, 1173, or 1273 K). The oxidation with nitric acid increased the acid groups (656 µmol g−1), but decreased the surface area (469 m2 g−1) and micropore volume (0.18 cm3 g−1). On the other hand, the thermal treatment at 1073 K increased the surface area (1127 m2 g−1), micropore volume (0.42 cm3 g−1), and the level of basic groups (1270 µmol g−1). The thermal treatment at 1173 and 1273 K decreased textural parameters and increased the basicity. • The increase of the oxygenated groups in the activated carbon CAO increased the surface polarity, which favored the adsorbent–solvent interactions (water). Using the calorimetric data, it was determined that the immersion enthalpy in water was −66.6 J g−1, while for the activated carbons with thermal treatment the immersion enthalpy in water was −30.4 J g−1. C 2018, 4, 62 18 of 20

• It was determined that the adsorption capacity of phenol, salicylic acid, acetaminophen, and methylparaben in the modified activated carbons decreases with increasing the molecular weight of the substituent. For thermally modified activated carbons and CAG, the adsorption capacity decreased by 0.02 mmol g−1 per gram of substituent; in the activated carbon CAO, the value decreased by 0.025 mmol g−1 due to the decreases in the pore volume and surface area. • The adsorption capacity of phenol, salicylic acid, and acetaminophen increased with the basic groups present on the adsorbent. Phenol had the highest adsorption capacities in all activated carbons (2.28–2.60 mmol g−1) because it has the smallest molecular size and easy access to micropores. In adsorbates with a larger size, the adsorption capacity depended mainly on the adsorbate-adsorbent interactions. In the case of acetaminophen, the increase in oxygenated groups decreased the adsorption capacity (0.71 mmol g−1) while for methylparaben it increased (2.20 mmol g−1); this was due to the type of interactions that were formed with the adsorbate when it presented an ionic charge, a relevant factor in the adsorption of salicylic acid, which was ionized at the solution pH. • The immersion enthalpy allowed the association of energy changes in the system with the formation of adsorbate–adsorbent and solvent-adsorbent interactions. Depending on the surface chemistry, the increase of oxygenated groups in the adsorbent increased the immersion enthalpy due to the formation of interactions between activated carbon and water. However, when increasing the concentration of the adsorbate, and increase in enthalpy was observed, which indicated the displacement of the solvent and the formation of activated carbon–adsorbate interactions. The immersion enthalpy values were between −9.42 and −56.0 J g−1.

Author Contributions: V.B., L.G. conceived and designed the experiments; V.B., L.G., and J.C.M.-P. performed the experiments; V.B., L.G., and J.C.M.-P. analyzed the data; V.B., L.G., and J.C.M.-P. wrote the paper; L.G. and J.C.M.-P. critically revised the manuscript. Funding: This research received no external funding. Acknowledgments: The authors thank the Framework Agreement between the Universidad de Los Andes and the National University of Colombia and the act of agreement established between the Chemistry Departments of the two universities. The authors also thank the Faculty of Sciences of Universidad de Los Andes for the partial funding through the call “short projects/additional product” and the DIEB of National University of Colombia Project 37348. Conflicts of Interest: The authors declare no conflict of interest.

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