SQU Journal for Science, 2023, 28(1), 42-52 DOI:10.53539/squjs.vol28iss1pp42-52 Sultan Qaboos University 42 Adsorption of Methylene Blue onto Hydrophobic Activated Carbon Said M. Al-Mashaikhi, El-Said I. EL-Shafey*, Saleh Al-Busafi, FakhrEldin O. Suliman Department of Chemistry, College of Science, Sultan Qaboos University, P.O. 36, Al-Khod 123, Muscat, Sultanate of Oman. *Email: elshafey@squ.edu.om. ABSTRACT: Activated carbon (AC) was prepared from date palm leaflets using NaOH activation. AC was oxidized using concentrated HNO3 to produce oxidized activated carbon (ON). Alkylamines including methylamine (M), dimethylamine (DM), ethylamine (E), diethylamine (DE), and diisopropylamine (DIP) were covalently immobilized onto ON to produce ONM, ONDM, ONE, ONDE, and ONDIP hydrophobic activated carbons, respectively. The surface area of AC (588 m 2 /g) decreased on oxidation with a further decrease on functionalization. These carbons were tested for methylene blue (MB) adsorption. An initial pH 7 was found to be optimum for the adsorption of MB. The hydrophobic activated carbons showed faster adsorption (except for ONDE), than AC with adsorption data following a pseudo-second-order kinetic model. The rate of adsorption increased with increasing temperature. The activation energy, Ea, was less than 42 kJ/mol indicating physical adsorption. Equilibrium adsorption follows an L-type isotherm with the adsorption data following the Langmuir model. The adsorption capacity of MB follows the order of ONDIP > ONE > ONDM > AC > ON > ONM > ONDE. Thermodynamic parameters indicate spontaneous and endothermic adsorption. Carbon reuse for MB adsorption was more efficient for ONDIP, ONE, and ONDM than AC. Keywords: Activated carbon; Methylene blue; Alkylamine; Hydrophobic; Adsorption. المثيليين األزرق على الكربون النشط الكاره للماء امتزاز ، صالح البوصافي، فخر الدين سليمان ييد الشافععسعيد المشايخي، الس حمض باستخدام تم أكسدة الكربون المنشطومن وريقات نخيل التمر باستخدام تنشيط هيدروكسيد الصوديوم. (AC) تم تحضير الكربون المنشط :صلخمال ، وثنائي ميثيل أمين (M) بما في ذلك ميثيل أمينإلنتاج الكربون الكاه للماء أمينات األلكيل تثبيت. تم (ON) مؤكسدالكربون ال المركز إلنتاج النيتريك (DM) وإيثيل أمين ، (E) وثنائي إيثيل أمين ، (DE) وثنائي أيزوبروبيل أمين ، (DIP) نتاجإلتساهميًا ONM و ، ONDM و ، ONE و ، ONDE ، متر 855وتبين أن مساحة السطح للكربون النشط ). علي التوالي ل ONDIP و 2 تم /جرام( قد انخفضت باألكسدة وانخفضت أكثر مع تثبيت األلكيل أمين. و ق. تُظهر الكربونات المنشطة الميثيلين األزر زازاألمثل المتهو 7الرقم الهيدروجيني األولي .ووجد أن الميثيلين األزرق زالةال الناتج اختبار الكربون معدل . وقد زادحركي من الدرجة الثانيةالنموذج ال زازبيانات االمتاتباع مع AC من (ONDE) باستثناءللميثلين األزرق أسرع زازاالكارهة للماء امت L النوع تزانالفيزيائي. يتبع امتزاز اإل زازلى االمتكيلو جول / مول مما يشير إ 22طاقة التنشيط أقل من وتبين أناالمتزاز مع ارتفاع درجة الحرارة. ONE> ONDM> AC> ON> ONM ب :ترتيالتتبع الميثيلين األزرق امتزازسعة النجمير.ووجد أن نموذجلبيانات االمتزاز واتباع ONDE ا أسرع وأكثر لـتزازالكربون امت تدويردراسة ص للحرارة. أظهرت اوم تلقائي زازالديناميكية الحرارية إلى امت دراسةتشير .و ONDIP و ONE و ONDM مقارنةً بالكربون المنشط. إمتزاز -الكيل أمين -الميثيلين االزرق -الكاره للمياه -الكربون النشط :مفتاحيةالكلمات ال ADSORPTION OF METHYLENE BLUE ONTO HYDROPHOBIC ACTIVATED CARBON 43 1. Introduction yes are toxic pollutants and can cause a threat to aquatic organisms and human health. Recent data shows that there are more than 100,000 commercial dyes, with an annual production of over 70,000 tonnes/year [1,2]. The textile industry utilizes more than 10,000 tonnes of dyes every year from which 100 tonnes are discharged into the environment [2,3]. Most of these dyes are toxic to the aquatic environment. Their complex chemical structures make them resistant to degradation by light, heat, and oxidizing agents, increasing the danger of this type of chemicals [4-6]. The toxicity of azo dyes arises from the presence of amines, while the color of anthraquinone dyes remains in effluents for a long time as they are resistant to degradation. In addition, the high solubility of reactive dyes and their chemical stability in water causes serious problems for the aquatic environment [7]. In aquatic ecosystems, photosynthetic activity is affected by decreasing light penetration [2]. Moreover, these harmful materials are carcinogenic to various fish species. Dyes can cause damage to human health and reduce the effectiveness of the reproductive system, kidneys, brain, liver, and central nervous system [6, 8]. Typical methods used for the removal of dyes from water include ozonation [9], photocatalytic degradation [10,11], advanced oxidation [12], microbial biodegradation [13], coagulation and flocculation [14], filtration and membrane-filtration processes [15]. Such methods have the essential limitations of being complex and uneconomic [16]. Adsorption has proven superior to other techniques in terms of flexibility, simplicity of design, and initial cost [17]. In previous studies activated carbon prepared from pepper stems showed an adsorption capacity of 174 mg/g with the best performance at pH 8.0 [18] and activated carbon prepared from date palm leaves using KOH activation showed 270 mg/g, while after ethylamine functionalization the capacity increased to 393 mg/g with best performance at pH 7 [19]. Dehydrated carbon at pH 7 had a capacity of 196 mg/g, and after functionalization using ethylamine showed 296 mg/g [20]. Other adsorbents have also been used including clay [21], zeolite [22], bentonite [6], and biosorbents [23]. In this study, AC was prepared from date palm leaflets using NaOH activation. AC was oxidized, using nitric acid, and was surface functionalized using alkylamines with different sizes of the alkyl groups via amide coupling to produce hydrophobic activated carbons. These carbons were tested for the removal of methylene blue (MB) from the aqueous solution in terms of kinetics and equilibrium. The effect of the different sizes of alkyl amines on the extent of functionalization and its performance towards methylene blue adsorption is a novel area of study and is discussed in this paper. 2. Materials and methods 2.1 Materials The chemicals used were of analytical grade. Activated carbon (AC) was prepared from clean dried date palm leaves using NaOH activation. AC was oxidized using concentrated nitric acid at 80 o C to produce oxidized activated carbon (ON). ON was covalently functionalized using methylamine (M), dimethylamine (DM), ethylamine (E), diethylamine (DE), and diisopropylamine (DIP) via amide coupling to produce the hydrophobic activated carbons: ONM, ONDM, ONE, ONDE, and ONDIP, respectively. The study of AC preparation, surface functionalization, and characterization was carried out in our laboratories, and published previously [24]. The stock solution of methylene blue (MB) (1000 mg/L) was prepared in deionized water. The standards and adsorption test solutions were prepared by suitable dilution in deionized water. 2.1 Effect of pH on MB adsorption To investigate the effect of pH on MB adsorption, ~ 0.06 g of carbon was mixed with 25 mL of MB solution (100 mg/L) in clean glass vials at different initial pH values of 3.0-9.0. Before the addition of the pre-weighed adsorbent, the pH was adjusted using a few drops of dilute NaOH or HCl. At 20 o C, the samples were shaken regularly until equilibrium was reached. Residual MB was analyzed using a Varian/Cary/50 Conc UV-Visible spectrophotometer at λmax 665 nm with deionized water as blank. For all carbons, initial pH 7 was found optimal for MB adsorption and, thus, was selected for the kinetic, equilibrium, and carbon recycle studies of MB adsorption. 2.2 Kinetics of MB adsorption In the kinetic experiments, 0.12 g of carbon was mixed with 50 mL of MB solution (100 mg/L) at initial pH of 7. The adsorption vessels were shaken mechanically for 48 hours. At different time intervals, aliquots of supernatant were withdrawn and analyzed. The kinetic experiments were carried out at 20 and 40 o C. 2.3 Equilibrium adsorption The equilibrium studies were carried out by mixing ~ 0.06 g of carbon with 25 mL of MB solution (50 – 1000 mg/L) at initial pH 7 at two temperatures of 20 and 40 o C. The adsorption vessels were kept under mechanical agitation until equilibrium was reached. Residual MB was withdrawn and analyzed. D SAID AL-MAHAIKHI ET AL. 44 2.4 Adsorbent recycle For MB recovery, 0.12 g of carbon was added to 50 mL of MB solution (100 ppm) at initial pH 7 and was kept for 48 hours under agitation until equilibrium was reached. After equilibrium, samples were separated for analysis. The adsorbent was washed with a boiling solution of sodium hydroxide at pH 10 followed by deionized water. The samples were washed with dilute HCl to neutralize residual NaOH, followed by deionized water until the adsorbent was acid- free. The carbon was partially dried and transferred carefully to a new adsorption vessel containing 50 mL of MB solution under the same conditions as the first adsorption cycle (100 ppm, pH 7). The adsorption mixture was left under agitation until equilibrium was reached and samples of MB were separated for analysis. Residual MB concentration was determined using a Varian/Cary/50 Conc UV-visible spectrophotometer at λmax of 665 nm with deionized water as blank. All the experiments and analysis were carried out at least twice. 3. Results and discussion The study of AC preparation, surface oxidation, and functionalization using different alkylamines was carried out in our laboratories and published elsewhere, and included the carbons tested in this study [24]. A schematic representation of the seven carbons under investigation is presented in Figure 1. Fourier transform infrared (FTIR), carbon, hydrogen, and nitrogen analysis (CHN), and thermogravimetric analysis (TGA) showed that surface functionalization was successful [24]. The surface areas for the carbons were 588, 259.3, 217.6, 150.3, 108.8, 93.2, and 72.0 for AC, ON, ONM, ONDM, ONE, ONDE, and ONDIP, respectively. The extent of functionalization followed the order ONM > ONDM > ONE > ONDE > ONDIP [24] that corresponds to the size of the alkylamine immobilized. Figure 1. Schematic representation of (A) AC, (B) ON, (C) ONM, (D) ONDM, (E) ONE, (F) ONDE, and (G) ONDIP [24]. OH OH OH N N N N O O O O O N OH (A) (B) (C) (D) (E) (F) (G) (F) ADSORPTION OF METHYLENE BLUE ONTO HYDROPHOBIC ACTIVATED CARBON 45 As the size of alkylamine increases, the functionalization decreases due to steric effects [24]. These seven carbons were tested for MB adsorption in this study. 3.1 Effect of pH on methylene blue adsorption The adsorption of MB depends on the pH of the aqueous solution. The amount of MB adsorbed was calculated from Equation 1. qe = (Co-Ce)V/m Eq. (1) where qe is MB adsorbed (mg/g) at equilibrium, Co, and Ce are initial and equilibrium MB concentration (mg/L), V is the volume in L, and m is the mass of adsorbent (g). As MB is a cationic dye regardless of the pH of the solution, the variation of MB adsorption with initial pH is mostly related to the surface properties at different pH values. As shown in Figure 2A, MB adsorption remains almost constant for AC, ONM, ONDM, ONE, and ONDIP regardless of the initial pH values. However, for ON and ONDE, a slight decrease at low pH was observed. The insignificant variation of MB adsorption onto ONM, ONDM, ONE, and ONDIP is related to the dominance of hydrophobic interaction forces between the hydrophobic surface and MB. However, MB adsorption onto AC is dominated by van der Waals’ interaction forces, and  interactions [25]. MB adsorption onto ON (pHzpc 3.38 [24]) shows low adsorption at pH 3. At this pH, the surface is protonated and neutral. However at higher pH, adsorption increases and remains constant in the range of pH 7-9. In that range of pH, electrostatic interaction between the negatively charged ON surface and MB cations dominate the adsorption process. Thus, pH 7.0 was selected for further studies. ONE and ONDIP show higher MB adsorption than AC or ON. ONDE shows less adsorption due to the limited extent of functionalization, self- adsorption, and entangling of immobilized diethylamine chains [24]. The final pH against the initial pH is presented in Figure 2B. There is a slight increase in the final pH in the acidic range and this could be due to the protonation of unfunctionalized carboxylic groups. The final pH seems to be less than the initial pH in the alkaline initial pH range and this could be due to OH - adsorption onto adsorbed MB molecules as a second layer. Figure 2. (A) Effect of pH on MB adsorption, (B) Initial pH vs final pH for MB adsorption. 3.2 Kinetics of MB adsorption The kinetics of MB adsorption is presented in Figure 3. At 20 o C, approximate equilibrium was reached within 35 hours for AC and ON, and ~ 24 hours for hydrophobic activated carbons. At 40 o C, the equilibrium time was reached faster for ONDIP, ONDE (12 hours), and AC and ON (24 hours). MB adsorption was enhanced with the temperature rise. MB adsorption was found to vary almost linearly with the half power of time, in the initial stage of adsorption. This was observed by applying the pore diffusion model [26] (Equation 2). qt = kd t 0.5 Eq. (2) where qt is MB adsorbed at time t and kd is the pore diffusion constant (mg/g / h 0.5 ). As the temperature increases, kd values increase for all carbons investigated (Table 1). kd (mg/g/hr 0.5 ) follows the order of ONDIP> ONE > ONDM > ON>ONM> AC > ONDE at 20 o C. The same order is followed at 40 o C except for ONM, which is slightly higher than ON. SAID AL-MAHAIKHI ET AL. 46 Figure 3. Kinetics of MB adsorption at (A) 20 o C and (B) 40 o C. MB adsorption kinetic data were investigated for pseudo-first-order and pseudo-second-order kinetic models (Equations 3 & 4). log(qe-qt) = log qe – k1t/2.303 Eq. (3) t/qt = 1/k2qe 2 + t/qe Eq. (4) Where k1 and k2 are pseudo-first-order rate constant (hr -1 ), and pseudo-second-order constant (g/mg/hr). The initial adsorption rate can be obtained from Equation 5. h = k2qe 2 Eq. (5) The plots of log (qe − qt) versus t show straight lines from which qe and k1 are determined. The linear plots of the pseudo-first-order model for MB adsorption show low correlation values (R 2 ) indicating poor fit to the model, Table 1, and thus, the kinetic parameters of this model are not discussed. The linear plots of t/qt versus t for the pseudo-second- order kinetic model show straight lines with high correlation values (R 2 ), Table 1. Such good fit suggests that the adsorption of MB complies very well with the pseudo-second-order kinetic reaction between MB and the carbons’ surfaces. This reflects that the rate of MB adsorption depends on both the adsorbent and adsorbate and the limiting step in the adsorption process involves sharing or exchange of electrons between the carbon surface and MB [27,28]. qe, h, and k2 values from the pseudo-second-order model are found to increase with temperature increase similarly to kd increase. The rise in temperature decreases the solvation layers around the adsorbent and adsorbate allowing faster diffusion and more access to active sites and consequently more MB uptake [29,30]. k2, h and qe follow the order of ONDIP> ONE > ONDM > ON  ONM> AC > ONDE. AC shows fewer values of the rate constants because MB molecules diffuse through a network of porous structures within AC particles. ONDIP, ONE, ONDM, and ONM show faster adsorption than AC because the active sites here are externally immobilized chains. ONDIP and ONE show higher kinetic parameters of kd, k2, and h. ONDE shows the slowest MB adsorption due to the self-adsorption or entangling of immobilized chains [24]. ON shows a faster kinetic process than AC and this is related to a different mechanism of electrostatic interaction involved between the negatively charged ON and the positively charged MB. The activation energies, Ea (kJ/mol) for MB adsorption were calculated considering the rate constant (k2) due to the good fit to the pseudo-second-order model. The energy of activation, Ea, was calculated from the Arrhenius equation for two temperatures (Equation 6). ' 2 2 1 2 1 1 ln( ) ( ) a Ek k R T T   Eq. (6) R is the gas constant (8.314 J mol -1 K -1 ), k2 is the rate constant at T1 (298 K) and k'2 is the rate constant at T2 (213 K). The lower values of Ea indicate physical adsorption processes (Ea <42 kJ/mol), whereas higher values of Ea (Ea > 42 kJ/mol) indicate chemically controlled processes [30]. The Ea values for MB adsorption lie between 5.85 and 36.12 kJ/mol (Table 1) indicating physisorption domination. (B) (A) ADSORPTION OF METHYLENE BLUE ONTO HYDROPHOBIC ACTIVATED CARBON 47 Table 1. Kinetic parameters of MB adsorption on AC, ON, and ON hydrophobic carbons at 20 and 40 o C. 3.3 Equilibrium studies and temperature effect MB adsorption follows an L-type adsorption isotherm with better performance as the temperature rises (Figure 4). MB uptake increases with the increase in dye concentration reaching a plateau at high MB concentrations. The concentration of the dye acts as a driving force to overcome mass transfer resistance for the transport of dye molecules from the aqueous solution to the carbon surface. When the surface is saturated with the dye molecules, available active sites decrease, obstructing further uptake of the dye molecules and reaching a plateau [31]. The increased MB adsorption is related to the desolvation of the carbon particles and dye molecules by temperature rise allowing more access to active sites and consequently, more MB uptake [29,30]. Figure 4. Adsorption of MB at (A) 20 o C and (B) at 40 o C (initial pH 7). The adsorption isotherms are used to characterize the adsorption systems. Despite the availability of different isotherm equations, the most commonly used isotherms are the Langmuir and Freundlich isotherms (Equations 7 and 8), respectively. Ce/qe = 1/b.q + Ce/q Eq. (7) log qe = log K + (1/n) log Ce Eq. (8) where q is the monolayer capacity (mg/g), and b is the Langmuir constant (L/mg). K (l 1/n mg 1-1/n /g) and 1/n are the Freundlich constants, which are relative indicators of adsorption capacity and adsorption intensity, respectively. Sorbent Temp. Pore diffusion constant, kd (mg/g / h 0.5 ) Pseudo-first-order model Pseudo-second-order model Ea, (kJ/mol) k1 (h -1 ) qe (mg/g) R 2 rate const. k2, (g/mg/h) Initial adsorption rate, h, (mg/g/h) qe, (mg/g) R 2 AC 20 o C 5.34 0.081 4.29 0.9560 0.0029 4.99 41.32 0.9969 5.85 40 o C 9.53 0.169 6.12 0.8609 0.0034 8.10 43.78 0.9972 ON 20 o C 9.28 0.149 4.96 0.9211 0.0061 8.25 36.90 0.9989 6.06 40 o C 12.17 0.158 5.15 0.9367 0.0711 13.21 43.10 0.9979 ONM 20 o C 8.90 0.107 4.40 0.9520 0.0069 8.22 34.48 0.9944 16.43 40 o C 12.90 0.104 4.26 0.9645 0.0106 16.89 39.84 0.9989 ONDM 20 o C 10.1 0.104 3.88 0.9153 0.0102 14.71 38.02 0.9988 26.47 40 o C 19.54 0.118 3.27 0.9512 0.0204 39.53 44.05 0.9998 ONE 20 o C 11.49 0.191 4.56 0.9611 0.0116 20.33 41.84 0.9994 28.81 40 o C 20.01 0.098 3.60 0.9325 0.0247 49.26 44.64 0.9999 ONDE 20 o C 0.95 0.094 2.93 0.8906 0.0039 0.662 13.09 0.9985 14.11 40 o C 2.51 0.137 3.75 0.9910 0.0056 2.37 20.58 0.9984 ONDIP 20 o C 13.63 0.139 3.85 0.9532 0.0137 25.19 42.92 0.9997 36.12 40 o C 27.45 0.126 2.78 0.8709 0.0353 73.53 45.66 0.9999 (A) (B) SAID AL-MAHAIKHI ET AL. 48 The Langmuir isotherm model assumes the formation of monolayer coverage of adsorbates at equilibrium as adsorption sites become occupied by the adsorbate, after which no adsorption can take place at the occupied sites [32]. In contrast, for surfaces having non-uniform energy distribution, the Freundlich isotherm is extensively used [33]. Adsorption data were found to fit the Langmuir model well with high values of R 2 (Table 2). On the other hand, the adsorption data do not show good fit to the Freundlich model showing low correlation values of R 2 (Table 2). MB adsorption favors monolayer formation at equilibrium following the Langmuir adsorption model well. The adsorption capacity (q) of MB increased with the rise in temperature for all carbons tested. This could be due to the desolvation of dyes and the decrease in the thickness of boundary aqueous layers of carbons and MB molecules with temperature rise [29,30]. As a result, the mass transfer resistance of dye in the boundary layer decreases. q follows the order of ONDIP > ONE > ONDM > AC > ON > ONM > ONDE. ONDIP, ONE, and ONDM are hydrophobic activated carbons with incomplete surface functionalization due to the large size of the alkylamine immobilized. The unfunctionalized content of carboxylic groups acts as a barrier between the immobilized hydrophobic chains disallowing hydrophobic interaction between the immobilized chains or their entangling together [24]. This allows more interaction of MB molecules with the hydrophobic active sites and thus enhanced MB uptake. On the other hand for ONM, surface functionalization extent is high and this provides small hydrophobic active sites (methyl group). Thus, the large MB molecules can be adsorbed onto multiple active sites simultaneously. This restricts further access to the inner surface active sites and, thus, MB adsorption decreases. For ONDE, the long diethyl chains immobilized are probably self-adsorbed onto the carbon surface [24] or entangled with adjacent diethyl chains minimizing the uptake of MB dye. For AC, despite having a large surface area (588 m 2 /g), it shows less adsorption of MB compared with ONDIP, ONE, and ONDM which possess lower surface area. It is expected that the immobilized hydrophobic chains onto ONDIP, ONE, and ONDM can adsorb multiple molecules of MB. MB adsorption onto AC takes place mostly via van der Waals forces and - interactions [19, 34, 35]. Table 2. Equilibrium parameters for MB adsorption at different temperatures. ON shows less MB adsorption than AC and this is probably related to the incomplete saturation of the ON surface by the MB molecule due to its larger size. Another characteristic of the Langmuir isotherm is the separation factor, Rs, (Equation 9). Rs = 1/(1+bCo) Eq. (9) The effectiveness of the adsorption process can be predicted from the values of the separation factor. For Rs = 0, the adsorption is irreversible, and when Rs lies between 0 and 1, the adsorption is favorable [36]. If Rs is larger than 1, the adsorption is unfavorable but if Rs = 1, the adsorption is linear [36]. Table 2 shows that Rs is between 0 and 1 indicating that the adsorption of MB is favorable. As the values of Rs approach zero, this indicates strong adsorption tending to irreversibility. The monolayer adsorption capacity of MB on adsorbents, in this study, can be compared with different adsorbents in other studies. For AC, prepared by phosphoric acid activation from coconut shell, MB capacity was 200 mg/g [37]. Other adsorbents showed different capacities including AC prepared by potassium hydroxide activation (270 mg/g) [19], dehydrated carbon (196.1 mg/g) [20], ethylamine hydrophobic activated carbon (393 m 2 /g), lignin (34 mg/g) [38], D-fructose (83 mg/g) [39], and anaerobic activated sludge (91 mg/g) [40]. In another study, AC showed a Sorbent Sorption Temp. o C Langmuir constants Separation factor, Rs R 2 Freundlich constants R 2 q (mg/g) b (L/mg) 1/n K AC 20 o C 88.5 0.040 0.04-0.33 0.9983 0.333 13.2 0.9719 40 o C 102.0 0.153 0.01-0.12 0.9999 0.288 24.2 0.8388 ON 20 o C 81.3 0.052 0.03-0.28 0.9983 0.298 15.0 0.9681 40 o C 94.3 0.064 0.03-0.24 0.9985 0.367 40.7 0.9698 ONM 20 o C 75.8 0.038 0.05- 0.35 0.9970 0.303 12.9 0.9838 40 o C 86.2 0.075 0.02-0.21 0.9986 0.270 19.5 0.9678 ONDM 20 o C 91.8 0.044 0.04-0.32 0.9968 0.318 14.9 0.9796 40 o C 112.4 0.129 0.01-0.14 0.9985 0.302 24.2 0.9328 ONE 20 o C 96.2 0.055 0.03-0.27 0.9981 0.314 16.73 0.9707 40 o C 122.0 0.117 0.02-0.15 0.9990 0.308 25.8 0.9371 ONDE 20 o C 19.04 0.032 0.06-0.38 0.9970 0.180 1.20 0.986 40 o C 30.86 0.058 0.03-0.26 0.9994 0.180 10.6 0.9277 ONDIP 20 o C 99.0 0.061 0.03-0.25 0.9981 0.308 18.14 0.9748 40 o C 131.6 0.158 0.01-0.11 0.9988 0.283 32.2 0.8543 ADSORPTION OF METHYLENE BLUE ONTO HYDROPHOBIC ACTIVATED CARBON 49 capacity of 127.6 mg/g; however, on modification with sodium lauryl sulfate the capacity increased to 195.7 mg/g [2]. In this study, AC and ONDIP show adsorption capacities of 131.6 and 102 mg/g of MB, respectively. 3.5 Thermodynamic parameters The thermodynamic parameters were calculated for the adsorption of MB onto the carbons under investigation. The parameters were calculated from the change in equilibrium constant, Kc, at 20 and 40 o C. Kc can be calculated using Equation 10. Kc = CAe/ Ce Eq.(10) where CAe is the amount of MB adsorbed (mg) per liter of solution, and Ce is the equilibrium concentration of MB solution (mg/L). Kc is estimated from the initial part of the adsorption isotherm where qe versus Ce is linear. The Gibbs free energy change of the adsorption (ΔG°), is related to Kc as shown in Equation 11 [41]. ΔG o = -RTlnKc Eq. (11) where R is the gas constant (8.314 J mol -1 K -1 ) and T is the absolute temperature (K). The enthalpy (ΔHº) can be calculated from the Van’t Hoff equation (Equation 12) [41]. 2 1 1 2 1 1 ln( ) ( ) o c c k H k R T T    Eq. (12) where kc1 and kc2 are the equilibrium constants at T1 and T2, respectively. From ΔGº and H o , the entropy change, ΔSº, is calculated from Equation 13. G o = H o - TS o Eq. (13) Thermodynamic parameters for the adsorption of MB are shown in Table 3. Kc values increase with temperature indicating an endothermic nature of MB adsorption [41]. The negative values of ΔG o indicate spontaneous adsorption processes, which is usually the case for the adsorption of organic compounds from solution [42]. In addition, ΔG o values decrease further with the increase in temperature indicating that higher temperature enhances MB adsorption. ONDE shows a positive G o at 20 o C, indicating a nonspontaneous process at this temperature. Table 3. Thermodynamic parameters of MB adsorption on different carbons. Carbon Temp.(K) Kc G o (kJ/mol) H o (kJ/mol) S o (J/mol) AC 293 9.07 -5.37 34.95 137.6 313 22.68 -8.12 ON 293 10.39 -5.70 27.05 111.8 313 21.11 -7.94 ONM 293 8.43 -5.19 36.90 143.3 313 22.20 -8.07 ONDM 293 15.19 -6.63 34.78 146.3 313 37.83 -9.45 ONE 293 18.43 -7.10 35.75 143.8 313 47.09 -10.02 ONDE 293 0.96 0.09 38.46 131.0 313 2.64 -2.53 ONDIP 293 21.63 -7.49 31.55 133.3 313 49.49 -10.15 The positive values of ΔH o thermodynamically indicate an endothermic nature of MB adsorption. ΔH o values are less than 40 kJ/mol, indicating the physical adsorption nature of the MB. ΔH o values higher than 40 kJ/mol represent chemisorption processes [43]. The adsorption of methylene blue on AC showed an endothermic spontaneous process [43]. The positive value of ΔS o reveals an increase in the degree of randomness for all tested carbons at the solid- solution interface for MB adsorption. Previous studies showed similar trends for MB adsorption [43-45]. SAID AL-MAHAIKHI ET AL. 50 3.6 Reuse of adsorbents for dye adsorption AC, ONDM, ONE, and ONDIP were selected for this study (Figure 5). AC shows less uptake in the second adsorption cycle showing 79 % uptake compared with the first adsorption cycle. It is known that AC has limited reuse because it is difficult to desorb the adsorbate molecules from deep locations inside the porous structure [41]. On the other hand, ONDIM, ONE, and ONDIP reached 98.6, 99.5, and 99.6 % of MB adsorption in the second adsorption cycle compared with the first adsorption cycle. Figure 5. Carbon reuse for MB adsorption. 4. Conclusion AC, ON, and ON functionalized with different alkylamines (ONM, ONDM, ONE, ONDE, and ONDIP) were tested for MB adsorption. pH 7 was found to be optimum for MB adsorption. MB adsorption follows a pseudo-second- order kinetic model with the rate of adsorption increasing with temperature rise. 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