Adsorption of methylene blue onto activated carbon produced from tea (Camellia sinensis L.) seed shells: kinetics, equilibrium, and thermodynamics studies

Adsorption of methylene blue onto activated carbon produced from tea ( Camellia sinensis L.) seed shells: kinetics, equilibrium, and thermodynamics studies 1 first-author 2 1 3 1 corresp ‡Corresponding Authors ‡Corresponding Authors †E-mail:zdxp@zju.edu.cntyfwang@gmail.com †E-mail:zdxp@zju.edu.cntyfwang@gmail.com zdxp@zju.edu.cn 1 1 4 corresp ‡Corresponding Authors ‡Corresponding Authors †E-mail:zdxp@zju.edu.cntyfwang@gmail.com †E-mail:zdxp@zju.edu.cntyfwang@gmail.com zdxp@zju.edu.cn Department of Tea Science, Zhejiang University, Hangzhou 310058, China Department of Tea Science, Zhejiang University, Hangzhou 310058, China Crop Management Bureau, Agricultural Department of Zhejiang Province, Hangzhou 310029, China Crop Management Bureau, Agricultural Department of Zhejiang Province, Hangzhou 310029, China Zhejiang Academy of Agricultural Sciences, Hangzhou 310021, China Zhejiang Academy of Agricultural Sciences, Hangzhou 310021, China Key Laboratory of Horticultural Plant Growth, Development and Quality Improvement, Ministry of Agriculture, Hangzhou 310029, China Key Laboratory of Horticultural Plant Growth, Development and Quality Improvement, Ministry of Agriculture, Hangzhou 310029, China Tea ( Camellia sinensis L.) seed shells, the main byproduct of the manufacture of tea seed oil, were used as precursors for the preparation of tea activated carbon (TAC) in the present study. A high yield (44.1%) of TAC was obtained from tea seed shells via a one-step chemical method using ZnCl 2 as an agent. The Brunauer-Emmett-Teller (BET) surface area and the total pore volumes of the obtained TAC were found to be 1 530.67 mg 2 /g and 0.782 6 cm 3 /g, respectively. The equilibrium adsorption results were complied with Langmuir isotherm model and its maximum monolayer adsorption capacity was 324.7 mg/g for methylene blue. Adsorption kinetics studies indicated that the pseudo-second-order model yielded the best fit for the kinetic data. An intraparticle diffusion model suggested that the intraparticle diffusion was not the only rate-controlling step. Thermodynamics studies revealed the spontaneous and exothermic nature of the sorption process. These results indicate that tea seed shells could be utilized as a renewable resource to develop activated carbon which is a potential adsorbent for methylene blue. Activated carbon Adsorption Tea seed shells Methylene blue Introduction 1 Synthetic dyestuffs used extensively in textile, paper, printing industries and dye houses are mostly discharged into surface water resources as industrial wastes. This has turned out to be a significant source of aquatic ecosystem pollutants (Aksu, 2005 2005 ; Attia et al., 2008 2008 ). Dyeing industry effluents constitute one of the most problematic wastewaters to be treated not only for their high chemical and biological oxygen demands, suspended solids, and content in toxic compounds but also for colour, which are the first contaminant to be recognized by the human eye (Aksu, 2005 2005 ). Thus, dye wastewater needs to be treated for the sake of the protection of human health and environmental safety. However, the treatment of dyes in industrial wastewater presents several problems since dyes usually have a synthetic origin and complex aromatic molecular structures which make them very stable and generally difficult to be biodegraded and photodegraded (Lorenc-Grabowska and Gryglewicz, 2007 2007 ; Gupta, 2009 2009 ). Recently, the removal of dyes from wastewater through adsorption techniques has proven promising due to its simplicity, inexpensiveness, and efficiency (Wang et al., 2005 2005 ; Baccar et al., 2010 2010 ; Dural et al., 2011 2011 ; Kyzas et al., 2012 2012 ). Adsorption is the process by which a solid adsorbent can attract a component in water to its surface and form an attachment via a physical or chemical bond, thus removing the component from the fluid phase (Demirbas et al., 2008 2008 ). A successful adsorption process not only depends on dye adsorption performance of the adsorbents, but also on the constant supply of the materials for the process (Han et al., 2008 2008 ). Activated carbon is known for its unique properties including porous structure, high specific surface areas, and large sorption capacities (Dural et al., 2011 2011 ). It is also an excellent adsorbent, and can be used for a variety of purposes, such as purification and separation, in the abatement of hazardous contaminants, in municipal and industrial wastewater treatment, as a catalyst or catalyst support in medicine, and in the recovery of valuable metals, etc. (Sun and Jiang, 2010 2010 ). However, the costs of preparing activated carbon have historically been high. Currently agro-industrial waste, a renewable resource, is considered to be a promising candidate for the production of activated carbon and would make activated carbon widely available for adsorption. Previously, many activated carbons have been prepared from agro-industrial waste biomasses, including coconut shell (Azevedo et al., 2007 2007 ), waste apricot (Nal, 2006 2006 ), olive stone (Kula et al., 2008 2008 ), coffee residue (Boudrahem et al., 2011 2011 ), piassava fiber (Avelar et al., 2010 2010 ), rice husk (Kalderis et al., 2008 2008 ), and pomegranate seed (Uçar et al., 2009 2009 ). It is estimated that over a million tons of tea seed shells are produced annually in China. Driving the popularity of tea seed oil in recent years is the fact that it is an edible oil of high quality, containing more than 84% unsaturated fatty acid while the predominant fatty acids are oleic acid and linoleic acid (Sahari et al., 2004 2004 ; Demirbas, 2009 2009 ). As the popularity of tea seed oil has increased, the amount of tea seed shells, the main byproduct during manufacture of seed oil, has greatly increased as well. However, tea seed shells are not typically utilized and the majorities are discarded as agro-industrial waste. Due to the large quantities of material available and the consistent accumulation of tea seed shells, there is a need for efficient management of them. Therefore, the aim of this research was to prepare activated carbon from tea seed shells by using a ZnCl 2 activation method, to investigate its textural and chemical surface characterization, and to study its adsorption of methylene blue (MB) including kinetics, equilibrium, and thermodynamics studies. Materials and methods 1 Raw materials 2 Tea seed used for the preparation of activated carbon was collected from the Panban tea garden (Zhejiang, China). The seed shell was removed from the seed, dried at 110 °C for 12 h, ground and sieved to a particle size range of 40–60 meshes, and then stored in sealed containers for experimentation. Adsorbate 2 A cationic dye MB was chosen for the adsorption experiments. Stock solutions of 500 mg/L dye were prepared with distilled water and the required concentrations were obtained by diluting. Preparation of the activated carbon 2 A one-step method of preparation of the activated carbon was selected in this paper. The processed raw material was soaked in ZnCl 2 solution with an impregnation ratio of 1:1 for 24 h and then dried at 110 °C for 6 h. The dried mixture was put into a furnace and heated to 500 °C at the rate of 20 °C/min under a constant N 2 (99.99%) flow of 120 cm 3 /min. The activation process was maintained for 1 h. Then, the activated substance was repeatedly washed with 0.1 mol/L HCl followed by distilled water until the pH reached 7. Finally, the product was dried at 110 °C for 8 h, ground and sieved to a particle size of 180 meshes for further studies. The yield of activated carbon is defined as the weight after activation, washing, and drying. The percent yield was calculated by $${\text{Yield}} = \frac{m}{M} \times 100\%$$ , where m and M are the dry weights (g) of the activated carbon and the precursor, respectively. Preparation of the activated carbon 2 Textural characterization of the activated carbon was measured by N 2 adsorption at 77 K (AUTOSORB-1-C QUANTACHROME, America). Surface functional groups were examined using Fourier transform infrared analysis (AVA TAR370, America) with a scanning range of 4000–400 cm −1 . The surface morphology was detected by a scanning electron microscope (SEM; HITACHI S-3000N, Japan). The point of zero charge (pH pzc ) was determined according to the method described by Foo and Hameed ( 2011a 2011a ). Batch adsorption studies 2 MB kinetic and isotherm adsorption experiments were carried out to evaluate the adsorption performance. The equilibrium adsorption research was completed by adding a fixed amount of activated carbon into 25 ml different initial concentrations of MB. The kinetic adsorption studies were performed by adding 0.2 g activated carbon into 200 ml different initial concentrations of MB. The aqueous samples were taken at pre-set time intervals and their concentrations were determined. Concentration determination of all the samples was filtered before they were measured by UV-6100A spectrophotometer at the maximum absorption wavelength of 664 nm. The amount of MB adsorption at equilibrium was calculated by $${q_{\text{e}}} = ({C_{\text{o}}} - {C_{\text{e}}})V/W$$ , where C o and C e (mg/L) are the initial and equilibrium concentrations of MB, respectively, V (L) is the volume of the solution, and W (g) is the mass of adsorbents used. The uptake of MB at time t , q t (mg/g) was calculated by $${q_t} = ({C_{\text{o}}} - {C_t})V/W$$ , where C t is the concentration of MB at any time. Results and discussion 1 Yield 2 As can be seen from Table 1 1 , a high yield of tea activated carbon (TAC) was obtained with a desiring characteristic, this should be attributed to the lower temperature used in the chemical activation, which reduced the amount of compounds liberated by volatilization (Avelar et al., 2010 2010 ). S BET : Brunauer-Emmett-Teller (BET) surface area; V T : total pore volume; V mic : micropore volume; V mes : mesopore volume; D ap : average pore diameter Parameter Value S BET (m 2 /g) 1530 V T (cm 3 /g) 0.7826 V mic (cm 3 /g) 0.5989 V mes (cm 3 /g) 0.1837 D ap (nm) 2.045 Yield (%) 44.10 pH pzc 6.30 Characterization of TAC 2 Textural characterization 3 Fig. 1 1 shows the N 2 adsorption and desorption at 77 K of TAC, from which it can be seen that a sharp increase of adsorption volume occurred at low relative pressure, and at high relative pressure the adsorption volume reached a plateau. According to the International Union of Pure and Applied Chemistry (IUPAC) classification for adsorption isotherm, the isotherm is classified as type I, indicating the presence of large fractions of micropores and in some cases mesopores (Dural et al., 2011 2011 ). Pore size distribution is a very important property of adsorbents because the difference in the pore size affects the adsorption capacity for molecules of different sizes and shapes, and also this is one of the criteria by which carbon adsorbents are selected for a particular application (Xiao et al., 2012 2012 ). The pore size distribution of the TAC inillustrates that the sizes of the pores are smaller than 4.0 nm, in which pores less than 2.0 nm account for a large percentage (Fig. 2 2 ). From the data presented in Table 1 1 , it can be seen that the D ap value is 2.045 nm which is very near 2.0 nm indicating a great development of micropores based on the classification adopted by IUPAC: micropores ( < 2 nm), mesopores (2–50 nm), and macropores ( > 50 nm). Table 2 2 sums up activated carbon samples prepared from various agriculture waste biomass with ZnCl 2 as an agent. Though the Brunauer-Emmett-Teller (BET) surface areas of activated carbons produced from sugar beet bagasse and Enteromorpha prolifera are higher than that in the present work, the BET surface area of the TAC is much higher than others listed in Table 2 2 , from which it can be detected that it is a good choice to utilize tea seed shells for producing activated carbons of high specific surface area with the modification of ZnCl 2 . (a) Micropore size distribution by Horvath-Kawazoe (HK) method; (b) Mesopore size distribution by Barrett-Joyner-Halenda (BJH) method Precursor Activation temperature (°C) Activation time (min) BET surface area (m 2 /g) Reference Terminalia arjuna nut 500 60 1 260 Mohanty et al. ( 2005 2005 ) Coconut shell 500 180 1 266 Azevedo et al. ( 2007 2007 ) Rice husk 700 30 750 Kalderis et al. ( 2008 2008 ) Olive stone 650 120 790 Kula et al. ( 2008 2008 ) Pomegranate seed 600 60 979 Uçar et al. ( 2009 2009 ) Piassava fiber 500 240 1 190 Avelar et al. ( 2010 2010 ) Sugar beet bagasse 700 90 1 826 Demiral and Gündüzoğlu ( 2010 2010 ) Enteromorpha prolifera 500 60 1 688 Li et al. ( 2010 2010 ) Coffee residue 600 60 889 Boudrahem et al. ( 2011 2011 ) Tea seed shell 500 60 1 530 Present study SEM micrographs of TAC under different magnifications are given in Fig. 3 3 . It can be observed that the rough surface is filled with cavities and irregular holes, which resulted from the evaporation of ZnCl 2 during carbonization, leaving the space previously occupied by the reagent (Demiral and Gündüzoğlu, 2010 2010 ). Chemical surface characterization 3 Surface functional groups within activated carbon contribute significantly to its adsorption ability, as ion exchangers, adsorbents, catalysts, and catalyst supports (Ren et al., 2011 2011 ). Fourier transform infrared spectroscopy (FTIR) spectra of the precursor and TAC are presented in Fig. 4 4 . The bands located at 3 420–3 385 cm −1 in both of spectra were typically attributed to O–H stretching vibration of hydroxyl functional groups. The precursor had a peak at 2 925 cm −1 which was attributed to the C–H, yet there was a decrease in the spectrum of TAC. Significant differences occurred at the bands from 1 700–1 000 cm −1 . For the spectrum of the precursor, the band at 1 739 cm −1 is ascribed to C=O. The band at 1 618 cm −1 is ascribed to the olefinic (C=C) vibrations. The band at 1 258 cm −1 may be attributed to esters, ethers or phenol groups and the band at 1 048 cm −1 corresponded to the alcohol groups (R–OH) (Baccar et al., 2009 2009 ). For the spectrum of the precursor, the band around 1 560 cm −1 arose from C=C stretching in an aromatic ring. The band at 1 380 cm −1 could be assigned to the C=C stretching of aromatic rings polarized by oxygen atoms bound near one of the C atoms (Njoku and Hameed, 2011 2011 ). The peak occurring at 1 120 cm −1 was ascribed to the presence of C–O. So, it can be concluded that the surface functional groups were significantly different between samples treated with ZnCl 2 and those not treated. The changes in the FT-IR spectrum of TAC were evidence of the formation of structures containing multiple carbon-carbon bonds as well as the elimination of originally present oxygen and hydrogen atoms (Chen et al., 2011 2011 ). As can be seen from Table 1 1 , the pH pzc value was 6.30 indicating the acidic nature of the TAC. Adsorption kinetics 2 Kinetic study is important to the adsorption process because it depicts the uptake rate of adsorbate and controls the residual time of the whole adsorption process (Demirbas et al., 2009 2009 ). To explore the controlling mechanism of the adsorption process, the experimental data were fitted to the pseudo-first-order, pseudo-second-order, and intraparticle diffusion models. The linear forms of the pseudo-first-order and the pseudo-second-order are represented by $$\ln ({q_{\text{e}}} - {q_t}) = \ln {q_{\text{e}}} - {K_1}t$$ , $$\frac{t}{{{q_t}}} = \frac{1}{{{K_2}q_{\text{e}}^2}} + \frac{t}{{{q_{\text{e}}}}}$$ , where K 1 (min −1 ) and K 2 (g·mg −1 ·min −1 ) are the rate constants of the pseudo-first-order and pseudo-second-order, respectively. The parameters of kinetic models parameters and the correlation coefficient ( R 2 ) values for different initial concentrations are listed in Table 3 3 . As can be seen, the R 2 values obtained from the pseudo-second-order were consistently higher than those from the pseudo-first-order. In addition, the q e values coincided with the expected q e values ( q e , exp ). That indicates that the adsorption perfectly obeys the pseudo-second-order model meaning the controlling rate step is chemisorption. Meanwhile, the result showed that the rate of adsorption depended on the availability of adsorption sites on the surface of adsorbent rather than MB concentration in bulk solution (Liu, 2008 2008 ). 2 C o (mg/L) 2 q e,exp (mg/g) 3 Pseudo-first-order 3 Pseudo-second-order K 1 (min −1 ) q e (mg/g) R 2 K 2 (g·mg −1 ·min −1 ) q e (mg/g) R 2 150 149.9 0.014 23 3.600 0.867 8 0.012 700 150.2 1.000 200 199.7 0.017 00 15.01 0.916 0 0.004 040 200.4 1.000 250 248.8 0.013 30 46.11 0.822 0 0.001 090 250.0 1.000 To evaluate the contribution of intraparticle diffusion to the sorption mechanism, an intraparticle diffusion model is introduced in this paper. The equation is expressed as $${q_t} = {K_{\text{p}}}{t^{0.5}} + C$$ , where K p represents the rate constant (mg·g −1 ·min −0.5 ), and C (mg/g) is a constant reflecting the significance of the boundary layer or mass transfer effect. As can be seen from Fig. 5 5 , the plot of q t vs. t 0.5 can be divided into two straight portions for different initial concentrations. In the first portion, it is attributed to the instantaneous utilization of the adsorbing sites on the adsorbent surface and the second portion is ascribed to a very slow diffusion of the MB from the surface film into the micropores, which are the least accessible sites of adsorption (Ma et al., 2011 2011 ). This suggested that the intraparticle diffusion was not the only rate-controlling step. Meanwhile, this also indicated that external mass transfer of MB molecules onto sorbent particles is also significant in the sorption process, especially at the initial reaction period (Hameed et al., 2009 2009 ). Adsorption isotherms 2 Adsorption isotherms describe qualitative information of the nature of the solute-surface interaction as well as the specific relation between the concentration of adsorbate and its degree of accumulation onto adsorbent surface at constant temperature (Li and Wang, 2009 2009 ). The equilibrium data were analyzed by fitting them to two isotherm models, namely the Langmuir and Freundlich isotherm models. Langmuir isotherm model 3 The Langmuir model assumes monolayer adsorption onto a surface containing a finite number of adsorption sites of uniform energies of adsorption with no transmigration of adsorbate in the surface plane (Baccar et al., 2010 2010 ). The model is represented by $$\frac{{{C_{\text{e}}}}}{{{q_{\text{e}}}}} = \frac{1}{{{q_{\text{m}}}{K_{\text{L}}}}} + \frac{{{C_{\text{e}}}}}{{{q_{\text{m}}}}}$$ , where q m (mg/g) and K L (L/mg) are related to the maximum adsorption capacity and Langmuir constant, respectively. The Langmuir dimensionless constant separation factor or equilibrium parameter R L is calculated by (Hall et al., 1966 1966 ) $${R_{\text{L}}} = 1/(1 + {K_{\text{L}}}{C_{\text{o}}})$$ , where R L indicates the shape of the isotherm to be either unfavorable ( R L > 1), linear ( R L =1), favorable (0 < R L < 1), or irreversible ( R L =0). It can be observed from Fig. 6 6 that all values of R L are between 0 and 1 indicating the adsorption process was favorable. The values of R L at low temperatures are smaller than those at high temperatures, from which it can be deduced that the adsorption behavior is more effective at low temperature. Freundlich isotherm model 3 The empirical Freundlich model, which is known to be satisfactory for low concentrations and based on sorption on a heterogeneous surface (Baccar, et al., 2010 2010 ), is expressed by $$\lg {q_{\text{e}}} = \lg {K_{\text{f}}} + \frac{1}{n}\lg {C_{\text{e}}}$$ , where n and K f ((mg/g)(L/mg) 1/ n ) are Freundlich constants related to the favorability of adsorption process and the adsorption capacity of the adsorbate, respectively. If 1/ n < 1, it indicates a favorable adsorption. Table 4 4 lists the values of 1/ n at different temperatures, it can be seen that all the values of 1/ n are smaller than 1. The favorability of the adsorption process was further confirmed by the result. 2 Temperature (K) 3 Langmuir isotherm model 3 Freundlich isotherm model q m (mg/g) K L (L/mg) R 2 K f ((mg/g)·(L/mg) 1/ n ) 1/ n R 2 298 324.7 2.000 0.9997 221.8 0.1010 0.9481 308 319.5 1.204 0.9992 206.1 0.1102 0.9342 318 313.5 1.016 0.9991 205.1 0.09785 0.9530 The parameters of the isotherm models calculated from the experimental data and the values of correlation coefficient ( R 2 ) are presented in Table 4 4 . According to the results, the R 2 values for the Langmuir are higher than those in Freundlich isotherm model. It is clear that the equilibrium adsorption data comply with Langmuir isotherm model, which suggests that the adsorption is a process which occurs in a homogeneous surface. Additionally, these results also demonstrated no interaction and transmigration of dyes in the plane of the neighboring surface (Foo and Hameed, 2011c 2011c ). As can be seen from Table 4 4 , the value of K L decreased with temperature increase, which implied that low temperature favored the adsorption process. For the sake of comparison, Table 5 5 lists the maximum monolayer adsorption of MB onto activated carbons derived from different precursors. Based on the data in Table 5 5 , it was found that TAC displayed a relatively high adsorption capacity of MB. Precursor Agent Adsorption capacity (mg/g) Reference Posidonia oceanica dead leaves ZnCl 2 285.7 Dural et al. ( 2011 2011 ) Cotton stalk ZnCl 2 193.5 Deng et al. ( 2009 2009 ) Date stones KOH 316.1 Foo and Hameed ( 2011b 2011b ) Oil palm empty fruit bunch KOH 344.8 Foo and Hameed ( 2011d 2011d ) Cashew nut shell KOH 68.7 Kumar et al. ( 2011 2011 ) Vetiver roots H 3 PO 4 423.0 Altenor et al. ( 2009 2009 ) Durian shell K 2 CO 3 289.3 Chandra et al. ( 2007 2007 ) Tea seed shells ZnCl 2 342.7 This work Adsorption thermodynamics 2 The thermodynamic parameters such as free energy (Δ G 0 ), enthalpy (Δ H 0 ), and entropy (Δ S 0 ) give a description of adsorption behavior related to temperature. Parameters can be calculated from $$\Delta {G^0} = - RT\ln {K_{\text{L}}}$$ , $$\ln {K_{\text{L}}} = \frac{{\Delta {S^0}}}{R} - \frac{{\Delta {H^0}}}{{RT}}$$ , where K L (L/mol) is from Langmuir equation, R is the gas constant (8.314 J/(mol·K), and T (K) is the temperature in Kelvin. As can be seen from Table 6 6 , the values of Δ G 0 were consistently negative at all temperatures measured, which revealed the adsorption process was feasible and spontaneous in nature. Furthermore, the absolute values of Δ G 0 decrease with the increase of the temperature indicating that the sorption process is more favorable at low temperature. The negative value of Δ H 0 was indicative of the exothermic nature of the adsorption process. The negative value of Δ S 0 suggested the decreased randomness at the solid-solute adsorption systems, which indicated that gain of entropy caused by the water molecules displaced is less than that lost by the MB molecules. 2 Δ H 0 (kJ/mol) 2 Δ S 0 (J/(mol·K)) 3 Δ G 0 (kJ/mol) 298 K 308 K 318 K −26.80 −84.64 −1.720 −0.467 −0.0420 Conclusions 1 In the present study, tea seed shells were modified by using ZnCl 2 in a one-step method at relatively low energy consumption. The kinetic adsorption complied well with the pseudo-second-order model. The results of the intraparticle diffusion model suggest that intraparticle diffusion was not the sole rate determining step. It was found that the Langmuir isotherm model yielded the best for equilibrium data with the maximum monolayer adsorption of 324.7 mg/g for methylene blue. Thermodynamics studies indicated the spontaneous and exothermic nature of the sorption process. References Z Aksu . 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This article does not contain any studies with human or animal subjects performed by any of the authors. 10.1631/jzus.B12a0225 Project supported by the Science and Technology Department of Zhejiang Province (No. 2010C32051) and the Ministry of Science and Technology (No. 2011BAD01B03-5-1), China 2012-09-10 2013-06-17 2013-04-01 2022-01-05 Copyright © Zhejiang University and Springer-Verlag Berlin Heidelberg 2013 2013 Journal of Zhejiang University-SCIENCE B (Biomedicine and Biotechnology) 07 14