Removal of organic contaminants from aqueous solution by cattle manure compost (CMC) derived activated carbons

Applied Surface Science 255 (2009) 6107–6114

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Removal of organic contaminants from aqueous solution by cattle manure compost (CMC) derived activated carbons Qingrong Qian a,b,*, Qinghua Chen b, Motoi Machida c,d, Hideki Tatsumoto c,d, Kazuhiro Mochidzuki a, Akiyoshi Sakoda a a

Institute of Industrial Science, the University of Tokyo, 4-6-1, Komaba, Meguro-ku, Tokyo 153-8505, Japan College of Chemistry and Materials Science, Fujian Normal University, Fuzhou, Fujian 350007, China Graduate School of Engineering, Chiba University, Yayoi-cho 1-33, Inage-ku, Chiba 263-8522, Japan d Safety and Health Organization, Chiba University, Yayoi-cho 1-33, Inage-ku, Chiba 263-8522, Japan b c

A R T I C L E I N F O

A B S T R A C T

Article history: Received 12 August 2008 Received in revised form 28 December 2008 Accepted 20 January 2009 Available online 29 January 2009

The activated carbons (ACs) prepared from cattle manure compost (CMC) with various pore structure and surface chemistry were used to remove phenol and methylene blue (MB) from aqueous solutions. The adsorption equilibrium and kinetics of two organic contaminants onto the ACs were investigated and the schematic models for the adsorptive processes were proposed. The result shows that the removal of functional groups from ACs surface leads to decreasing both rate constants for phenol and MB adsorption. It also causes the decrement of MB adsorption capacity. However, the decrease of surface functional groups was found to result in the increase of phenol adsorption capacity. In our schematic model for adsorptive processes, the presence of acidic functional groups on the surface of carbon is assumed to act as channels for diffusion of adsorbate molecules onto small pores, therefore, promotes the adsorption rate of both phenol and MB. In phenol solution, water molecules firstly adsorb on surface oxygen groups by H-bonding and subsequently form water clusters, which cause partial blockage of the micropores, deduce electrons from the p-electron system of the carbon basal planes, hence, impede or prevent phenol adsorption. On the contrary, in MB solution, the oxygen groups prefer to combine with MB+ cations than water molecules, which lead to the increase of MB adsorption capacity. ß 2009 Elsevier B.V. All rights reserved.

Keywords: Activated carbon Phenol Methylene blue Adsorption Schematic model

1. Introduction Activated carbons are profusely used as adsorbents for decontamination processes because of their extended surface area, high adsorption capacity, microporous structure and special surface reactivity. One of the most important uses of activated carbons is in water treatments [1–4]. Water is very frequently contaminated with organic compounds including phenol and methylene blue (MB). Phenol and related compounds are toxic to humans and aquatic life, create oxygen demand in receiving water [5], cause unpleasant taste and odor of drinking water and can exert negative effects on different biological processes. The demand for the removal of this organic compound has been increased. Though different methods have been proposed to remove phenols including aerobic and anaerobic biodegradation, oxidation by ozone, and uptakes by ion exchange resins, adsorption by activated carbons is the best and most frequently used method [6,7]. The release of colored wastewater into the

* Corresponding author. Tel.: +86 8344 1949; fax: +86 8344 1949. E-mail address: [email protected] (Q. Qian). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.01.060

ecosystem is a dramatic source of aesthetic pollution, eutrophication and perturbation in aquatic life. Methylene blue is an important dye, which is the most commonly used dye for coloring among all dyes of its category. Though methylene blue is not strongly hazardous, it can cause some harmful effects. Acute exposure to methylene blue can cause increased heart rate, vomiting, shock, Heinz body formation, cyanosis and tissue necrosis in humans [8]. Therefore, the removal of such dye from process effluent becomes environmentally important. Recently, the removal of methylene blue by activated carbons prepared from agricultural wastes including bagasse [9], jute fiber [10], rice husks [11], coconut coir dust [12], durian shell [13], rattan sawdust [8] and coir pith [14] and other precursors has been investigated. On the other hand, phenol and methylene blue are commonly used to characterize the adsorption capacity of activated carbons [15]. They have been widely employed as a model compound for small and medium size molecule organic contaminants and dyes, respectively [16]. It was found that the adsorption capacity is significantly affected by activated carbons characteristics, including texture (surface area, pore size distributions), surface chemistry (surface functional groups, hetero elements incorporated in graphite layer) and ash content. It also depends on

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adsorbate characteristics such as molecular weight, polarity, pKa, molecular size and functional groups. Finally, the solution conditions such as pH, adsorbate concentration and the presence of other possible adsorbate are other factors to be taken into account [4]. However, the binding theories for adsorption of phenol and MB onto the surface of activated carbon are still not elucidated very clearly. In addition, there is still no schematic model proposed to simultaneously analyze the adsorptive processes for the two compounds onto activated carbon. In this study, the adsorption kinetics and capacity of phenol and MB onto activated carbons in aqueous phase were investigated. The influence of textural and surface chemical nature of the activated carbons on phenol and MB adsorption kinetics and capacity were discussed. The schematic models for the adsorptive processes of two adsorbates onto activated carbons were also proposed.

2.3. Kinetic procedures Kinetic experiments were performed using the initial concentration of 200 mg/L for both phenol and MB. Fifty milliliter of aqueous solution was piped in Erlenmeyer flask and kept in a thermostatic shaking water bath at 298 K. After attaining the temperature, 30 mg of activated carbon was added into the flask followed by agitation with speed of 100 rpm. The solution was separated from the activated carbons by centrifugation at 2000 rpm at desired agitation time and the adsorbate concentration was measured by direct ultraviolet absorbance method using an UV/Vis spectrometer (Shimadzu UV-2550) at the wavelength of 270 nm for phenol and 665 nm for MB, respectively. 2.4. Equilibrium experiments The experiments of phenol and methylene blue adsorption onto the activated carbons were carried out using batch equilibration

2. Materials and methods 2.1. Adsorbents The preparation of activated carbons from cattle manure compost (CMC) by zinc chloride activation has been described in detail elsewhere [17]. Two activated carbons with different mesopore contents from the ZnCl2/CMC ratios of 1.5 and 2.5 by 400 8C for 1 h activation were selected for this study and referred as CZ1.5 and CZ2.5, respectively. Furthermore, in order to obtain various surface chemical characteristics, CZ1.5 and CZ2.5 were outgassed under a helium flow at the temperature of 400 or 900 8C for 2 h, and denoted as CZ1.5–4, CZ1.5–9, CZ2.5–4 and CZ2.5–9, respectively. The textural and surface properties of the six activated carbons tabulated in Table 1 have been investigated and discussed elsewhere [18]. 2.2. Adsorbates Phenol used in the study is of analytical grade and was purchased from Kanto Chemical Co., Inc. (Japan). Three dimensional molecular sizes of phenol are 0.58 nm  0.42 nm  0.15 nm. A stock solution of phenol with concentration of 1000 mg/L was prepared by mixing appropriate amount of phenol with de-ionized water. The solution was suitably diluted to the desired initial concentration by using de-ionized water. Methylene blue (CI No. 52015) with a molecular formula of C16H18N3SCl
xH2O (x = 2–3) was purchased from Merck (Germen). Three dimensional molecular sizes of MB are 1.42 nm  0.62 nm  0.16 nm. An MB stock solution with concentration of 1000 mg/L was prepared by mixing appropriate amount of MB with KH2PO4/ Na2HPO4 buffer solution (pH 7.8). The solution was suitably diluted to the desired initial concentration by using buffer solution. Table 1 Physical and surface properties of ACs. Parameters

CZ1.5

Pore structure SBET (m2/g) Vtotal (mL/g) Vmeso/Vtotal (%) Dp (nm)

2110 1960 1150 2000 2006 1339 1.244 1.134 0.633 1.715 1.689 0.986 39.8 38.3 34.5 59.1 58.4 52.8 2.35 2.32 2.20 3.38 3.42 2.95

CZ1.5–4

Surface chemistry (by Boehm titration) Caroxylic (mmol/g) 0.260 0.030 Lactonic (mmol/g) 0.238 0.150 Phenolic (mmol/g) 0.540 0.521 Total acidic (mmol/g) 1.138 0.701 Total basic (mmol/g) 0.784 1.080

CZ1.5–9

0.000 0.000 0.079 0.079 1.150

CZ2.5

0.400 0.241 0.438 1.079 0.437

CZ2.5–4

0.030 0.150 0.300 0.480 1.020

CZ2.5–9

0.000 0.000 0.078 0.078 1.160

Fig. 1. Adsorption kinetics of phenol onto activated carbons (a) qt-t profiles and (b) t/qt-t plots; Black line for CZ1.5, CZ1.5–4 and CZ1.5–9, dotted line for CZ2.5, CZ2.5–4 and CZ2.5–9.

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Table 2 Kinetics parameters for phenol and MB adsorption. Pseudo-second-order qe (mg/g) Phenol

Methylene blue (MB)

Intraparticle diffusion k2 (g/mg min)

CZ1.5 CZ1.5–4 CZ1.5–9 CZ2.5 CZ2.5–4 CZ2.5–9

76 88 154 86 100 192

6.76  10 3.36  10 2.49  10 6.52  10 3.67  10 2.00  10

CZ1.5 CZ1.5–4 CZ1.5–9 CZ2.5 CZ2.5–4 CZ2.5–9

331 339 300 316 321 284

7.29  10 3.57  10 1.45  10 3.57  10 1.44  10 1.15  10

3 3 3 3 3 3 5 5 5 4 4 4

R2

kint (mg/g min1/2)

C

0.9999 0.9999 0.9994 0.9999 0.9990 0.9998

2.74 3.71 8.22 2.12 3.65 8.74

54.83 56.32 48.52 65.35 63.08 55.49

0.9997 0.9995 0.9946 0.9999 0.9998 0.9997

17.60 15.34 7.67 22.47 14.87 7.03

61.52 19.79 6.79 131.60 122.87 107.91

technique [19]. Thirty milligrams of activated carbon was mixed with 50 mL solution in Erlenmeyer flask, sealed and agitated at 298 K with the speed of 100 rpm until the equilibrium was attained. The flask was settled without agitation for several minutes to allow the carbon to reside on the bottom of the flask for sampling. The equilibrium concentration of adsorbate in solution was measured by the same method as that of kinetic studies. The amount of adsorbate adsorbed at equilibrium was calculated using the equation of qe = (C0 Ce)V/W, where V and W are the volume of solution and the dosage of adsorbent, respectively. 3. Results and discussion 3.1. Phenol adsorption kinetics Several kinetics models including the pseudo-first-order [20], the pseudo-second-order [21], and the intraparticle diffusion [22] were used to examine the controlling mechanism of organic contaminants adsorption process such as chemical reaction, diffusion control and mass transfer, respectively. The kinetic profiles for phenol adsorption from aqueous solution onto various activated carbons are shown in Fig. 1. The adsorptions of phenol onto the prepared activated carbons are found to be rapid at the initial period and then become slow with passing contact time. It was determined that equilibration times for phenol adsorption onto CZ1.5, CZ1.5–4, CZ2.5, and CZ2.5–4 are about 240 min and onto CZ1.5–9 and CZ2.5–9 are 960 min, respectively. The experimental data do not fit pseudo-first-order model, but can be predicted by pseudo-second-order model very well, indicating that the rate determining step is not the external diffusion of phenol from the bulk liquid to the carbon surface, but may involve one or more of the internal steps [23]. The parameters of pseudo-second-order model and intraparticle diffusion model are calculated in Table 2. k2 is rate constant of pseudo-second-order adsorption, revealing phenol adsorption rate on whole process, whereas kint is the intraparticle diffusion rate constant, showing the initial adsorption rate at far from equilibrium. It is obvious in Table 1 the CZ2.5 series (including CZ2.5, CZ2.5–4 and CZ2.5–9) possess higher proportion of mesopore volume than CZ1.5 series (including CZ1.5, CZ1.5–4 and CZ1.5–9), and heat treatment at 400 8C selectively removes some surface functional groups while at 900 8C eliminates most part of them. The values of k2 and kint for CZ1.5 series are comparable to those of CZ2.5 series correspondingly, indicating that mesopore concentration in carbon matrix has little effect on phenol adsorption rate. The decrease of k2 from CZ1.5, CZ1.5–4 to CZ1.5–9 and that from CZ2.5, CZ2.5–4 to CZ2.5–9 shows that the presence of surface acidic functional groups is operative for increase of phenol adsorption rate.

Fig. 2. Adsorption kinetics of MB onto activated carbons (a) qt-t profiles and (b) t/qt-t plots; Black line for CZ1.5, CZ1.5–4 and CZ1.5–9, dotted line for CZ2.5, CZ2.5–4 and CZ2.5–9.

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3.2. MB adsorption kinetics The kinetics of methylene blue adsorption onto the prepared activated carbons was performed at 25 8C in a buffer solution of pH 7.8 using the initial MB concentration of 200 mg/L. As depicted in Fig. 2a, the adsorption of MB onto the ACs is also found to be rapid at the initial period and then becomes slow with increasing contact time. The first rapid and the subsequent slow adsorption are attributed to switching the adsorbed MB molecules from the external diffusion to the pore and surface diffusion [12]. The equilibrium is reached at around 1500 min for CZ1.5 and CZ1.5–4 and CZ2.5 series, respectively, whereas at least more than 3000 min is required for CZ1.5–9. Fig. 2b shows the curve-fitting plots of pseudo-second-order model. All coefficient values of R2 are higher than 0.99 indicating availability of the pseudo-second-order model and chemisorption of MB onto the activated carbons [14]. As also shown in Table 2, the adsorption rate constants, k2 for the CZ2.5 series and for the CZ1.5 series are in the range of 3.57  10 5–1.15  10 4 g/mg min and 1.45  10 5–7.92  10 5 g/mg min, respectively. The former series always have greater k2 than the latter ones, revealing that high mesopore content leads to high MB adsorption rate from aqueous solution [24]. The k2 significantly drops from CZ1.5 to CZ1.5–4 and so as does from CZ2.5 to CZ2.5–4 even though no considerable textural change is observed between the two pairs of activated carbons, suggesting that the presence of acidic oxygen functional groups on activated carbon could significantly enhance the MB adsorption rate. 3.3. Phenol adsorption equilibrium isotherm The study of the adsorption equilibrium is helpful in determining the maximum adsorption capacity of adsorbate for a given adsorbent. Adsorption equilibrium is a dynamic concept achieved when the rate at which molecules adsorb onto a surface is equal to the desorption rate. At equilibrium, there will be no more change in the concentration of the solute on the solid surface or in the bulk solution. In this study, Langmuir [25] and Freundlich equations [26] were used to model the experimental isotherms. The phenol adsorption equilibrium isotherms are represented in Fig. 3. Adsorption isotherms for CZ1.5–9 and CZ2.5–9 exhibit L-shape according to the Giles classification [27,28], and their values of RL are found to be below 1.0. The results suggest that the two activated carbons are favorable for phenol adsorption, in such a way that the aromatic ring of phenol is adsorbed parallelly to the surface of adsorbent. However, isotherms for other activated carbons show F-shape, revealing the heterogeneous surface of activated carbons, and there is strong competition between phenol and water molecules for occupying the adsorption sites [29]. The Langmuir and Freundlich plots for phenol adsorption on activated carbons are also shown in Fig. 3b and c. All correlation coefficients, R2 are better than 0.96 indicating that the two models fit to the experimental data. It is also observed that the Freundlich model is better for predicting the experimental data for CZ1.5, CZ1.5–4, CZ2.5 and CZ2.5–4, while the Langmuir model fitted to CZ1.5– 9 and CZ2.5–9 more satisfactorily. The parameters of the two equations are given in Table 3. In general, for Freundlich model, 1/n ranging from 0 to 1 is considered to represent the surface heterogeneity. The surface textural of adsorbent becomes more heterogeneous as 1/n closed to 1 [29]. Table 3 demonstrates that the heterogeneity of carbon decreases with the order of CZ1.5 > CZ1.5–4 > CZ1.5–9, and as well as CZ2.5 > CZ2.5–4 > CZ2.5–9, indicating that high treatment temperature causes low surface heterogeneity. CO2 was expected to evolve from C–O complexes on the carbon surface in the process [30]. The result agrees well with that of Table 1. Table 3 also shows that CZ2.5–9 and CZ1.5–9 have the

Fig. 3. Equilibrium isotherms of phenol adsorption onto activated carbons (a) Qe–Ce plots, (b) Langmuir plots and (C) Freundlich plots; Black line for CZ1.5, CZ1.5–4 and CZ1.5–9, dotted line for CZ2.5, CZ2.5–4 and CZ2.5–9.

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Table 3 Equilibrium isotherm model parameters for phenol and MB adsorption. Carbon

Langmuir model

Freundlich model

qm (mg/g)

KL (L/mg)

R2

Phenol

CZ1.5 CZ1.5–4 CZ1.5–9 CZ2.5 CZ2.5–4 CZ2.5–9

149 172 238 159 167 250

0.008 0.013 0.061 0.010 0.016 0.100

0.9490 0.9580 0.9900 0.9670 0.9610 0.9930

Methylene blue (MB)

CZ1.5 CZ1.5–4 CZ1.5–9 CZ2.5 CZ2.5–4 CZ2.5–9

495 479 316 519 516 418

0.841 0.651 0.480 1.049 1.291 0.468

0.9997 0.9997 0.9999 0.9996 0.9999 0.9993

KF 4.1 8.95 57.6 4.9 10.2 61.7 243.3 234.7 190.4 295.7 279.1 232.6

1/n

R2

0.562 0.475 0.249 0.559 0.456 0.253

0.999 0.999 0.995 0.997 0.999 0.982

0.154 0.152 0.099 0.152 0.152 0.116

0.9726 0.9651 0.8914 0.9429 0.9301 0.9666

highest phenol adsorption capacities in each series, suggesting that the removal of surface functional groups from activated carbons will enhance the amount of phenol adsorption. 3.4. MB adsorption equilibrium isotherm As shown in Fig. 4a, each isotherm for MB adsorption onto the prepared activated carbons is of the L-type with a sharp knee and a fairly horizontal plateau, suggesting that the adsorption of MB onto activated carbon was site-specific and thus of monolayer [31]. Langmuir and Freundlich equations were used for deeper interpretation of the adsorption data. The results shown in Table 3

Fig. 4. Equilibrium isotherms of MB adsorption onto activated carbons; (a) Qe–Ce plots; (b) Langmuir plots and (c) Freundlich plots; Black line for CZ1.5, CZ1.5–4 and CZ1.5–9, dotted line for CZ2.5, CZ2.5–4 and CZ2.5–9.

Fig. 5. Schematic model for heat treatment of activated carbons.

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reveal that the correlation coefficient of R2 for the Langmuir model is always greater than those for the Freundlich model, suggesting that the monolayer coverage of the surface of activated carbons by MB molecules [32]. MB adsorption capacity, qm and adsorption affinity, KL of CZ2.5 series are always superior to those of CZ1.5 series, indicating that increase mesopore proportion is available for raising MB adsorption capacity. Kasaoka et al. [33] found that at the presence of micropores, adsorption can only occurred when the average micropore diameter is about 1.7 times larger than the molecule second widest dimension. Accordingly, methylene blue is generally considered not to enter the pores with average diameter less than 1.0 nm. Comparing CZ1.5 with CZ1.5–4 and CZ1.5–9, or CZ2.5 with CZ2.5–4 and CZ2.5–9, the Langmuir adsorption capacity, qm is proportional to both BET surface area and concentration of surface functional groups. This suggests that increase of BET surface area or acidic functional groups can enhance the capacity of MB adsorption on activated carbon.

3.5. Schematic model for adsorption The experimental results described above can be summarized as follows: (1) The mechanisms of phenol and methylene blue adsorption onto the prepared activated carbons are different. Phenol is predominantly adsorbed by pore filling and it enters most micropores, whereas MB is mainly adsorbed by surface coverage and generally considered not to enter the pores with diameter less than 1.0 nm. (2) Increase of mesopore proportion in carbon matrix leads to enhance MB adsorption rate prominently but causes no effect on phenol adsorption kinetic rate. (3) Presence of acidic functional groups increases both rate and capacity for MB adsorption from aqueous solution onto activated carbon, while it accelerates rate but decrease capacity for phenol adsorption. According to these results, the schematic models for adsorption of phenol and MB onto various activated carbons were proposed as mentioned in the following sections.

Fig. 6. Schematic models for adsorption of (a) phenol and (b) MB from aqueous solution onto original activated carbon (CZ1.5 and CZ2.5).

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Fig. 7. Schematic models for adsorption of (a) phenol and (b) MB from aqueous solution onto heat treated activated carbons.

3.5.1. Schematic model of heat treating process As presented previously, the prepared activated carbons indicate slit-like porous solids with large amount of micropores and some extent mesopores; heat treatment in a controlled condition can remove acidic functional groups from activated carbon surface without changing their pore structure [17,18,34]. Accordingly, the schematic model for the heat treating process can be proposed in Fig. 5. The structure and surface chemistry of original ACs (CZ1.5 and CZ2.5) can be represented as model (a) and heat treated ACs (CZ1.5–4, CZ1.5–9, CZ2.5–9 and CZ2.5) are simply shown as (b). 3.5.2. Schematic model for original ACs In general, adsorption may be described as a series of steps: mass transfer from the fluid phase to the particle surface across the boundary layer, diffusion within the porous particle and adsorption onto the surface [3]. Based on the experimental results, the schematic model for phenol and MB adsorption onto (a) (CZ1.5 or CZ2.5), which possesses high concentration of acidic functional groups can be concluded in Fig. 6. It is believed that the functional groups are located at the edges of the graphene layers or the pore entrances [35,36]. As shown in Fig. 6a, when an activated carbon with high concentration of acidic functional groups contacts with a phenol solution, water will be first adsorbed on the hydrophilic polar oxygen groups located at the pore entrances, for water molecules are more competitive than phenol towards the adsorption sites by forming H-bonding with these functional groups. The adsorbed water molecules will be further associated with each other to form water clusters, which are remarkably stabilized at the entrance of micropores [37,38]. The presence of water clusters causes partial blockage of the micropores and prevents phenol molecules enter in the micropores. Although oxygen functional groups possibly react with phenol molecules via ester formation route causing chemisorption on the carbon surface and enhance phenol adsorption rate, physical adsorption of phenol has been shown to be predominant based on the p–p dispersion interaction between the aromatic

ring of phenol and the carbon basal planes. It is well known that acidic groups on porous carbon surface can remove electrons from the p-electron system of the carbon basal planes, creating positive holes in the conducting p-band of the graphitic planes, leading to weaker interactions between the p-electrons of phenol aromatic ring and the carbon basal planes, thus lowering phenol adsorption amount. As shown in Fig. 6b, in MB solution, MB+ cations will be first adsorbed on the hydrophilic polar oxygen groups for it possesses positive charge. No water cluster forms on the entrance of pores to cause any blockage. Moreover, it was suggested that the adsorption of MB on surface functional groups could subsequently act as channels for diffusion of MB molecules onto basal planes and micropore wall [3]. As a result, the presence of acidic groups simultaneously promotes both rate and capacity for MB adsorption on activated carbon [39]. 3.5.3. Schematic model for heat treated ACs Schematic model for adsorption of phenol and MB onto thermal treated activated carbons is proposed in Fig. 7. There is no water cluster formed at pore entrances due to the elimination of surface groups by heat treatment. As a result, no blockage occurs for phenol pore filling, and no electron is removed from the p-electron system of the carbon basal planes. Moreover, the increase in polyaromatic characters for treated carbon surface can increase the pelectron density in the graphene layers, thus causing an enhancement of carbon dispersive adsorption. As a result, the capacity of phenol adsorption onto activated carbon is obviously promoted by heat treatment. On the other hand, the removal of surface groups eliminates channels of adsorbate diffusion into micropore, causing the decrease of adsorption rate. However, as illuminated in Fig. 7b, removing acidic functional groups concentration on carbon surface and the decrement of surface area caused by heat treatment eliminate diffusion channels and significantly diminish the sites for the MB adsorption from aqueous solution onto the carbon matrix. Therefore, both adsorption rate and capacity are found to decrease by heat treatment.

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4. Conclusions The present study shows that activated carbon obtained from cattle manure compost is an effective adsorbent for the removal of phenol and methylene blue from aqueous solution due to its high surface area and high proportion of mesopores. Kinetics experimental results are well fitted by the pseudo-second-order model, confirming the chemisorption of phenol or methylene blue onto CMC-based activated carbons. The adsorption rate is controlled by both textural characteristics and surface chemistry. The presence of acidic surface functional groups accelerates the rate for both phenol and MB adsorption onto activated carbons, revealing that the adsorption sites can act as channels for diffusion of these two adsorbates onto small pores. The phenol adsorption equilibrium data agree well with Freundlich model, illuminating that the pore filling of the process, whereas the Langmuir model is more adequate to describe MB adsorption, indicating the monolayer coverage of the surface of activated carbons by MB molecules. Adsorption capacities of phenol and MB on activated carbons are principally determined by surface area, mesopore volume and surface chemistry, while adsorption affinity is influenced by mesopore content and p-electron density relating to surface oxygen functional groups. The presence of acidic functional groups on the surface of activated carbon is favorable for basic dye adsorption but not favorable for phenol adsorption. Acknowledgements The authors thank Dr. Masami Aikawa, Kisarazu National College of Technology and Ms. Yoko Fujimura, Chiba Prefectural Environmental Research Center, for their discussion and suggestions in the study. References [1] A.P. Terzyk, J. Colloid Interface Sci. 268 (2003) 301–329. [2] M. Franz, H.A. Arafat, N.G. Pinto, Carbon 38 (2000) 1807–1819. [3] D.M. Nevskaia, A. Santianes, V. Munoz, A. Guerrero-Ruiz, Carbon 37 (1999) 1065–1074.

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