Removal of malachite green (MG) from aqueous solutions by native and heat-treated anaerobic granular sludge

Biochemical Engineering Journal 39 (2008) 538–546

Removal of malachite green (MG) from aqueous solutions by native and heat-treated anaerobic granular sludge Wen Cheng a , Shu-Guang Wang a,∗ , Lei Lu a , Wen-Xin Gong a , Xian-Wei Liu a , Bao-Yu Gao a , Hua-Yong Zhang b b

a School of Environmental Science and Engineering, Shandong University, Jinan 250100, PR China Energy and Environmental Research Center, North China Electric Power University, Beijing 102206, PR China

Received 9 June 2007; received in revised form 24 September 2007; accepted 11 October 2007

Abstract The performance of native and heat-treated anaerobic granular sludge in removing of malachite green (MG) from aqueous solution was investigated with different conditions, such as pH, ionic strength, initial concentration and temperature. The maximum biosorption was both observed at pH 5.0 on the native and heat-treated anaerobic granular sludge. The ionic strength had negative effect on MG removal. Kinetic studies showed that the biosorption process followed pseudo-second-order and qe for native and heat-treated anaerobic granular sludge is 61.73 and 59.17 mg/g at initial concentration 150 mg/L, respectively. Intraparticle diffusion model could well illuminate adsorption process and faster adsorption rate of native anaerobic granular sludge than heat-treated anaerobic granular sludge. The equilibrium data were analyzed using Langmuir and Freundlich model, and well fitted Langmuir model. The negative values of G◦ and H◦ suggested that the interaction of MG adsorbed by native and heat-treated anaerobic granular sludge was spontaneous and exothermic. Desorption studies revealed that MG could be well removed from anaerobic granular sludge by 1% (v/v) of HCl–alcohol solution. © 2007 Elsevier B.V. All rights reserved. Keywords: Biosorption; Malachite green; Anaerobic granular sludge; Heat-treatment; Kinetics

1. Introduction The presence of dyes and pigments in water, even at very low concentrations, is highly visible and undesirable [1]. It not only affects aesthetic merit, but also inhibits sunlight penetration and reduces photosynthetic action within ecosystem [2]. The complex aromatic molecular structures of dyes and synthetic origin make them more stable to light, heat and oxidizing agents, and are usually biologically non-degradable [3,4]. As a widely used dye in China, malachite green (MG) has been used as a strong anti-fungal, anti-bacterial and antiparasitical agent in fish farming [5]. It is also used for the dyeing of cotton, wool, silk, paper, and leather. It is known to be highly toxic to mammalian cells and acts as a tumor-enhancing agent. This dye may enter into the food chain and could possibly cause carcinogenic, mutagenic and teratogenic effect on humans [6,7].

∗

Corresponding author. Tel.: +861 531 88362802; fax: +86 531 88364513. E-mail address: [email protected] (S.-G. Wang).

1369-703X/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.bej.2007.10.016

Various physical-chemical processes have been extensively used in effective treatment of the dye-containing wastewater, which include conventional chemical coagulation/flocculation, precipitation, ozonation, oxidation, adsorption, ion-exchange, reverse osmosis and ultra filtration. However, their initial and operational costs are so great that they constitute an inhibition to dyeing and finishing industries, especially in developing countries [8]. Among these technologies, adsorption has appeared as the most effective process for the removal of dyestuff from aqueous effluents. Activated carbon is the most popular and widely used adsorbent, but it is hard to regeneration and need high operating costs [9]. Present day many investigators have made search for the feasibility of using low cost and efficient adsorbents. The use of biomass as adsorbents for dyes also offers a potential alternative to existing methods for detoxification [10]. And, since 1980s, the adaptation of live or dead biomass or their derivatives into adsorption studies has successfully been made, especially for the removal of heavy metals and other pollutants from wastewater [11]. In recent years, anaerobic granular sludge is widely used for treating high strength wastewaters. It has been proved

W. Cheng et al. / Biochemical Engineering Journal 39 (2008) 538–546

that it could efficiently adsorption of heavy metals and hazardous organic pollutants [12–14]. Due to the excellent solid/liquid separation ability, anaerobic granular sludge has better practical operation than other biosorbents, such as alga [15], floc sludge [16] and fungus [17]. The aim of present study was to examine optimum adsorption conditions, such as pH, ionic strength and temperature of a cost-effective biosorbent. The adsorption kinetic, isotherm and thermodynamic properties were also explored. Moreover, it also checked the effect of sterilization for the adsorption efficiency. 2. Methods

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The IR spectra of native and heat-treated anaerobic granular sludge were obtained by using a FT-IR spectrophotometer (Vector 22, Germany). 2.4. Biosorption studies The biosorption of MG on the native and heat-treated anaerobic granular sludge were investigated in a batch system. All adsorption experiments were conducted using 250 mL flasks, adding 100 mL of MG solution and weighted sludge were added. The flasks were closed with rubber plugs, flushed with N2 /CO2 (70/30) to ensure the anaerobic condition during the biosorption process.

2.1. Preparation of the biosorbent The anaerobic granular sludge used in this study was collected from Shandong Meiquan Environmental Protection Technology Ltd., China, which was used for treating starch wastewater. This anaerobic granular sludge was stored in a sealed container at 4 ◦ C until batch experiments. Heat-treated form of anaerobic granular sludge was prepared by autoclaving the sludge at 116 ◦ C and 110 kPa for 30 min. Prior to use, the sludge was washed with deionized water for three times to remove dirty particles. 2.2. Preparation of MG solutions All chemicals used in this study were of analytical-grade. The dye used in all the experiments was MG (C.I. 42000), a basic (cationic) dye, which was obtained from Tianjin Chemical Co., China. Its molecule structure is shown in Fig. 1. The dye stock solutions were prepared by dissolving accurately weighted 1 g of MG (99% purity) in 1 L deionized water. The experimental solutions were obtained by diluting the stock solutions in accurate portions to different initial concentrations. 2.3. Characterization of the anaerobic granular sludge The morphological and structural characteristics of anaerobic granular sludge were observed with a scanning electronic microscope (SEM) (Hitachi S-570, Japan). Granule samples were first washed with a phosphate buffer and fixing with 2% glutaraldehyde overnight at 4 ◦ C. Fixed granules were washed with 0.1 mol/L sodium cacodylate buffer and dewatered with a graded ethanol series (10, 25, 75, 90 and 100%). The dewatered samples were then dried with a critical point dryer. The samples were further sputter coated with gold for SEM observation.

2.4.1. Effect of pH on biosorption The influence of the initial solution pH on the biosorption amount of the anaerobic granular sludge was investigated in the pH range of 2–7 (which was adjusted with HCl or NaOH at the beginning of the experiment and not controlled afterwards) at a constant temperature of 25 ◦ C. The initial concentration of MG in the solution is 90 mg/L and the amount of adsorbent is 2.4 g dried weight per liter. 2.4.2. Effect of ionic strength on biosorption The effect of ionic strength on the equilibrium uptake of MG was examined by NaCl and MgCl2 at a constant temperature of 25 ◦ C. Aqueous dye solutions of 90 mg/L dye concentration were prepared and the amount of adsorbent is dried weight of 0.24 g dried weight per 100 mL. Ionic concentration of these dye solution varied from 0 to 0.20 mol/L. 2.4.3. Equilibrium studies Biosorption equilibrium studies were carried out by adding 0.24 g of dried weight anaerobic granular sludge in a series of 250 mL flasks containing 100 mL of MG solution of different dye concentrations at four different temperatures (25, 35, 45 and 60 ◦ C). 2.4.4. Kinetic studies Adsorption kinetic experiment were carried out by agitating 4.8 g dried weight anaerobic granular sludge in a series of beaker containing 2 L MG solution of known concentration using magnetic stirrers running at constant. 2 mL samples were taken at suitable time intervals. 2.4.5. Desorption tests In order to assess the practical utility of the adsorbent, desorption experiments were conducted. 0.24 g anaerobic granular sludge with adsorbed MG was treated by the desired concentration of alcohol, HCl aqueous solutions and their mixed solutions for a predetermined time. 2.5. Analysis

Fig. 1. Molecular structure of MG.

The concentration of MG remaining in solution was measured colorimetrically using a spectrophotometer (UV-754, Shanghai, China) at an absorbance wavelength of 615 nm. All

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the samples were filtered through 0.45 ␮m membranes before measure. 2.6. Dye uptake The amount of dye adsorbed onto unit weight of adsorbent, qe (mg/g), was calculated from the mass balance equation given by: qe =

(C0 − Ce )V m

(1)

where C0 is the initial dye concentration in liquid phase (mg/L), Ce represents the liquid phase dye concentration at equilibrium (mg/L), V is the volume of dye solution used (L) and m is the mass of adsorbent used (g). In order to compare the applicability of different models in fitting to data, the root mean square error (RMSE) was calculated  n 1/2 1 2 RMSE = (2) (qexp − qcal ) n i=1

where n is the number of data points; qexp the experimental vales; and qcal the calculated values by model. 3. Results and discussion 3.1. Properties of anaerobic granule sludge The surface morphology of the native and heat-treated anaerobic granular sludge was exemplified by SEM in Fig. 2. The surface of granular sludge is rough, uneven and porous. The surface structure of the anaerobic granular sludge has become loosen after heat treatment for sterilization. The FT-IR analysis of granular sludge was given in Fig. 3. The intense peaks at a frequency level of 3500–3200 and 1540 cm−1 represents amino groups stretching vibrations. The amino groups stretching vibrations bands of granular sludge are superimposed on the side of the hydroxyl group at 3500–3300 cm−1 . The strong peaks at around 1650, 1400 and 1240 cm−1 are caused by the C O stretching band of carbonyl groups. The strong peak at 2924 cm−1 represents asymmetric vibration of CH2 , 2850 cm−1 is symmetric vibration of CH2 . The band between 1130 and 1000 cm−1 is vibration of carboxylic acids and stretching OH vibration polysaccharides. A band <800 cm−1 is finger print zone which is phosphate and sulphur functional groups. As shown in Fig. 3, an unapparent shift can be seen before and after heat-treatment, indicating that heat-treatment process for sterilization has little effective for the modification of main function group of the anaerobic granular sludge. 3.2. Effect of initial pH Acids and alkalis are used in bleaching, desizing, scouring and mercerizing. And dyes, salts and other additive also affected the pH of dye-containing wastewater. The pH of initial solution plays an important role in the adsorption process. The removal

Fig. 2. Typical SEM micrograph (magnification: 5000×): (a) native and (b) heat-treated anaerobic granular sludge.

percentage of MG by native and heat-treated anaerobic granular sludge at different pH values is plotted in Fig. 4. The process of adsorption of MG on anaerobic granular sludge is highly pH-dependent. The removal percentage of MG by native and heat-treated anaerobic sludge at various pH has a similar trend and the removal percentage has no marked disparity. Removal percentages of MG by both of adsorbents were high at high pH. When pH was more than 5.0, removal percentage had no significant change. The final pH (5.13–7.22) was higher than initial pH (3.0–7.0).

W. Cheng et al. / Biochemical Engineering Journal 39 (2008) 538–546

Fig. 3. FT-IR spectra of: (a) native and (b) heat-treated anaerobic granular sludge.

The pH of the dye solution affected not only the surface charge of the adsorbent, the degree of ionization of the materials and the dissociation of functional groups on the active sites of the adsorbent, but also the structure of dye molecule [18]. When pH is lower than isoelectric point of biomass (the isoelectric point of anaerobic sludge would be usually between pH 1 and 3 [19]), the number of negatively charged adsorbent sites decreased and the number of positively charged surface sites increased, which did not favor the adsorption of positively charged dye cations. However, MG (pKa = 10.3) gets protonated in lower pH and deprotonated at higher pH [20]. Consequently, the dye molecule has high positive charge density at a lower pH. Therefore, with the decreases of the pH in the initial solution, electrostatic repulsion exists between the positively charged surface and the positively charged dye molecule will increase, resulting the decreasing of the amount of adsorbed MG onto adsorbents. This illustrates the amount of adsorption decreased with decreasing of pH. Also, at lower pH, the H+ ions compete effectively with dye cations causing a decrease of adsorption of MG [5,21]. 3.3. Effect of ionic strength Generally, various salts and metal ions exist in the dyecontaining wastewater. The slats lead to high ionic strength, which may affect the dye adsorption onto adsorbents.

Fig. 4. Effect of pH on the removal percentage of MG. Symbols: native (); heat-treated (), at 25 ◦ C.

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Fig. 5. Effect of NaCl and MgCl2 concentrations on biosorption amount of MG. Symbols: NaCl–native (); NaCl-heat-treated (); Mg Cl2 –native (䊉); Mg Cl2 -heat-treated (), pH 5.0, at 25 ◦ C.

Fig. 5 showed that the ion existing in the solution affected the MG adsorption onto native and heat-treated anaerobic sludge. It was seen that adsorption amount of MG decreased with the increasing of Na+ and Mg2+ concentrations in the solution. This could be attributed to the competitive effect between dye ions and cations from the salt for the sites available for the biosorption process. And, with the ionic strength increasing, the activity of MG and the active sites decreases, so the adsorption amount of MG decreases [22]. When ionic concentration increases from 0 to 0.2 mol/L, the adsorption amount sharply decreases, after that, the adsorption amount have tiny change with the increasing of concentration of salt. From Fig. 5, it was also seen that the effect of monovalent and divalent ions on the sorption potential of adsorbents. For divalent electrolytes (MgCl2 ) has more contribution to ionic strength and more positive charge than univalent electrolytes (NaCl) [23]. So the effect of Mg2+ on adsorption is more serious than Na+ . 3.4. Biosorption kinetics 3.4.1. Effect of initial dye concentration and time Fig. 6 showed the plots between quantities of dye adsorbed qt (mg/g) versus time t (min) at different dye concentrations on native (a) and heat-treated (b) anaerobic granular sludge, respectively, where qt represents the amount of dye adsorbed at any time t. All the plots had the same general features, a very rapid initial adsorption over a few minutes, followed by a longer period of much slower uptake. When the initial dye concentration increases from 90 to 150 mg/L, the amount of dye adsorbed on native and heat-treated anaerobic granular sludge increases from 37.26 to 61.18 mg/g and from 36.72 to 58.55 mg/g, respectively. The effect of initial dye concentration on the biosorption amount has been found to be of considerable significance for the dye used. The reason is that a higher initial concentration provides an important driving force to overcome all resistances of the dye between the aqueous and solid phases, thus increasing the uptake [24]. In addition, increasing the initial dye concentration increases the number of collisions between dye ions and the anaerobic granular sludge, which enhances the adsorption amount.

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The pseudo-first-order rate equation of Lagergren may be represented as follows [25]: dqt (3) = K1 (qe − qt ) dt After integration, the integrated form of Eq. (3) becomes: qt = qe (1 − e−K1 t )

(4)

where K1 is the rate constant of pseudo-first-order biosorption (min−1 ), qe and qt (mg/g) are the amounts of biosorption at equilibrium and time t (min), respectively. The pseudo-second-order kinetic model proposed by Ho and McKay can be expressed as [26]: dqt = K2 (qe − qt )2 dt The integrated form of Eq. (5) becomes: qt =

(5)

K2 qe2 t 1 + K 2 qe t

(6)

where K2 is the rate constant of pseudo-second-order adsorption model (g/mg min). The constant K2 is used to calculate the initial sorption rate h (mg/g min) used: h = K2 qe2 Fig. 6. Biosorption kinetics for MG onto: (a) native anaerobic sludge and (b) heat-treated anaerobic sludge. Symbols: initial concentration 90 mg/L (), 120 mg/L () and 150 mg/L (), pH 5.0, at 25 ◦ C.

The equilibrium time of native and heat-treated anaerobic granular sludge was established about 40 and 80 min, respectively. From Fig. 6, it was rapid for the entire sorption period from t = 0 to equilibrium. After equilibrium, the amount of adsorbed dye did not change significantly with time. It is seen that heat-treated anaerobic sludge need more time to reach equilibrium than that of native anaerobic sludge. It may be that heat-treatment process for sterilization affected the adsorption rate of anaerobic granular sludge. 3.4.2. The first- and second-order kinetic model Two classical kinetics models including the pseudo-firstorder and pseudo-second-order are used to examine the controlling mechanism of adsorption process.

(7)

The sorption uptake kinetics for MG by anaerobic granular sludge was analyzed by non-linear curve fitting analysis method, using Microcal(TM) Origin software, to fit the pseudo-first and second-order equations. The kinetic parameters for the biosorption of the dye on native and heat-treated anaerobic granular sludge were tabulated in Table 1. From Table 1, it was also observed that the values of the initial adsorption rates h increased with an increase in the initial dye concentration. It could be attributed to the increase in the driving force for the mass transfer, allowing more dye molecules to reach the surface of the adsorbents in a shorter period of time. The RMSE of second-order kinetic model was lower than that of first-order kinetic model. On the other hand, the theoretical values of qe calculated from the second-order kinetic also agreed with the experimental value of maximum biosorption amount (qex ). Thus, the second-order kinetic model was suitable for description of biosorption kinetics for the removal of MG molecule from aqueous solution onto native and heat-treated anaerobic granular sludge. This suggests that the biosorption of

Table 1 First-order and second-order adsorption rate constants obtained by using the non-linear method at different concentration, pH 5.0, at 25 ◦ C Experimental qex (mg/g)

C0 (mg/L)

Native

37.27 49.32 61.73

Heat-treated

36.72 50.99 58.55

Granular sludge

First-order constants

Second-order constants

qe (mg/g)

K1 (1/min)

R2

RMSE

qe (mg/g)

K2 (g/mg min)

h (mg/g min)

R2

RMSE

90 120 150

35.667 44.007 51.158

0.945 1.171 0.764

0.9734 0.9033 0.9021

4.348 5.766 7.178

37.346 48.736 56.056

0.047 0.035 0.034

65.552 81.898 106.837

0.9944 0.9574 0.9261

0.7076 2.3831 3.7378

90 120 150

30.094 38.793 50.803

0.281 0.325 0.277

0.8319 0.9169 0.9071

2.313 2.741 2.826

36.217 49.239 57.065

0.0088 0.0059 0.0065

11.543 14.304 21.167

0.9278 0.9471 0.9664

1.859 2.5279 2.6274

W. Cheng et al. / Biochemical Engineering Journal 39 (2008) 538–546

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MG onto anaerobic granular sludge is presumably a chemisorption process involving exchange or sharing of electrons mainly between dye cations and functional groups (mainly hydroxyl and carboxyl groups) of the biomass cells [27]. 3.4.3. Intraparticle diffusion model The pseudo-first-order and pseudo-second-order kinetic models could not identify the diffusion mechanism. An intraparticle diffusion model has been used to predict the ratecontrolling step. In general, the process of adsorption is divided into three stages: (1) mass transfer across the external boundary layer film of liquid surrounding the outside of the particle; (2) dye molecule diffusion into the micropores of adsorbent; (3) biosorption process in activated positions of inner surface [28]. The intraparticle diffusion equation is expressed as follows: qt =

ki +C t 1/2

(8)

where ki is the intraparticle diffusion rate constant (mg/g min1/2 ), and C is the intercept. The plots of qt against t1/2 were showed in Fig. 7. The constants of intraparticle diffusion model were given in Table 2. From this study, multi-linearities were observed, indicating that there was three-stage diffusion of MG onto anaerobic granular sludge. The first stage is the external mass transfer, followed by intraparticle diffusion in macro, meso, and micropore, finally it reach equilibrium stage. If the plots passed through the origin, the intraparticle diffusion is the only rate-controlling step; if not, external mass transfer controlled the adsorption to some degree [29,30]. From Fig. 7, only first stage passed through the origin, the others not. It could be deduced that the second and third stage were controlled by both external mass transfer and intraparticle diffusion. From Table 2, the order of intraparticle diffusion rate was k1 > k2 > k3 , which were calculated from the slopes of the multilinearties. Adsorption rate of external mass transfer was faster than that of intraparticle diffusion, and the adsorption rate of equilibrium stage was the lowest. At the beginning, dye molecule diffused through the solution to the external surface, and a portion of the substance can be adsorbed by the outer surface of sludge. This process was very fast. When the external surface adsorption reached saturation, the dye molecule entered into the inner of the anaerobic granular sludge through the pore within the particle and adsorbed to the activated positions of anaerobic granular sludge. With the increase of the adsorption amount on the interior surface of the particle, the diffusion resistance

Fig. 7. Intraparticle diffusion kinetics for adsorption of MG onto (a) native anaerobic sludge and (b) heat-treated anaerobic sludge. Symbols: initial concentration 90 mg/L (), 120 mg/L () and 150 mg/L (), pH 5.0, at 25 ◦ C.

increased. It caused the intraparticle diffusion rate to decrease. Finally, the dye concentration became extremely low in the solution, the intraparticle diffusion rate slowed down markedly; it means that the adsorption reached the equilibrium. Therefore, the change of k1 , k2 and k3 could be attributed to the adsorption stages of external mass transfer, intraparticle diffusion and equilibrium, respectively [31]. Table 2 also shown that constant k3 of native granular sludge was lower than that of heat-treated. The reason was that the residual MG concentration of heat-treated sludge at this stage was higher than that of native sludge. However, constant k1 and k2 of heat-treated anaerobic sludge was much lower than that of native anaerobic sludge, implying that the adsorption rate of the native anaerobic sludge is faster than that of heat-treated sludge both in the first and second step. It may be that the process of heat-treatment for sterilization changes the property of the

Table 2 Intraparticle diffusion constant at different initial dye concentration, pH 5.0, at 25 ◦ C Granular sludge

C0 (mg/L)

k1 (mg/g min1/2 )

R2

RMSE

k2 (mg/g min1/2 )

R2

RMSE

k3 (mg/g min1/2 )

R2

RMSE

Native

90 120 150

21.984 23.032 24.61

0.9875 0.9521 0.9145

0.4078 0.9540 2.7942

3.1135 3.4709 4.4573

0.547 0.8514 0.9543

1.1953 1.7761 3.0130

0.4247 0.7191 1.0291

0.6758 0.9934 0.9998

0.3739 0.4121 0.3624

Heat-treated

90 120 150

10.046 13.621 15.556

0.7325 0.9363 0.8526

2.9298 3.8667 3.8264

2.9227 3.1609 3.8119

0.6885 0.9474 0.9946

1.8360 3.0796 2.3343

0.5469 0.8293 1.101

0.7355 0.5513 0.8274

0.1044 1.0824 1.2679

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anaerobic granular sludge, or native anaerobic granular sludge provided active transport to increasing dye molecular adsorption rate. From kinetics models, heat-treatment process for sterilization has little effect on adsorption amount, but it prolonged the time to reaching equilibrium.

process of MG onto anaerobic granular sludge [27]. The essential characteristics of a Langmuir isotherm can be expressed in terms of a dimensionless separation factor, RL which describes the type of isotherm and is defined by:

3.5. Adsorption isotherms

If

3.5.1. Langmuir and Freundlich isotherm In this study, Langmuir and Freundlich isotherms are used to describe the adsorption equilibrium of MG onto native and heat-treated anaerobic granular sludge. The Freundlich isotherm is derived by assuming a heterogeneous surface with a nonuniform distribution of heat of adsorption over the surface, whereas in the Langmuir theory the basic assumption is that the sorption takes place as specific homogeneous sites within the adsorbent [32]. The Langmuir isotherm is valid for monolayer adsorption onto a surface with a finite number of identical sites. It can be expressed [33] as: qe =

Q◦ bCe 1 + bCe

(9)

where Ce is the equilibrium concentration of the dye solution (mg/L), qe is the amount of adsorbed dye onto per mass of adsorbent at equilibrium (mg/g), Q◦ is the maximum amount (mg/g) and represents a practical limiting sorption amount when the adsorbent surface is fully covered with monolayer sorbate molecule (mg/g) and b is the constant related to the affinity of the binding sites (L/mg). The Freundlich equation is given as: qe = KF Ce1/n

(10)

where KF is the indicators of adsorption density and n is the adsorption intensity. The non-linear Langmuir and Freundlich adsorption isotherm constants obtained at four different temperatures are given in Table 3. The same as the kinetic model, the best-fit isotherm model was determined by the RMSE. From Table 3, it is observed that the equilibrium adsorption data were very well presented by Langmuir isotherms. It confirmed the monolayer adsorption

RL =

1 (1 + bC0 )

(11)

RL > 1, unfavorable; RL = 1, linear; 0 < RL < 1, favorable; RL = 0, irreversible. The values of RL for C0 = 60 mg/L obtained were found to be less than 1 for MG adsorption on both of the adsorbents. It confirmed that the adsorption process is favorable. And, the values of n were also found to be larger than unity for both of the adsorbents showing, which means adsorption is favorable at all of the temperature studied [34,35]. Both the Langmuir constants Q◦ and the Freundlich constants KF decreased with increasing temperature, indicating that amount of adsorption decreased with the increase of temperature. This was in agreement with the experimental observation. From both of experiments and models, the adsorption amount increased with temperature decreased. 3.5.2. Thermodynamic of biosorption To estimate the effect of temperature on the biosorption of MG onto anaerobic granular sludge, the free energy change (G◦ ), enthalpy change (H◦ ) and entropy change (S◦ ) were determined. Thermodynamic parameters can be calculated from the variation of the thermodynamic equilibrium constant K0 with the change in temperature. For adsorption reactions, K0 is defined as follows [36]: K0 =

as vs qe = ae ve Ce

(12)

where as is the activity of adsorbed MG, ae is the activity of the MG in solution at equilibrium, qe is the amount of MG adsorbed by per mass of adsorbent (mg/g) and Ce is the MG concentration in solution at equilibrium (mg/L), vs is the activity coefficient of the adsorbed MG and ve is the activity coefficient of the MG in solution. As the MG concentration in the solution

Table 3 Langmuir and Freundlich isotherm constants obtained by using the non-linear method at different temperatures, pH 5.0 Granular sludge

T (◦ C)

Langmuir constants Q◦

(mg/g)

Freundlich constants

b (L/mg)

R2

RMSE

KF

1/n

R2

RMSE

Native

25 35 45 60

85.59 81.32 72.02 72.26

0.477 0.429 0.632 0.466

0.8913 0.9200 0.8234 0.9158

4.355 3.601 3.877 4.207

31.61 29.46 28.75 28.59

0.303 0.294 0.326 0.264

0.8519 0.8956 0.7949 0.8836

4.412 3.812 4.274 4.877

Heat-treated

25 35 45 60

135.23 133.95 149.05 119.19

0.034 0.3148 0.063 0.034

0.9668 0.9932 0.9929 0.9195

2.985 3.087 1.786 3.136

10.69 9.64 8.41 8.16

0.544 0.558 0.622 0.584

0.9812 0.9915 0.9918 0.9102

2.342 3.551 1.895 4.849

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Table 4 Thermodynamic parameters of MG biosorption on anaerobic granular sludge Granular sludge

Native

(◦ C)

T G◦ (kJ/mol) H◦ (kJ/mol) S◦ × 10−3 (kJ/mol)

Heat-treated

25 −1.987 −16.215 −47.747

35 −1.625 −47.34

45 −1.712 −45.6

60 −0.316 −47.74

25 −3.884 −4.304 −1.406

35 −3.658

45 −3.647

60 −3.835

−2.096

−2.064

−1.406

decreases and approaches to zero, K0 can be obtained by plotting ln(qe /Ce ) versus qe and extrapolating qe to zero. The straight line obtained is fitted to the points based on a least-squares analysis. Its intercept with the vertical axis gives the values of K0 . The adsorption standard free energy changes (G◦ ) can be calculated according to G◦ = −RT ln KC0

(13)

where R is the universal gas constant (8.314 J/mol k) and T is absolute temperature (K). The average standard enthalpy change (H◦ ) is obtained from Van’t Hoof equation:   −H ◦ 1 1 (14) − ln K0(T4 ) − ln K0(T1 ) = R T4 T1 where T4 and T1 represent two different temperatures. The standard entropy change (S◦ ) can be obtained by S ◦ = −

G◦

− H ◦

(15) T The thermodynamic parameters are listed in Table 4.G◦ < 0 shows that the adsorption process is spontaneous. A negative standard enthalpy (H◦ ) change suggests that the interaction of MG adsorbed by adsorbent is exothermic reaction, which supported by the increasing adsorption of MG with the decrease in temperature. And the negative S◦ values confirm the decreased randomness at the solid–solute interface during biosorption. 3.6. Desorption studies The repeated availability is an important factor for an adsorbent. Such adsorbent should not only possess higher adsorption capability, but also show better desorption, which will significantly reduce the overall cost for adsorbent. The desorption tests were carry out by utilizing various concentrations of alcohol, HCl aqueous solutions and their mixed solutions for 3.0 h. The results are listed in Table 5. The highest desorption of dye was observed in 1% (v/v) of HCl–alcohol solution. Table 5 Results of desorption and recovery tests, at 25 ◦ C Eluant

Native (%)

Heat-treated (%)

1% (v/v) of HCl 2% (v/v) of HCl Alcohol 1% (v/v) of HCl–alcohol solution 2% (v/v) of HCl–alcohol solution

44.12 18.13 36.14 90.24 53.76

33.64 17.67 32.19 94.16 50.39

Fig. 8. Adsorption capacities of MG to anaerobic granular sludge at five cycles of adsorption/desorption process. (a) Native and (b) heat-treated anaerobic granular sludge, at 25 ◦ C.

Five consecutive cycles of sorption–desorption experiments were carried out by 1% (v/v) of HCl–alcohol solution (Fig. 8). A reduction in adsorption amount was noticed for each new cycle after desorption with five cycles. The adsorption amount on native and heat-treated anaerobic granular sludge reduced from 34.31 to 23.19 mg/g and 33.43 to 22.36 mg/g, respectively. 4. Conclusions The removal of MG from aqueous solution using native and heat-treated anaerobic granular sludge was studied at different conditions in batch experiment. From the experiment, the adsorption amount was highly dependent of operating variables such as pH, ionic strength, temperature, contact time and initial dye concentration. The adsorption amount increased with the increasing of pH value from 2 to 7, the maximum removal of dye was observed at pH 5.0. And it was seen that the increase in the ionic strength resulted in a decrease of MG adsorption onto anaerobic granular sludge. Biosorption of MG onto anaerobic sludge best fitted the pseudo-second-order kinetic model and qe for native and heattreated anaerobic granular sludge is 61.73 and 59.17 mg/g at initial concentration 150 mg/L, respectively. Intraparticle diffusion model could well explain the adsorption process. And it also illustrated heat-treatment process have effect on the adsorption rate, but adsorption amount. The equilibrium data were found to be better represented by the Langmuir isotherm than Freundlich isotherm, the negative values of thermodynamic study showed that the adsorption of dyes on native and heat-treated granular sludge is spontaneous and exothermic process. Desorption experiments proved that 1% (v/v) of HCl–alcohol solution was

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