Biosorption thermodynamic and kinetic of direct dye from aqueous solutions on bacterial cellulose

In recent years, dyes pollution has become one of t he most serious environmental problems. Biosorption as a biotechnology for removal of dyes pollution from aqueous solutions has been extensively studied and most biosorption research m ainly focused on the process kinetics and thermodynamics. Bacterial cellulose is receiving gr eat attention and presently being widely investigated as a new type of cost-efficient biosor bent due to its nanofibers network, biocompatibilit y, nontoxicity, biodegradability and high water holdin g capacity. The aim of this study was to determine the thermodynamic and kinetic adsorption of direct blue 15 dye from aqueous solutions with bacterial cellulose as a biosorbent. The effects of pH, conta ct time and temperature on adsorption of direct dye by bacterial cellulose were also evaluated. Kinetic study of direct blue 15 on bacterial cellulose, we re carried out under sorption conditions of pH 3.0, ML R 1:500 and an initial dye concentration 100 mg/L. Kinetic analyses were conducted using pseudo firstand second-order models. The regression results showed that the adsorption kinetic was more accurat ely represented by a pseudo second-order model. Changes in free energy of adsorption ( ∆G), enthalpy ( ∆H) and entropy ( ∆S) as well as the activation energy (E a) were determined. ∆H , ∆S and E a with pH control were -26.5, -230 and 43.5 kJ/mol. The result showed that the bacterial cellulose could be employed as an effective sorbent for the removal o f direct dye from aqueous solution and the values of ∆H, ∆G and E a indicate that the adsorption of direct dye onto bacterial cellulose was a physisorp tion process.

into rivers and lakes results in a reduced dissolved oxygen concentration causing anoxic conditions, which subsequently affect aerobic organisms (Chander and Arora, 2007).The use of direct dyes has continuously increased in the textile industry and finishing processes since the development of synthetic fibers (Chao et al., 2008).To minimize the risk of pollution generated by such effluent, this effluent must be treated before discharged into the environment.
In recent years, research attention has been focused on biological methods for the treatment of effluents, some of which are in the process of commercialization (Prasad and Freitas, 2003).There are three principle advantages of biological technologies for the removal of pollutants; first, biological processes can be carried out in situ at the contaminated site; Second, bioprocess technologies are usually environmentally benign (no secondary pollution) and third, they are cost effective.Of the different biological methods, bioaccumulation and biosorption have been demonstrated to possess good potential to replace conventional methods for the removal of dyes (Volesky et al., 2003).
Biosorbents for the removal of dyes mainly come under the following categories: bacteria, fungi, algae, industrial wastes, agricultural wastes and other polysaccharide materials (Vijayaraghavan and Yeoung-Sang, 2008).Bacterial cellulose (BC) is homopolysaccharide having wide industrial applications and it could be Biotechnology's next high-value product (Pansar et al., 2009).
BC is synthesized by the acetic bacterium Acetobacter xylinum.The fibrous structure of bacterial cellulose consists of a three-dimensional network of nanofibers containing glucan chains bound together by hydrogen bonds (Sutherland, 1998;Czaja et al., 2006;Hestrin andAshjaran et al. 1271 Schramm, 1954).It has established a long record of applications in various areas such as the textile industry, the paper industry, and the biomedical field as tissue engineering materials due to their good biocompatibility, mechanical properties similar to those of hard and soft tissue and easy fabrication into a variety of shapes with adjustable interconnecting porosity (Ross et al., 1991;Meftahi et al., 2010).Recently, BC has been investigated as a potential scaffold for tissue engineering.nanofibers of BC are about several times thinner than that of plant cellulose making it a highly porous, large surface area and high water holding capacity material.Bacterial cellulose is one of the new biosorbent, most abundant natural, renewable, biodegradable, and biocompatible polymers (Richmond, 1991).BC applied to adsorb metal ions has been reported in the previous literature (Evans et al., 2003), but dyes biosorption of it from aqueous solution is not reported.The main aim of this study was to investigate the potentiality of using BC as biosorbent for the adsorption of direct blue 15 dye from aqueous solution.The effects of temperature, contact time and pH on BC adsorption were studied.Biosorption thermodynamic and kinetics parameters were also calculated and discussed.

Chemicals
All the chemicals and reagents used in the present investigation were of analytical grade and mainly purchased from Sigma-Aldrich (USA) and Merck (Germany).

Preparation of biosorbent
A. xylinum ATCC 23768 was used for BC production.The microbe was provided from the medical sciences faculty tarbial modares university, Tehran, Iran.The bacterium was grown in SH medium at 28ºC under static culture conditions.SH medium was composed of 2% (w/v) glucose, 0.5% (w/v) yeast extract, 0.5% peptone, 0.27% (w/v) Na2HPO4 and0.115%(w/v) citric acid.Preinoculum for all experiments was prepared by transferring a single colony grown on SH agar medium into a 50 ml Erlenmeyer flask filled with liquid SH medium.After 7 days of cultivation at 28°C, the ce llulose pellicle formed on the surface of the culture broth. 10 ml of the cell suspension was introduced into a 500 ml Erlenmeyer flask containing 100 ml of a fresh SH medium.The culture was carried out statically for 72 h and the cell suspension derived from the synthesized cellulose pellicle was used as the inoculums for further cultures (Kimura et al., 2001;Rezaee et al., 2005).The cellulose sheets were removed after cultivation and rinsed with distilled water.They were cut into 0.05 g and used for dye biosorbent.

Preparation of aqueous dye solutions
In the present investigation, direct blue 15 dye (obtained from Nordex International D.Z.E Dye Co. in UK) was used without further purification.Stock solutions of both dyes were prepared by dissolving 0.1 g of dye in 1000 ml of double distilled water.Standard curves were developed through the measurement of the dye solution absorbance by UV/visible spectrophotometer.The general characteristics and chemical structure of this dye is shown in Table 1 and Figure 1.

Instruments
A thermostated shaker bath (Heto-Holten A/S Denmark, Type SBD-50 cold), operated at 100 rpm, was used to study the kinetic adsorption of direct blue 15 dye onto BC.A pH meter (Mettler Delta 320, UK) was used to measure the pH values of the dye solutions.A Cary 100 Bio UVevisible spectrophotometer (VARINA-UK) was employed for absorbance measurements using quartz cells of path length of 1 cm.

pH of dye solutions and batch kinetic experiments
The initial pH value of the solution is an important factor which must be considered during sorption studies (Aksu, 2005).The effect of pH on the amount of dye removal was analyzed over the pH range from 2 to 10 (Solid/Liquid ratio= 0.05/25 ml, temperature= 25°, contact time= 48 h, agitation rate= 100 rpm).Direct blue 15 dye was dissolved in deionized water to the required concentrations.For batch kinetic experiments, the pH of the dye solutions was adjusted to 3.0 with glacial acetic acid.The dye solution (25 ml) in each conical flask (100 mL) was shaken in a thermostated shaker bath operated at 100 rpm.After 30 min, the BC (0.050 g), which had been pre-warmed in the thermostated bath for 30 min, was immersed in the dye solution.The BC samples were then rapidly withdrawn after different immersion times.Dye concentrations were determined at time zero and at subsequent times using a calibration curve based on absorbance at λ max 596 nm versus dye concentration in standard dye solutions.The amount of dye adsorbed per gram of BC (mg/g BC) at any time (qt) was calculated by a mass-balance relationship equation (1) as follows: (1) Where C0 is the initial dye concentration (mg/L) and Ct is the dye concentration after dyeing time t (mg/L), V is the volume of dye solution (ml) and W is the weight of BC (g) used.

RESULTS AND DISCUSSION
The effect of pH, temperature and contact time on the biosorption of dye on BC The most important parameter for the adsorption experiments, effect of pH, was examined (Shukla et al., 2002).The pH values were varied between 2 and 10, keeping the other parameters constant.Figure 2 shows the influence of pH on the adsorption of dye on BC.The amount of adsorption increases with decreasing pH to a maximum value (pH 3).This may be ascribed to the protonation of the hydroxyl groups at the acidic conditions.But when the pH value is above 3, the adsorption capacity declined.The reason may be explained that the very unstable and weak bond between dye and BC when pH <3.So in the later experiments, the solution pH on the adsorption test was adjusted to about 3.
The effect of temperature on the biosorption of dye on BC was investigated.It was found that a higher dyeing temperature resulted in higher initial dye adsorption rate (hi) on BC before equilibrium as shown in Figure 3. Near the equilibrium time, the dye adsorbed by the BC decreased with increasing temperature, indicating an exothermic process.As can be observed from Figure 3, the time required to reach equilibrium was shorter at higher dyeing temperatures, that is, 120, 60 and 5 min at 30, 45 and 60°C, respectively.The dyeing condition of pH 3.0, initial dye concentration of100 mg/L and MLR of 1:500 was subsequently used to study the adsorption kinetics of direct blue 15 dye on BC.

Kinetic of adsorption
In order to analyze the adsorption kinetics of direct blue 15 dye onBC, the pseudo first-and second-order kinetic models were used to analyze the experimental data.
A simple kinetic analysis of adsorption is the Lagergren equation.The Lagergren equation, a pseudo first-order equation, describes the kinetics of the adsorption process as follows (Ho and McKay, 1996;Sun and Yang, 2003;Chiou et al., 2004): (2) Where k 1 is the rate constant of pseudo first-order adsorption(s -1 ), and q e and q t are the amounts of dye adsorbed per gram of BC (mg/g BC) at equilibrium and at time t.
In many cases, the first-order equation of Lagergren does not fit well for the whole range of contact times and is generally applicable over only the initial stage of the adsorption (Chiou and Li, 2002).After definite integration by applying the initial conditions q t =0 at t = 0 and q t = q t at t = t, Equation (2) becomes: (3) A straight line of ln (q e -q t ) versus t suggests the applicability of this kinetic model to fit the experimental data.The first-order rate constant k 1 and equilibrium adsorption density q e were calculated from the slope and intercept of this line (Figure 4).
The pseudo second-order kinetic model (Ho and McKay, 1996;Chiou and Li, 2002) is based on adsorption equilibrium capacity and can be expressed as follows: (4) Where k 2 (g BC/mg min) is the rate constant for pseudo second-order adsorption.Integrating Equation 4 and applying the initial conditions give: (5) or equivalently,  Where h i (Chiou and Li, 2003) is the initial dye adsorption rate (mg/g BC min).If pseudo second-order kinetics are applicable, the plot of (t/q t ) versus t would show a linear relationship.The slope and intercept of (t/q t ) versus t were used to calculate the pseudo second order rate constant k 2 and q e .It is likely that the behavior over the whole range of adsorption is in agreement with the chemisorption mechanism being the rate-controlling step (Chiou and Li, 2002).
Kinetic data obtained from dye adsorption in the present study, was analyzed using the pseudo first-order kinetic model proposed by Lagergren (Ho and Chiang, 2001) according to Equation 3. The results are listed in Table 2. Based on the correlation coefficients obtained, the adsorption of dye on BC is not likely to be a first-order reaction.
The pseudo second-order kinetic model was also used to test the experimental data using Equation 6, and plots of (t/q t ) against t for the adsorption of dye on BC are given in Figure 5.The slopes and intercepts of these plots were used to calculate the adsorption capacity (q e,cal ) and the rate constant (k 2 ).The experimental data showed a good compliance with the pseudo secondorder equation and the correlation coefficients for the linear plots were higher than 0.99 for all the experimental data.Also, the calculated q e,cal values agreed very well with the experimental data.Figure 6 shows the comparison of experimental and calculated q e values.These results suggested that the experimental data for the adsorption kinetics of dye on BC were fitted by the pseudo second-order kinetic model.

Validity of kinetic models
The applicability of both pseudo-first order and pseudosecond order models for the biosorption of dye onto BC equation (Chiou et al., 2004) given by ( 8): (8)  Where, N is the number od data point used in the linear plot of each model.The validity of these models was compared by judging the low value of SSE (%) which indicates the better fit.The values of (SSE, %) obtained for two models listed in Table 2.It is indicated that the pseudo second order kinetic model yield the lowest SSE (%) values (to).Whereas, the first order model led to very high values of SSE (%) (to).This agrees with the pervious values of both R 2 and q e, cal obtained earlier for the pseudo second order (Table 2) to further prove the suitability of pseudo second order kinetic to describe the biosorption process of direct blue 15 dye onto BC.

Activation parameters
From the rate constant k 2 (Table 2), the activation energy (E a ) for the adsorption of direct blue 15 dye on BC was determined using the Arrhenius equation ( 9) (Do˘gan and Alkan, 2003): Where E a , R and A refer to the Arrhenius activation energy, the gas constant and the Arrhenius factor, respectively.
The Arrhenius plot of ln k against 1/T for the adsorption of dye on BC is shown in Figure 7 and the activation energy value is listed in Table 3  were also calculated using the Eyring equation ( 10) (Do˘gan and Alkan, 2003) as follows: (10) Where k b and h refer to Boltzmann's constant and Planck's constant, respectively.
The enthalpy (∆H # ) and entropy (∆S # ) were calculated from the slope and intercept plot of ln (k/T) versus 1/T (y=-3.9937+3163.5X;R 2 = 0.9933) in Figure 8, while the free energy of activation (∆G # ) was obtained from Equation ( 11): (11) The calculated values are listed in Table 3.The ∆G # values were negative at all tested temperatures (30 to 60ºC) verifying that the biosorption of direct dye onto BC was spontaneous and thermodynamically favorable.In other words, more negative ∆G # implies a greater driving force of adsorption, resulting in increased adsorption capacity.As temperature increased from 30 to 60ºC, ∆G # decreased negative, suggesting that adsorption capacity decreased at high temperatures.The negative ∆H # values indicate that adsorption of direct dye onto BC is an exothermic process, which is supported by the decrease in adsorption of direct dye as temperature increase.Furthermore, the negative ∆S # indicates that the degrees of freedom decreased at the solid-liquid interface during adsorption of direct dye onto BC.Physisorption and chemisorptions can be classified, to a certain extent, by the magnitude of enthalpy change.Bonding strengths of < 84 kJ/mol are typically considered as those of physisorption bonds.Chemisorptions bond strengths can be 84-420 kJ/mol.Generally, ∆G # for physisorption is less than that for Chemisorptions.The former is between -20 and 0 kJ/mol and the latter is between -80 and -400 kJ/mol (Yahya et al., 2008).Additionally, Chao et al., 2008 demonstrated that the physisorption process normally had activation energy of 5-40 kJ/mol, while Chemisorption had a relatively higher activation 40 to 800 kJ/mol.Therefore, the values of ∆H # , ∆G # and E a all suggest that biosorption of direct blue 15 dye onto BC by a physisorption process.

Conclusion
The removal of colored and colorless organic pollutants from industrial wastewater is considered as an important application of adsorption processes.As BC fibrils are much thinner than fibers of plant cellulose, much more reactive hydroxyl groups on the surface of BC can be functionalized.The small size of microbial fibrils seems to be a key factor that determines its remarkable performance as an effective adsorbent.This survey showed that BC with highly water holding capacity as a biosorbent could be employed for the removal of dye from aqueous solution.
This study investigated the biosorption kinetics of direct blue 15 dye on BC.The dye adsorption rates onto BC decreased at higher dyeing temperatures which indicated the process was exothermic.The adsorption kinetics of dye on BC was found to follow the pseudo second-order kinetic model.The activation energy for the biosorption process on BC was found to be 43.5 kJ/mol.The values of Gibbs free energy ∆G # showed that reaction was spontaneous.Also the values of ∆H # , ∆G # and E a all suggested that the biosorption of direct dye onto bacterial cellulose was by physisorption.

Figure 3 .
Figure 3.The effect of contact time and temperature of dye on BC at an initial dye concentration 100 mg/L, MLR 1:500 and pH 3.0, amount of BC 0.05 g.

Figure 4 .
Figure 4. Plot of the pseudo first-order equation at different temperatures for the adsorption of dye on BC.

Figure 5 .
Figure 5. Plot of the pseudo second-order equation at different temperatures for the adsorption of dye on BC.

Figure 6 .
Figure 6.Comparison of experimental and calculated qe , mg/g of BC.

Figure 7 .
Figure 7. Arrhenius plot for the biosorption of direct dye on BC.

Figure 8 .
Figure 8. Eyring plot for the biosorption of direct dye on BC.

Table 1 .
Characteristics of Direct Blue 15 Dye.

Table 2 .
Comparison of the pseudo first-and second-order adsorption rate constants of dyeing onto BC at an initial dye concentration 100 mg/L, MLR 1:500 and pH 3.0.