Rapid industrialization has led to increased various types of pollutants into aquatic systems that have been identified as among the main causes of environmental pollution and degradation. Dyes, which are widely used in different industries, can impede light penetration into water, retard photosynthetic activity, inhibit the growth of biota and have a tendency to chelate metal ions  . Accordingly, the treatment of colored dye effluent streams has been attracted the attention of environmentalists, technologists, and entrepreneurs because of its socioeconomic and political dimensions. Almost one million tons of dyes are annually produced in the world, of which azo dyes, characterized by an azo bond (R1 - N = N - R2), represent approximately 70 wt%  . Azo dyes are the most common synthetic colorants released to the environment by textile, paint, varnish, paper, plastic, food, pharmaceutical, and cosmetic industries. The release of azo dyes in aquatic environment is challenging not only for aesthetic reasons but also because azo dyes and their cleavage products (aromatic amines) are carcinogenic   . Numerous technologies, such as adsorption     , membrane filtration  , coagulation  , ozonation  , advanced chemical oxidation   , and biological processes  , have been developed and applied worldwide for the scavenging and/or degradation of dyestuff wastewater.
The adsorption process is an efficient method for the removal of dyes from effluent due to its low initial cost, simplicity of design, ease of operation and insensitivity to toxic substances. Activated carbon is the most widely used as an adsorbent due to its large surface area, microporous structure and high adsorption capacity  . However, its use is limited due to its high cost. Therefore, the use of low-cost, easily obtained, high-efficiency and eco-friendly adsorbents has been investigated as an ideal alternative to the current, more expensive methods for removing dyes from wastewater  . A number of reports on the application of low cost adsorbents to eliminate reactive dyes from aqueous solution appeared in literatures  -  . Chitin and its partially deacetylated derivative chitosan are biocompatible and biodegradable polymers. Chitosan have large numbers of amine and hydroxyl functional groups (Figure 1(a)). These functional groups are very effective and promising to isolate toxic heavy metals from aqueous solution   . The opportunity of utilizing chitosan as a biosorbent to eliminate azo dyes from aqueous solution has also been taken into significant consideration       . However, the related reports on utilizing chitosan to remove the reactive red 239 (RR239; Figure 1(b)) azo dye from aqueous solution are comparatively insufficient.
The aim of this study was to examine the ability of chitosan 8B (80% deacetylated chitin) as a low cost adsorbent to separate reactive red 239 (RR239) from aqueous solution. The influences of significant aspects, such as solution pHs, initial dye concentration, ionic strengths and temperatures on RR239 adsorption onto chitosan 8B were studied in batch mode. The potential rate controlling step and adsorption mechanisms were analyzed by several kinetic equations (i.e., pseudo first-, pseudo second-order equations, Elovich and intraparticle diffusion models). Freundlich, Temkin, Dubinin-Radushkevich and Langmuir adsorption isotherms were applied to compute the adsorption equilibrium. Activation and apparent thermodynamic parameters were also evaluated.
Katokichi Bio Co., Ltd., Japan provided chitosan 8B (80% deacetylated chitin) which was used without further purification. A laser scattering particle size analyzer (LDSA-2400A, Tonichi Computer Applications, Japan) equipped with a dry dispersing apparatus (PD-10S, Tonichi Computer Applications, Japan) was used to determine the particle size of chitosan 8B. The mass median diameters of the chitosan 8B flakes were found to be 247 ± 9 µm.
Reactive red 239 (RR239; Figure 1(b)) dye was obtained from Sigma-Aldrich, Germany and was utilized without further purification. Other reagents used in the present work were analytical grade. Deionized water was prepared by circulating distilled water through a deionizing column (Barnstead, Syboron Corporation, Boston, USA).
2.2. Spectroscopic Measurements
For spectroscopic measurements, the samples were prepared by mixing solid RR239, chitosan 8B, and chitosan 8B flake-RR239 complex with solid KBr. The diffuse reflectance electronic absorption spectra of solid samples were recorded by a Varian, Cary 5000 UV-visible-NIR spectrometer (Varian Inc., USA) in the wavelength region of 200 - 800 nm.
Figure 1. The structure of (a) chitosan 8B and (b) reactive red 239 (RR239).
2.3. Batch Adsorption Experiments
Batch adsorption experiments were conducted with slight modifications of previously described method for the adsorption of reactive yellow 145 (RY145) onto chitosan in aqueous solution  . The adsorption of RR239 onto chitosan 8B was explored in 125 mL stoppered bottles containing a fixed amount (0.05 g) of chitosan 8B with 25 mL of dye solution (100 µmol/L). The primary pH of the dye solution was adjusted with 1 mol/L HCl or 1 mol/L NaOH solution. The pH of dye solution was measured by using a pH meter (HANNA instruments microprocessor pH meter). The stoppered bottles were shaken in a thermostated shaker at room temperature (30˚C ± 0.2˚C) with a speed of 120 r/min until reaching equilibrium. Each bottle was capped to avoid evaporation at high temperature. Samples were withdrawn at predetermined intervals of time for analyzing the concentration of RR239 in solution. The samples were centrifuged using a centrifuge machine (Labofuge 200, D-37520 Osterods, Germany) at a speed of 4000 rpm for 5 min. The concentration of RR239 in the supernatant was determined by spectrophotometric methods using a Shimadzu UV-1601PC spectrophotometer (Shimadzu, Japan) at λmax value of 541 nm. The λmax (541 nm) of RR239 solution was found to be constant at the pH ranges between 4 and 10. The apparent molar absorptivity of RR239 was estimated to be 15 × 103 L/mol/cm at 541 nm and pH 4 - 10.
The amount of RR239 adsorbed onto chitosan 8B at time t, qt (µmol/g) was determined by
where C0 (µmol/L) and Ct (µmol/L) are the liquid-phase concentrations of RR239 at initial and any time t, respectively; V(L) is the volume of RR239 solution and m (g) is the amount of dry chitosan 8B used.
The adsorption kinetics was also performed varying initial concentration of dye solutions (30 - 200 µmol/L), ionic strengths (0.01 - 0.04 mol/L) and temperatures (30˚C, 35˚C, 40˚C and 45˚C), respectively. The ionic strength of dye solutions was adjusted with 1 mol/L Na2SO4 solution. The equilibrium adsorption was carried out at four different temperatures (30˚C, 35˚C, 40˚C and 45˚C), and at pH 4 in absence of Na2SO4.
The amount of RR239 adsorbed onto chitosan 8B at equilibrium time, qe (µmol/g) was determined by
where Ce (µmol/L) is the liquid-phase concentrations of RR239 at equilibrium time; C0, V and m remain same as described above.
In the desorption study, 25 mL of 0.1 mol/L NaOH solution was used to desorb RR239 from the chitosan 8B flakes. The adsorbent was contacted with 25 mL of 100 µmol/L RR239 solution for 120 min, filtered and the adsorbent was dried at room temperature (30˚C) overnight. The dye loaded-adsorbent was transferred into 25 mL desorbing solution and the mixture was stirred for 90 min. The amount of adsorption was determined in the same way as described above. All data presented in this paper are the mean of double measurements.
3. Results and Discussion
3.1. Spectral Studies
The chitosan 8B-RR239 complex was formulated by combining chitosan 8B flakes with a solution of RR239 at pH 4. The dye adsorption onto chitosan 8B flakes was confirmed by diffuse reflectance electronic absorption spectra. In Figure 2, the absorption maxima of chitosan 8B-RR239 complex were observed at around 298 and 540 nm (Figure 2(c)), whereas free chitosan 8B flakes exhibited two absorption peaks at 220 and 300 nm (Figure 2(a)), respectively. RR239 dye revealed absorption peaks at 292 and 558 nm (Figure 2(b)), respectively. Hence the blue shift 18 nm of the absorption peak at 540 nm, and the red shift 6 nm of the absorption peak at 298 nm must be attributed the adsorption of RR239 onto chitosan 8B flakes. However, the absorption maximum of free RR239 was found in the visible region at 541 nm in aqueous solution (pH 4 - 10).
3.2. Adsorption Kinetics
3.2.1. Effect of pH on Adsorption Kinetics
The surface charge of adsorbents and the configuration of dye molecules are significantly affected by solution pH  . Therefore, the pH of aqueous solution is a critical issue for the period of dye adsorption process. At present study, the effect of pH on the RR239 dye adsorption capacities of chitosan 8B was conducted at varying pH (pH 4 - 10) with 100 μmol/L fixed initial dye concentrations and adsorbent dosage 0.05 g for 120 min. As can be seen from Figure 3, the adsorption of RR239 dye onto chitosan 8B is intimately dependent on initial pH of aqueous solution. It is noticed that the dye adsorption rate was fast for initial 10 min and subsequently it progressed at a slower rate and lastly reached equilibrium within ~60 min. However, data were received for 120 min to confirm overall equilibrium. The initial rate of dye adsorption, h (µmol/g min), decreased significantly with increasing solution pH (Table 1). After 120 min of sorption, the extent of equilibrium dye adsorption (qe) was observed to be 49.70 µmol/g at pH 4 and 19.20 µmol/g at pH 10 (Table 1), respectively. These results can be rationalized based on the changes in surface charges of adsorbent and the structure of dye ions with changing the pH of aqueous solution. The pHzpc of chitosan is 6.3. It assumes that the 99% of chitosan is protonated at pH 4.3  . Hence the positively charged exteriors of chitosan 8B particles are magnificently adsorbed negatively charged dye anions in aqueous at pH 4. On the other hand, the surfaces of chitosan 8B particles turn into negative charges with rising the solution pH, causing in repulsive forces between adsorbent and anionic dye adsorbate  , which ultimately causes in a smaller adsorption. The suitable solution pH was found to be 4 within the examined solution pH 4 - 10. Similar results were also observed in the adsorption of tartazine onto agricultural by-product  and reactive black 5 (RB5) onto chitosan  in aqueous solutions.
Table 1. Comparison of experimental and calculated qe values, and kinetic parameters obtained from various kinetic models applied for the batch adsorption of RR239 dye onto chitosan 8B in aqueous solution.
Figure 2. Diffuse reflectance electronic absorption spectra of (a) chitosan 8B flakes, (b) RR239, and (c) chitosan 8B-RR239 complex in KBr.
Figure 3. The effects of solution pHs on the adsorption kinetics of RR239 onto chitosan 8B in aqueous solution ([RR239]0 dye: 100 µmol/L; solution volume: 25 mL; chitosan 8B: 0.05 g; temperature: 30˚C ± 0.2˚C; solution pHs: ○: pH 4; ●: pH 5; ◊: pH 6; ♦: pH 7; ∆: pH 8; ▲: pH 9; □: pH 10). All solid lines are numerically generated by using pseudo second-order adsorption kinetic Equation (4) and the values of qe(cal) and k2 listed in Table 1.
3.2.2. Effect of Initial Dye Concentration on Adsorption Kinetics
The initial concentration supplies a significant pouring strength to overcome all mass transfer resistance of the dye between the aqueous and solid phases  . In this work, the influences of initial concentration of dye solutions on adsorption were studied in aqueous solution (pH 4) at 30˚C for 120 min and the amount of chitosan 8B was 0.05 g. Remarkably, the amount of equilibrium RR239 dye adsorption, qe (μmol/g), increases (Figure 4), but the initial rate of RR239 dye adsorption, h (µmol/g min), reduces with increasing initial dye concentration (Table 1), indicating that the RR239 dye adsorption onto chitosan 8B is strongly concentration dependent. As can be seen from Figure 4, when the initial dye concentration increases from 30 to 200 μmol/L, the extent of dye sorption capacities of chitosan 8B increase from 14.57 to 71.67 μmol/g. These results are in accordance with obtained findings by other researchers   .
3.2.3. Effect of Ionic Strength on Adsorption Kinetics
The solution ionic strength is an essential issue that influences both electrostatic and non-electrostatic interactions between dyes and adsorbent surfaces. The effect of ionic strength on the adsorption kinetics of RR239 onto chitosan 8B in aqueous solution (pH 4) was successively inspected by the adding various amounts of Na2SO4 salt to the 100 µmol/L dye solution at 30˚C. Results presented in Figure 5 obviously showed a decrease in both the initial dye adsorption rate, h (µmol/g min), and the extent of dye adsorption, qe (µmol/g), with increasing solution ionic strengths (Table 1). The results indicate that RR239 dye anions and (from Na2SO4) are competing for the active sorption sites, which is good agreement with previous reports   . This also confirms the electrostatic interactions between adsorbent and dye molecules.
3.2.4. Effect of Temperature on Adsorption Kinetics
The kinetics of RR239 dye adsorption onto chitosan 8B was investigated in aqueous solution (pH 4) at four different temperatures, 30˚C, 35˚C, 40˚C and 45˚C and the results are depicted in Figure 6 where the initial concentration of dye solution was 100 μmol/L. These results indicate that the initial rate of dye adsorption, h (µmol/g min), and the amount of equilibrium dye adsorption, qe (µmol/g), increased with increasing the solution temperature (˚C). Therefore,
Figure 4. The effects of initial dye concentration on the adsorption kinetics of RR239 onto chitosan 8B in aqueous solution (pH 4) (solution volume: 25 mL; chitosan 8B: 0.05 g; temperature: 30˚C ± 0.2˚C; [RR239]0: ○: 30 µmol/L; ●: 50 µmol/L; ◊: 100 µmol/L; ♦: 150 µmol/L; ∆: 200 µmol/L). All solid lines are numerically generated by using pseudo second-order adsorption kinetic Equation (4) and the values of qe(cal) and k2 listed in Table 1.
Figure 5. Adsorption kinetics of RR239 dye onto chitosan 8B in aqueous solution at various ionic strengths ([RR239]0: 100 µmol/L; solution volume: 25 mL; chitosan 8B: 0.05 g; temperature: 30˚C ± 0.2˚C; ionic strength: ○: 0.01 mol/L; ●: 0.02 mol/L; ◊: 0.03 mol/L; ♦: 0.04 mol/L). All solid lines are numerically generated by using pseudo second-order adsorption kinetic Equation (4) and the values of qe(cal) and k2 listed in Table 1.
Figure 6. Adsorption kinetics of RR239 onto chitosan 8B in aqueous solution at various temperatures ([RR239]0: 100 µmol/L; solution volume: 25 mL; chitosan 8B: 0.05 g; temperature: ○: 30˚C; ●: 35˚C; ◊: 40˚C; ♦: 45˚C). All solid lines are numerically generated by using pseudo second-order adsorption kinetic Equation (4) and the values of qe(cal) and k2 listed in Table 1.
the RR239 dye molecules interact more efficiently with functional groups of chitosan 8B at higher temperatures because the mobility of the dye molecules increased with rising solution temperature. This is expected that rising of the solution temperature may generate a swelling effect within the internal structure of chitosan 8B  . Similar results were also observed in adsorption of RY145 and RB5 onto chitosan in aqueous solutions, respectively   .
3.3. Modeling of Adsorption Kinetics
The adsorption kinetics illustrates the dye uptake rate onto the adsorbent in adsorption reaction. This is an essential feature in expressing the efficacy of adsorption. Various kinetic models have been utilized to investigate the dye adsorption mechanisms in aqueous solutions. Here, pseudo first-  , second-order  , Elovich  , and intraparticle diffusion  kinetic models were employed to calculate the adsorption kinetic data obtained from batch experiments. The pseudo first-order adsorption kinetics is usually stated by Equation (3):
where k1 (1/min) is the pseudo first-order sorption rate constant estimated from the slope of a plot versus t.
The pseudo second-order kinetic model is expressed as:
where k2 (g/μmol min) is the pseudo second-order adsorption rate constant calculated from a linearized form of this equation, represented by Equation (5):
The plot of t/qt versus t would exhibit a straight line if pseudo second-order kinetics is applicable. The values of k2 and qe were obtained from intercept and slope of the straight line. The initial rate of adsorption, h (µmol/g min), is able to know by Equation (6):
The Elovich model is generally expressed as:
where α (µmol/g min) is an initial rate of dye adsorption and β (g/μmol) is associated to the degree of surface coverage and the activation energy for chemisorption. The Elovich coefficients can be estimated from the plot of qt versus lnt.
The values of correlation coefficients (R2) and kinetic parameters obtained from various applied kinetic models are presented in Table 1. The values of R2 obtained from pseudo first-order (≤0.995) and Elovich (≤0.986) kinetic models are insignificant compare to the values of R2 (≥0.999) found in the cases of pseudo second-order kinetic model. Moreover, the calculated qe(cal) values from pseudo second-order kinetic model and the experimental qe(exp) values are comparable (Table 1) suggesting a promising condition for pseudo second-order adsorption kinetics. Similar results were also observed in the adsorption of RY145 and RB5 onto chitosan, respectively   .
The intraparticle diffusion model equation can be written as:
Intraparticle diffusion model: (8)
where kid (μmol/g min0.5) is the intraparticle diffusion rate constant and I (μmol/g) is the intercept. The typical intraparticle diffusion plots for the effect of initial dye concentration on the adsorption of RR239 onto chitosan 8B were shown in Figure 7. From this figure, it was observed that there were two linear portions. The intraparticle diffusion constants, kid1 (μmol/g min0.5) and kid2 (μmol/g min0.5), are calculated from the slope of the corresponding linear regions of Figure 7. The values of kid1 (μmol/g min0.5) and kid2 (μmol/g min0.5) express diffusion rates of the different stages in the adsorption (Table 2).
The changes of kid1 (μmol/g min0.5) and kid2 (μmol/g min0.5) represent to the adsorption levels of the exterior and interior surfaces of adsorbent  . The dye adsorption rate was very fast at the early stage because the RR239 was adsorbed by the exterior surface of chitosan 8B particle. When exterior surface of chitosan 8b was saturated then RR239 dye molecules entered into chitosan 8B particles and were adsorbed by the interior surface of the chitosan particles. The diffusion rate became slow. From Figure 7, it is also be noticed that none of the straight lines at any concentration of RR239 solution passes through the origin, which suggests that the dye adsorption mechanism was complex and both surface adsorption as well as intraparticle diffusion contributed to the actual sorption process. Similar results were also observed in the adsorption of RY145 onto chitosan in aqueous solution  .
Figure 7. Typical intraparticle diffusion model plots for the adsorption of RR239 dye on Chitosan 8B at various initial concentrations (solution volume: 25 mL; chitosan 8B: 0.05 g; temperature: 30˚C ± 0.2˚C; initial concentrations of RR239: ○: 30 µmol/L; ●: 50 µmol/L; ◊: 100 µmol/L; ♦: 150 µmol/L; ∆: 200 µmol/L). The values of kid1 (μmol/g min0.5) and kid2 (μmol/g min0.5) are listed in Table 2.
Table 2. Diffusion rate parameters (kid1 and kid2) for the adsorption of RR239 onto chitosan 8B in aqueous solution at various conditions.
3.4. Activation Parameters
The values of k2 at various temperatures recorded in Table 1 were utilized to determine the activation energy for the adsorption of RR239 dye onto chitosan 8B in aqueous solution. According to Arrhenius equation, the rate constant (k2), temperature (T) and activation energy (Ea) can be expressed by Equation (9):
where R (8.314 J/mol K) is the gas constant. The value of Ea was determined be 23.30 kJ/mol from the slope of straight line obtained from lnk2 versus 1/T plot (R2 = 0.928) in temperature range 30˚C - 45˚C. Literature survey showed that the values of Ea were obtained to be 33.35 kJ/mol for the reactive dye adsorption on carbon nanotubes  , 19.72 kJ/mol for the RY145 dye adsorption onto chitosan  and 33.96 kJ/mol for the maxilon blue GRL dye adsorption onto sepiolite  . It is well known that the extent of the activation energy produces evidence on whether the adsorption is mainly physical or chemical. Wu reported that the activation energy of 5 - 40 kJ/mol is normally for physisorption process and a higher activation energy (40 - 800 kJ/mol) is for chemisorption process  . The value of Ea (23.30 kJ/mol) suggests that the present adsorption is a physisorption process.
The other thermodynamic activation parameters such as changes in entropy of activation (ΔS‡), changes in enthalpy of activation (ΔH‡), and changes in Gibbs free energy of activation (ΔG‡), are expressed by the following relations  :
where k2 (g/mol min), R and T are same as described before, kB is the Boltzman constant (kB = 1.381 × 10-23 J/K) and hP is the Plank constant (hP = 6.626 × 10-34 Js). The values of ΔH‡ and ΔS‡ were determined from the slope and y-intercept of the straight line obtained from the plot ln(k2/T) versus 1/T (R2 = 0.911). The ΔH‡ value was estimated to be 20.72 kJ/mol. The low value of ΔH‡ shows that the interactions between RR239 and chitosan 8B are weak. The value of ΔS‡ was determined to be -102.33 J/mol K, which indicates that the RR239 anions at activated state and at the interface are always more organized than those in the bulk solution phase  . Similar results were also observed in the removal of malachite green from wastewater using activated carbon and activated slag  . The value of TavΔS‡ was calculated to be -31.77 kJ/mol where Tav represents the average value of four temperatures used for adsorption studies. The value of ΔH‡ was found to be smaller than that of TavΔS‡ (ΔH‡ < TavΔS‡) which indicates that the impact of entropy is more significant than enthalpied in activation  . Moreover, the values of ΔG‡ were found to be 51.72, 52.23, 52.75 and 53.26 kJ/mol at 30˚C, 35˚C, 40˚C and 45˚C, respectively. The positive values of ΔG‡ suggest the existence of an energy barrier in the adsorption process  .
3.5. Reuse of the Chitosan
The typical phenomena of RR239 dye adsorption-desorption-adsorption onto chitosan 8B flakes in aqueous are shown in Figure 8. In adsorption step, the initial dye concentration was 100 µmol/L with a solution pH 4.0 and a temperature 30˚C whereas the release of RR239 from dye-loaded chitosan 8B was carried out in 0.1 M NaOH solution at pH 12.5 and temperature 30˚C. It is observed that the amount of equilibrium dye adsorption was 49.70 µmol/g in the first adsorption step which was persisted for 120 min. The dye desorption rate was rapid for first 20 min and 85% of RR239 dye was released from dye-loaded chitosan within 35 min. After 90 min of desorption, the liberated dye from RR239-loaded chitosan was 89%. This may be due to the fact that in a basic solution, the positively charged amino groups of chitosan 8B are deprotonated, and the electrostatic interaction between chitosan and dye molecules becomes much weaker  . Following the desorption step, the second adsorption step took place and it repeated the similar dynamical shape of the first adsorption step. It takes 120 min to adsorb the similar amount of dye RR239 as that of desorption, and the total amount of dye adsorption is the same as that of the first adsorption step. These results show that our chitosan flakes can be reused for further dye adsorption.
3.6. Adsorption Isotherm
The analysis of the adsorption isotherm data is important to know the nature of the interaction between adsorbate and the adsorbent used for the eliminating of organic pollutants. A plot of qe versus Ce at various temperatures is presented in Figure 9. The values of qe increased with increasing solution temperature from 30˚C to 45˚C indicating that the present adsorption is an endothermic process.
The adsorption isotherm data were examined by using four isotherm models such as Ferundlich  , Temkin  , Dubinin-Radushkevich  , and Langmuir  . Nonlinear and linear forms of all the applied models are presented as follows.
Nonlinear form (12)
Linear form (13)
Nonlinear form (14)
Linear form (15)
Nonlinear form (16)
Linear form (17)
Nonlinear form (18)
Linear form (19)
where Ce (µmol/L) is the equilibrium RR239 dye concentration in solution, KF ((µmol/g) (µmol/L)−1/n) and n are Freundlich isotherm constants indicating the capacity and intensity of the adsorption, respectively. KT (µmol/L) is the Temkin isotherm constant, b (J/mol) is a constant related to heat of adsorption, R (8.314 J/mol K) is an ideal gas constant and T is the absolute temperature (K). The qe (µmol/g) is the amount of equilibrium RR239 dye adsorbed per unit weight of adsorbent, qDR is the maximum adsorption capacity, K (mol2/kJ2) is the activity coefficient useful in obtaining the mean sorption energy E (kJ/mol) and ε is the Polanyi potential. KL (L/g) and aL (L/µmol) are the characteristic constants of the Langmuir equation, and the ratio of KL/aL gives the maximum dye adsorption capacity qm (µmol/g) of chitosan 8B. The values of isotherm parameters are given in Table 3. In all cases, Langmuir isotherm has highest correlation coefficients (R2), which is considerably a better fit compared with Freundlich, Temkin and Dubinin-Radushkevich adsorption isotherms.
Figure 8. Adsorption and desorption kinetics of RR239 on chitosan 8B in aqueous solution at 30˚C and 100 µmol/L initial dye concentration with three steps: adsorption step 1 at initial pH 4.0, desorption step at initial pH 12.5, and adsorption step 2 at initial pH 4.0.
Figure 9. Adsorption isotherm of RR239 onto chitosan 8B in aqueous solution at various temperatures ([RR239]0: 50 - 1500 µmol/L; solution volume: 25 mL; chitosan: 0.05 g; solution: pH 4; temperatures: ○: 30˚C; ●: 35˚C; ◊: 40˚C; ♦: 45˚C). All solid lines are numerically generated by using the Langmuir model in Equation (18) and the Langmuir isotherm constants listed in Table 3.
Table 3. Freundlich, Temkin, Dubinin-Radushkevich and Langmuir isotherm constants at different temperatures and thermodynamic parameters for the adsorption of dye RR239 onto chitosan 8B in aqueous solution at pH 4.
It is stated that the characteristics of the Langmuir isotherm can be expressed in terms of separation factor (RL)  , which is expressed in Equation (20):
where aL (L/µmol) is the Langmuir equilibrium constant and C0 (μmol/L) is the highest initial dye concentration. The shape of the isotherm can be interpreted as shown in Table 4 based on the value of RL. The values of RL were found to be from 0 to 1 at 30˚C, 35˚C, 40˚C and 45˚C, respectively, which indicates that the adsorption is favorable at all temperatures  .
The applicability of the Langmuir model suggests homogeneous surfaces of the chitosan 8B and monolayer coverage of RR239 dye onto the adsorbent. According to Langmuir, the maximum dye adsorption capacity of chitosan 8B was found to be 123.46 µmol/g at 30˚C and 163.93 µmol/g at 45˚C, respectively. This value is much higher than that of previously reported adsorbents as shown in Table 5    .
Thermodynamic parameters related to the adsorption process, i.e., Gibb’s free energy change (∆G, kJ/mol), enthalpy change (∆H, kJ/mol), and entropy change (∆S, J/mol K) are determined by using the values of Langmuir binding constant (aL, L/mol) and the following equations:
Table 4. The relation between the value of RL and type of adsorption.
Table 5. The maximum adsorption capacity (qm) for RR239 of various adsorbents.
where R (8.314 J/mol K) is the universal gas constant and T is the absolute temperature (K). ∆H and ∆S values can be calculated from the slope and y-intercept of the linear polt of lnaL versus 1/T (R2 = 0.978). The thermodynamic results are presented in Table 3. Enthalpy changes (∆H) indicate that the present adsorption followed endothermic processes. The positive values of ∆S confirm that there was increase in the randomness at the solid-liquid interface during the adsorption of RR239 onto chitosan 8B, which results from the higher translational entropy developed by the displaced water molecules as compared to that lost as a result of dye uptake. Negative values of ∆G indicate that the RR239 dye adsorption by chitosan 8B adsorbents is a spontaneous process and more favorable process at higher temperature. The increase in the adsorption capacities of Chitosan 8B at higher temperatures may be attributed to the enhanced mobility and penetration of dye molecules within the adsorbent porous structures by overcoming the activation energy barrier and enhancing the rate of intraparticle diffusion  .
In the present study, RR239 dye adsorption onto chitosan 8B was investigated in an aqueous solution. The effect of working parameters such as solution pHs, initial dye concentrations, ionic strengths and temperatures on the adsorption of RR239 was investigated. The obtained results suggested that all the parameters had a strong effect on the adsorption kinetics and equilibrium adsorption of RR239 onto the adsorbent. The batch adsorption kinetics was well explained by pseudo second-order model while intraparticle diffusion performed significant role in the rate limiting step. The batch adsorption kinetic profiles were absolutely reproduced by numerical analysis based on the pseudo second-order kinetic model in Equation (4) using the values of k2 and qe(cal) given in Table 1. Adsorption isotherms were fitted well with the Langmuir model rather than the Freundlich, Temkin and Dubinin-Radushkevich models. The maximum RR239 dye adsorption capacity of chitosan 8B was found to be 163.93 μmol/g at 45˚C. Thermodynamic parameters showed that the RR239 dye adsorption onto chitosan 8B was spontaneous endothermic physisorption process. The significant amount (89%) of RR239 dye was released from dye-loaded chitosan in 0.1 M NaOH solution (pH 12.5) and the chitosan flakes could be reused. The present works confirm that the easily obtainable chitosan 8B can be utilized as an efficient biosorbent to remove reactive dyes from industrial wastewater.
The authors have declared no conflict of interest. We are indebted to the authority of the Ministry of Science and Technology, Government of the People’s Republic of Bangladesh for giving a research grant to carry out this work (FY 2013-2014). We gratefully acknowledge the support by the Alexander von Humboldt Foundation (George Forster Research Fellowship to T.K.S at Göettingen University, Germany where he has taken the diffuse reflectance electronic absorption spectra of solid samples). Authors are thankful to Prof. Yoshinobu Fukumori and Prof. Hideki Ichikawa (Kobe Gakuin University, Kobe, Japan) for providing the sample of chitosan 8B and measuring its particle size.