BATCH ADSORPTION STUDIES FOR MALACHITE GREEN DYE REMOVAL FROM WASTE WATER USING BIOMASS BASED ADSORBENT

Adsorption of malachite green dye over the adsorbents (Ad) derived from Sapindus seed hull (S) and Camelina (C) after treatment with sulphuric acid is studied. Batch adsorption study on both Ad is compared as a function of various parameters such as pH, time, initial concentration and temperature. The isotherm data is found to be best presented by Langmuir model for both adsorbents. Separation factor (R L ) revealed a favorable adsorption. Further, the rate of adsorption followed pseudo-first order kinetics. The intraparticle diffusion model facilitates to understand mass transfer in adsorption of dye molecules. Thermodynamic analysis on both Ad revealed the spontaneity of the phenomena at higher temperature. The comparative study illustrated higher adsorption capacity of CAd in comparison to SAd.


Introduction
Adsorbate: Malachite green supplied by Thermofischer was used as such without any treatment. Distilled water was prepared by borosilicate distillation unit indigenously and used for the preparation of solutions. Initially, a stock solution of 100 mg.L −1 was prepared and remaining working solutions were prepared by dilution.
Adsorption studies: The adsorption isotherms and kinetics studies were carried out in batch wise fashion. Equilibrium study was conducted by taking 50 ml of different concentrations (10−100 mg·L −1 ) of malachite green solution at pH 6 and 100 mg of AC was added. The mixture was then stirred for 360 min at 120 rpm to ensure equilibrium. The solution concentration was analyzed through uv-vis spectrophotometer (Varian Carry-100) at 617 nm. The amount of adsorption (q e ) at equilibrium was calculated as: where m is the mass of dry adsorbent used (g), C i -initial and C e -equilibrium concentration (mg·L −1 ) of malachite green. The same expression was used for kinetics studies by replacing q e and C e with q t and C t , respectively. Kinetic studies were carried out in the similar fashion with those of equilibrium experiments, wherein the samples were taken at different time intervals and analyzed as above. Four different concentrations of malachite green solutions (10,40,70 and 100 mg·g −1 ) were taken for the kinetic analysis at three different temperatures 30, 40, and 50C. Table 1 presents CHNSO content of all the four samples, where, 'O' content was calculated on the difference basis as: O, % = 100-(C%+H%+N%+S%). H/C ratio depicts significant fall in both CAd and SAd in contrast to their respective precursors. It has been observed that although sample S is richer in carbon content than C; yet, CAd has shown greater extent of enrichment than SAd. The reason is the ease of volatiles excursion in the case of sample C; sapindus seeds are hard and the escalation of volatiles is not that much easier. This finding facilitates to understand the better adsorption ability of CAd in comparison to SAd. The study of temperature effect on adsorption facilitates evaluation of thermodynamic parameters including enthalpy change, entropy change, Gibbs free energy likewise [25]. So, temperature dependent study was performed for malachite green dye adsorption at 30, 40 and 50C. The extent of adsorption on CAdand SAdis presented in the Fig. 1 where each curve represent a study at particular temperature. Both materials proved higher percent of dye removal with increase in temperature. The percent dye removal from solution at 30C by SAd and CAd are found to be 6.5 and 19.5%, respectively, at initial concentration of 100 mg·L −1 . These values, further, increased up to 19.5 and 30.8% for SAd and CAd, respectively, with increase in temperature to 50C. This is clear from the experimental data that at higher temperature dye molecules must have been excited enough to reach adsorption site. Further, it is expected that a fall in viscosity of the solution at higher temperature improves boundary layer diffusion [10].

Results and Discussion
It is important to study the effect of initial concentration on the adsorption rate. This study proves that at higher concentration the uptake of dye molecules increased for both CAd and SAd. The initial concentration provides driving force between aqueous solution and adsorbent [26]. At every temperature the same finding is observed. Although, the percent removal shows a drop in value due to the saturation of active sites in solid substrate; the uptake in mg.g -1 improves. The pH of solution affects degree of ionization and hence, the extent of adsorption as explained [25]. In case of CAd, the adsorption uptake is found to increase from 12.7 to 20.6 mg·g −1 when pH of aqueous solution changes from 2 to 6 as shown in Fig. 2. Alike trend has been reported elsewhere [27]. It, further, dropped to 18.4 mg·g −1 as pH is increased from 6 to 10. Similarly, q e for SAd increases from 9.2 to 14.7 mg·g −1 with an increase in pH from 2 to 6. Again, q e lowered to 12.4 mg·g −1 as pH exceeds 6 up to 10. Analogous behavior has been reported in the literature [10]. It proves that pH influence the surface charge of the adsorbents. The effect of contact time on adsorption of malachite green is shown in Fig. 3 for CAd and SAd. It is clear from the figure that initially adsorption takes place at faster rate and after 150 min this rate slows down. The reason behind so is the availability of numerous sites for dye molecules to get adsorbed at an early stages of adsorption [10]. Later, decrease in diffusion rate as well as concentration gradient results in slow down of adsorption as mentioned [28]. Both adsorbents showed an equilibration state after 180 min. Further, Fig. 3 depicts the possibility of monolayer adsorption of dye molecules due the smoothness and continuity of the curve [29]. The adsorption isotherm tells the distribution of adsorbate in solid to liquid phase and hence, presents a relation among the quantity adsorbed q e to the concentration in the solution at equilibrium, C e . Among the various models in literature, the most commonly employed are Langmuir and Freundlich models. Langmuir model is used to understand the equilibrium with an important assumption of homogeneity of adsorption on the surface. The mathematical expression of Langmuir model is presented below [10]: where, q e is the amount of adsorbate adsorbed (mg·g −1 ) at equilibrium and evaluated as presented elsewhere, C e is the concentration at equilibrium (mg·L −1 ), q max represents monolayer capacity (mg·g −1 ) and K L in the above equation corresponds to adsorption equilibrium constant (L·mg −1 ). A graph between C e /q e versus C e yields a straight line as shown in Fig. 4. The slope and intercept of this line produces q max and K L values and are presented in Table 2.

Fig. 4. Langmuir isotherms for CAd and SAd at temperatures 30, 40 and 50°C for initial concentration 40 mg·L −1
It has been observed that q max for CAd and SAd increases from 14.41 to 23.04 and 4.83 to 14.64 mg·g −1 , respectively, when temperature was raised from 30 to 50C. This proves that higher temperature favors adsorption of malachite green on C and S derived adsorbents. Some lesser value of adsorption capacity has already been reported elsewhere [1,7,19,29,30]. Further, K L values at 30C are found to be 0.92 and 0.59 L·mg −1 for CAd and SAd, respectively, and found to be raised to 1.85 and 11.78 L·mg −1 , respectively, at 50C. Separation factor, R L indicates the nature of adsorption process and is evaluated using the relation as stated in [10]: Here, C o is the highest initial dye concentration. R L value between 0 and 1 signifies a favorable process of dye adsorption. During the adsorption of malachite green on CAd and SAd, this value varies within the range 0.0107−0.0009 and 0.0167−0.0008, respectively, at all temperatures under study. The following linear expression for Freundlich isotherm is used [10]: where, K f (mg·g −1 ·(mg·L −1 ) 1/n ) and n represents bonding energy [28] and process favorability, respectively. Freundlich constants, K f and n are evaluated from the plot between log q e and log C e as shown in Fig. 5 and are reported in Table 2 for CAd and SAd. It is found that temperature has positively affected and resulted in the rise of both constants K f and n for CAd as well as SAd. The value of n1 indicates that the dye is favorably adsorbed favoring normal Langmuir isotherm on CAd and SAd. The correlation coefficient, R 2 close to 1 depicts best fit of the equilibrium model. In this study, Langmuir model is best fitted to the equilibrium data for the malachite green dye adsorption on both adsorbents CAd and SAd.

Fig. 5. Freundlich isotherms for CAd and SAd at temperatures 30, 40 and 50C for initial concentration 40 mg·L −1
In order to understand the controlling step involved in the adsorption of malachite green such as diffusion, adsorption and chemical reaction; various kinetic models are employed to the experimental results. The pseudo-first-order expression is presented below: ln(q − q ) = ln q + k t where, q e and q t represents amount of dye adsorbed at equilibrium and at time, t; both in mg·g −1 . The first order rate constant, k 1 (min −1 ) is evaluated from the slope of straight line between ln(q e -q t ) and t as shown in Fig. 6. The values of parameters k 1 , q e and R 2 at different initial concentrations are listed in Table 3. The values of R 2 are found to be inconsistent and so, concluded that adsorption of malachite green dye does not follow pseudo-first-order kinetic model.  The pseudo-second-order kinetic model is presented below: where, q e represents maximum adsorption in mg·g −1 through second order adsorption kinetics and k 2 is the rate constant at equilibrium in g·(mg·min) −1 . Both parameters -q e and k 2 are evaluated from the plot between t/q t and t as shown in Fig. 7 and are presented in Table 3 at various initial concentrations. The correlation coefficient R 2 for pseudo-second-order kinetic model is close to unity; this implies that adsorption phenomenon is best represented by second-order mechanism. Alike results for pseudo-second order kinetics are presented elsewhere [10]. To study the mechanism of intraparticle diffusion Weber and Morris model is used: where, K p is the intraparticle diffusion constant (mg·g −1 ·min −0.5 ) and 'c' is a constant (mg·g −1 ). Graph between q t versus t 0.5 produces a straight line as shown in Fig. 8. K p and 'c' are evaluated from slope and intercept, respectively, and are presented in Table 3. In the Weber and Morris model, 'c' represents boundary layer thickness [10,28] and higher value indicates diffusion through this layer is the rate controlling step. The value of 'c' is found to increase from 0.05 to 0.14 mg·g −1 for SAd and from 0.06 to 0.35 mg·g −1 in case of CAd with an increase in initial concentration from 10 to 100 mg·L −1 . This change in value is attributed to the internal mass transfer that becomes prominent at higher concentration and external mass transfer decreases subsequently [28]. Further, the values of K p at higher concentration is observed to be more than that at lower concentration. It lies in the range 0.29−0.82 and 0.09−0.14 mg·g −1 ·min −0.5 for CAd and SAd, respectively, for an increase in initial concentration from 10 to 100 mg·L −1 . It is also concluded that CAd offers larger boundary layer effect in comparison to SAd as clear from Table 4; dye molecules have to overcome this extra resistance before they actually get adsorbed on the surface of an adsorbent. Several thermodynamic parameters are determined from the basic relations as given below: where, ∆G°= ∆H°− T∆S°.
In above relations, K L is the distribution coefficient, R is the universal gas constant and T is the temperature in K. The values for H and S are evaluated from slope and intercept, respectively, from the graph between ln K L vs. 1/T. All the thermodynamic parameters evaluated from the above equations are presented in Table 4. Temperature increment boosts negative free energy change of both solid substrates. The experimental value for G changes its trend from positive at 303 K to negative values at 313 and 323 K as clear from Table 4. It implies that the process was non-spontaneous at lower temperature andbecame spontaneous at higher temperature.Higher temperature enhances reaction rate; hence, G shows a negative trend [31,32]. Again, H for CAd and SAd is found to be 100.26 and 120.16 kJ·mol −1 , respectively. The positive values indicate that the process is endothermic in nature with SAd showing higher value. This finding is in agreement with the above findings. For CAd and SAd, the value for S is observed to be 330.22 and 387.10 J·mol −1 ·K −1 , respectively. The positive values of S reflect an increase in randomness at solid-solution interface as well as affinity of adsorbents for the dye molecules.

Conclusions
The potential of two adsorbents -CAd and SAd has been explored in this work. The following are the observations:  Camelina and Sapindus seeds based adsorbents showed maximum adsorption for pH 6 at 30°C.  Increased initial concentration resulted in higher removal percentage of dye from water solution.  Both found to fit well in Langmuir model for isotherm.  Rate is governed by the pseudo-first order kinetic model for CAd and SAd.  Process spontaneity rose with temperature.  Camelina being the softer precursor facilitated more escalation of volatiles during treatment with acid than Sapindus.
 CAd proved better adsorption capacities in comparison to SAd. Hence, study illustrates the use of Camelina and Sapindus, for the production of adsorbents is quite promising as agricultural wastes are cheap, easily available and renewable sources.