Removal of brilliant green (BG) from aqueous solution by using low cost biomass Salix alba leaves (SAL): thermodynamic and kinetic studies

The removal of brilliant green dye (BGD) from aqueous solution by using Salix alba leaves (SAL) was carried out via batch studies. The maximum removal ef ﬁ ciency was found to be 95.2% with initial dye concentration 50 mg/L at 0.15 g adsorbent dosage, pH ¼ 6, and 298 K temperature, and the equilibrium was observed within 3½ hours. The adsorption capacity increased (2.21 – 15.89 mg/g) from 10 to 50 mg/L of dye concentration. Kinetic and isotherm studies were also carried out. The results showed that pseudo-second order model better describes the adsorption mechanism. The isotherm equilibrium data analysis was carried out by using Freundlich and Langmuir models and the sorption process was observed to conform with the Langmuir isotherm with linear correlation coef ﬁ cient (R 2 ¼ 0.99). The thermodynamic properties Δ G (cid:2) , Δ H (cid:2) , and Δ S (cid:2) delineated that BGD adsorption over SAL was feasible, spontaneous, and endothermic between 303 and 323 K temperature.


INTRODUCTION
Dyes are color organic compounds which are discharged in sewage water of many industries, such as paper, fabrics, leather, cosmetics, and printing. The long-time adverse environmental effects caused by discharging these dyes change water color, sunlight permeation, and are unpleasant for drinking and domestic uses. Annually, a huge amount of dyes and pigments (over 7 × 10 5 tons) are being produced throughout the world (Machado et al. ) and 90% of this production is used in the textile industry (Mane & Babu ). Wastewater discharge increases with the development of printing, textile, and tanning industries (Nandi et al. ). The textile industry is responsible for twothirds of the production of total dye stuff and effluent discharges (10-15%) of the used dyes in water bodies (Mittal et al. ). The treatment of dye wastewaters is difficult due to their organic molecular composition, dye resistance to aerobic digestion, stability to oxidizing agents, heat and light (Ertugrul et  methylene blue, brilliant green, and methyl orange. The most widely used adsorbent for dye removal is activated carbon, but its use is restricted due to higher cost and regeneration problems (Mall et al. ). Nowadays scientists are focusing on low cost nonconventional and efficient adsorbents as an alternative for costly adsorbents. Some of these include bagasse fly ash (Aroguz et al. ), powdered peanut (Gong et al. ), raw pine cone (Dawood & Sen ), and palm kernel coat (Oladoja & Akinlabi ).
Brilliant green dye (BGD) appears as a golden crystal and organic dye that belongs to the triphenylmethane family. It has many uses, for example, dermatological agent, biological stain, veterinary medicine, to inhibit mold propagation in poultry feed, fungus, and intestinal parasites (Nandi et al. ). Brilliant green .comis toxic when injected into humans and animals. Common harmful effects of BG on humans are irritation to the gastrointestinal tract with long-time exposure resulting in organ damage (Mittal et al. ). As a result of its decomposition, sulphur oxides, nitrogen oxides, and carbon dioxide are produced which further pollute the environment (Kismir & Aroguzet 2011).
In this study, Salix alba leaf (SAL) powder has been investigated as an adsorbent for BGD removal. SAL is an easily and freely available biomass. SAL has not been used for adsorption of BG previously. However, in this work, batch adsorption studies have been used for the removal of BGD from aqueous solution. Different parameters such as the effect of temperature, contact time, effect of pH, initial dye concentration, and adsorbent dosage have been studied for efficient removal of dye from wastewater. The kinetic and thermodynamic mechanism of adsorption has also been studied.

Preparation of stock solution
Brilliant green (C 27 H 34 N 2 O 4 S) by BDH Chemicals was used as received. The standard solution of BG (1,000 mg/L) was prepared by mixing 1 g of BG per 1,000 mL of distilled water. Further dilution of the mixture was carried out to prepare 10-50 mg/L solutions.

Preparation of adsorbent
Locally collected Salix alba leaves from the village Rairban, Bagh Azad Jammu and Kashmir, Pakistan were washed with deionized H 2 O to eliminate dirt and dried in a thermostatically controlled oven at 100 C for 24 hours. The desiccated biomass was crushed and ground with a mortar and pestle then passed through sieves to obtain particles of 70 mesh size.

Characterization of Salix alba leaves
The SAL powder was characterized by different techniques such as Fourier transform infrared (FTIR) spectrophotometer and UV-vis-spectrophotometer. A scanning electron microscope (SEM) was applied for the delineation of surface morphology. TGA/DSC studies were done by using instrument universal V 4.5A TA, SDT Q600 V 20.9. The surface area of the SAL powder was delineated by employing Quatachrome Novawin V. 11.04. The bulk density and moisture contents were also calculated and the acquired results are presented in Table 1.

Desorption
Desorption studies were also carried out for the determination of regeneration of the used adsorbent. The experiments were conducted using 0.1 molar solutions of desorbing agents (distilled water, KCl, NaOH, and HCl) for the used SAL regeneration possibility. The BG solution (50 mL of 50 mg/L) was treated with 0.15 g of SAL for 3½ hours. The dye loaded adsorbent was shaken with 25 mL of desorbing agents for 5 hours. The desorption percentage was calculated by employing a relation: where m a (mg/L) denotes adsorbed and m d (mg/L) desorbed amounts of BG.

Batch equilibrium and kinetic studies
The batch studies were carried out by preparation of 50 mL dye solution in 250 mL conical flasks with 0.15 g of the adsorbent (SAL). The three flasks were agitated and heated at 303, 313, and 323 K, repectively. The samples were filtered and the absorbance of the filtrate was determined with UV-Vis spectrophotometer at ƛ max 625 nm for brilliant green. The samples' equilibrium concentrations were determined by the application of the Beer-Lambert law while adsorption capacity was found using the following equation: where m denotes SAL mass in (g). C o initial, C e initial, and equilibrium concentrations of BG (mg/L) and V designates volume of solution (L). The removal efficiency was found using a relation given below (Ahmad & Kumar ): The data analysis was carried out by employing the isotherm adsorption models (Langmuir and Freundlich) using the relation: where Q o is the adsorption capacity.
The dimensionless factor of Langmuir isotherm is expressed as: where C o (mg/L) denotes the initial concentration of adsorbate (BG). The R L value is helpful in the determination of the nature of the adsorption process. The process is favor- for determination of heterogeneity of system using an empirical isotherm: where K F (mg/g (L/mg) 1/n ) and n in the above equation are Freundlich constants. The thermodynamic studies were carried out for determination of the nature of the adsorption process by using various mathematical relations such as entropy change (ΔS J/K mol), free energy change (ΔG kJ/mol), and enthalpy change (ΔH kJ/mol) (Equations (7) and (8)); In the above equation, equilibrium constant is K c , R stands for gas constant (8.314 J/K mol) and T (Kelvin).

RESULTS AND DISCUSSION
Characterization of Salix alba leaves

FTIR analysis
The FTIR analysis was carried out from 4,000 and 500 cm À1 using FTIR spectrophotometer (Perkin Elmer spectrum 100 series) to investigate surface functional groups. The prominent peaks before adsorption spectra at 3,417 cm À1 is due to stretching vibrations of amine and water on the adsorbent surface, the peak observed at 1,620 cm À1 represents the presence of aromatic C ¼ C bendings, the prominent peak at 1,041 cm À1 is due to carboxylic acids, C-O stretching of alcohols, esters, and ethers present on the adsorbent surface.
The stretching vibrations of C-S appeared at 611 cm À1 ( Figure 1(a)). After the adsorption of BG on the SAL (adsorbent) surface, significant changes in the frequencies were observed at 1,652 and 1,382 cm À1 (Figure 1

Effect of preliminary dye concentration
The dye uptake capacity also affected initial dye concentration. The effect of preliminary dye concentration was

Effect of pH
The pH of the solution affects greatly the BG percentage removal of SAL, as shown in Figure 6.  (Tien ). The surface of SAL may become negatively charged, due to which, cationic species increase the adsorption through electrostatic attraction, thus resulting in increased adsorption. Hence, the dye uptake capacity (15.897 mg/g) was obtained at pH ¼ 6 and then started to decline at pH ¼ 13.

Effect of temperature
The effect of temperature on the process of adsorption was carried out by taking 50 mg/L initial dye concentration,

Adsorption thermodynamics
The adsorption process can be characterized by thermodynamic parameters such as enthalpy change (ΔH kJ/mol), change in free energy (ΔG kJ/mol), and entropy change (ΔS J/mol). These parameters can be calculated by  Equations (9) and (10): In Equation (9) The plot of lnK versus 1/T (Figure 8) was used for the determination of ΔH (kJ/mol) and ΔS (J/mol) by using Equation (10): The positive ΔH (kJ/mol) (56.634) indicated that adsorption of BG onto SAL is an endothermic process.
The negative values of ΔG (kJ/mol) in Table 2

Equilibrium studies
The equilibrium isotherm describes the amount of dye where Q o is monolayer formation adsorbate amount, C e is equilibrium concentration (mg L -1 ), Q e is adsorbed amount (mg) per adsorbent amount (g), and b is Langmuir constant. Hence, the straight-line plot of C e versus C e /q e ( Figure 9) is obtained having slope (1/Q e ) and an intercept (1/Q e b); the calculated values are presented in Table 3.   The dimensionless factor of the Langmuir isotherm is expressed as: where C o (mg/L) denotes the adsorbate (BG) initial concentration. The R L value is helpful in determination of the nature of the adsorption process. The process is favorable if The heterogeneous or non-ideal adsorption is determined by the Freundlich model. The model is applicable for a multilayer adsorption system (Ghaedi et al. ).
This model is expressed by the relation: where n and K F are empirical constant and Freundlich equilibrium constant, respectively. As such, the straight-line plot of ln Ce versus ln qe (Figure 10) is drawn where 1/n is slope and lnK intercept. The parameters determined for SAL adsorption are shown in Table 3 (Haghseresht & Lu 1998). In the Freundlich isotherm n represents the sorption intensity generally and n > 1 shows the favorable adsorption of adsorbate on adsorbent. Adsorption intensity increases with the increase in n value.

Adsorption kinetics
The mechanism and rate of adsorption process can be found by three kinetic models, namely, intraparticle diffusion, pseudo-first order model (PFO), and pseudo-second order model (PSO). Lagergren () proposed the PFO and it is written as: log (q e À q t ) ¼ log q e À K 1 2:303 t (14) (1/min). The graph ( Figure 11) of ln(qe À qt) versus t is used to calculate the k 1 given in Table 4. The PFO model has a linear relation as shown below (Ghaedi et al. ): The slope and intercept obtained from t versus t/q t ( Figure 12) are utilized for the determination of PSO model to determine the PSO rate constant (k 2 ; g/mg min) and q e (adsorption capacity) (Figures 11 and 12). Adsorption kinetics of BG by SAL were calculated at different conditions from respective plots as shown in

Error analysis
The statistical analysis, sum of squared error (SSE) was employed for error analysis. The following relation was used for error analysis: (q e calc À q e meas ) 2 The q e (meas) and q e (calc) in the aforementioned equation are the values of experimental and calculated adsorption capacity (mg/g), respectively. The low values of SSE demarcate best match with experimental data (Tables 3 and 4).
The comparison of q max with different adsorbent is also given in Table 5.

Intraparticle diffusion
The Weber and Moris plot was used for the determination of the intraparticle diffusion process (Weber & Morris 1963).  The relation is expressed as: where C is the intercept which can be calculated from the slope of the linear plot of t ½ against qt and k i (mg/g) denotes the diffusion intraparticle rate constant (Azubuike & Okhamafe ). According to this model, the involvement of diffusion in the adsorption mechanism, the plot between qt and t 1/2 (Figure 13), should be linear. The process will be rate controlling when the lines cross through the origin. The slope in a linear portion of the plots from Figure 13 is used for calculation of intraparticle rate constant and intercept (C) ( Table 3). The mass transfer rate difference between two stages (initial and final) of the adsorption process deviated the straight lines from the origin ( Figure 13).
Moreover, this deviation also indicated that the pore diffusion is not a sole rate step.

Desorption
Desorption studies were also carried out for the determination of regeneration of the used adsorbent. The experiments were conducted using various kinds of molar desorbing agents (distilled water, KCl, NAOH, and HCl) for used SAL regeneration possibility. The maximum desorption (53.10%) was observed with 0.1 M KCl. The percentage desorption of different reagents of BG loaded SAL powder is presented in Table 6.

CONCLUSIONS
The current study shows that low-cost adsorbent SAL (biomass) can be effectively used for the removal of brilliant green from its aqueous solutions. The removal efficiency of Salix alba increased as the contact time, initial dye concentration, adsorbent dosage, and temperature increased. The adsorption capacity of adsorbent was found to be increased with the rise in initial dye concentration, contact time, and temperature, but with the increasing adsorbent dosage, a decrease was observed. The adsorption kinetics results showed a good correlation coefficient R 2 > 0.98 with pseudo-second order kinetics. Equilibrium isotherm results conformed well with the Langmuir isotherm equation. The adsorption capacity (13.71 mg/g) of BG over SAL confirmed the monolayer adsorption capacity. The dimensionless separation factor (R L ) also denotes SAL adsorbent capacity for the BG removal from aqueous solutions. Thermodynamic parameters such as ΔH , ΔS , and ΔG of adsorption were also calculated and indicated the BG adsorption process over SAL is spontaneous and physical in nature. The adsorbent (SAL) is very cost-effective and easily available in large amounts. This adsorbent gives