Removal of Direct Yellow-12 Dye from Water by Adsorption on Activated Carbon Prepared from Ficus Racemosa L

The adsorption of direct yellow-12 dye (DY-12) by Atti leaf (Ficus racemosa) powder carbon (ATC) was carried out by varying the parameters such as agitation time, dye concentration, adsorbent dose, pH and temperature. Equilibrium adsorption data followed both Langmuir and Freundlich isotherms. Adsorption followed second-order kinetics. The adsorption capacity was found to be 6.7 mg dye per gram of the adsorbent. Acidic pH was favorable for the adsorption of DY-12. Desorption studies suggest that chemisorptions might be the major mode of adsorption.


Introduction
In developing countries like India, industries cannot afford to use conventional wastewater treatment chemicals like alum, ferric chloride, polymer flocculants and coal based activated carbon because they are not cost-effective.An inexpensive and more easily available adsorbent would make the removal of pollutants an economically viable alternative.As such, there has been a great deal of research in finding cost-effective methods for the removal of contaminants from wastewater.Interest in this area is exemplified by publication of several reviews worldwide [1][2][3][4][5][6][7] .
An economic sorbent is defined as one which is abundant in nature or is a by-product or waste from industry and requires little processing 8 .In recent past, considerable attention has been devoted to the study of removal of dyes from wastewater by adsorption using agricultural products and by-products [9][10][11] .Natural materials that are available in large quantities from agricultural operations may have potential to be used as low cost adsorbents, widely available and are environmental friendly 12 .The present study deals with the adsorption efficiency of Atti tree leaf powder carbon (ATC) for the removal of direct yellow-12 (DY-12) dye from aqueous solution.DY-12 is commonly used in textile and leather industries.
The Atti tree is a plant belonging to the family Moraceae (Ficus racemosa L.).It is an evergreen plant, which is extensively grown in India.The Atti tree leaves are agricultural waste which is arbitrarily discarded or set on fire.
The dye selected as sorbate was direct yellow-12 (DY-12) (λ max = 389).The effects of various operating parameters on adsorption such as sorbent dosage, particle size, contact time, initial dye concentration and pH were monitored and optimal experimental conditions were decided.
The properties of adsorbate and adsorbents are quite specific and depend upon their constituents.The constituents of adsorbents are mainly responsible for the removal of any particular pollutants from wastewater 13 .

Experimental
The dried Atti leaf (2.0 kg) was added in small portions to sulphuric acid (98%, 2.5 L) for 2 h followed by boiling for 3 h in a fume hood.It was cooled in an ice bath and the reaction mixture was poured onto cold water (8 L) and filtered.The obtained carbon was dried in an open oven at 180 °C for 2 h followed by immersed in 5% sodium bicarbonate (5.0 L) to remove any remaining acid and then filtered.The obtained carbon was then washed with distilled water until the pH of the activated carbon reached 6; it was dried at 150 °C for 3 h and sieved to the particle size ≤ 0.200 mm and kept in a glass bottle until used.

Preparation of dye solution
The dye direct yellow-12 (DY-12); 98% purity; molecular formula is C 30 H 26 N 4 Na 2 O 8 S 2 , C.I. No. 24895; molecular weight 680.66, (obtained from Sigma-Aldrich, Bangalore, India), in commercial purity, was used without further purification.The dye stock solutions were prepared by dissolving accurately weighed dye in distilled water to the concentration of 1000 mg/L.The experimental solutions were obtained by diluting the stock solution in accurate proportions to different initial concentrations.

Methods and measurements
Adsorption experiments were carried out in a rotary shaker at 150 rpm at 27 °C using 250 mL shaking flasks containing 100 mL different concentrations and initial pH values of dye solutions.The initial pH values of the solutions were previously adjusted with 0.1M HNO 3 or NaOH using pH meter.Different doses of sorbent were added to each flask.After shaking the flasks for predetermined time intervals, the samples were withdrawn from the flasks and the dye solutions were separated from the sorbent by filtration then centrifugation.Dye concentrations in the supernatant solutions were estimated by measuring absorbance at maximum wavelengths of dyes with a 752W Grating Spectrophotometer (Avatar, India) and computing from the calibration curves.The experiments were conducted in duplicate and the negative controls (with no sorbent) were simultaneously carried out to ensure that adsorption was by Atti leaves powder carbon (ATC) and not by the container.

Effect of sorbent dose
The effects of sorbent dose on the removal ratios of dyes were shown in Figure 1 and Table 1.The percentages of dyes sorbed increased as the sorbent dose was increased over the range 0.1-1.5 g/L.The adsorption ratios of dyes increased from 23.21 to 94.11% DY-12 dye.Above 1.0 g/L of sorbent dose, the adsorption equilibrium of dye was reached and the removal ratio of dye has almost no variation.So, the ATC of 1.0 g/L was chosen for subsequent experiments.

Effect of equilibration period
The graphical representation of the dependence of the retained amount of dye versus time is given by single and continuous curves, which proves the formation of a monolayer at the external surface of the sorbent (Table 2 & Figure 2).For DY-12 the equilibrium is attained in 120-210 min.It was observed that depending on equilibration period, the dye is retained after various time of contact with the sorbent.

Effect of initial pH
An important influencing factor for dye biosorption on agricultural by-products has been referred to pH as in most studies published in the literature 14 .The effects of initial pH on adsorption percentages of dyes were studied over a range of pH values from 2 to 10.
As elucidated in Table 3 & Figure 3, for the dye DY-12, the dye removal ratio was high at the initial pH 2. The ratio of dye sorbed increased as the initial pH was increased from 2 to 7, then the dye removal ratios were not significantly altered beyond pH 8.
From the Figure 3, it was observed that in the pH range of 7-8, the activated carbon adsorbs to the extent of 94.45 % of the colouring matter.Hence in all the studies an optimum pH of 7.0 was used.

Adsorption isotherm
Adsorption isotherms are prerequisites to understand the nature of the interaction between adsorbate and the adsorbent used for the removal of organic pollutants 15 .In the present study, the adsorption of DY-12 dye on ATC (Table 4 & Figure 4) shows that the adsorption of the dye increases with increase in dye concentration and tends to attain almost saturation at higher concentrations.The experimental data have also been analyzed by two well-known adsorption isotherm models Langmuir (Eq 1) and Freundlich (Eq 2).
C e /q e = (1/K L ) + (a L C e /K L ) (1) Where C e and q e are the equilibrium dye concentration in the solution (mg/L) and on sorbent (mg/g), respectively, a L and K L are Langmuir constants.
Figure 5 shows that adsorption of ATC fits well to the Langmuir isotherm model with a correlation coefficient of 0.998, supporting monolayer coverage of the adsorbate on the surface of adsorbent.The theoretical monolayer saturation capacities of ATC have been calculated to be 33.7 mg/g (0.051 mmol/g).This value is higher to that of other biosorbents such as GAC 16 , coir pith 17 , PAC 16 , banana peel and orange peel 18 .However, adsorption capacity of neem leaf powder 19,20, and waste coir pith 21 respectively towards DY-12 is higher to a small extent.The data was also analyzed by the linearized form of Freundlich isotherm model Where, C e and q e are the equilibrium concentration of dyes in the solution (mg/L) and on ATC (mg/g), respectively.K F is the Freundlich constant and 1/n is the heterogeneity factor.Figure 6 shows that Freundlich model also fits for DY-12 dye (correlation coefficient 0.997) adsorption process.

Adsorption kinetics
The rate as well as mechanism of adsorption process can be elucidated on the basis of kinetic study.

Influence of initial dye concentration
The initial concentration provides an important driving force to overcome all mass transfer resistance of dye anions between the aqueous and solid phases.In addition, increasing initial dye concentration increases the number of interactions between dye anions and sorbent, which enhances the sorption process.Hence a higher initial concentration of DY-12 dye will increase the biosorption rate.The effect of initial dye concentration on the dye sorption capacity of dried ATC was investigated between 30 and 60 mgL -1 (Table 5-8 and Figure 7-9).When the dye concentration was increased from 50 to 500 mg/L, the percentage of dye sorbed decreased.The sorption is very fast initially.It was observed that depending on concentration, the dye is retained after 20 min.time of contact with the sorbent.Table 5. Kinetic data (Initial dye concentration (q 0 ) = 30 ppm; O.D. for initial dye concentration = 0.214; equilibration dye concentration (q e ) = 0.007; q 0 -q e = 30.00-0.007= 29.993)Contact time, min (t) Final O.D. q t q 0 -q t log q 0 -q t t/qt log t  6. Kinetic data (Initial dye concentration (q 0 ) = 40 ppm; O.D. for initial dye concentration = 0.227; equilibration dye concentration (q e ) = 0.008; q 0 -q e = 40.00-0.008 = 39.992)Contact time, min (t) Final O.D. q t q 0 -q t log q 0 -q t t/q t log t Kinetic data (Initial dye concentration (q 0 ) = 50 ppm; O.D. for initial dye concentration = 0.312; equilibration dye concentration (q e ) = 0.010; q 0 -q e = 50.00-0.010 = 49.990)Contact time, min (t) Final O.D. q t q 0 -q t log q 0 -q t t/q t log t  8. Kinetic data (Initial dye concentration (q 0 ) = 60 ppm; O.D. for initial dye concentration = 0.384; equilibration dye concentration (q e ) = 0.011; q 0 -q e = 60.00-0.011= 59.989)Contact time, min Final O.D. q t q 0 -q t log q 0 -q t t/q t log t Dye adsorption on solid surface may be explained by two distinct mechanisms; (i) an initial rapid binding of dye molecules on the adsorbent surface followed by (ii) relatively slow intra-particle diffusion.
Progress of the adsorption of dye by sorbent with agitation time (Table 6-8 & Figure 7) shows that the rate of adsorption of the dye molecules was initially very fast and then gradually slows down to reach equilibrium within 1 h.

Figure 7. Elovich plot -Effect of contact time on DY-12 dye
To determine the rate controlling and mass transfer mechanism, experimental kinetic data were correlated to linear form of the pseudo first order (Eq.3) and second-order (Eq.4) rate models.k 1 t log (q e -q t ) = log q e -2.303 Where, q e is the equilibrium concentration, t is the time, k 1 and k 2 are the first-and second-order rate constants, respectively; q t is the uptake per gram of ATC activated carbon at time t.
On the other hand, linearity (Figure 8 & 9) with high correlation coefficients (0.999) indicates that the present sorption process follows second-order rate model.Transportation of the dyes from the solution phase into the pores of the adsorbent may also be considered as the rate controlling stage in batch experiments under rapid stirring condition

Conclusion
This study showed that Atti tree leaf powder carbon particle (ATC) could effectively remove DY-12 dye from aqueous solution.The optimal pH for favorable adsorption of dyes was 7 and above.The percentages of dyes sorbed increased then reached maximum values as

Figure 1 .
Figure 1.Effect of sorbent dose on adsorption of DY-12

Figure 5 .Figure 6 .
Figure 5. Langmuir plots for the adsorption of DY-12Langmuir equation can also be used to obtain, R L , the dimensionless equilibrium parameter or the separation factor from the expression: R L = 1/ (1+a L C o ) Where C 0 is the initial concentration of the adsorbate and a L is the Langmuir coefficient as described above.The value of R L has been determined to be 0.050 for DY-12, for an initial dye concentration of 250 mg/L indicating that the adsorption process is favourable.

Figure 8 .Fig. 9 .
Figure 8. Pseudo first order plot at different initial concentration of DY-12 dye was increased.The adsorption equilibrium was reached at about 12 h.The isothermal data fitted the Langmuir model and Freundlich model.The adsorption processes followed the pseudo-first-order rate kinetics.