1. INTRODUCTION:
Textile and dyeing industry are important sources for the continuous pollution of the environment. The effluents of these industries are highly coloured and the disposal of these waste into environment can be extremely dangerous1. Presence of colour in the wastewater is one of the main problems in textile industries. From the environmental point of view, the removal of synthetic dyes is of great concern, because some dyes and their degradation products are carcinogenic and toxic2.
There are many varieties of dyes such as acidic, basic, reactive, azo, disperse, vat and metal complex dyes. All of these dyes are harmful when in contact with living tissues for a long time. Removing colour from waste water can be done via several methods namely chemical, biological and physical methods. Chemical methods use coagulation or flocculation combined with floatation and filtration, precipitation –flocculation, electro-floatation, electro kinetic coagulation and ozonisation to remove colour3. Biological treatment utilizes fungi, bacteria or other biomass (either dead or alive) and is widely accepted due to its economical advantage. Physical methods often applied are membrane filtration and adsorption techniques.
Recently, adsorption process has offered most economical and effective treatment method for removal of dyes. Most conventional adsorption systems use activated carbon which is expensive and necessitates regeneration, so there was a search for low cost materials to remove organic contaminates such as dyes from industrial effluents4. Since commercially available activated carbon is very expensive, now the research is focused on the use of low cost adsorbents derived from agricultural and wood wastes. A number of agricultural waste and by- products of cellulosic origin have been analyzed for their capacity to remove dyes from waste water, such as coir pith5 maize bran6, rice husk 7, orange peel8, lemon peel9, saw dust10, barley straw11, egg shell 12, sunflower stalks 13and pea nut hulls14.
In the present research work, the adsorption of Acid Violet 49 onto activated carbon prepared from Thevetia peruviana wood by carbonization with Phosphoric acid (TPAC) was investigated. The thermodynamic and kinetic parameters such as free energy changes (ΔG), enthalpy changes (ΔH), entropy changes (ΔS), pseudo first order rate constant (k1) and second order constant (k2) were calculated for the adsorption of Acid dye.
2. MATERIALS AND METHODS:
2.1 Preparation of adsorbent
Thevetia peruviana wood was used as precursor for the preparation of activated carbon. The wood cut into pieces of 2 cm to 3 cm size, dried in sunlight for 10 days. The dried material soaked in a boiling solution of 40 % H3PO4 for one hour and kept at room temperature for 24 hours. After 24 hours the wood material separated, air dried and carbonized in muffle furnace at 400°C. The carbonized material was powdered and activated in a muffle furnace at 800°C for a period of 10 minutes. Then the material was washed with plenty of water to remove residual acid, dried, sieved to a desired particle size and stored in a tight lid container for further adsorption studies. The characteristics of the activated carbon are studied as per the standard procedures 15, 16 and given in Table 1.
2.2 Preparation of adsorbate
The dye used in this study was Acid Violet 49 having molecular formula C39H40N3NaO6S2 (Mol.Wt: 733.87) with CI No. 42640. All the chemicals were reagent grade. The structure of the dye is given in Fig. 1. The stock solution of 1000mg/L of the AV49 dye was prepared using double distilled water. The experimental solutions were obtained by diluting the dye stock solutions in accurate proportions to different initial concentrations.
Fig. 1 .Structure of Acid violet 49
2.3 Batch mode adsorption experiments
The adsorption experiments were carried out by agitating 100 mg adsorbent with 200 mL of dye solutions of 25 to 100 mg/L concentration at 150rpm on an Orbital shaker (Universal make). The mixture was withdrawn at specified intervals, centrifuged using electrical centrifuge (Universal make) at 5000rpm for 20 minutes and unadsorbed supernatant liquid was analyzed for the residual dye concentration using Elico make UV Spectrophotometer (CI 73) at 548nm for AV49. The effect of pH was studied by adjusting the pH of the adsorptive solutions by using dilute HCl and NaOH solutions. The effect of temperature was studied at four different temperatures (30, 35, 40 and 45°C). All experiments were carried out in duplicate and the mean values are reported, where the maximum deviation was within 4%.
The amount of dye adsorbed on TPAC adsorbent was calculated from the following equation
V (1)
Where, qt (mg/g) is the amount of dye adsorbed at time t, C0 and Ce (mg/L) are the concentrations of dye at initial and equilibrium respectively. V (L) and W (g) are the volume of the solution and the mass of dry adsorbent used, respectively.
2.4 Desorption Studies
The regeneration of the adsorbent may make the treatment process economical. After centrifugation, the supernatant was separated and the adsorbent was separated and allowed to agitate with 100 mL of distilled water at different pH (2 – 11) above the equilibrium time of adsorption. The desorbed dye solution was separated by centrifugation and estimated as given in the adsorption studies17.
3. RESULTS AND DISCUSSION:
3.1 Characterization studies
Characteristics of carbon prepared from Thevetia Peruviana are presented in Table 1. The surface morphology of TP carbon visualized via Scanning Electron Microscope (SEM), (Make -Jeol, Model 6390LA) at 2000X magnification. Examination of SEM micrograph of the TPAC particles showed rough areas of the surface of the carbon. The characteristics (Table 1) showed that carbon had a high porosity and more bulk density.
Table 1: Physico – chemical characteristics of TPAC
|
S.No |
Properties |
Values |
|
1. 2. 3. 4. 5. 6. 7. |
pH Moisture Content,% Bulk density, g/mL Porosity,% Methylene Blue Value , mg/g Iodine Number, mg/g Surface Area,m2 /g |
6.68 12.2 0.41 55.43 420 825 862.394 |
3.2 Effect of agitation time and initial dye concentration
The variation in amount of AV 49 dye adsorbed with contact time at different time initial dye concentrations ranging from 25 to 100mg/L at 30° C is shown in Fig. 2. It was observed that the maximum amount of dye adsorption taking place within the contact time of 30 min and it reached equilibrium at 50 minutes. After that no significant change was observed in the extent of adsorption. The adsorption curves are single, smooth and continuous till the saturation of dye on the carbon surface. The adsorption capacity at equilibrium increased from 46.84 mg/g to 173.41 mg/g for AV49 with an increase in the initial concentrations from 25 to 100mg/L.This is due to the increase in availability of the dye molecules near adsorbent.
The percentage of dye removal decreased from 93.68 % to 86.71 % for AV49 while increasing the initial dye concentrations from 25 to 100 mg/L. Adsorption and initial dye concentration correlation is exponential in the case of TPAC. Similar behaviors were reported for the adsorption of acid dyes onto activated bleaching earth18 and the adsorption of acidic dye onto sawdust, polymerized sawdust and sawdust carbon-II19.
Fig. 2 . Effect of agitation time on the adsorption of AV49 on to TPAC at 30° C
3.3 Effect of temperature
Fig. 3. Effect of temperature on the adsorption of AV49 on to TPAC.
To observe the effect of temperature on the percentage of dye removal, experiments were carried out for four different temperatures (30, 35, 40 and 45°C) at an initial dye concentration of 50 mg/L and the normal pH of dye solution (4.3). Fig. 3 showed that the percentage removal of AV49 by TPAC increases from 90.18 % to 95.54% while increasing the temperature. This indicates that the sorption of AV49 on to TPAC is endothermic in nature.
3.4 Effect of pH
The percentage of dye adsorption decreases with increase in pH from 2 to 5 and remains constant thereafter. In the acidic range (when pH is 1 – 3), the positive surface charge of adsorbent increases and this would attract the negatively charged functional groups on the reactive dyes. When the pH is increased, the number of negatively charged sites increases and there will be competition between the negatively charged hydroxyl ions and anionic dye for the sorption sites and the adsorption rate get decreased.
3.5 Adsorption kinetics
Adsorption kinetics is quite significant as it decides the rate of the adsorption process. In this present study, the following kinetic models were applied for the experimental data.
Pseudo first –order kinetic model assumes that the rate of change of solute uptake with time is directly proportional to difference in saturation concentration and the amount of solid uptake with time. It was proposed by Lagergren20. The rate constant of adsorption is expressed as a first – order rate expression given as:
(2)
where qt and qe are the amount of dye adsorbed (mg/g) at contact time t (min) and at equilibrium, and k1 is the pseudo-first-order rate constant (min-1). After integrating with the boundary conditions at initial time (t = 0), qt = 0 and at any time (t >0), amount of dye adsorbed is qt and rearranging Eq. (2), the rate law for a pseudo- first-order reaction becomes:
log
(3)
The plot of log (qe-qt) versus t should give a straight line with slope of –k1/2.303 and intercept log qe which allows calculation of adsorption rate constant k1and equilibrium adsorption capacity qe. It was observed that the experimental data point does not fit a straight line. Calculated values of k1 and qe are summarized in Table 2. The pseudo first-order kinetic model of Lagergren does not fit well with the experimental data over the whole range of initial concentrations studied. Hence the adsorption of both AV49 onto TPAC does not follow the pseudo first-order model.
The pseudo second-order kinetic equation is expressed as
(4)
Where, k2 is the rate constant of pseudo second –order adsorption (g/mg min)
The initial adsorption rate, h, (mg/g min) is expressed as
h = (5)
and qe is the equilibrium adsorption capacity (mg/g).
When the pseudo second order kinetic is applicable, the plot of t/qt against t should give a linear relationship from which k2 and qe can be determined respectively from the intercept and slope of the plot. The correlation coefficients are higher than pseudo first order model. Further, the rate constant, k2 decreases with increase in initial dye concentration. It can be shown from Table 2 that the adsorption of AV 49 dye onto TPAC fitted well to the pseudo second order kinetic model than the pseudo first order kinetic model.
Adsorption is a multi step process involving transport of the solute molecules from the aqueous phase to the surface of the solid particulates followed by diffusion into the interior of the pores. Assuming that the rate is controlled by pore and intra particle diffusion, the amount adsorbed (qt) is proportional to the t1/2
+ I (6)
Where qt is the amount of dye adsorbed (mg/g) at time t (min) and I is the intercept (mg/g). kid and I values were obtained from the slopes and intercepts of the linear plot. Fig. 4 represents the plot of qt versus t1/2 for adsorption of AV 49 onto TPAC for the initial dye concentration of 25 to 100mg/L.
It seems that the plots are non linear for all the concentrations but careful observations shows the data points can be represented by double linear. In first straight line, the sudden increase in slope signifies that the dye molecules are transported to the external surface of the adsorbent through film diffusion. Intra -particle diffusion of dye molecules through pore is represented by second straight line21. The linear portion does not pass through the origin which concludes that the pore diffusion is the only rate determining step and not the film diffusion. The values of kid and r2 values are given in Table 2.
Fig. 4 . Intra particle diffusion plot for the adsorption AV 49 on to TPAC at 30° C
