Kinetic Approach for the Adsorption of Organophosphorous Pesticides from Aqueous Solution Using “ Waste ” Jute Fiber Carbon

Chemically activated “Waste” Jute Fiber carbon has been effectively used for the removal of five organophosphorous pesticides (malathion, monocrotophos, methylparathion, phosphamidon and dimethoate) from aqueous solutions. The prepared activated jute fiber carbon was characterized by using Elemental analyzer and proximate analysis methods. The adsorption equilibrium was examined at 28 oC. Three different kinetic models, the pseudo first order, pseudo second order and Elovich kinetic models were selected to analyses the adsorption process. To compare the fitness of pseudo first order and pseudo second order, sum of the squares of the errors and correlation coefficient, r values were calculated. The Elovich model was used to confirm the chemisorptions.


Introduction
A pesticide gives potential hazards to animals, humans and aquatic life 1.They also cause deleterious effect on soil fertility and crop productivity.They enter into cyclic environment process such as absorption by soil, leaching by water 2 .Pesticides are very dangerous and harmful because of their toxic and carcinogenic in nature 3 .Organophosphorous pesticides (OPPs) are frequently used to control the pest in the agricultural field.Common members of the family are methylparathion, malathion, dimethoate, phosphamidon, phorate, fenitrothion, monocrotophos etc.Therefore, the removal of OPPs from water is one of the major environmental concerns.Some treatment methods such as biodegraation 4 , extraction 5 , solidphase extraction 6 , photocatalysis 7 , photo-fenton 8 , flocculation 9 , ion-exchance 10 , chlorination and ozonisation 11 and adsorption have been used to remove toxic materials from aqueous solutions.
In the recent years, adsorption has become one of the most important processes for the removal of organic pollutant from water [12][13][14][15][16][17][18] .Currently, the use of new low cost agricultural by product materials such as tea leaves 19 , biomass 13 , chickpea husk 20 , olive stones 21 , cotton stalk 22 , rice bran, rice husk, bagasse fly ash 23,24 , date stone 25,26 , biowaste 27 , coconut tree male flower 28,29 and Jute fiber 28,30 is being considered as adsorbent for the removal of toxic chemicals from aqueous solutions.In this study, we prepared sulphuric acid activated "Waste" jute fiber carbon (JFC) was used as a low-cost material for the removal of five OPPs from aqueous solution.Recently, our research group successfully used "waste" jute fiber carbon (JFC) as adsorbent for the dye removal 30 .

Adsorbent
Sun dried "waste" Jute Fiber (JF) was carbonized using sulphuric acid (SA) in the weight ratio 4:3 (JF:SA) and kept in hot air oven, maintained at 160 o C for a period of 12 h.The carbon was then dried for 4 h at 100 o C. The resulting carbons were washed with distilled water until a constant pH of the slurry.The dried materials were used as adsorbent for the removal of OPPs.

Elemental and proximate analysis
Carbon / hydrogen / oxygen / nitrogen / sulfur (C/H/O/N/S) contents of our carbon were analyzed by using elemental analyzer (model CHNO-RAPID, Heraeus Co., Germany).For each analysis, the standard sample was first analyzed for checking the experimental error.All measurements were carried out in duplicate.The proximate analysis was made according to ASTM 3172-3175 standards and the results are given in Table 1.

Batch studies
In equilibrium experiments, a fixed amount of "Waste" Jute Fiber Carbon (JFC) (0.5 g/L) and fifty milliliters of different initial pesticides concentrations (10, 20, 30 and 40 mg/L) were taken in a 100 mL conical flasks.The flasks were agitated at room temperature (28 o C) at 120 rpm for a pre-determined time intervals using thermostatic rotary orbital shaker with an initial pH 6.57.At time t = 0 and equilibrium, the concentration of pesticides were measured using Shimadzu UV-Vis spectrophotometer (Shimadzu Model: UV 1601).

Adsorption kinetics
Pseudo first order equation is generally expressed as follows 31 , Where, k 1 is the pseudo first order rate constant.After integration by applying conditions, q t = 0 at t = 0 and q t = q t at t = t, then equation (1) becomes Where, q e is the amount adsorbed at equilibrium of pseudo second order equation.If the rate of adsorption is a second order mechanism, the pseudo second order kinetic rate equation ( 3) is expressed as 32   ) ( Where, k 2 is the pseudo second order rate constant.On integrating the equation ( 3)

Elovich model
The most interesting model to describe the activated carbon adsorption 33 is Elovich equation ( 5), ) exp( Where, 'a' and 'b' are constants.The constant 'a' is considered as the initial adsorption rate (mg/g min) and 'b' is related to the extent of surface coverage and activation energy for adsorptions (g/mg).(dq t / dt) → a as q t → 0 and given that q t = 0 at t = 0, the integrated form of equation ( 5) becomes, Where, t o = (1/ ab).If t >> t o , equation 6 is simplified as, Where, 'a' is the initial adsorption rate (mg/g), and the parameter 'b' is related to the extent of surface coverage.Sum of the squares of the errors, SSE are calculated using the expression 34 Where, N is the number of data points.

Effect of contact time and initial malathion concentration on adsorption
Adsorption of pesticides onto waste JFC at different concentrations (10, 20, 30 and 40 mg/L) with fixed adsorbent dose (0.025 g/50 mL) was carried out at room temperature (28 °C).The effects of pesticide initial concentration with contact time are shown in Figures 1-5, respectively for malathion, monocrotophos, methylparathion, phosphamidon and dimethoate.It shows that, the percent adsorption decrease with increase in the initial pesticide concentration.But the actual amount of pesticides adsorbed per unit mass of carbon increases with increase in pesticides concentration.The curves in the Figures 1-5 are smooth and continuous, suggesting the possible monolayer coverage of pesticides on JFC surface.Similar results were reported by various researchers for the adsorption of pesticides onto different adsorbents 23,30,35   The SSE values calculated from the plot of q t vs. ln(t+t o ) (Figures 16-20) for the Elovich model are lies within 0.930 -0.940, 0.1212-1.795,0.2854-0.6399,0.0632-0.148and 0.127-1.274for the adsorption of malathion, monocrotophos, methylparathion, phosphamidon and dimethoate onto JFC.The correlation coefficient r 2 , which are closer to one in the case of Elovich model.The agreement of the Elovich equation with experimental data may be explained as follows: The successful applications of the Elovich equation to heterogeneous catalyst surfaces helps to explain its success in predicting the sorption of all the pesticides studied on JFC 39,40 .The general explanations for this form of kinetic equation involves a variation of the energetic of chemisorption with the active sites are heterogeneous in JFC.This supports that the heterogeneous sorption mechanism likely to be responsible for the pesticides uptake.Elovich model basically supports chemisorption.

Conclusion
• The adsorption of the pesticide was carried with "Waste" Jute Fiber Carbon as an adsorbent.• The prepared activated carbon was analyzed by elemental and proximate method.
• From the effect of JFC dose study, adsorption increases with increase of JFC dose up to the equilibrium.malathion, monocrotophos and methylparathion adsorption onto JFC follows the first order kinetic model.The adsorption of phosphamidon and dimethoate onto JFC, which obey the pseudo second order kinetic model.• The adsorption was supported by Elovich model. ,

Table 1 .
Elemental and proximate analysis of JFC

Table 2 .
Comparison of correlation coefficients and SSE values obtained from different kinetic models