Yengkhom B Kumar, Neera Singh and Shashi B Singh
Adsorption-desorption of atrazine, metribuzin, alachlor and metolachlor on granular carbon were studied in a batch study. All herbicides were highly sorbed on the granular carbon (93.8-100%). Adsorption data were subjected to the Langmuir and the Freundlich adsorption isotherms, and fitted well to the Freundlich adsorption isotherm (r2>0.96). Except alachlor, where the isotherm was nearly linear, 1/n values for atrazine, metribuzin and metolachlor isotherms were <1, suggesting a decrease in herbicide adsorption with increasing concentration in the aqueous phase. The Freundlich adsorption parameter [Kf(1/n)] values were 6813.6, 2981.8, 1420.7 and 5661.4 for atrazine, metribuzin, metolachlor and alachlor, respectively. Thus, granular carbon showed the highest adsorption capacity for atrazine followed by alachlor, metribuzin and metolachlor. However, a desorption study suggests that, except alachlor (<3%), none of the herbicides were desorbed during three desorption cycles.
Although alachlor adsorption coefficients were higher than those of metribuzin and metolachlor, it was desorbed. The Freundlich constant Kf showed a positive correlation with solubility (S) while no correlation was observed with the octanol-water partition coefficient (Kow). Results of three sequential adsorption studies suggest that the order of herbicide adsorption was: atrazine>metribuzin>metolachlor>alachlor. This is in agreement with the desorption results, which suggested that granular carbon had poor retention capacity for alachlor. Among the herbicides used in this study, granular carbon has maximum retention capacity for atrazine.
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