ADSORPTION OF THE HERBICIDE ACIFLUORFEN ON HUMIC ACIDS

In this study, the sorptive capabilities of two humic acids for removing the acifluorfen from herbicide solutions were evaluated. Various experiments at constant pH (3.5, 5.0, 7.0) and constant humic acid concentration but variable acifluorfen concentration were used to prepare plots representative of the adsorption of the herbicide by humic acids. The amounts of acifluorfen adsorbed were dependent on pH, being considerably higher (up to 63%) at low pH, near the pK(a) of the herbicide (3.5). Amounts less than 30% were desorbed with water, suggesting that physical binding was partially involved in the acifluorfen-humic acids interaction. Adsorption and desorption data were fitted to the Langmuir and Freundlich isotherms and model parameters and correlation coefficients were calculated and interpreted. The adsorption by the two humic samples gave different non-linear isotherms which looked like the L- and S-type, thus indicating that humic acids from different sources may show a different affinity for the same substrate, probably due to their differences in chemical, physical and conformational properties.