Sorption of organic compounds to humin from soils irrigated with reclaimed wastewater

Humin is an important fraction of soil organic matter that is known to tightly bind many organic compounds. The objective of this study was to evaluate the role of humin as a natural sorbent in soils irrigated with freshwater and treated wastewater. To meet this goal, the sorption-desorption behavior of phenanthrene, atrazine and chlorotoluron was studied with humin isolated from Basra and Akko soils sampled from plots irrigated with treated wastewater or freshwater. For the Basra samples, the sorption affinities (K_OC values) of all tested compounds were higher for the humin sample isolated from the freshwater-irrigated soil than for that from the wastewater-irrigated soil. The opposite trend was recorded for the Akko humin samples. Desorption hysteresis was observed with atrazine but not with chlorotoluron or phenanthrene. Lipid extraction significantly decreased phenanthrene sorption affinities (K_OC values) for both Basra humin samples. For the humin isolated from the Akko freshwater-irrigated sample, lipid removal significantly decreased the sorption linearity, but sorption affinity was less affected. For the humin isolated from the Akko wastewater-irrigated sample, lipid extraction did not affect sorption affinity or linearity. Our data suggest that humin lipids constitute a powerful sorption domain and probably block pores or other high-energy adsorption sites within the humin matrix; therefore, their removal promotes desorption hysteresis. Our data show significantly small differences in sorption affinity between the humin and the source soil samples. We therefore suggest that the role of humin as a natural sorbent in soils should be studied on humin samples isolated under mild conditions to avoid the removal of matrix components and alteration of the humin.