The role of natural organic matter (humic substances) on adsorption of pesticides possessing endocrine disruptor activity

In this paper an overview is provided of adsorption processes of representative persistent organic pollutants (POPs), in particular atrazine, alachlor and cis- and trans-chlordane pesticides with endocrine disruptor activity, by humic acids (HAs), the most abundant and chemically and biochemically active fractions of humified natural organic matter in soil. Endocrine disruptor compounds (EDCs) are hormone-like substances able to alter, i.e., disrupt, the normal endocrine functions in animals and humans. The estrogenic risk of EDCs is generally related to their distribution and speciation in the various soil phases, in which adsorption processes play a very important role. Adsorption kinetics of atrazine and adsorption isotherms of atrazine, alachlor and cis- and trans-chlordane onto HA samples isolated from various soil sets were determined using a batch equilibrium method and the HPLC technique with various detectors. Adsorption of atrazine onto HAs occurs rapidly within the first 4 h of contact. Experimental adsorption data were best fitted in a linear isotherm or a nonlinear, L-shaped Freundlich isotherm for atrazine, a linear isotherm for alachlor, and a nonlinear, S-shaped isotherm for cis- and trans-chlordane. Thus, no limiting adsorption is observed for any pesticide adsorbate onto any HA sorbent examined over the concentration range tested. In general, the adsorption capacity of HAs for chlordane is much larger than that for atrazine and alachlor, which are adsorbed at almost the same extent by HAs. The carboxylic and phenolic hydroxyl group content, the organic free radical concentration, and the aromaticity and humification degrees appear to be the most important chemical properties of HAs affecting atrazine adsorption, whereas it is not clear which compositional, structural and functional features of HAs may affect adsorption of alachlor and chlordane.