Adsorption onto granular activated carbons of a mixture of pesticides and their metabolites at trace concentrations in groundwater
Résumé
The presence of pesticides (alachlor, metolachlor and atrazine) is a major concern for the production of drinking water. However, their main metabolites, namely oxanilic acid (OA) and ethanesulfonic acid (ESA) for alachlor and metolachlor, and desethyl-and deisopropyl-atrazine for atrazine, are also encountered in natural waters. The adsorption of these micropollutants onto two types of granular activated carbons under dynamic (fixed bed) conditions was studied. The performance of the adsorbers was evaluated under operating conditions comparable to those found for the production of drinkable water, i.e. inlet concentrations of 1.0 μg L−1 for each pesticide and metabolite added to groundwater containing 1.5 mgC L−1 of natural organic matter. Regardless of the location in the column or the duration of the treatment, humic acids were preferentially removed compared to fulvic acids. As for micropollutants, a pseudo-steady state was reached after filtration of approximately 2.0 m3 of water (6000 equivalent bed volumes). The retention of micropollutants onto granular activated carbons is driven by their adsorption kinetics. Moreover, in the case of alachlor and metolachlor, but not atrazine, different adsorption profiles were observed between the parent pesticide and its metabolites. Desethyl- and deisopropyl-atrazine, were adsorbed quickly; the mass transfer zone was narrow (approximately 60 cm) with a superficial velocity of the liquid phase equal to 10mh−1. By contrast, the OA and ESA metabolites reached breakthrough when the mass transfer zones were larger than the height of the columns. The Amundson model was used to determine the adsorption rate and adsorption capacity each pollutant.
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