Long-term experiments to investigate irreversibility in sorption of pesticides to soil

Experiments investigated irreversibility in pesticide sorption to soil. Sorption behaviour under abiotic conditions was quantified for chlorotoluron, prometryn and hexaconazole in three soils over periods of up to 274 days. An isotope-exchange procedure was used whereby sorption of 12C- and 14C-pesticide isotopes in shaken suspensions of three soils (7-168 days shaking) was followed by substitution of the isotopes in the liquid phase and a 14-day exchange phase. This was followed by forced isotope exchange where the sorbed 14C material was desorbed by adding an excess of non-radiolabelled compound. Experiments were concluded with solvent extraction and soil combustion to determine remaining radioactivity. The pesticide-soil systems took around four months to approach sorption equilibrium, resulting in strong asymmetry between the profiles of exchange for isotopes of all three compounds. Under the steep concentration gradient of forced isotope exchange, physically entrapped residues were released back into solution and small amounts of radioactivity were still being released at the termination of the experiment. Results were shown not to deviate markedly from ideal behaviour based on the assumption that sorption is fully reversible. Whilst the timescales for release of sorbed residues back into solution were very long, soil combustion at study termination only yielded <1-2% of applied radioactivity; this confirms that sorption processes under abiotic soil conditions were overwhelmingly reversible for these compounds and set of soils.