Methidathion degradation in soil amended with biosolids and a cationic surfactant: use of different kinetic models

The influence of the addition of a municipal biosolid and the cationic surfactant tetradecyl trimethyl ammonium bromide (TDTMA) on the degradation of an organophosphorus insecticide, methidathion, in a soil was investigated. Before incubation, top soil samples were amended with 4% of biosolid (SB), TDTMA (SS) and both amendments (SBS). Two additional samples of top soil, unamended (Ss) and amended with the biosolid (SBs), were sterilised to check for abiotic degradation. The amendments did not markedly affect insecticide degradation in any sample, with minute changes attributed to small sorption differences. Measurements of the dehydrogenase activity (DHA) seem to support the hypothesis that methidathion degradation is carried out by soil micro-organisms. The presence of the biosolid increases methidathion persistence in the sterile samples. Experimental data were fitted to three mathematical equations: a first-order one (monophasic model), a biexponential model and the Hoerl function (biphasic models). Biphasic models better predict the results obtained in top soil amended or not, while for sterile samples all the proposed equations are good descriptors.