Role of a non-ionic surfactant and carboxylic acids on the leaching of aged DDT residues in undisturbed soil columns

Abstract

Purpose: Irrigation or flooding events with riverine waters containing compounds such as surfactants, colloids, and dissolved organic carbon and plant growth will modify pesticide availability and leaching. This work studies the effect of a non-ionic surfactant (Tween 80: polyethylene glycol sorbitan monooleate, Tw-80), carboxylic acids (CA), and the role of dissolved organic matter on the lixiviation, availability, and degradation of aged DDT (p,p′-DDT, p,p′-DDE and p,p′-DDD) residues in soil. Materials and methods: Undisturbed soil columns were collected in a known polluted area and irrigated under saturation conditions with 4 L of riverine water (control), Tween 80 at 0.5 times the critical micellar concentration (Tw-80), and a 0.05 M solution of oxalic and citric acids (CA). After elution, columns were air-dried (24 h), opened, and divided into 5-cm sections for pesticide residue analysis in soil by GC-ECD. Batch desorption studies were conducted to assess pesticide availability before treatments. Metabolite ratios were analyzed in relation with the soil microbial activity measured by the dehydrogenase activity (DHA) assay. Soil organic matter and dissolved organic matter were analyzed, and humic substances were characterized in terms of humic and fulvic acid content by spectrophotometric ratios. Results and discussion: The p,p′-DDE>p,p′-DDT>p,p′-DDD pattern was found in all soil sections and treatments with decreased levels with depth. Levels of p,p′-DDE ranged between 8 and 280, between 8 and 80, and between 13 and 250 ng g −1 dry weight in control, Tw-80, and CA, respectively. Lixiviation of p,p′-DDE was enhanced in Tw-80 or CA. Compared to control, elution with Tw-80 diminished effectively soil DDT levels, while with CA results showed an enhancement of the transference to water. Higher (p,p′-DDE + p,p′-DDD)/p,p′-DDT ratio in Tw-80 or CA soils, with respect to control, were related with higher p,p′-DDD and DHA levels in CA and with lower p,p′-DDE and p,p′-DDT levels in Tw-80. Control and CA showed a positive correlation between p,p′-DDE and p,p′-DDT soil levels and desorption (r > 0.8, p < 0.02). Losses of fulvic acid content in soil after Tw-80 agree with the enrichment in fulvic acids in the desorption solution. Conclusions: The influence of Tw-80 and CA on the availability of DDTs was related with the mobilization of organic matter fractions. Results highlight the role of the non-ionic surfactant Tw-80 in the mobilization of fulvic acids as well as on DDT desorption from soil. Both solutions might interact differentially with pesticides and with the soil microbial population controlling the fate of DDT residues in the soil matrix.