Nitrate and natural organic matter in aqueous solutions irradiated by simulated sunlight - Influence on the degradation of the pesticides dichlorpropand terbutylazine

By means of simulated sunlight, the influence of natural organic matter (NO M) on the photochemical degradation of halogenated pesticides in the absenc e and presence of nitrate as a precursor of the highly reactive OH radicals in aqueous solutions and freshwater was investigated. Solutions of dichlor prop or terbutylazine (a) in phosphate-buffered demineralized water contain ing nitrate and/or NOM and (b) in natural freshwaters were irradiated by a 1000-W Xe short-are lamp. The collimated beam was filtered using a combinat ion of optical filters (WG 320 and WG 295) to simulate the solar spectrum u nder summer midday conditions. In the absence of nitrate and NOM, the pesti cides were degraded photolytically by simulated sunlight. The degradation r ates depended on the absorption spectrum in the UVB range and the quantum y ield of the degradation. The photochemical degradation of the pesticides wa s faster in the presence of nitrate due to the sunlight-induced formation o f OH radicals. In the absence of nitrate, low concentrations of NOM of a br own-water lake accelerated the degradation due to the formation of reactive species by NOM. At higher concentrations of NOM, the inner filter effect o f NOM lowered the degradation rates. In the presence of: 50 mg 1(-1) nitrat e, NOM decreased the degradation rate significantly. In case the natural wa ter samples were used as a matrix for the experiments (nitrate concentratio ns between 2 mg l(-1) and 15 mg l(-1), DOC concentrations below 2.3 mg l(-1 )), NOM acted mainly as a radiation filter and as a scavenger of OH radical s. As a consequence, in most freshwater systems, the accelerating effect of NOM by the formation of reactive species is of minor importance compared t o the inner filter effect and to radical scavenging.