Low pressure UV/H2O2 treatment of the pesticides metaldehyde, clopyralid and mecoprop and matrix constituents in drinking water

Abstract

Advanced oxidation processes (AOPs) are increasingly being applied in water treatment to degrade recalcitrant micro-pollutants such as pesticides, pharmaceuticals, and endocrine-disrupting compounds. This research focused on the combination of low pressure ultraviolet light with hydrogen peroxide (LP-UV/H2O2) and determined, for the first time, (i) the degradation, kinetics and formation of reaction products of three pesticides (mecoprop, clopyralid, metaldehyde) selected because of their current relevance to the water sector and different chemical structures, (ii) the formation of nitrite and nitrated compounds in waters containing natural organic matter (NOM) and nitrate, and (iii) the mutagenicity of LP-UV/H2O2–treated waters, both synthetic and real waters obtained from a UK water treatment works applying LP-UV/H2O2 at full-scale. Based on the findings of this research, it is concluded that effective degradation of the selected recalcitrant pesticides can be achieved using practical UV fluences and H2O2 doses. The degradation and kinetic order was mecoprop > metaldehyde > clopyralid, with the latter being the most affected by the presence of a background matrix (NOM, nitrate, bromide), and their main reaction products were identified, with reaction mechanisms proposed. Nitrite formed from nitrate photolysis was directly proportional to nitrate concentration, H2O2 dose, pH and NOM presence. Three different types of NOM were considered, but the chemical composition of the NOM was not a significant factor in nitrite formation by LP-UV/H2O2 under the conditions tested. The regulatory limit (i.e. 0.1 μg/L NO2- in the EU) was exceeded in the presence of NOM only at the highest fluence tested (2000 mJ/cm2). Nitrophenol formation was observed and attributed to NOM-nitrate interactions. Mutagenic activity below the accepted level of concern was detected by the Ames II test in both synthetic and full-scale LP-UV/H2O2-treated waters. Overall therefore, the findings of this research support the use of LP-UV/H2O2 in water treatment applications.