Micropollutant removal by advanced oxidation processes

The use of pesticides in agriculture has been associated to high concentrations found in surface waters and ultimately to the tightening of drinking water regulations. Whilst traditional granular activated carbon filtration or ozone are effective barriers for the large majority of pesticides, new polar pesticides such as clopyralid or metaldehyde are not readily removed by such technologies. The use of advanced oxidation processes (AOPs) is suggested as an effective alternative for metaldehyde removal. Although metaldehyde’s reactivity towards •OH appears in the last third of an extensive list of various pesticides’ reactivity, it was still well removed by the two AOPs tested: UV/H2O2 and UV/TiO2 in pure systems at similar rates of degradation under optimised doses of 100 mg.L-1 of TiO2 and 8 mM of H2O2. The presence of scavengers in natural waters such as natural organic matter (NOM) and carbonate ions reduced the effectiveness of metaldehyde removal. Experiments were undertaken using model compounds in order to evaluate the influence of these scavengers by directly monitoring the competition for •OH between background organic matter and metaldehyde. It was found that the concentration of background scavengers rather than the concentration of micropollutant had the greatest impact on both AOPs. In addition, whilst the alkalinity did not significantly influence the UV/H2O2 process, it totally inhibited the UV/TiO2 photocatalytic process due to the aggregation of TiO2 particles and further investigations are critical in order to break these aggregates for UV/TiO2 photocatalysis to become a competitive alternative to traditional treatments. Finally, a cost analysis showed that AOPs already appear as an economically viable technology for metaldehyde removal. Development of UV/LEDs will provide a lower energy option which will be economically competitive within the next 7-8 years.