Elimination of neonicotinoids by ozone-based advanced oxidation processes: Kinetics and performance in real water matrices

Abstract

The Decision 2018/840 included some neonicotinoid insecticides (thiamethoxam (TMX), imidacloprid (ICP), clothianidin (CTD), thiacloprid (TCP), and acetamiprid (AMP)) within the watch list of substances for Union-wide monitoring in the field of water policy due to their potential risk to human health and the environment. In this work, the study of the elimination of these five neonicotinoids present in waters by ozone-based oxidation treatments has been carried out. While AMP and TCP presented very low rate constants with ozone and were hardly degraded, CTD (ozone rate constant of 116 M−1 s−1 at pH ≥ 7) and TMX (ozone rate constant of 22.7 M−1 s−1 for the studied pH range) were partially degraded by ozone. Furthermore, ICP ozonation showed a great pH dependence, with increasing ozone rate constant values from 2.7 M−1 s−1 at pH 5 to 160 M−1 s−1 at pH 9. In addition, rate constants for the reaction of neonicotinoids with hydroxyl radicals (•OH) were determined to be in the range (2.0–4.5)·109 M−1 s−1. The specific contribution of direct ozonation and radical pathways to the overall neonicotinoid degradation was evaluated, being the indirect oxidation with •OH dominant over direct ozone reactions. The influence of real water constituents such as dissolved organic matter (DOM) and bicarbonate, both acting as radical scavengers, on the ozonation process was negative. The ozone dose and the specific ozone dose required to remove neonicotinoids from real water matrices were higher than 5 mg/L and 1 mg O3 mg DOC-1, respectively, typically applied in water treatment. However, the application of the ozone-based advanced oxidation processes O3/H2O2 and O3/activated carbon provided an appreciable improvement in the elimination of neonicotinoids compared to ozone alone. Therefore, ozone-based advanced oxidation processes are needed to sufficiently abate neonicotinoids in contaminated waters.