Removal of neonicotinoids present in secondary effluents by ferrate(VI)-based oxidation processes

Abstract Neonicotinoids are a new class of insecticides derived from nicotine. However, their persistence in the environment and potential harmful effects have raised some concerns and proposed several measures for remediation. The main five neonicotinoids mainly used are: thiamethoxam (THM), imidacloprid (IMI), clothianidin (CLO), thiacloprid (THC), and acetamiprid (ACE). The aim of this work was to study the reactivity of these neonicotinoids with ferrate (Fe(VI)), focused on the determination of the second-order rate constants and the effect of pH and the organic matter present, and to evaluate the improvements in neonicotinoid removal reached by the ferrate-based processes Fe(VI)/peroxymonosulfate and Fe(VI)/sulfite. The most reactive compound with Fe(VI) was THC, followed by CLO and THM. ACE did not significantly react with Fe(VI). The second-order rate constants obtained at different pH confirm this trend. The optimum pH for efficient removal of the compounds was around 8, considering the predominance of the most reactive species HFeO4- and the decreasing self-decomposition of Fe(VI) with pH. The oxidation of the selected pollutants in secondary effluents by single Fe(VI) was rather slow, and only THC could be efficiently removed. The additional presence of peroxymonosulfate (Fe(VI)/PMS system) slightly increased the removal of neonicotinoids due to the formation of hydroxyl and sulfate radicals, being both the main reactive oxygen species. Finally, the additional presence of sulfite (Fe(VI)/sulfite system) considerably increased the oxidation rate of selected pollutants, with enhanced formation of hydroxyl and, especially, sulfate radicals. Results suggest that Fe(VI)/sulfite system can be applied efficiently to remove all the neonicotinoids.