A review of ecotoxicity reduction in contaminated waters by heterogeneous photocatalytic ozonation.

The widespread deterioration of our water systems requires new wastewater treatment technologies to ensure environmental protection. Conventional wastewater treatments were not designed for, and are therefore ineffective, at removing contaminants of emerging concern (CECs) such as pharmaceuticals, personal care products, pesticides, and industrial chemicals. Furthermore, treatment processes capable of breaking down CECs may produce toxic transformation products more harmful than the parent chemicals. Heterogeneous photocatalytic ozonation provides a promising option with high degradation and mineralization of organic compounds. The aim of the present paper is to review ecotoxicity reduction in water treated by heterogeneous photocatalytic ozonation as a measure of process viability. The discussion investigates changes in toxicity based on a variety of toxicity tests performed to evaluate potential effects on ecosystems, the types of catalysts and radiation sources used, the nature of the target contaminants, and the type of water matrix treated. Acute toxicity testing, TiO2 catalysts, and mercury-vapour lamps including blacklights were dominant in the reviewed studies, investigated in 86%, 84% and 79% of the papers, respectively. Pharmaceuticals were the main group of chemicals treated and the water matrices used were predominantly pure water and secondary effluent. Overall, the findings of these studies provide evidence that photocatalytic ozonation is an efficient process to remove persistent organic compounds while, most of the time, not increasing the toxicity of the effluent (as reported by 86% of the studies). Due to the wide variation in experimental set-ups, no clear correlation between reaction conditions and toxicity was determined, however, V. fischeri acute toxicity assays and chronic/sublethal tests appeared most sensitive to transformation products. Future studies need to a) incorporate multiple toxicity tests to produce a more reliable and inclusive ecotoxicity assessment of treated effluent and b) investigate immobilized catalysts and energy efficient radiation sources (i.e. solar and LEDs) for industrial applications.