Efficient ultrasound-assisted piezo-photocatalytic degradation of diclofenac by ZnO

In the last years, the growth and aging of world population has led to a high consumption of drugs with negative consequences on the quality of surface waters, leading to pollution due to either residues or metabolites of the drugs themselves. Traditional wastewater treatment plants (WWTPS) show low removal efficiency towards numerous recalcitrant pollutants, such as pharmaceutical and personal care products, fire retardants, plasticizers, steroidal hormones, antibiotic resistance genes and many other groups of compounds, also known as contaminants of emerging concern (CECs). The European Union started to face up the question of CECs with the Directive 2013/39/EU, including new substance in the priority list of the Directive 2000/60/EC and fixing a watch list of 10 substances that have to be monitored to guarantee water quality (Decision 2015/495/EU, Decision 2018/840/EU) [1]. In order to overcome the disadvantages related to low efficiency of WWTPs, alternative post-treatment technologies have been developed. Among them, advanced oxidation processes represent a class of treatment methods that not only remove hazardous pollutants but also degrade them. The use of combined techniques can positively affect the efficiency of the process, speeding up the degradation and reducing noxious by-product formation. The combination of ultrasound with photocatalysis has been reported to enhance the formation of highly active radical species, promote mass transfer and contribute to continuously regenerating the surface of the photocatalyst. So far, TiO2 has been the most investigated photocatalyst for sonophotocatalytic water remediation. However, our recent results demonstrated that, when drinking water is used instead of ultrapure water, its photoactivity strongly decreases [2]. Piezoelectric-assisted photocatalysis represents a new concept in the realm of innovative pollutant degradation methods. By this approach a mechanical energy (e.g., ultrasonic vibration) enhances the separation of photoinduced charge carriers by piezoelectric effect [3]. In the present research we report our recent results in degradation of diclofenac, a non-steroidal and anti-inflammatory recalcitrant contaminant, both in ultrapure and simulated tap water by a piezo-enhanced sonophotocatalytic approach based on ZnO, demonstrating that the sonophotocatalysis has a positive effect in the mineralization process, leading to 70% of complete degradation of diclofenac. These promising experimental results promote this process as a good alternative to traditional degradation approaches for remediation of real water matrices. [1] O.J.E.U.L. (2018) 9–12. Directive, COMMISSION IMPLEMENTING DECISION (EU) 2018/840 of 5 June 2018 establishing a watch list of substances for Union-wide monitoring in the field of water policy pursuant to Directive 2008/105/EC of the European Parliament and of the Council and rep, Decision 2018/840/EU, (n.d.). https://eur-lex.europa.eu/eli/dec_impl/2018/840/oj. [2] D. Meroni, M. Jiménez-Salcedo, E. Falletta, B.M. Bresolin, C. Fai Kait, D.C. Boffito, C.L. Bianchi, C. Pirola, Ultrason. Sonochem. 67 (2020) 105123. https://doi.org/10.1016/j.ultsonch.2020.105123. [3] A. Marino, and R. Becker, Nature, 228 (1970) 473–474. https://doi.org/10.1038/228473a0