Sonochemical degradation of diclofenac: byproduct assessment, reaction mechanisms and environmental considerations

Abstract

The study covers a thorough assessment of the overall degradation of diclofenac-Na (DCF) by high-frequency ultrasound, focusing particularly on identification, interpretation, and characterization of the oxidation byproducts and their reaction mechanisms. It was found that sonication of 5 mg L(-1) DCF at near neutral pH rendered complete conversion of the compound, 45 % carbon, 30 % chlorine, and 25 % nitrogen mineralization. Density functional theory (DFT) calculations confirmed the experimentally detected major byproduct 2,6-dichloroaniline, the formation of which was explained by OH• addition to the ipso-position of the amino group. The stability of UV absorption at around 276-280 nm throughout reaction was in agreement with the detected byproduct structures, i.e., the presence of amino/amine groups and phenolic, aniline, benzene, and quinine-type derivatives, which all absorbed at around the same band. Microtox toxicity of the reactor aliquots at early reaction showed that initially the reaction products, specifically 1-(2,6-dichlorophenyl)-2-indoline-one, were very toxic; subsequently toxicity exhibited a fluctuating pattern, and a steady declination towards the "non-toxic" level was observed only after 90 min. Oxygen uptake analysis also revealed the formation of harmful products at early reaction, but the reactor was totally biodegradable upon 1-h sonication.

A review on pharmaceuticals removal from waters by single and combined biological, membrane filtration and ultrasound systems

“…This is the case of DCF, where upon sonication the toxicity of the samples increases within the first ~ 20–30 min of the reaction. By prolonging sonication time, toxic by-products gradually degrade (mineralise) and toxicity reduces [31] , [118] . A similar behaviour has been reported for CBZ [130] , amoxicillin [163] , NOR [54] and CIPRO [116] in distilled water (synthetic waters).…”

“…Ultrasound has proven effective in the degradation of pharmaceuticals from water and wastewater, however, despite reaching 100% degradation for certain pharmaceuticals, sonication alone is not sufficient to achieve high or complete mineralisation [29] , [30] . Sonication has shown to lead to the formation of intermediates with a higher toxicity than the parent compounds [31] , [32] , but in some cases these intermediates can increase biodegradability or reduce antimicrobial resistance. NF, FO and RO have shown high pharmaceutical rejection but do not serve as an absolute barrier against all pharmaceuticals [38] , producing concentrated solutions that need to be further treated prior to discharge.…”

“…It is important to mention that US and US-based AOPs are effective in the removal of pharmaceuticals from water streams as tertiary treatment methods [29] , [30] . However, the low mineralisation percentages achieved with US-based AOPs [29] , [30] leads to the formation of intermediates that can increase water toxicity [31] , [32] . As above mentioned, US can be coupled with other AOPs (e.g., photochemical systems) to increase degradation effects.…”

A Plug flow reactor model for UV-based oxidation of amoxicillin

“…10), which indicates the formation of phenolic, anilinic, benzene, or quinone derivatives [42]. However, these distinct absorbance increases were not observed in the UV/H 2 O 2 -treated effluents due to the higher mineralization and lower amounts of observed phenolic, anilinic, benzene, or quinone derivatives.…”

“…However, the monitored byproducts were limited, and the proposed byproduct formation pathway describes only the opening of the β-lactam ring and the thiazole ring. Conversely, the proposed byproduct pathway for UV-based oxidation of amoxicillin results in further rearrangements and cleavage of the molecule [29,30,42]. A survey in the literature [30], which was carried out using E.coli and Bacillus subtilis, indicates that the byproducts formed with the UV and UV/H 2 O 2 oxidation also possess an antibacterial activity similar to the original compound (amoxicillin).…”