Ultrasonic degradation, mineralization and detoxification of diclofenac in water: Optimization of operating parameters

Abstract

The 20 kHz ultrasound-induced degradation of non-steroidal, anti-inflammatory drug diclofenac (DCF) was investigated. Several operating conditions, such as power density (25–100 W/L), substrate concentration (2.5–80 mg/L), initial solution pH (3.5–11), liquid bulk temperature and the type of sparging gas (air, oxygen, argon), were tested concerning their effect on DCF degradation (as assessed measuring absorbance at 276 nm) and hydroxyl radicals generation (as assessed measuring H 2O 2 concentration). Sample mineralization (in terms of TOC and COD removal), aerobic biodegradability (as assessed by the BOD 5/COD ratio) and ecotoxicity to Daphnia magna and Artemia salina were followed too. DCF conversion is enhanced at increased applied power densities and liquid bulk temperatures, acidic conditions and in the presence of dissolved air or oxygen. The reaction rate increases with increasing DCF concentration in the range 2.5–5 mg/L but it remains constant in the range 40–80 mg/L, indicating different kinetic regimes (i.e. first and zero order, respectively). H 2O 2 production rates in pure water are higher than those in DCF solutions, implying that decomposition basically proceeds through hydroxyl radical reactions. Mineralization is a slow process as reaction by-products are more stable than DCF to total oxidation; nonetheless, they are also more readily biodegradable. Toxicity to D. magna increases during the early stages of the reaction and then decreases progressively upon degradation of reaction by-products; nevertheless, complete toxicity elimination cannot be achieved at the conditions in question. Neither the original nor the treated DCF samples are toxic to A. salina.