Degradation of metronidazole by heat-activated persulfate: mechanism, water matrix, ecotoxicity removal, and energy-efficiency analysis.

Metronidazole (MTZ), an extensively used antibiotic and antiparasitic drug for treating infections, has been reported as a hazardous micropollutant that has detrimental ecotoxic effects on aquatic species and the environment. Therefore, effective treatment to remove hazardous MTZ from water is needed. Sulfate radical anion-based advanced oxidation processes (S-AOPs) are promising water treatment techniques for the degradation of organic pollutants. Hence, we aimed to study the ability of heat-activated persulfate (HA/PS)-based S-AOP for the degradation and detoxification of MTZ. The pseudo-first-order rate constant value for the degradation of MTZ by HA/PS was calculated to be 0.059 min^-1 in reverse osmosis (RO) water, achieving 99.2% of MTZ within 80 min under optimum conditions. As the initial concentration of MTZ was increased (60-2000 µg/L), the degradation efficiency decreased, whereas with an increase in initial PS (0.1 to 0.8 mM) concentration, the degradation efficiency increased. The thermal activation energy was determined to be 134.47 kJ·mol^-1. The HA/PS system was effective in the wide pH range (2.5 to 10.5). The ${\mathrm{SO}}_4^{\bullet -}$ and ${\mathrm{HO}}^{\bullet }$ were attributed to the degradation of MTZ with ${\mathrm{SO}}_4^{\bullet -}$ as a prominent reactive species. Chloride and humic acid water constituents reduced the degradation efficiency by 48.66% and 28.75%, respectively, as compared to RO water. Further, density functional theory (DFT) identified the susceptible sites of MTZ for the radical attack. Degradation byproducts were analyzed by liquid chromatography mass spectrometry (LC-MS), and degradation pathways were presented. Importantly, more than 95% mineralization was observed in 240 min of treatment. ECOSAR analysis predicted MTZ as toxic, whereas the degradation byproducts were non-toxic. Electrical energy per order (EEO) and cost for the degradation of MTZ were determined to be 19.394 kWh/m³/order and $1.520/m³, respectively. Degradation of MTZ occurred in various water matrices, namely, tap water (TW) and wastewater (WW), revealing that the HA/PS system effectively removed MTZ micropollutant. Therefore, the HA/PS system is efficient, energy-saving, and cost-effective, and the study provides insight to augment the HA/PS system as an effective S-AOP for degradation and detoxification of hazardous organic micropollutants.