Metal-free carbocatalytic treatment of analgesics - an atypical non-radical pathway and degradation enhancement by ultrasound

Metal-free carbonaceous materials from rice husk were obtained via a single-step pyrolysis at temperatures of 250, 500, and 800 °C, then their carbocatalytic abilities to deal with acetaminophen, ibuprofen, and diclofenac in water were explored. The material prepared at 800 °C (BRH-800) efficiently activated peroxymonosulfate-PMS. The three target analgesics showed different degradation efficiencies, denoting a dependence on their chemical structure, with the acetaminophen elimination being more favorable. Through diverse analytical/instrumental and theoretical/computational tools, the predominance of a non-radical pathway in the carbocatalytic process was evidenced. It was proposed that the defects close to Si-containing functional groups of the material lead to the formation of an initial complex (named PMS-BRH-800*), which withdraws electrons from the pharmaceuticals, thus leading to the oxidation process. In the case of acetaminophen, a primary by-product was identified, which, according to the ECOSAR analysis and phytotoxicity assays, had a lower toxicity for living organisms. Additionally, the BRH-800/PMS system was stable to pH changes, with the carbonaceous material exhibiting good performance even after the third reuse cycle. The combination of carbocatalysis with ultrasound (BRH-800/PMS/US) improved the degradation of ACE and IBU via mechanical effects (carbonaceous particles disaggregation) mainly. Finally, the sonocarbocatalytic treatment of the analgesics in the spiked hospital wastewater and tap water showed that, despite the interfering effects of the matrix components, a partial degradation of pollutants was found. Hence, this work showed that carbocatalysis and sonocarbocatalysis with metal-free materials are promising treatments for degrading some specific analgesics in aqueous samples.