The acid and base catalytic oxidation of permanganate towards antibiotic pollutants

Antibiotics have attracted increasing attention as an emerging class of environmental pollutants in recent years, mainly due to their usage for preventing infection, as well as their inevitable leakage into various environmental media. Although the reported concentration levels are commonly low, the persistence and long-term exposure of these antibiotics may induce the development of antibiotic-resistant bacteria/genes, subsequently threatening the ecological environment and human health.

Chemical oxidation using permanganate has been extensively applied in water and wastewater treatment processes for controlling taste, odor, and algae growth. Recently, permanganate as a powerful oxidant has shown great potential in the degradation of multiple micropollutants including antibiotics. However, the acid/base catalytic mechanism of permanganate remains unclear, although it is very important to effectively control the antibiotic pollutants in aqueous solutions.

From the kinetic and mechanistic points of view, the acid catalysis of permanganate towards sulfamethoxazole could be attributed to the formation of neutral HMnO₄, which showed a four orders of magnitude higher reactivity than anionic MnO₄^-. The rate-limiting step was determined to be simple N-H bond oxidation, which was also studied computationally using DFT calculations. It is the first time that the Brønsted acid catalysis of permanganate could not only produce the stronger electrophile HMnO₄, but also change the reaction mode by avoiding bond cleavage in the electron transfer process.

Although base catalysis was also considered to, similarly, add a hydroxyl ion to permanganate, the kinetic salt effect and DFT analysis indicated that the hydroxyl ion could attract the proton of tetracycline toward itself to form a complex-like structure with a highly reactive phenolate-type moiety. The base-catalyzed effect was finally explained by the HOMO orbital and electrostatic potential, whereby the hydroxyl ion could make the phenolic group a more electron-rich moiety for electrophilic attack.