Complex wastewater matrices present a major environmental concern. Besides the biodegradable organics, they may contain a great variety of toxic chemicals, heavy metals, and other xenobiotics. The electrochemically activated persulfate process, an efficient way to generate sulfate radicals, has been widely applied to the degradation of such complex effluents with very good results.
This review presents the fundamentals of the electro-persulfate processes, highlighting the advantages and limitations, followed by an exhaustive evaluation on the application of this process for the treatment of complex industrial effluents.
An overview of the main relevant experimental parameters/details and their influence on the organic load removal is presented and discussed, having in mind the application of these technologies at an industrial scale.
Finally, the future perspectives for the application of the electro-persulfate processes in the treatment of complex wastewater matrices is outlined.
Keywords: sulfate radical-based processes, electrochemical persulfate activation, hybrid persulfate activation, recalcitrant wastewaters.
Increasing industrialization and urbanization results in the generation of large volumes of wastewaters, which represent a major environmental concern, due to their possible release in the environment without the appropriate treatment. Depending on its origin, municipal, livestock, refinery, industrial or food-processing activities, among others, wastewaters can present, in its composition, biodegradable organics, a great variety of toxic chemicals, heavy metals, and other xenobiotics. Most of these compounds are persistent and adversely affect human health and aquatic biota, imparting genotoxicity, endocrine disruption, and bioaccumulation, which makes it imperative to treat the wastewaters before their discharge into water courses.
Conventional wastewater treatments often lack any capability to treat complex wastewaters that contain a mixture of refractory and non-biodegradable compounds. Thus, in the past years, several studies have been focused on the development of new technologies that enable the efficient treatment of these complex wastewater matrices to comply with the discharge regulations. Among the studied technologies, advanced oxidation processes (AOPs) have gained increasing attention, due to their good performance in the removal of recalcitrant pollutants. Traditionally, AOPs are based on the in situ generation of the non-selective strong oxidant hydroxyl radical (HO•), with a redox potential of 1.8–2.7 V, which unselectively promotes the partial or complete mineralization of a wide range of organic compounds. However, more recently, sulfate radical-based AOPs have emerged, as a sub-category of the AOPs, where the generation of the sulfate radical (SO4•−) is promoted, alone or combined with hydroxyl radicals.
Among the different SO4•− generation processes, the electrochemical ones, usually designated as electro-persulfate processes, were extensively investigated and considered the most efficient for the formation of sulfate radicals. There are several studies reporting the application of electro-persulfate processes in the treatment of wastewaters with recalcitrant properties, with very promising results, making this technology a possible solution for the remediation of complex wastewater matrices.
The aim of this paper is to present a general review of the most relevant applications of the electro-persulfate processes in the treatment of complex wastewater matrices. To better understand their advantages and limitations for wastewater treatment, the fundamentals are briefly presented. The studies found in the literature revision for the application of such technologies to the treatment of complex wastewaters are discussed, to point out the most relevant conclusions. Finally, the future perspectives for application of the electro-persulfate processes at an industrial scale are presented.