In-situ chemical oxidation: Principle and applications of peroxide and persulfate treatments in wastewater systems

Hydrogen peroxide (H₂O₂) and persulfate are the most efficient and commonly used oxidants in in-situ chemical oxidation (ISCO) of organic contaminants. This review focuses on the principle and activation techniques used in H₂O₂ and persulfate based ISCO processes. It is crucial to understand the effect of activation techniques on process chemistry and free radicals behaviour in order to achieve high degradation efficiency. The chemistry of interaction of activated H₂O₂ and persulfate with organic contaminants is complex and many parameters influence the performance of ISCO processes, namely non-productive reactants, reaction intermediates, oxygen and pH. The poor understanding of interaction behaviour and reaction chemistry of oxidants with organic contaminants prevents the utilization of full potential of the process. Therefore, particular attention has been given to the factors affecting degradation efficiency and the performance of ISCO processes. Further, the mechanism of contaminant degradation using activated H₂O₂ and persulfate significantly differ from each other. The interaction of SO₄·⁻ radical usually involves electron transfer reactions whereas HO· radical involve electron-transfer and hydrogen-atom abstraction reactions. Moreover, the research gaps have been identified based on the knowledge of current research and recommendations are made for further understanding of ISCO processes. - Highlights: • H₂O₂ and persulphate based ISCO is an efficient technique for contaminant removal. • Method of activation affects ISCO process degradation efficiency. • HO* and SO₄*^- radicals play a central role in contaminant degradation. • Chlorides and carbonates impact reaction pathways and degradation efficiency. • Mechanism of contaminant degradation differ depending on nature of oxidant.