Mechanism of Base Activation of Persulfate

TLDR

Base is the most commonly used activator of persulfate for in situ chemical oxidation of contaminated groundwater. The study proposes a mechanism for base activation of persulfate involving base‑catalyzed hydrolysis to hydroperoxide anion and sulfate, followed by reduction of another persulfate molecule by hydroperoxide. The proposed mechanism entails hydrolysis of persulfate to hydroperoxide anion and sulfate, then hydroperoxide reduces a second persulfate to yield sulfate radical, sulfate anion, and superoxide. Kinetic and stoichiometric analyses confirm base‑catalyzed hydrolysis of persulfate to hydroperoxide and sulfate, with hydroperoxide reacting 1:1 with persulfate to generate sulfate radical, hydroxyl radical, and superoxide, as demonstrated by ESR and probe experiments, supporting the mechanism.

Abstract

Base is the most commonly used activator of persulfate for the treatment of contaminated groundwater by in situ chemical oxidation (ISCO). A mechanism for the base activation of persulfate is proposed involving the base-catalyzed hydrolysis of persulfate to hydroperoxide anion and sulfate followed by the reduction of another persulfate molecule by hydroperoxide. Reduction by hydroperoxide decomposes persulfate into sulfate radical and sulfate anion, and hydroperoxide is oxidized to superoxide. The base-catalyzed hydrolysis of persulfate was supported by kinetic analyses of persulfate decomposition at various base:persulfate molar ratios and an increased rate of persulfate decomposition in D2O vs H2O. Stoichiometric analyses confirmed that hydroperoxide reacts with persulfate in a 1:1 molar ratio. Addition of hydroperoxide to basic persulfate systems resulted in rapid decomposition of the hydroperoxide and persulfate and decomposition of the superoxide probe hexachloroethane. The presence of superoxide was confirmed with scavenging by Cu(II). Electron spin resonance spectroscopy confirmed the generation of sulfate radical, hydroxyl radical, and superoxide. The results of this research are consistent with the widespread reactivity reported for base-activated persulfate when it is used for ISCO.