Revisiting the Role of Dissolved Silicate in Catalyzing the Hydrogen Peroxide Process with Iron-Bearing Minerals

Abstract

Injecting H2O2 into aquifers is a widely used in situ chemical oxidation (ISCO) technology for groundwater remediation. Dissolved silicate has been reported to decrease the reactivity of iron (III)-bearing minerals toward H2O2. In this study, the effect of naturally occurring levels of dissolved silicate (≤1 mM) on the catalyzing hydrogen peroxide (CHP) with Fe(II) minerals is revaluated, and new observations that contradict previous studies are reported. Specifically, dissolved silicate enhances the CHP process with Fe(II) minerals. In the presence of Fe(II) minerals, siderite and ferrous oxide (FeO), which had a stronger dissolution tendency than Fe(III) minerals, dissolved silicate could prevent the dissolved iron species from precipitation through a coordinating effect, therefore reinforcing the homogeneous CHP process and the degradation of 2,4-dichlorophenol. The solution pH decreased due to the generation of degradation intermediates, and the solution acidification in turn promoted further dissolution of Fe(II) minerals. FeO particles exhibited the strongest silicate adsorption among the minerals; therefore, a higher initial silicate concentration of 1 mM was needed to observe the enhancing effect. This study redefines the role of dissolved silicate in the CHP process and provides clues to the design of an efficient H2O2-based ISCO system for the remediation of groundwater.