Inhibitory Effect of Dissolved Silica on H2O2 Decomposition by Iron(III) and Manganese(IV) Oxides: Implications for H2O2-Based In Situ Chemical Oxidation

The decomposition of H2O2 on iron minerals can generate (OH)-O-center dot, a strong oxidant that can transform a wide range of contaminants. This reaction is critical to In Situ Chemical Oxidation (ISCO) processes used for soil and groundwater remediation, as well as advanced oxidation processes employed in waste treatment systems. The presence of dissolved silica at concentrations comparable to those encountered in natural waters decreases the reactivity of iron minerals toward H2O2, because silica adsorbs onto the surface of iron minerals and alters catalytic sites. At circumneutral pH values, goethite, amorphous iron oxide, hematite, iron-coated sand, and montmorillonite that were pre-equilibrated with 0.05-1.5 mM SiO2 were significantly less reactive toward H2O2 decomposition than their original counterparts, with the H2O2 loss rates inversely proportional to SiO2 concentrations. In the goethite/H2O2 system, the overall (OH)-O-center dot yield, defined as the percentage of decomposed H2O2 producing (OH)-O-center dot, was almost halved in the presence of 1.5 mM SiO2. Dissolved SiO2 also slowed H2O2 decomposition on manganese(IV) oxide. The presence of dissolved SiO2 results in greater persistence of H2O2 in groundwater and lower H2O2 utilization efficiency and should be considered in the design of H2O2-based treatment systems.