Hydroxyl radical scavenging by solid mineral surfaces in oxidative treatment systems: Rate constants and implications.

Advanced oxidation treatment processes used in various applications to treat contaminated soil, water, and groundwater involve powerful radical intermediates, including hydroxyl radicals (•OH). Inefficiency in •OH-driven treatment systems involves scavenging reactions where •OH react with non-target species in the aqueous and solid phases. Here, •OH were generated in iron (Fe)- and UV-activated hydrogen peroxide (Fe-AHP, UV-AHP) systems where the loss of rhodamine B served as a quantitative metric for •OH activity. Kinetic analysis methods were developed to estimate the specific •OH surface scavenging rate constant (k≡S). In the Fe-AHP system, k≡S for silica (2.85 × 106 1/m2 × s) and alumina (3.92 × 106 1/m2 × s) were similar. In the UV-AHP system, estimates of k≡S for silica (4.50 × 106 1/m2 × s) and alumina (7.45 × 106 1/m2 × s) were higher. k≡S for montmorillonite (MMT) in the UV-AHP system was ≤4.22 × 105 1/m2 × s. Overall, k≡S,silica ∼ k≡S, alumina > k≡S,MMT indicating k≡S is mineral specific. Radical scavenging was dominated by surface scavenging at 10-50 g/L silica, alumina, or MMT, in both Fe-AHP and UV-AHP systems. The experimentally-derived surface •OH scavenging rate constants were extended to in-situ chemical oxidation (ISCO) treatment conditions to contrast •OH reaction rates with contaminant and aqueous phase reactants found in aquifer systems. •OH reaction was dominated by solid surfaces comprised of silica, alumina, and montmorillonite minerals relative to •OH reaction with trichloroethylene, the target compound, and H2O2, a well-documented radical scavenger. These results indicate that solid mineral surfaces play a key role in limiting the degradation rate of contaminants found in soil and groundwater, and the overall treatment efficiency in ISCO systems. The aggressive •OH scavenging measured was partially attributed to the relative abundance of scavenging sites on mineral surfaces. Published by Elsevier Ltd.