Advanced oxidation processes used in various applications to treat contaminated soil, water, and groundwater involve powerful radical intermediates including hydroxyl radicals (.OH) and sulfate radicals (SO4.-). A major source of inefficiency in radical-driven treatment systems involves scavenging reactions where radicals react with non-target species. Kinetic studies of radical scavenging have solely been focused on soluble constituents in the aqueous phase and not on solid phase media also found in oxidative treatment systems. Quantifying radical scavenging by solid surfaces in heterogeneous systems is a key requirement for a comprehensive understanding of contaminant transformation efficiency. Detailed laboratory and kinetic analysis methods were developed to estimate the specific .OH and SO4.- surface scavenging rate constants (k≡S) for several minerals (alumina, silica, montmorillonite). Radicals were generated under a range of catalytic conditions including Fe- and UV-activated hydrogen peroxide, UV- and thermal-activated persulfate. Contrasting degradation rates of the target compound between solids-free homogeneous systems and solids-amended heterogeneous systems, in conjunction with several kinetic parameters permitted the surface scavenging rate constants to be estimated. The .OH scavenging rate constants were found to be mineral-specific k≡S,silica ~ k≡S,alumina &gt; k≡S,MMT; validation of k≡S was obtained by similar estimates using various radical generating systems. Radical scavenging in lab reactors was dominated by mineral surfaces, relative to aqueous phase scavenging in both the .OH and SO4.- oxidative treatment systems. The laboratory-derived k≡S for .OH and SO4.- were used to predict reaction rates between target contaminants and aqueous and solid phase scavengers in an idealized in-situ chemical oxidation scenario. Consequently, the rates of .OH and SO4.- reaction with mineral surfaces dominated the fate of these radicals by several orders of magnitude, relative to the target compound (i.e., TCE, 1,4-dioxane), and other aqueous phase radical scavengers including hydrogen peroxide and persulfate.

Hydroxyl and sulfate radical scavenging by solid phase mineral species: Rate constants, implications, future directions

ACS Spring Meeting 2021

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