Peroxide stabilizers remarkably increase the longevity of thermally activated peroxydisulfate for enhanced ISCO...

Thermally activated peroxydisulfate In Situ Chemical Oxidation (TAP-ISCO) is often applied for the remediation of soil-sorbed hydrophobic organic contaminants (HOCs) and nonaqueous phase liquids (NAPLs), which act as long-term sources of groundwater contamination. TAP-ISCO benefits from improved desorption/dissolution of organic contaminants into the aqueous phase and efficient activation of peroxydisulfate at elevated temperatures, but the primary limitation of TAP-ISCO is the short lifetime of peroxydisulfate (therefore the availability of reactive radical species). To resolve this problem, coupling of peroxide stabilizers with TAP were tested. The compatibility of seven representative commercial organic and inorganic peroxide stabilizers, including sodium stannate, trisodium phosphate, sodium pyrophosphate, sodium silicate, sodium citrate, ethylene diamine tetra methylene phosphonic acid and ethylenediaminetetraacetic acid disodium salt, with TAP in aqueous solutions and solutions containing goethite or soil particles were first studied. The effects of stabilizers on the formation, distribution and reactivity of reactive oxygen species were then investigated through electron paramagnetic resonance (EPR) spin-trapping experiments using 5,5-dimethyl-1-pyrroline-N-oxide, chemical probe experiments using anisole, nitrobenzene and hexachloroethane, and biphasic trichloroethylene (TCE) dense nonaqueous phase liquids (DNAPLs) TAP-ISCO mimicking experiments. The results indicate that organic stabilizers significantly accelerate peroxydisulfate decomposition at both ambient and elevated temperatures. In contrast, inorganic stabilizers can markedly increase peroxydisulfate longevity by suppressing the acid-catalyzed peroxydisulfate decomposition, quenching radical-chain acceleration, and sequestering transition metal species. In addition, TAP systems containing inorganic stabilizers can effectively generate a variety of reactive radical species, including SO₄^•-, HO^•, and O₂^•-, and improve the oxidation of anisole and nitrobenzene, though suppressing the reduction of hexachloroethane to some extent. Especially, suitable inorganic stabilizers (e.g., trisodium phosphate) can effectively improve TAP oxidation of TCE DNAPL while suppressing peroxydisulfate decomposition. Overall, this study provides the fundamental basis of coupling TAP-ISCO with peroxide stabilizers.