Oxidation of Intermediate Sulfur Species (Thiosulfate) by Free Chlorine to Increase the Bed Life of Tailored Granular-Activated Carbon Removing Perchlorate

Abstract

Abstract

Thiosulfate (S₂O₃²⁻) hosts redox stability in time frame of days, rendering it a candidate ion exchange competitor in anoxic groundwaters during the brief time frames of treatment processes. Indeed, this research determined that the oxyanion thiosulfate (S₂O₃²⁻) competed with perchlorate (ClO₄⁻) for sorption onto activated carbons that had been preloaded with a quaternary ammonium surfactant. Moreover, this research showed that when proper prechlorination oxidized this S₂O₃²⁻ to sulfate (SO₄²⁻), this competition was diminished. When rapid small-scale column test employed Redlands, CA, groundwater that contained a native 30 μg/L ClO₄⁻, this exhibited a 6 μg/L ClO₄⁻ breakthrough after 33,000 bed volumes (BVs) when processed through bituminous-based granular-activated carbon that had been preloaded with a quaternary ammonium surfactant, namely, 0.24 g/g dicocoalkyldimethylammonium chloride (Arquad 2C-75). When this same water was spiked with 1,000 μg/L S₂O₃²⁻, 6 μg/L of ClO₄⁻ broke through at 17,000 BV. However, when S₂O₃²⁻ spiked Redlands groundwater was also spiked with 2,500 μg/L chlorine, this reactant stoichiometrically oxidized the S₂O₃²⁻ so as to diminish this competition, such that 6 μg/L ClO₄⁻ broke through at 31,000 BV. Similar rapid small-scale column test trends were exhibited when using deionized distilled water that had been spiked with ClO₄⁻, S₂O₃²⁻, and chlorine.