Oxidation of intermediate sulfur species (thiosulfate) by free chlorine to increase the bed life of tailored granular-activated carbon removing perchlorate

Thiosulfate (S₂O₃^2-) 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 (S₂O ₃^2-) 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 S₂O₃^2- to sulfate (SO₄^2-), 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 S₂O₃^2-, 6 μg/L of ClO₄^- broke through at 17,000 BV. However, when S ₂O₃^2- spiked Redlands groundwater was also spiked with 2,500 μg/L chlorine, this reactant stoichiometrically oxidized the S₂O₃^2- 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^-, S ₂O₃^2-, and chlorine.