AWWA WQTC69286

This paper describes research into S₂O₃^2- oxidation by chlorine. Free chlorine was used as a pretreatment method to remove S₂O₃^2- from groundwater in Redlands, California. Chlorination in the form of hypochlorous acid (HOCl) and hypochlorite ion (OCl^-) was used to oxidize S₂O₃^2- to sulfate prior to treatment with tailored granular activated carbon (TGAC). Thiosulfate, RSS, was seen to compete with ClO₄ ^- for adsorption sites on the TGAC. Experiments were first conducted synthetically using deionized distilled (DI) water spiked with different concentration of ClO₄ ^-, S₂O₃^2- and chlorine depicting a variety of treatment conditions. Conditions include concentrations of 1 and 0.5 mg/L ClO₄ ^- and 1, 0.5 and 0.1 mg/L of S₂O₃^2-. These experiments were carried out as pseudo reactions where free chlorine was in excess. Redlands groundwater was also spiked with S₂O₃^2-, the same concentration as background ClO₄ ^- concentration that is found in the groundwater. Results showed that DI water spiked with only 1 mg/L ClO₄ ^- broke through at 15,000 bed volumes, while DI water spiked with both 1 mg/L S₂O₃^2- and 1 mg/L ClO₄ ^- broke through at 8,000 bed volumes, representing a 47% capacity reduction. Moreover, deionized distilled water that was spiked with 1 mg/L ClO₄ ^- and just a mere 0.1 mg/L S₂O₃^2- showed breakthrough at 9,400 bed volumes, representing a 37% reduction. Next, the authors also pre-chlorinated DI water with 0.25mg/L of free chlorine as HOCl, while this water also contained 1 mg/L ClO₄ ^- and 0.1 mg/L S₂O₃^2-; and in this case, ClO₄ ^- broke through at 12,000 bed volumes. This indicates that chlorine oxidation of S₂O₃^2- increased the TGAC capacity to remove ClO₄ ^- from 63% to over 80%. As follow-up, Redlands groundwater that contained 30 µg/L ClO₄ ^- was spiked with 40 µg/L S₂O₃^2-, plus the 0.25 mg/L of HOCl. This RSSCT exhibited no difference for ClO₄ ^- breakthrough between the RSSCT that was spiked, 30,000 bed volumes, and the RSSCT that had no S₂O₃^2- present, 31,000 bed volumes. Significant difference in bed volumes to breakthrough was however observed when S₂O₃^2- was more than 10 times greater than the ClO₄ ^- concentration. The TGAC capacity to remove ClO₄ ^- decreased to 17% and 50% when S₂O₃^2- concentration was 250µg/L and 1mg/L respectively. Results to date suggest that S₂O_3<