Hexavalent chromium Cr­(VI), typically existing as the oxyanion form of CrO4 2–, is being considered for more stringent drinking water standards by regulatory agencies. Cr­(VI) can be inadvertently produced via the oxidation of trivalent chromium Cr­(III) solids. This study investigated the kinetics and mechanisms of Cr­(III) solids oxidation by chlorine in drinking water and associated Cr­(VI) formation. Batch experiments were carried out with three Cr­(III) solids of environmental relevance, i.e., chromium hydroxide Cr­(OH)3(s), chromium oxide Cr2O3(s), and copper chromite Cu2Cr2O5(s). Impacts of water chemical parameters including pH (6.0–8.5) and bromide concentration (0–5 mg/L) were examined. Results showed that the rapid oxidation of Cr­(III) solid phases by chlorine was accompanied by Cr­(VI) formation and an unexpected production of dissolved oxygen. Analysis of reaction stoichiometry indicated the existence of Cr intermediate species that promoted the autocatalytic decay of chlorine. An increase in pH modestly enhanced Cr­(VI) formation due to changes of reactive Cr­(III) surface hydroxo species. Bromide, a trace chemical constituent in source waters, exhibited a catalytic effect on Cr­(VI) formation due to an electron shuttle mechanism between Cr­(III) and chlorine and the bypass of Cr intermediate formation. The kinetics data obtained from this study suggest that the oxidation of Cr­(III) solids by chlorine in water distribution systems can contribute to Cr­(VI) occurrence in tap water, especially in the presence of a trace level of bromide.

Removal of excess nitrate is critical to balance the nitrogen cycle in aquatic systems. This study investigated a novel denitrification process by tailoring photochemistry of nitrate with formate. Under UV light irradiation, short-lived radicals (i.e., HO • , NO 2• , and CO 3 •− ) generated from nitrate photolysis partially oxidized formate to highly reductive formate radical (CO 2•− ). CO 2 •− further reduced nitrogen intermediates generated during photochemical denitrification (mainly NO • , HNO, and N 2 O) to gasphase nitrogen (i.e., N 2 O and N 2 ). The degradation kinetics of total dissolved nitrogen was mainly controlled by the photolysis rates of nitrate and nitrite. The distribution of final products was controlled by the reaction between CO 2•− and N 2 O. To achieve a simultaneous and complete removal of dissolved nitrogen (i.e., nitrate, nitrite, and ammonia) and organic carbon, the formate-to-nitrate stoichiometry was determined as 3.1 ± 0.2 at neutral pH in deionized water. Solution pH impacted the removal rates of nitrate and nitrite but not that of total dissolved nitrogen or formate. The presence of dissolved organic matter at levels similar to those in groundwater had a negligible impact on the photochemical denitrification process. A high denitrification efficiency was also achieved in a synthetic groundwater matrix. Outcome from this study provides a potential denitrification technology for decentralized water treatment and reuse facilities to abate nitrate in local water resources.

Chromium can exist as both trivalent Cr(iii) and hexavalent Cr(vi) in typical drinking water conditions, with Cr(vi) being of particular concern due to its high toxicity and new regulatory perspectives.

The occurrence of chromium (Cr) as an inorganic contaminant in drinking water is widely reported. One source of Cr is its accumulation in iron-containing corrosion scales of drinking water distribution systems as Cr(III)–Fe(III) hydroxide, that is, FexCr(1–x)(OH)3(s), where x represents the Fe(III) molar content and typically varies between 0.25 and 0.75. This study investigated the kinetics of inadvertent hexavalent chromium Cr(VI) formation via the oxidation of FexCr(1–x)(OH)3(s) by chlorine as a residual disinfectant in drinking water, and examined the impacts of Fe(III) content and drinking water chemical parameters including pH, bromide and bicarbonate on the rate of Cr(VI) formation. Data showed that an increase in Fe(III) molar content resulted in a significant decrease in the stoichiometric Cr(VI) yield and the rate of Cr(VI) formation, mainly due to chlorine decay induced by Fe(III) surface sites. An increase in bicarbonate enhanced the rate of Cr(VI) formation, likely due to the formation of Fe(III)-carbonato surface complexes that slowed down the scavenging reaction with chlorine. The presence of bromide significantly accelerated the oxidation of FexCr(1–x)(OH)3(s) by chlorine, resulting from the catalytic effect of bromide acting as an electron shuttle. A higher solution pH between 6 and 8.5 slowed down the oxidation of Cr(III) by chlorine. These findings suggested that the oxidative conversion of chromium-containing iron corrosion products in drinking water distribution systems can lead to the occurrence of Cr(VI) at the tap, and the abundance of iron, and a careful control of pH, bicarbonate and bromide levels can assist the control of Cr(VI) formation.

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