Kinetics and Mechanisms of Cr(VI) Formation via the Oxidation of Cr(III) Solid Phases by Chlorine in Drinking Water

Abstract

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.

Novel synthesis of Ni/Fe layered double hydroxides using urea and glycerol and their enhanced adsorption behavior for Cr(VI) removal

“…Adsorbents chemical stability. The chemical stability of the prepared LDH materials was tested by analyzing XRD patterns of these adsorbents at different pHs (3)(4)(5)(6)(7)(8)(9). For Ni/Fe LDH (Fig.…”

Resilient biological hexavalent chromium removal with a two‐stage, fixed‐bed biotreatment system

“…Although the biological reduction of Cr(VI) is thought to primarily result in Cr(III) hydroxides with low solubility, the presence of organic and inorganic Cr(III)‐chelating ligands in groundwater can result in soluble Cr(III) complexes at circumneutral pH (Dong et al, ; Puzon, Roberts, Kramer, & Xun, ), which differ in mobility and recalcitrance depending on the ligand (Cao, Guo, Mao, & Lan, ; Puzon et al, ). Furthermore, chlorine (Chebeir & Liu, ; Lindsay, Farley, & Carbonaro, ) and, to a lesser extent, hydrogen peroxide (H 2 O 2 ) (Luo & Chatterjee, ; Ye et al, ) may oxidize Cr(III) from both organic and inorganic complexes. Given the potential formation of Cr(III)–organic complexes and their reoxidation during downstream disinfection to potentially hazardous Cr(VI), the possibility of total chromium removal needs to be carefully assessed, along with contaminant removal, degradation kinetics, and process limitations when evaluating a biological treatment system.…”

Electrochemical removal of hexavalent chromium using electrically conducting carbon nanotube/polymer composite ultrafiltration membranes

“…Chromium is a common element that is both naturally ubiquitous and widely used in a variety of industrial applications, such as electroplating, textile processing, oil refining, corrosion protection, and pigment manufacturing [1]. A combination of poor wastewater management practices and natural geological formations have led to its wide occurrence in many communities" drinking water resources [2][3][4][5][6]. In the environment, chromium is generally found in one of two stable redox states: hexavalent chromium (Cr (VI)) and trivalent chromium (Cr(III)) [7,8].…”