Coupling of Oxidation-Reduction Reactions of Chromium, Iron and Manganese: Implications for the Fate and Mobility of Chromium in Aquatic Environments
Date of Award
Doctor of Philosophy (PhD)
Both within the United States and internationally, hexavalent chromium (Cr(VI)) is a contaminant of concern in drinking water supplies. The U.S. Environmental Protection Agency is considering a Cr(VI)-specific standard. Thus improved technologies for Cr(VI) removal in drinking water are needed. Iron electrocoagulation for Cr(VI) removal was examined at conditions directly relevant to drinking water treatment, and humic acid (HA) affects the performance of electrocoagulation in multiple ways. The success of the chromium treatment or remediation also relies on the stability of the Cr(III)-containing solids with respect to reoxidation under groundwater conditions. Manganese is ubiquitous in aquatic and terrestrial environments, and the redox cycling of manganese may significantly impact the fate and transport of chromium. Coupling of redox reactions of chromium, iron and manganese involves multiple interaction pathways that occur in the aqueous phase as well as at solid-water interfaces. A mechanistic and quantitative understanding of these processes is needed to establish input parameters for kinetic and transport models and to enable decision-making for chromium treatment strategies.
Iron electrocoagulation (EC) is a technology that can successfully achieve low concentrations of Cr(VI) in treated drinking water. In our research we have applied iron electrocoagulation (EC) with iron serving as the sacrificial anode to treat simulated drinking water solutions. Experiments have evaluated the effects of pH, dissolved oxygen, and common anions on Cr(VI) removal during batch EC treatment. In addition, the presence of humic acid (HA) inhibited the rate of Cr(VI) removal in electrocoagulation, with slower Cr(VI) removal at higher pH. This is due to dissolved oxygen competing with Cr(VI) for the oxidation of Fe(II) released from the anode. As determined using dynamic light scattering and wet chemistry experiments, the presence of HA resulted in the formation of Cr(III)-Fe(III)-HA colloids during electrocoagulation, which is difficult to remove in following water treatment steps of sedimentation and granular media filtration. Characterization of the solids by X-ray diffraction indicates that the iron oxides produced are lepidocrocite at pH 8, with more ferrihydrite in the presence of HA.
Building on previous knowledge of MnO2 as an oxidant for Cr-containing solids, we systematically evaluated the rates and products of the oxidation of Cr(III) in iron oxides by MnO2. We found that Cr(III) dissolution from CrxFe1-x(OH)3 greatly influenced the Cr(VI) production rates. A multi-chamber reactor was used to assess the role of solid-solid mixing in CrxFe1-x(OH)3-MnO2 interactions. A dialysis membrane divided the reactor into two chambers, eliminating the possibility of direct contact of the solids in each chamber but allowing dissolved species to diffuse across the membrane. The Cr(VI) production rate was much lower in multi-chamber experiments (CrxFe1-x(OH)3||MnO2) than in completely mixed batch experiments under the same condition, indicating that the redox interaction is greatly accelerated by mixing of the two solids. The model was first established to predict Cr(VI) release in Cr(OH)3||MnO2 multichamber experiments, as dissolved Cr(III) concentration in equilibrium with Cr(OH)3 is higher at low pH and it’s easy to observe the behavior of Cr(VI) dynamics with more Cr(VI) generation. While solid phase Mn(IV) is well known oxidants of Cr(III)-containing solids, the localized oxidation of adsorbed Mn(II) by dissolved oxygen can also promote the oxidation of Cr(III) contained within CrxFe1-x(OH)3. The promotional effects was likely due to Mn redox cycling in which oxidized forms of Mn species were generated as oxidants of CrxFe1-x(OH)3 that were more potent than O2.
Daniel E. Giammar
John Fortner, Young-Shin Jun, Jeffrey G. Catalano, Jay Turner,
Permanent URL: https://doi.org/10.7936/K73T9GMQ