Arsenic Oxidation by Flavin-Derived Reactive Species under Oxic and Anoxic Conditions: Oxidant Formation and pH Dependence.

Flavins are ubiquitous redox-active compounds capable of producing reactive oxygen (O2•-, •OH, and H2O2) and flavin radical species in natural environments, yet their roles in the redox transformations of environmental contaminants, such as arsenic (As), remain to be investigated. Here, we show that reduced flavins can be a source of effective oxidants for As(III) under both oxic and anoxic conditions. For instance, in the presence of 15 µM reduced riboflavin (RBFH2), 22% of 30 µM As(III) is oxidized in aerated solution at pH 7.0. The co-oxidation of As(III) with RBFH2 is pH-dependent, with a faster reaction rate under mildly acidic relative to alkaline conditions. Quencher tests with 2-propanol (for •OH) and catalase (for H2O2) indicate that As(III) oxidation under oxic conditions is likely controlled by flavin-derived •OH at pH 5.2 and 7.0, and by H2O2 at pH 9.0. Kinetic modeling further implies that flavin-derived reactive oxygen species are mainly responsible for As(III) oxidation under oxic conditions, whereas oxidation of As(III) under anoxic conditions at pH 9.0 is attributed to riboflavin radicals (RBFH•) generated from co-existing oxidized and reduced riboflavin. The demonstrated ability of flavins to catalyze As(III) oxidation has potential implications for As redox cycling in the environment.

XANES-Based Determination of Redox Potentials Imposed by Steel Corrosion Products in Cement-Based Media.

The redox potential (Eh) in a cementitious nuclear waste repository is critical to the retardation behavior of redox-sensitive radionuclides (RNs), and largely controlled by embedded steel corrosion but hard to be determined experimentally. Here, we propose an innovative Eh determination method based on chemical/spectroscopic measurements. Oxidized nuclides (UVI, SeIV, MoVI, and SbV) were employed as species probes to detect the Eh values imposed by steel (Fe0) and steel corrosion products (magnetite/hematite, and magnetite/goethite couples) in cement pore water. Nuclides showed good sorption affinity, especially toward Fe0, in decreasing Kd order for U > Sb > Se > Mo under both N2 and H2 atmospheres. The reduced nuclide species were identified as UO2, U4O9, FeSe, FeSe2, Se0, Sb0, and Sb2O3, but no redox transformation occurred for Mo. Eh values were obtained by using the Nernst equation. Remarkably, their values fell in a small range centered around -456 mV at pH ∼ 13.5 for both Fe0 and Fe-oxyhydroxides couples. This Eh value appears to be controlled by the nanocrystalline Fe(OH)2/Fe(OH)3 or (Fe1- x,Ca x)(OH)2/Fe(OH)3 couple, whose presence was confirmed by pair distribution function analyses. This approach could pave the way for describing the Eh gradient in reinforced concrete where traditional Eh measurements are not feasible.

On-off mobilization of contaminants in soils during redox oscillations.

Near-surface biogeochemical systems can oscillate between oxic and anoxic conditions. Under such periodic changes many redox-sensitive inorganic contaminants undergo speciation, mobility and toxicity changes. We investigated the changes to chromium (Cr), arsenic (As), selenium (Se), antimony (Sb) and uranium (U) mobility during a series of laboratory experiments where argillaceous substrates were subjected to successive cycles of oxidizing and reducing conditions. The EH oscillated between -320 and +470 mV, induced via both abiotic and microbial forcings. Chemically induced cycles of oxidation and reduction were achieved via a combination of gas (N2:CO2 vs compressed air) and carbon (ethanol) addition, to stimulate the metabolism of a natively present microbial community. The contaminants were added either alone or as contaminant mixtures. Results show clear on-off switch mobility behavior for both major elements such as carbon (C), iron (Fe) and manganese (Mn) and for contaminants. Mn, Fe, and As were mobilized under anoxic conditions, whereas Sb, Se, and U were mobilized under oxic conditions. While As, Sb, and U were reversibly sorbed, Se and Cr were irreversibly sequestered via reductive precipitation. When present in aqueous solutions at high concentrations, Cr(VI) prevented the reduction of Mn and Fe, and inhibited the mobilization of elements with lower EH(o). To improve remediation strategies for multiple contaminants in redox-dynamic environments, we propose a mixed kinetic-equilibrium biogeochemical model that can be forced by oscillating boundary conditions and that uses literature rates and constants to capture the key processes responsible for the mobilization of contaminants in soils.