Iron-Mediated Oxidation of Methoxyhydroquinone under Dark Conditions: Kinetic and Mechanistic Insights

Despite the biogeochemical significance of the interactions between natural organic matter (NOM) and iron species, considerable uncertainty still remains as to the exact processes contributing to the rates and extents of complexation and redox reactions between these important and complex environmental components. Investigations on the reactivity of low-molecular-weight quinones, which are believed to be key redox active compounds within NOM, toward iron species, could provide considerable insight into the kinetics and mechanisms of reactions involving NOM and iron. In this study, the oxidation of 2-methoxyhydroquinone (MH₂Q) by ferric iron (Fe­(III)) under dark conditions in the absence and presence of oxygen was investigated within a pH range of 4–6. Although Fe­(III) was capable of stoichiometrically oxidizing MH₂Q under anaerobic conditions, catalytic oxidation of MH₂Q was observed in the presence of O₂ due to further cycling between oxygen, semiquinone radicals, and iron species. A detailed kinetic model was developed to describe the predominant mechanisms, which indicated that both the undissociated and monodissociated anions of MH₂Q were kinetically active species toward Fe­(III) reduction, with the monodissociated anion being the key species accounting for the pH dependence of the oxidation. The generated radical intermediates, namely semiquinone and superoxide, are of great importance in reaction-chain propagation. The kinetic model may provide critical insight into the underlying mechanisms of the thermodynamic and kinetic characteristics of metal–organic interactions and assist in understanding and predicting the factors controlling iron and organic matter transformation and bioavailability in aquatic systems

Effects of Fulvic Acid on Uranium(VI) Sorption Kinetics

This study focuses on the effects of fulvic acid (FA) on uranium­(VI) sorption kinetics to a silica sand. Using a tritium-labeled FA in batch experiments made it possible to investigate sorption rates over a wide range of environmentally relevant FA concentrations (0.37–23 mg L^–1 TOC). Equilibrium speciation calculations were coupled with an evaluation of U­(VI) and FA sorption rates based on characteristic times. This allowed us to suggest plausible sorption mechanisms as a function of solution conditions (e.g., pH, U­(VI)/FA/surface site ratios). Our results indicate that U­(VI) sorption onto silica sand can be either slower or faster in the presence of FA compared to a ligand-free system. This suggests a shift in the underlying mechanisms of FA effects on U­(VI) sorption, from competitive sorption to influences of U­(VI)-FA complexes, in the same system. Changes in metal sorption rates depend on the relative concentrations of metals, organic ligands, and mineral surface sites. Hence, these results elucidate the sometimes conflicting information in the literature about the influence of organic matter on metal sorption rates. Furthermore, they provide guidance for the selection of appropriate sorption equilibration times for experiments that are designed to determine metal distribution coefficients (<i>K</i>_d values) under equilibrium conditions

Uranium(VI) Reduction by Iron(II) Monosulfide Mackinawite

Reaction of aqueous uranium­(VI) with iron­(II) monosulfide mackinawite in an O₂ and CO₂ free model system was studied by batch uptake measurements, equilibrium modeling, and <i>L</i>_III edge U X-ray absorption spectroscopy (XAS). Batch uptake measurements showed that U­(VI) removal was almost complete over the wide pH range between 5 and 11 at the initial U­(VI) concentration of 5 × 10^–5 M. Extraction by a carbonate/bicarbonate solution indicated that most of the U­(VI) removed from solution was reduced to nonextractable U­(IV). Equilibrium modeling using Visual MINTEQ suggested that U was in equilibrium with uraninite under the experimental conditions. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy showed that the U­(IV) phase associated with mackinawite was uraninite. Oxidation experiments with dissolved O₂ were performed by injecting air into the sealed reaction bottles containing mackinawite samples reacted with U­(VI). Dissolved U measurement and XAS confirmed that the uraninite formed from the U­(VI) reduction by mackinawite did not oxidize or dissolve under the experimental conditions. This study shows that redox reactions between U­(VI) and mackinawite may occur to a significant extent, implying an important role of the ferrous sulfide mineral in the redox cycling of U under sulfate reducing conditions. This study also shows that the presence of mackinawite protects uraninite from oxidation by dissolved O₂. The findings of this study suggest that uraninite formation by abiotic reduction by the iron sulfide mineral under low temperature conditions is an important process in the redistribution and sequestration of U in the subsurface environments at U contaminated sites

Sorption and Redox Reactions of As(III) and As(V) within Secondary Mineral Coatings on Aquifer Sediment Grains

Important reactive phenomena that affect the transport and fate of many elements occur at the mineral–water interface (MWI), including sorption and redox reactions. Fundamental knowledge of these phenomena are often based on observations of ideal mineral–water systems, for example, studies of molecular scale reactions on single crystal faces or the surfaces of pure mineral powders. Much less is understood about MWI in natural environments, which typically have nanometer to micrometer scale secondary mineral coatings on the surfaces of primary mineral grains. We examined sediment grain coatings from a well-characterized field site to determine the causes of rate limitations for arsenic (As) sorption and redox processes within the coatings. Sediments were obtained from the USGS field research site on Cape Cod, MA, and exposed to synthetic contaminated groundwater solutions. Uptake of As­(III) and As­(V) into the coatings was studied with a combination of electron microscopy and synchrotron techniques to assess concentration gradients and reactive processes, including electron transfer reactions. Transmission electron microscopy (TEM) and X-ray microprobe (XMP) analyses indicated that As was primarily associated with micrometer- to submicrometer aggregates of Mn-bearing nanoparticulate goethite. As­(III) oxidation by this phase was observed but limited by the extent of exposed surface area of the goethite grains to the exterior of the mineral coatings. Secondary mineral coatings are potentially both sinks and sources of contaminants depending on the history of a contaminated site, and may need to be included explicitly in reactive transport models

Production of Hydrogen Peroxide in Groundwater at Rifle, Colorado

The commonly held assumption that photodependent processes dominate H₂O₂ production in natural waters has been recently questioned. Here, we present evidence for the unrecognized and light-independent generation of H₂O₂ in groundwater of an alluvial aquifer adjacent to the Colorado River near Rifle, CO. In situ detection using a sensitive chemiluminescent method suggests H₂O₂ concentrations ranging from lower than the detection limit (<1 nM) to 54 nM along the vertical profiles obtained at various locations across the aquifer. Our results also suggest dark formation of H₂O₂ is more likely to occur in transitional redox environments where reduced elements (e.g., reduced metals and NOM) meet oxygen, such as oxic–anoxic interfaces. A simplified kinetic model involving interactions among iron, reduced NOM, and oxygen was able to reproduce roughly many, but not all, of the features in our detected H₂O₂ profiles, and therefore there are other minor biological and/or chemical controls on H₂O₂ steady-state concentrations in such aquifer. Because of its transient nature, the widespread presence of H₂O₂ in groundwater suggests the existence of a balance between H₂O₂ sources and sinks, which potentially involves a cascade of various biogeochemically important processes that could have significant impacts on metal/nutrient cycling in groundwater-dependent ecosystems, such as wetlands and springs. More importantly, our results demonstrate that reactive oxygen species are not only widespread in oceanic and atmospheric systems but also in the subsurface domain, possibly the least understood component of biogeochemical cycles

Evaluating Chemical Extraction Techniques for the Determination of Uranium Oxidation State in Reduced Aquifer Sediments

Extraction techniques utilizing high pH and (bi)­carbonate concentrations were evaluated for their efficacy in determining the oxidation state of uranium (U) in reduced sediments collected from Rifle, CO. Differences in dissolved concentrations between oxic and anoxic extractions have been proposed as a means to quantify the U­(VI) and U­(IV) content of sediments. An additional step was added to anoxic extractions using a strong anion exchange resin to separate dissolved U­(IV) and U­(VI). X-ray spectroscopy showed that U­(IV) in the sediments was present as polymerized precipitates similar to uraninite and/or less ordered U­(IV), referred to as non-uraninite U­(IV) species associated with biomass (NUSAB). Extractions of sediment containing both uraninite and NUSAB displayed higher dissolved uranium concentrations under oxic than anoxic conditions while extractions of sediment dominated by NUSAB resulted in identical dissolved U concentrations. Dissolved U­(IV) was rapidly oxidized under anoxic conditions in all experiments. Uraninite reacted minimally under anoxic conditions but thermodynamic calculations show that its propensity to oxidize is sensitive to solution chemistry and sediment mineralogy. A universal method for quantification of U­(IV) and U­(VI) in sediments has not yet been developed but the chemical extractions, when combined with solid-phase characterization, have a narrow range of applicability for sediments without U­(VI)

Speciation and Reactivity of Uranium Products Formed during <i>in Situ</i> Bioremediation in a Shallow Alluvial Aquifer

In this study, we report the results of <i>in situ</i> U­(VI) bioreduction experiments at the Integrated Field Research Challenge site in Rifle, Colorado, USA. Columns filled with sediments were deployed into a groundwater well at the site and, after a period of conditioning with groundwater, were amended with a mixture of groundwater, soluble U­(VI), and acetate to stimulate the growth of indigenous micro­organisms. Individual reactors were collected as various redox regimes in the column sediments were achieved: (i) during iron reduction, (ii) just after the onset of sulfate reduction, and (iii) later into sulfate reduction. The speciation of U retained in the sediments was studied using X-ray absorption spectroscopy, electron microscopy, and chemical extractions. Circa 90% of the total uranium was reduced to U­(IV) in each reactor. Noncrystalline U­(IV) comprised about two-thirds of the U­(IV) pool, across large changes in microbial community structure, redox regime, total uranium accumulation, and reaction time. A significant body of recent research has demonstrated that noncrystalline U­(IV) species are more suceptible to remobilization and reoxidation than crystalline U­(IV) phases such as uraninite. Our results highlight the importance of considering noncrystalline U­(IV) formation across a wide range of aquifer parameters when designing <i>in situ</i> remediation plans