Influence of Divalent Cation Inhibition and Dissolved Organic Matter Enhancement on Phenol Oxidation Kinetics by Manganese Oxides.

Manganese oxides can oxidize organic compounds, such as phenols, and may potentially be used in passive water treatment applications. However, the impact of common water constituents, including cations and dissolved organic matter (DOM), on this reaction is poorly understood. For example, the presence of DOM can increase or decrease phenol oxidation rates with manganese oxides. Furthermore, the interactions of DOM and cations and their impact on the phenol oxidation rates have not been examined. Therefore, we investigated the oxidation kinetics of six phenolic contaminants with acid birnessite in ten whole water samples. The oxidation rate constants of 4-chlorophenol, 4-tert-octylphenol, 4-bromophenol, and phenol consistently decreased in all waters relative to buffered ultrapure water, whereas the oxidation rate of bisphenol A and triclosan increased by up to 260% in some waters. Linear regression analyses and targeted experiments demonstrated that the inhibition of phenol oxidation is largely determined by cations. Furthermore, quencher experiments indicated that radical-mediated interactions from oxidized DOM contributed to enhanced oxidation of bisphenol A. The variable changes between compounds and water samples demonstrate the challenge of accurately predicting contaminant transformation rates in environmentally relevant systems based on experiments conducted in the absence of natural water constituents.

Elevated Radium Activity in a Hydrocarbon-Contaminated Aquifer.

Hydrocarbon spills that reach the subsurface can modify aquifer geochemical conditions. Biogeochemical zones typically form proximal to the source zone that include iron (Fe(III)) and manganese (Mn(III/IV)) (hydr)oxide reduction, with potential to release associated geogenic contaminants to groundwater. Here, multi-level monitoring systems are used to investigate radium (226Ra, 228Ra) activities in an aquifer contaminated with a mixture of chlorinated solvents, ketones, and aromatics occurring as a dense non-aqueous phase liquid in the source zone. 226Ra activities are up to 10 times higher than background 60 m downgradient from the source zone, where pH is lower, total dissolved solid concentrations are higher, and conditions are methanogenic. Correlations indicate that Fe and Mn (hydr)oxide reduction and sorption site competition are likely responsible for elevated Ra activities within the dissolved phase plume. 226Ra activities return to background within the Fe(III)/SO42--reducing zone 600 m downgradient from the source, near the middle of the dissolved phase plume. Geochemical models indicate that sorption to secondary phases (e.g., clays) is important in sequestering Ra within the plume. Although maximum Ra activities within the plume are well below the U.S. drinking water standard, elevated activities compared to background emphasize the importance of investigating Ra and other trace elements at hydrocarbon-impacted sites.

Aqueous Co removal by mycogenic Mn oxides from simulated mining wastewaters.

Naturally occurring manganese (Mn) oxide minerals often form by microbial Mn(II) oxidation, resulting in nanocrystalline Mn(III/IV) oxide phases with high reactivity that can influence the uptake and release of many metals (e.g., Ni, Cu, Co, and Zn). During formation, the structure and composition of biogenic Mn oxides can be altered in the presence of other metals, which in turn affects the minerals' ability to bind these metals. These processes are further influenced by the chemistry of the aqueous environment and the type and physiology of microorganisms involved. Conditions extending to environments that typify mining and industrial wastewaters (e.g., increased salt content, low nutrient, and high metal concentrations) have not been well investigated thus limiting the understanding of metal interactions with biogenic Mn oxides. By integrating geochemistry, microscopic, and spectroscopic techniques, we examined the capacity of Mn oxides produced by the Mn(II)-oxidizing Ascomycete fungus Periconia sp. SMF1 isolated from the Minnesota Soudan Mine to remove the metal co-contaminant Co(II) from synthetic waters that are representative of mining wastewaters currently undergoing remediation efforts. We compared two different applied remediation strategies under the same conditions: coprecipitation of Co with mycogenic Mn oxides versus adsorption of Co with pre-formed fungal Mn oxides. Co(II) was effectively removed from solution by fungal Mn oxides through two different mechanisms: incorporation into, and adsorption onto, Mn oxides. These mechanisms were similar for both remediation strategies, indicating the general effectiveness of Co(II) removal by these oxides. The mycogenic Mn oxides were primarily a nanoparticulate, poorly-crystalline birnessite-like phases with slight differences depending on the chemical conditions during formation. The relatively fast and complete removal of aqueous Co(II) during biomineralization as well as the subsequent structural incorporation of Co into the Mn oxide structure illustrated a sustainable cycle capable of continuously remediating Co(II) from metal-polluted environments.

Investigation of ICP-MS/MS for total sulfur quantification in freshwater dissolved organic matter.

Sulfur-containing functional groups in dissolved organic matter (DOM) interact with trace metals, which in turn affects trace metal mobility and bioavailability in aquatic environments. Typical methods for identification and quantification of sulfur in DOM are costly, complex, and time intensive. Triple quadrupole inductively coupled plasma tandem mass spectrometry (ICP-MS/MS) is capable of part per billion-level sulfur quantification in environmental samples and is a more accessible analytical technique compared with other available methods. This study is the first published investigation of ICP-MS/MS for the direct quantification of sulfur in freshwater DOM. Sulfur (32 S) detection occurs at a mass-to-charge ratio of 48 as 32 S16 O+ after removal of interferences and reaction with oxygen gas. We compare three commonly used DOM preparation methods to assess variability among replicate samples. Preparation of freshwater DOM samples by solid phase extraction followed by evaporation overnight and dissolution in 2% nitric acid results in the most accurate quantification of sulfur. Analysis of sulfur in Suwannee River Fulvic Acid standard serves as method validation, measuring a carbon-normalized sulfur concentration that is ∼20% higher than previously reported methods. We apply the ICP-MS/MS analysis method to determine sulfur concentrations in DOM from nine lakes in the northern Midwest. Carbon-normalized sulfur concentrations in the selected lakes are in general agreement with previously reported percentages of sulfur-containing formulas in DOM found by Fourier transform-ion cyclotron resonance-mass spectroscopy.

Selective Reactivity and Oxidation of Dissolved Organic Matter by Manganese Oxides.

Dissolved organic matter (DOM) varies widely across natural and engineered systems, but little is known about the influence of DOM composition on its reactivity with manganese oxides. Here, we investigate bulk and molecular transformations of 30 diverse DOM samples after reaction with acid birnessite (MnO2), a strong oxidant that may react with DOM in Mn-rich environments or engineered treatment systems. The reaction of DOM with acid birnessite reduces Mn and forms DOM that is generally more aliphatic and lower in apparent molecular weight. However, the extent of reaction depends on the water type (e.g., wastewater, rivers) and highly aromatic DOM undergoes greater changes. Despite the variability in reactivity due to the DOM composition, aqueous products attributable to the oxidation of phenolic precursors are identified in waters analyzed by high-resolution mass spectrometry. The number of matched product formulas correlates significantly with indicators of DOM aromaticity, such as double-bond equivalents (p = 2.43 × 10-4). At the molecular level, highly aromatic, lignin-like carbon reacts selectively with acid birnessite in all samples despite the variability in initial DOM composition, resulting in the formation of a wide range of aqueous products. These findings demonstrate that DOM oxidation occurs in diverse waters but also suggest that reactivity with acid birnessite and the composition of the resulting aqueous DOM pool are composition-dependent and linked to the DOM source and initial aromaticity.

Simultaneous Kinetics of Selenite Oxidation and Sorption on δ-MnO2 in Stirred-Flow Reactors.

Selenium (Se) is an essential and crucial micronutrient for humans and animals, but excessive Se brings negativity and toxicity. The adsorption and oxidation of Se(IV) on Mn-oxide surfaces are important processes for understanding the geochemical fate of Se and developing engineered remediation strategies. In this study, the characterization of simultaneous adsorption, oxidation, and desorption of Se(IV) on δ-MnO2 mineral was carried out using stirred-flow reactors. About 9.5% to 25.3% of Se(IV) was oxidized to Se(VI) in the stirred-flow system in a continuous and slow process, with the kinetic rate constant k of 0.032 h-1, which was significantly higher than the apparent rate constant of 0.0014 h-1 obtained by the quasi-level kinetic fit of the batch method. The oxidation reaction was driven by proton concentration, and its rate also depended on the Se(IV) influent concentration, flow rate, and δ-MnO2 dosage. During the reaction of Se(IV) and δ-MnO2, Mn(II) was produced and adsorbed strongly on Mn oxide surfaces, which was evidenced by the total reflectance Fourier transform infrared (ATR-FTIR) results. The X-ray photoelectron spectroscopy (XPS) data indicated that the reaction of Se(VI) on δ-MnO2 produced Mn(III) as the main product. These results contribute to a deeper understanding of the interface chemical process of Se(IV) with δ-MnO2 in the environment.

Neutralization of high pH and alkalinity effluent from recycled concrete aggregate by common subgrade soil.

Use of recycled concrete aggregate (RCA) as highway basecourse material conserves virgin aggregate, reduces energy consumption and CO2 emissions, and may also decrease costs during construction. However, concerns remain over possible negative environmental impacts associated with high pH (>11) effluent from RCA in contact with water. This study examines the reactive transport of high-pH and high-alkalinity water, modeled on RCA leachate, through model subgrade soils. By developing an understanding of the reactions controlling effluent neutralization, this study aims to quantify the change in pH from the discharge site through surrounding subgrade soils. Four types of subgrade soils with a range of mineral composition, Atterberg limits, and cation exchange capacities (CECs) are examined. They include a clayey sand (SC10), low-plasticity clays (M14, SC25), and a high-plasticity clay (CH38). Batch reaction experiments are used to develop kinetic parameters describing the neutralization of high-pH and -alkalinity leachate by clay minerals through mineral dissolution and reprecipitation. Given this information, a reactive transport model incorporating advection, diffusion, and reaction is used to model the change in pH as a function of distance traveled through model subgrade soils and is applied to laboratory-scale column experiments. The rate at which the high pH front travels is directly related to a soil's clay mineral content. Soils with high CECs effectively delay the propagation of hydroxide front by the dissolution of clay minerals. This study demonstrates that common subgrade soils with moderate clay content will effectively neutralize high pH leachate initially produced by RCA.

Effects of Co(II) ion exchange, Ni(II)- and V(V)-doping on the transformation behaviors of Cr(III) on hexagonal turbostratic birnessite-water interfaces.

Natural birnessite-like minerals are commonly enriched in various transitional metals (TMs), which greatly modify the mineral structure and properties. However few studies are yet conducted systematically on the effects of TM doping on birnessite reactivity towards Cr(III) oxidation. In the present study, the transformation behaviors of Cr(III) on Co-, Ni-, V-containing birnessites were investigated. Co and Ni doping generally decrease the mineral crystalline sizes and hydrodynamic sizes (DH) while V-doping greatly decreases the crystalline sizes but not the DH, owing to particle aggregation. Co and Ni firstly decrease and then increase the mineral zeta potentials (ζ) at pH4 while V decreases ζ. Electrochemical specific capacitances for Co-containing birnessites are gradually reduced, while those for Ni-doped birnessites are slightly reduced and for V-doped birnessites increased, which have a positively linear relationship with the amounts of Cr(III) oxidized by these samples. Cr(III) removal efficiencies from solution by these Co-, Ni- and V-containing birnessites are 26-51%, ∼62-72% and ∼96-100%, respectively, compared to ∼92% by pure birnessite. Cr(III) oxidation kinetics analysis demonstrates the gradual decrease of Mn(IV) and concurrent increase of Mn(III) and the adsorption of mainly Cr(III) on mineral surfaces. A negatively linear relationship exists between birnessite lateral sizes and the proportions of Mn(IV/III) consumed to oxidize Cr(III). Apparent initial Cr(III) oxidation rate (kobs) for Co-containing birnessites are greatly reduced, while those for Ni-doped samples moderately decreased and for V-doped samples first increased and then decreased. A positively or negatively linear relationship exists between kobs or the amount of Mn(II) released and the mineral Mn(IV) content respectively. Cr(III) oxidation probably initiates from layer edge sites of Ni-doped birnessites but the vacancies of Co- and V-containing birnessites. These results provide insights into the reaction mechanisms of Cr(III) with natural birnessite-like minerals.

Identifying the mechanisms of cation inhibition of phenol oxidation by acid birnessite.

Many phenolic compounds found as contaminants in natural waters are susceptible to oxidation by manganese oxides. However, there is often variability between oxidation rates reported in pristine matrices and studies using more environmentally relevant conditions. For example, the presence of cations generally results in slower phenolic oxidation rates. However, the underlying mechanism of cation interference is not well understood. In this study, cation co-solutes inhibit the transformation of four target phenols (bisphenol A, estrone, p-cresol, and triclosan) by acid birnessite. Oxidation rates for these compounds by acid birnessite follow the same trend (Na+ > K+ > Mg2+ > Ca2+) when cations are present as co-solutes. We further demonstrate that the same trend applies to these cations when they are absent from solution but pre-exchanged with the mineral. We analyze valence state, surface area, crystallinity, and zeta potential to characterize changes in oxide structure. The findings of this study show that pre-exchanged cations have a large effect on birnessite reactivity even in the absence of cation co-solutes, indicating that the inhibition of phenolic compound oxidation is not due to competition for surface sites, as previously suggested. Instead, the reaction inhibition is attributed to changes in aggregation and the mineral microstructure.

Recycled concrete aggregate in base course applications: Review of field and laboratory investigations of leachate pH.

The prevalence of construction and demolition (C&D) waste and the concurrent demand for construction aggregate presents the opportunity to recycle C&D waste materials as substitutes for virgin aggregate. Commonly, recycled concrete aggregate (RCA) is used as base course in pavement construction. Environmentally responsible applications of RCA must consider the high pH leachate and trace element leaching risks reported in the literature. This review presents the methodology, results, and limitations of existing laboratory and field investigations of RCA leachate chemistry. Long-term highway field studies of RCA leachate illustrate that an initially high leachate pH approaches neutral within approximately one to two years of construction. Conversely, laboratory investigations of RCA leachate pH using batch reactor leaching tests and column leaching tests measure consistently high leachate pH (pH > 10). The discrepancies between field and laboratory measurements of RCA leachate pH suggest that the current laboratory methodology inadequately describes leachate conditions in the field. The authors recommend that future laboratory investigations consider intermittent wetting and drying cycles, eliminate particle abrasion, employ relevant contact times, and consider additional environmental processes that reduce leachate pH such as soil acidity and carbonation.

Impact of bisphenol A influent concentration and reaction time on MnO2 transformation in a stirred flow reactor.

Bisphenol A (BPA) is an endocrine disrupting compound commonly found in natural waters at concentrations that are considered harmful for aquatic life. Manganese(iii/iv) oxides are strong oxidants capable of oxidizing organic and inorganic contaminants, including BPA. Here we use δ-MnO2 in stirred flow reactors to determine if higher influent BPA concentrations, or introduction rates, lead to increased polymer production. A major BPA oxidation product, 4-hydroxycumyl alcohol (HCA), is formed through radical coupling, and was therefore used as a metric for polymer production in this study. The influent BPA concentration in stirred flow reactors did not affect HCA yield, suggesting that polymeric production is not strongly dependent on influent concentrations. However, changes in influent BPA concentration affected BPA oxidation rates and the rate of δ-MnO2 reduction. Lower aqueous Mn(ii) production was observed in reactors at higher BPA introduction rates, suggesting that single-electron transfer and polymer production are favored under these conditions. However, an examination of Mn(ii) sorption during these reactions indicated that the length of the reaction, rather than BPA introduction rate, caused enhanced aqueous Mn(ii) production in reactors with low introduction rates and longer reaction times due to increased opportunity for disproportionation and comproportionation. This study demonstrates the importance of investigating both the organic and inorganic reactants in the aqueous and solid phases in this complex reaction.

Stability of Ferrihydrite-Humic Acid Coprecipitates under Iron-Reducing Conditions.

Recent studies have suggested the potential for release of iron (hydr)oxide-bound organic carbon (OC) during dissimilatory iron oxide reduction (DIR). However, the stability of iron (hydr)oxide-bound OC in the presence of a natural microbial consortium capable of driving both OC metabolism and DIR has not been resolved. Pure ferrihydrite (Fhy) and Fhy-humic acid coprecipitates (Fhy-HA) were inoculated with a small quantity of freshwater sediment and incubated under anoxic conditions in the presence and absence of H2 or glucose as electron donors for DIR. H2 promoted DIR led to release of ca. 1 mM dissolved organic carbon (DOC). However, comparable amounts of DOC were released from both pure Fhy and Fhy-HA, similar to DOC levels in mineral-free, inoculum-only controls. These results suggest that the observed DOC release during H2-promoted DIR originated from OC contained in the inoculum as opposed to the much larger pool (ca. 38 mM) of OC in the Fhy-HA. Thus, DIR preferentially released sorbed OC with low aromaticity (inoculum OC) versus highly aromatic OC (HA) coprecipitated with iron oxide. Our findings provide new insight into the extent and mechanisms by which DIR is likely to influence aqueous/solid-phase OC partitioning in anoxic soils and sediments.

Use of Routine Soil Tests to Estimate Pb Bioaccessibility.

Soil lead (Pb) hazard level is contingent on bioavailability, but existing assays that estimate Pb bioavailability for human health risks are too expensive or otherwise inaccessible to many people that are impacted by Pb-contaminated soil. This study investigated the use of routine soil nutrient tests to estimate soil-Pb bioaccessibility as a surrogate measure of Pb bioavailability. A silt loam soil was spiked to a target concentration of 2000 mg Pb kg-1 with Pb(NO3)2 and amended with H3PO4 (varying P-to-Pb molar ratios) and KCl (Cl-to-P molar ratio of 2:5) to generate soils with similar total Pb concentrations but a range of Pb bioavailability (and bioaccessibility). Soils were extracted using Mehlich 3, Mehlich 1, Bray P1, Olsen, and  micronutrient (DTPA) methods, and the results were compared to U.S. Environmental Protection Agency method 1340 data as well as to extended X-ray absorption fine structure (EXAFS) spectroscopy. The Mehlich 3 and method 1340 treatment effect ratios were well-correlated ( r2 = 0.88, p ≤ 0.05), whereas Bray P1, DTPA, and Olsen results were more reflective of EXAFS data. Preliminary animal-feeding trials suggest that the Mehlich 3 is as effective as method 1340 at predicting the impact of P treatment on Pb relative bioavailability; however, both methods over-estimated the Pb hazard to mice in P-amended soil. Other routine soil tests that have heightened sensitivity to P amendment (e.g., Bray P1) may be promising candidates for Pb bioaccessibility assessment.

Dual Role of Humic Substances As Electron Donor and Shuttle for Dissimilatory Iron Reduction.

Dissimilatory iron-reducing bacteria (DIRB) are known to use humic substances (HS) as electron shuttles for dissimilatory iron reduction (DIR) by transferring electrons to HS-quinone moieties, which in turn rapidly reduce Fe(III) oxides. However, the potential for HS to serve as a source of organic carbon (OC) that can donate electrons for DIR is unknown. We studied whether humic acids (HA) and humins (HM) recovered from peat soil by sodium pyrophosphate extraction could serve as both electron shuttles and electron donors for DIR by freshwater sediment microorganisms. Both HA and HM served as electron shuttles in cultures amended with glucose. However, only HA served as an electron donor for DIR. Metagenomes from HA-containing cultures had an overrepresentation of genes involved in polysaccharide and to a lesser extent aromatic compound degradation, suggesting complex OC metabolism. Genomic searches for the porin-cytochrome complex involved in DIR resulted in matches to Ignavibacterium/Melioribacter, DIRB capable of polymeric OC metabolism. These results indicate that such taxa may have played a role in both DIR and decomposition of complex OC. Our results suggest that decomposition of HS coupled to DIR and other anaerobic pathways could play an important role in soil and sediment OC metabolism.

Coordination geometry of Zn2+ on hexagonal turbostratic birnessites with different Mn average oxidation states and its stability under acid dissolution.

Hexagonal turbostratic birnessite, with the characteristics of high contents of vacancies, varying amounts of structural and adsorbed Mn3+, and small particle size, undergoes strong adsorption reactions with trace metal (TM) contaminants. While the interactions of TM, i.e., Zn2+, with birnessite are well understood, the effect of birnessite structural characteristics on the coordination and stability of Zn2+ on the mineral surfaces under proton attack is as yet unclear. In the present study, the effects of a series of synthesized hexagonal turbostratic birnessites with different Mn average oxide states (AOSs) on the coordination geometry of adsorbed Zn2+ and its stability under acidic conditions were investigated. With decreasing Mn AOS, birnessite exhibits smaller particle sizes and thus larger specific surface area, higher amounts of layer Mn3+ and thus longer distances for the first MnO and MnMn shells, but a low quantity of available vacancies and thus low adsorption capacity for Zn2+. Zn K-edge EXAFS spectroscopy demonstrates that birnessite with low Mn AOS has smaller adsorption capacity but more tetrahedral Zn (IVZn) complexes on vacancies than octahedral (VIZn) complexes, and Zn2+ is more unstable under acidic conditions than that adsorbed on birnessite with high Mn AOS. High Zn2+ loading favors the formation of VIZn complexes over IVZn complexes, and the release of Zn2+ is faster than at low loading. These results will deepen our understanding of the interaction mechanisms of various TMs with natural birnessites, and the stability and thus the potential toxicity of heavy metal pollutants sequestered by engineered nano-sized metal oxide materials.

Decreased Electron Transfer between Cr(VI) and AH2DS in the Presence of Goethite.

9,10-Anthraquinone-2,6-disulfonic acid (AQDS) is commonly used as a model species to examine the influence of quinones on different biogeochemical cycles. The reduced form of this quinone, AHDS, can donate electrons to the toxic metal species Cr(VI), leading to the precipitation of less soluble Cr(III) phases. Due to the environmental abundance of Fe(III) (oxyhydr)oxides, such as goethite (α-FeOOH), it is important to study the role of these mineral phases on the electron transfer reaction between AHDS and Cr(VI). In this study, this electron transfer reaction is examined in the presence and absence of goethite at three different ratios of AHDS/Cr(VI). Ultraviolet-visible spectroscopy is used to qualitatively assess the oxidation state of AQDS during reactions with goethite. Iron K-edge and Cr K-edge X-ray absorption spectroscopy are used to examine the role of goethite in electron transfer and identify Cr(III) phases that form. Goethite inhibits the extent of Cr(VI) reduction to Cr(III), most notably at the highest ratio of AHDS/Cr(VI) investigated. Production of semiquinone radical species may limit electron transfer and decrease the yields of Fe(II) and Cr(III), both in the presence and absence of goethite. Understanding abiotic electron transfer reactions that occur in systems with multiple redox active species is important to determine the contribution of abiotic redox reactions to Fe biogeochemical cycling in natural soils.

Colonization Habitat Controls Biomass, Composition, and Metabolic Activity of Attached Microbial Communities in the Columbia River Hyporheic Corridor.

Hydrologic exchange plays a critical role in biogeochemical cycling within the hyporheic zone (the interface between river water and groundwater) of riverine ecosystems. Such exchange may set limits on the rates of microbial metabolism and impose deterministic selection on microbial communities that adapt to dynamically changing dissolved organic carbon (DOC) sources. This study examined the response of attached microbial communities (in situ colonized sand packs) from groundwater, hyporheic, and riverbed habitats within the Columbia River hyporheic corridor to "cross-feeding" with either groundwater, river water, or DOC-free artificial fluids. Our working hypothesis was that deterministic selection during in situ colonization would dictate the response to cross-feeding, with communities displaying maximal biomass and respiration when supplied with their native fluid source. In contrast to expectations, the major observation was that the riverbed colonized sand had much higher biomass and respiratory activity, as well as a distinct community structure, compared with those of the hyporheic and groundwater colonized sands. 16S rRNA gene amplicon sequencing revealed a much higher proportion of certain heterotrophic taxa as well as significant numbers of eukaryotic algal chloroplasts in the riverbed colonized sand. Significant quantities of DOC were released from riverbed sediment and colonized sand, and separate experiments showed that the released DOC stimulated respiration in the groundwater and piezometer colonized sand. These results suggest that the accumulation and degradation of labile particulate organic carbon (POC) within the riverbed are likely to release DOC, which may enter the hyporheic corridor during hydrologic exchange, thereby stimulating microbial activity and imposing deterministic selective pressure on the microbial community composition.IMPORTANCE The influence of river water-groundwater mixing on hyporheic zone microbial community structure and function is an important but poorly understood component of riverine biogeochemistry. This study employed an experimental approach to gain insight into how such mixing might be expected to influence the biomass, respiration, and composition of hyporheic zone microbial communities. Colonized sands from three different habitats (groundwater, river water, and hyporheic) were "cross-fed" with either groundwater, river water, or DOC-free artificial fluids. We expected that the colonization history would dictate the response to cross-feeding, with communities displaying maximal biomass and respiration when supplied with their native fluid source. By contrast, the major observation was that the riverbed communities had much higher biomass and respiration, as well as a distinct community structure compared with those of the hyporheic and groundwater colonized sands. These results highlight the importance of riverbed microbial metabolism in organic carbon processing in hyporheic corridors.

Structural Transformation of MnO2 during the Oxidation of Bisphenol A.

Bisphenol A (BPA) is an endocrine-disrupting compound widely used in the plastic industry and found in natural waters at concentrations considered harmful for aquatic life. BPA is susceptible to oxidation by Mn(III/IV) oxides, which are commonly found in near-surface environments. Here, we quantify BPA oxidation rates and the formation of its predominant product, 4-hydroxycumyl alcohol (HCA), in tandem with transformation of a synthetic, Mn(III)-rich δ-MnO2. To investigate the effect of Mn oxide structural changes on BPA oxidation rate, 12 sequential additions of 80 µM BPA are performed at pH 7. During the additions, BPA oxidation rate decreases by 3 orders of magnitude, and HCA yield decreases from 40% to 3%. This is attributed to the accumulation of interlayer Mn(II/III) produced during the reaction, as observed using X-ray absorption spectroscopy, as well as additional spectroscopic and wet chemical techniques. HCA is oxidized at a rate that is 12.6 times slower than BPA and accumulates in solution. These results demonstrate that BPA degradation by environmentally relevant Mn(III/IV) oxides is inhibited by the buildup of solid-phase Mn(II/III), specifically in interlayer sites. Nevertheless, Mn oxides may limit BPA migration in near-surface environments and have potential for use in drinking and wastewater treatment.

Influence of Oxygen and Nitrate on Fe (Hydr)oxide Mineral Transformation and Soil Microbial Communities during Redox Cycling.

Oscillations between reducing and oxidizing conditions are observed at the interface of anaerobic/oxic and anaerobic/anoxic environments, and are often stimulated by an alternating flux of electron donors (e.g., organic carbon) and electron acceptors (e.g., O2 and NO3(-)). In iron (Fe) rich soils and sediments, these oscillations may stimulate the growth of both Fe-reducing bacteria (FeRB) and Fe-oxidizing bacteria (FeOB), and their metabolism may induce cycling between Fe(II) and Fe(III), promoting the transformation of Fe (hydr)oxide minerals. Here, we examine the mineralogical evolution of lepidocrocite and ferrihydrite, and the adaptation of a natural microbial community to alternating Fe-reducing (anaerobic with addition of glucose) and Fe-oxidizing (with addition of nitrate or air) conditions. The growth of FeRB (e.g., Geobacter) is stimulated under anaerobic conditions in the presence of glucose. However, the abundance of these organisms depends on the availability of Fe(III) (hydr)oxides. Redox cycling with nitrate results in decreased Fe(II) oxidation thereby decreasing the availability of Fe(III) for FeRB. Additionally, magnetite is detected as the main product of both lepidocrocite and ferrihydrite reduction. In contrast, introduction of air results in increased Fe(II) oxidation, increasing the availability of Fe(III) and the abundance of Geobacter. In the lepidocrocite reactors, Fe(II) oxidation by dissolved O2 promotes the formation of ferrihydrite and lepidocrocite, whereas in the ferrihydrite reactors we observe a decrease in magnetite stoichiometry (e.g., oxidation). Understanding Fe (hydr)oxide transformation under environmentally relevant redox cycling conditions provides insight into nutrient availability and transport, contaminant mobility, and microbial metabolism in soils and sediments.

A critical review of the reactivity of manganese oxides with organic contaminants.

Naturally occurring manganese (Mn(iii/iv)) oxides are ubiquitous in a wide range of environmental settings and play a key role in numerous biogeochemical cycles. In addition, Mn(iii/iv) oxides are powerful oxidants that are capable of oxidizing a wide range of compounds. This review critically assesses the reactivity of Mn oxides with organic contaminants. Initial work with organic reductants employed high concentrations of model compounds (e.g., substituted phenols and anilines) and emphasized the reductive dissolution of the Mn oxides. Studies with lower concentrations of organic contaminants demonstrate that Mn oxides are capable of oxidizing a wide range of compounds (e.g., antibacterial agents, endocrine disruptors, and pesticides). Both model compounds and organic contaminants undergo similar reaction mechanisms on the oxide surface. The oxidation rates of organic compounds by manganese oxides are dependent upon solution conditions, such as pH and the presence of cations, anions, or dissolved organic matter. Similarly, physicochemical properties of the minerals used affect the rates of organic compound oxidation, which increase with the average oxidation state, redox potential, and specific surface area of the Mn oxides. Due to their reactivity with contaminants under environmentally relevant conditions, Mn oxides may oxidize contaminants in soils and/or be applied in water treatment applications.