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.

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.

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.

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.

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.

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.

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.

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 bicarbonate-rich liquid condensed phase in non-saturated solutions in the absence of divalent cations.

Bicarbonate (HCO3 -) and sodium (Na+)-containing solutions contain droplets of a separate, bicarbonate-rich liquid condensed phase (LCP) that have higher concentrations of HCO3 - relative to the bulk solution in which they reside. The existence and composition of the LCP droplets has been investigated by nanoparticle tracking analysis, nuclear magnetic resonance spectroscopy, refractive index measurements and X-ray pair distribution function analysis. The bicarbonate-rich LCP species is a previously unaccounted-for, ionic phenomenon which occurs even in solutions with solely monovalent cations. Its existence requires re-evaluation of models used to describe and model aqueous solution physicochemistry, especially those used to describe and model carbonate mineral formation.

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.

Chromium(III) oxidation by three poorly crystalline manganese(IV) oxides. 2. Solid phase analyses.

Layered, poorly crystalline Mn(IV)O(2) phases are abundant in the environment. These mineral phases may rapidly oxidize Cr(III) to more mobile and toxic Cr(VI) in soils. There is still, however, little knowledge of how Cr(III) oxidation by Mn(IV)O(2) proceeds at the microscopic and molecular levels. Therefore, the sorption mechanisms of Cr(III) and Cr(VI) on Random Stacked Birnessite (RSB), δ-MnO(2), and Acid Birnessite (AB) were determined by Extended X-ray Absorption Fine Structure Spectroscopy (EXAFS). These three synthetic Mn(IV)O(2), which are poorly crystalline phases and have layered structures, were reacted with 50 mM Cr(III) at pH 2.5, 3, and 3.5 before being analyzed by EXAFS. The results indicated that Cr(VI) was loosely sorbed as an outer-sphere complex on Mn(IV)O(2), while Cr(III) was tightly sorbed as an inner-sphere complex. Further research is needed to understand why Cr(III) stopped being significantly oxidized by Mn(IV)O(2) after 30 min. This study, however, demonstrated that the formation of a Cr surface precipitate is not necessarily responsible for the cessation in Cr(III) oxidation. Indeed, no Cr surface precipitate was detected at the microscopic and molecular levels on Mn(IV)O(2) surfaces reacted with Cr(III) for 1 h, although the Cr(III) oxidation ceased before 1 h of reaction at most employed experimental conditions.

Chromium(III) oxidation by three poorly-crystalline manganese(IV) oxides. 1. Chromium(III)-oxidizing capacity.

The Cr(III)-oxidizing capacity of three layered poorly crystalline Mn(IV)O(2) phases, i.e. δ-MnO(2), Random Stacked Birnessite (RSB), and Acid Birnessite (AB), was determined in real-time and in situ, using Quick X-ray Absorption Fine Structure Spectroscopy (Q-XAFS). The results obtained with this technique, which allows the measurement of the total amount of Cr(VI) produced in the system, indicated that the Cr(III) oxidation reaction had ceased between 30 min and 1 h under most experimental conditions. However, this cessation was not observed with a traditional batch technique, which only allows the measurement of Cr(VI) present in solution and thus neglects the amount of Cr(VI) that may be sorbed to Mn(IV)O(2). This study also demonstrated that the Mn(IV)O(2) phase oxidizing the highest amount of Cr(III), which is positively charged in solution, was the mineral featuring the most negatively charged surface. Also, the results indicated that the presence of Mn(II) and/or Mn(III) impurities inside the Mn(IV)O(2) structure could enhance the mineral's capacity to oxidize Cr(III). The information provided in this study will be useful in predicting the capabilities of naturally occurring Mn oxide minerals, which are similar to the three synthetic Mn(IV)O(2) investigated, to oxidize Cr(III) to toxic and mobile Cr(VI) in the soil of contaminated sites.

Synthesis and evaluation of substrate analogue inhibitors of trypanothione reductase.

Trypanothione reductase (TR) is found in the trypanosomatid parasites, where it catalyses the NADPH-dependent reduction of the glutathione analogue, trypanothione, and is a key player in the parasite's defenses against oxidative stress. TR is a promising target for the development of antitrypanosomal drugs; here, we report our synthesis and evaluation of compounds 3-5 as low micromolar Trypanosoma cruzi TR inhibitors. Although 4 and 5 were designed as potential irreversible inhibitors, these compounds, as well as 3, displayed reversible competitive inhibition. Compound 3 proved to be the most potent inhibitor, with a K(i) = 2 µM.

Quantification of rapid environmental redox processes with quick-scanning x-ray absorption spectroscopy (Q-XAS).

Quantification of the initial rates of environmental reactions at the mineral/water interface is a fundamental prerequisite to determining reaction mechanisms and contaminant transport modeling and predicting environmental risk. Until recently, experimental techniques with adequate time resolution and elemental sensitivity to measure initial rates of the wide variety of environmental reactions were quite limited. Techniques such as electron paramagnetic resonance and Fourier transform infrared spectroscopies suffer from limited elemental specificity and poor sensitivity to inorganic elements, respectively. Ex situ analysis of batch and stirred-flow systems provides high elemental sensitivity; however, their time resolution is inadequate to characterize rapid environmental reactions. Here we apply quick-scanning x-ray absorption spectroscopy (Q-XAS), at sub-second time-scales, to measure the initial oxidation rate of As(III) to As(V) by hydrous manganese(IV) oxide. Using Q-XAS, As(III) and As(V) concentrations were determined every 0.98 s in batch reactions. The initial apparent As(III) depletion rate constants (t < 30 s) measured with Q-XAS are nearly twice as large as rate constants measured with traditional analytical techniques. Our results demonstrate the importance of developing analytical techniques capable of analyzing environmental reactions on the same time scale as they occur. Given the high sensitivity, elemental specificity, and time resolution of Q-XAS, it has many potential applications. They could include measuring not only redox reactions but also dissolution/precipitation reactions, such as the formation and/or reductive dissolution of Fe(III) (hydr)oxides, solid-phase transformations (i.e., formation of layered-double hydroxide minerals), or almost any other reaction occurring in aqueous media that can be measured using x-ray absorption spectroscopy.

Arsenite oxidation by a poorly-crystalline manganese oxide. 3. Arsenic and manganese desorption.

Arsenic (As) mobility in the environment is greatly affected by its oxidation state and the degree to which it is sorbed on metal oxide surfaces. Manganese oxides (Mn oxides) have the ability to decrease overall As mobility both by oxidizing toxic arsenite (As(III)) to less toxic arsenate (As(V)), and by sorbing As. However, the effect of competing ions on the mobility of As sorbed on Mn-oxide surfaces is not well understood. In this study, desorption of As(V) and As(III) from a poorly crystalline phyllomanganate (δ-MnO(2)) by two environmentally significant ions is investigated using a stirred-flow technique and X-ray absorption spectroscopy (XAS). As(III) is not observed in solution after desorption under any conditions used in this study, agreeing with previous studies showing As sorbed on Mn-oxides exists only as As(V). However, some As(V) is desorbed from the δ-MnO(2) surface under all conditions studied, while neither desorptive used in this study completely removes As(V) from the δ-MnO(2) surface.

Speciation and release kinetics of cadmium in an alkaline paddy soil under various flooding periods and draining conditions.

This study determined Cd speciation and release kinetics in a Cd-Zn cocontaminated alkaline paddy soil, under various flooding periods and draining conditions, by employing synchrotron-based techniques, and a stirred-flow kinetic method. Results revealed that varying flooding periods and draining conditions affected Cd speciation and its release kinetics. Linear least-squares fitting (LLSF) of bulk X-ray absorption fine structure (XAFS) spectra of the air-dried, and the 1 day-flooded soil samples, showed that at least 50% of Cd was bound to humic acid. Cadmium carbonates were found as the major species at most flooding periods, while a small amount of cadmium sulfide was found after the soils were flooded for longer periods. Under all flooding and draining conditions, at least 14 mg/kg Cd was desorbed from the soil after a 2-hour desorption experiment. The results obtained by micro X-ray fluorescence (µ-XRF) spectroscopy showed that Cd was less associated with Zn than Ca, in most soil samples. Therefore, it is more likely that Cd and Ca will be present in the same mineral phases rather than Cd and Zn, although the source of these two latter elements may originate from the same surrounding Zn mines in the Mae Sot district.

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.