Derek Peak

ATR-FTIR spectroscopic studies of boric acid adsorption on hydrous ferric oxide

Boron is an important micronutrient for plants, but high B levels in soils are often responsible for toxicity effects in plants. It is therefore important to understand reactions that may affect B availability in soils. In this study, Attenuated Total Reflectance Fourier transform Infrared (ATR-FTIR) spectroscopy was employed to investigate mechanisms of boric acid (B(OH)3) and borate (B(OH)4 ) adsorption on hydrous ferric oxide (HFO). On the HFO surface, boric acid adsorbs via both physical adsorption (outer-sphere) and ligand exchange (inner-sphere) reactions. Both trigonal (boric acid) and tetrahedral (borate) boron are complexed on the HFO surface, and a mechanism where trigonal boric acid in solution reacts to form either trigonal or tetrahedral surface complexes is proposed based upon the spectroscopic results. The presence of outer-sphere boric acid complexes can be explained based on the Lewis acidity of the B metal center, and this complex has important implications for boron transport and availability. Outer-sphere boric acid is more likely to leach downward in soils in response to water flow. Outer-sphere boron would also be expected to be more available for plant uptake than more strongly bound boron complexes, and may more readily return to the soil solution when solution concentrations decrease. Copyright

Spectroscopic studies of Pb(II)-sulfate interactions at the goethite-water interface

We used a combination of in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy and X-ray absorption fine structure (XAFS) spectroscopy to conduct molecular scale studies on Pb(II)-sulfate interactions at the solid-water interface of goethite at pH 4.5, 5.0, and 6.0. Both the ATR-FTIR studies (probing sorbed SO4 in a flow cell setup as a function of the Pb concentration) and the EXAFS studies (probing sorbed Pb at high levels of co-adsorbing SO4) indicated the formation Pb-SO4 ternary complexes at the goethite surface. Based on the combined information from the IR and XAFS studies, possible Pb-SO4 ternary complex configurations were presented and discussed by comparison to a set of reference sulfate FTIR spectra. In addition to forming ternary complexes with SO4, adsorption of Pb also promoted SO4 sorption to the goethite surface by changing the surface charge, leading to additional formation of inner- and outer-sphere SO4 sorption complexes not coordinated by Pb. The relative impacts of these mechanisms (i.e., ternary complex formation versus electrostatic effects) appeared to be a function of pH and the level of Pb addition. Formation of ternary complexes was promoted (relative to the importance of electrostatic effects) at low pH values and high Pb concentrations, whereas electrostatic effects were more pronounced at high pH values and low Pb concentrations. In addition, it was found that part of the SO4 initially sorbed at the goethite surface as inner-sphere complexes without being coordinated by Pb was transformed into SO4-Pb ternary complexes as the Pb concentration was increased, an effect most pronounced at low pH. This study shows that co-adsorption of SO4 and Pb may lead to changes in both the extent and mechanisms of the adsorption of these contaminants to the goethite surface relative to binary Pb/goethite and SO4/goethite systems. The presence of co-adsorbing metals or anions may therefore significantly impact the behavior of contaminants in environmental settings.

Direct Observation of Tetrahedrally Coordinated Fe(III) in Ferrihydrite

Ferrihydrite is a common iron hydroxide nanomineral commonly found in soils, sediments, and surface waters. Reactivity with this important environmental surface often controls the fate and mobility of both essential nutrients and inorganic contaminants. Despite the critical role of ferrihydrite in environmental geochemistry, its structure is still debated. In this work, we apply bulk sensitive Fe L edge X-ray absorption spectroscopy to study the crystal field environment of the Fe in ferrihydrite and other Fe oxides of known structure. This direct probe of the local electronic structure provides verification of the presence of tetrahedrally coordinated Fe(III) in the structure of ferrihydrite and puts to rest the controversy on this issue.

Application of XANES spectroscopy in understanding the metabolism of selenium in isolated rainbow trout hepatocytes: insights into selenium toxicity

Selenium (Se) is an essential element, but causes toxic effects in fish at a slightly elevated level beyond the threshold. However, the degree of Se toxicity differs depending on the chemical forms of Se (e.g., organic vs. inorganic) to which fish are exposed to. The mechanisms of Se metabolism and toxicity in fish, particularly at cellular level, are poorly understood. The present study was designed to examine the metabolic fate of different seleno-compounds, both inorganic and organic, in isolated hepatocytes of rainbow trout (Oncorhynchus mykiss) in primary culture using XANES spectroscopy. In cells exposed to 100 μM of selenate and selenite for 6-24 h, elemental Se was found to be the primary metabolite. Whereas, selenocystine appeared to be the major metabolite in cells exposed to 100 μM seleno-L-methionine for 6-24 h. Interestingly, we recorded L-methionine-γ-lyase activity in S9 fraction of cell lysate-an enzyme that directly catalyzes selenomethionine into methylselenol. We also found concurrent reduction of glutathione (GSH) concentration following reaction of seleno-L-methionine with cellular S9 fraction. Moreover, we observed a rapid increase in cellular reactive oxygen species (ROS) generation with increasing seleno-L-methionine exposure dose (100-1000 μM). These findings indicated the rapid cellular metabolism of seleno-L-methionine into methylselenol at higher exposure dose (≥100 μM), and the occurrence of GSH mediated redox cycling of methylselenol--a process that is known to produce reactive oxygen species (ROS). Overall, our results suggest that inorganic and organic selenium are metabolized through different metabolic pathways in rainbow trout hepatocytes. The findings of our study have important implications for understanding the chemical species-specific differences in Se toxicity to fish.

Rates and mechanisms of Zn2+ adsorption on a meat and bonemeal biochar.

Biochars produced from meat and bonemeal (MBM) waste materials contain large amounts of calcium phosphate and are potentially useful sorbents for the remediation of metals. Because the reactivity of biochars depends strongly upon the conditions used in their production, the objective of this study was to evaluate the rates and mechanisms of Zn sorption onto a commercially supplied MBM biochar prior to its application in a field-scale revegetation project. Sorption experiments varying pH, time, and Zn concentration found that, above pH 6.1, Zn adsorbed to MBM biochar quickly (within 5 h) with a maximum adsorption capacity of 0.65 mmol Zn g(-1). Synchrotron-based Zn K-edge extended X-ray absorption fine structure spectroscopy was consistent with a tetrahedral Zn bound to phosphate groups in a monodentate inner-sphere surface complex for all conditions tested. With an acidification pretreatment and at more acidic pH, MBM biochar causes precipitation of a ZnPO4 phase. On the basis of these results, this MBM biochar has a high capacity to rapidly adsorb Zn in adsorption experiments and can be considered a promising sorbent for Zn remediation of contaminated soils.

Tissue-specific accumulation and speciation of selenium in rainbow trout (Oncorhynchus mykiss) exposed to elevated dietary selenomethionine.

The toxicity of selenium in fish is influenced by its chemical speciation and the exposure route. In the natural environment, selenium exposure to fish occurs primarily in the form of selenomethionine in diet. Thus, the main objective of this study was to examine the tissue-specific selenium burden and speciation in fish exposed to elevated dietary selenomethionine. Rainbow trout (Oncorhynchus mykiss) were treated with dietary selenomethionine (40 μg g(-1) dry mass) for 2 weeks, and at the end of the exposure different tissue samples were collected to assess the tissue-specific distribution and speciation of selenium. We used synchrotron-based X-ray absorption near edge spectroscopy (XANES) to determine the selenium speciation profile. Selenomethionine, selenocysteine and selenocystine were found to be the predominant form of selenium in all of the tissues; however their relative proportion varied across different tissues. In general, the organs primarily involved in selenium handling in fish (e.g., liver, kidney) accumulated a higher percentage of selenocystine. We also found that dietary selenomethionine exposure resulted into a marked increase in selenium burden of all major tissues in fish including the brain. Collectively, our findings provide new insights into the tissue-specific distribution and speciation of selenium in fish exposed to selenomethionine via diet.

Soft X-ray induced photoreduction of organic Cu(II) compounds probed by X-ray absorption near-edge (XANES) spectroscopy.

Photoreduction is a major obstacle for using the X-ray absorption near-edge structure (XANES) fingerprint to perform metal speciation at the molecular level in biological and environmental samples, especially for metalloproteins. In this study, soft X-ray induced photoreduction was observed in organic Cu(II) compounds during XANES measurements in a third-generation synchrotron source. Next Cu L(3)-edge, O K-edge, and C K-edge XANES spectroscopy, together with the scanning transmission X-ray microscopy (STXM), were used to probe the specific radiation damage processes of Cu acetate with similar local structures to Cu metalloproteins. Breakup of the Cu-Cu bond was hypothesized for the initial photoreduction of Cu acetate. The following radiation damage of Cu acetate produced CuO and an organic Cu(I) compound with a C═C bond, and the further photoreduction of the resulting CuO to Cu metal was also demonstrated. Our results indicated the importance of consideration of photoreduction during soft XANES measurements for the solid state compounds with high valence metals. Reducing the radiation dose to ~0.1 MGy effectively prevented the photoreduction of organic Cu(II) compounds during these measurements. This proposed radiation damage mechanism in Cu acetate may be generally useful in explaining the photoreduction process in Cu metalloproteins.

Speciation and distribution of copper in a mining soil using multiple synchrotron-based bulk and microscopic techniques

Molecular-level understanding of soil Cu speciation and distribution assists in management of Cu contamination in mining sites. In this study, one soil sample, collected from a mining site contaminated since 1950s, was characterized complementarily by multiple synchrotron-based bulk and spatially resolved techniques for the speciation and distribution of Cu as well as other related elements (Fe, Ca, Mn, K, Al, and Si). Bulk X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy revealed that soil Cu was predominantly associated with Fe oxides instead of soil organic matter. This agreed with the closest association of Cu to Fe by microscopic X-ray fluorescence (U-XRF) and scanning transmission X-ray microscopy (STXM) nanoanalysis, along with the non-occurrence of photoreduction of soil Cu(II) by quick Cu L3,2-edge XANES spectroscopy (Q-XANES) which often occurs when Cu organic complexes are present. Furthermore, bulk-EXAFS and STXM-coupled Fe L3,2-edge nano-XANES analysis revealed soil Cu adsorbed primarily to Fe(III) oxides by inner-sphere complexation. Additionally, Cu K-edge μ-XANES, L3,2-edge bulk-XANES, and successive Q-XANES results identified the presence of Cu2S rather than radiation-damage artifacts dominant in certain microsites of the mining soil. This study demonstrates the great benefits in use of multiple combined synchrotron-based techniques for comprehensive understanding of Cu speciation in heterogeneous soil matrix, which facilitates our prediction of Cu reactivity and environmental fate in the mining site.

Bioaccessibility of Metal Cations in Soil Is Linearly Related to Its Water Exchange Rate Constant

Site-specific risk assessments often incorporate the concepts of bioaccessibility (i.e., contaminant fraction released into gastrointestinal fluids) or bioavailability (i.e., contaminant fraction absorbed into systemic circulation) into the calculation of ingestion exposure. We evaluated total and bioaccessible metal concentrations for 19 soil samples under simulated stomach and duodenal conditions using an in vitro gastrointestinal model. We demonstrated that the median bioaccessibility of 23 metals ranged between <1 and 41% under simulated stomach conditions and < 1 and 63% under simulated duodenal conditions. Notably, these large differences in metal bioaccessibility were independent of equilibrium solubility and stability constants. Instead, the relationship (stomach phase R = 0.927; duodenum phase R = 0.891) between bioaccessibility and water exchange rates of metal cations (k(H₂O)) indicated that desorption kinetics may influence if not control metal bioaccessibility.

Dark channel fluorescence observations result from concentration effects rather than solvent–solute charge transfer

Dark channel fluorescence observations result from concentration effects rather than solvent–solute charge transfer