Soil-plant-animal relationships and geochemistry of selenium in the Western Phosphate Resource Area (United States): A review.

While naturally found in trace quantities, several regions throughout the world have been designated as "seleniferous" or containing an overabundance of the trace element, selenium (Se), in soil. In particular, portions of the Western Phosphate Resource Area (WPRA) of the United States are considered seleniferous, notably due to past phosphate mining reclamation practices that have promoted Se release and accumulation in soil from weathering overburden waste rock. Concern over Se soil contamination in this region has been attributed to its high levels (ranging from 2.7 to 435 mg Se kg-1 soil), bioavailability, and subsequent hyperaccumulation in vegetation at toxic concentrations (exceeding 10,000 mg Se kg-1 plant tissue). The Se hyperaccumulator, western aster (Symphyotrichum ascendens (Lindl.)), is responsible for the vast majority of acute selenium livestock poisonings and fatalities throughout the region. This inherent bioavailability is largely controlled by soil redox chemistry and sorptive processes. The purpose of this review is to integrate information related to the unique site history of the WPRA from onset mining to current Se problems. This review will provide current details and connection of WPRA mining geology, soil Se geochemistry, plant hyperaccumulation, and related livestock fatalities. Soil remediation strategies will also be discussed along with their applicability and viability in this particular anthropogenically-influenced seleniferous region.

Adsorption of Selenite and Selenate on Ferrihydrite in the Presence and Absence of Dissolved Organic Carbon.

This study examines selenite [Se(IV)] and selenate [Se(VI)] adsorption on two-line ferrihydrite in the presence and absence of two low-molecular-weight dissolved organic carbon (DOC) species, citric and salicylic acid. Ferrihydrite surface potential measurements were also examined to identify shifts in isoelectric points, which suggest possible adsorption mechanisms. Sorption was completed in batch reactor systems at environmentally relevant pH. Our results indicate citric acid suppressed both Se(IV) and Se(VI) sorption on ferrihydrite, which may be caused by competition. This was especially evident at pH 5 to 7 for Se(IV) and pH 5 to 6 for Se(VI). Little sorption suppression was observed for both Se species in the presence of salicylic acid. In the presence of Se(IV) and Se(VI), citric acid adsorption was reduced (pH 5-8). Salicylic acid sorption was almost completely suppressed in the presence of Se(IV) throughout the entire pH range examined, with minimal sorption occurring at pH 5. In the presence of Se(VI), the largest reduction in salicylic acid sorption occurred at pH 5 to 6. Small shifts in the surface potential of ferrihydrite at higher pH suggest that Se(VI) and salicylic acid form weak, outer-sphere complexes. However, at pH 5 and 6, there is a shift in the surface potential measurements to more negative values, indicating possible formation of stronger, inner-sphere complexes. Larger surface potential shifts for Se(IV) and citric acid suggest the formation of strong, inner-sphere complexes. This work demonstrates the ability of low-molecular-weight DOC species (particularly for citric acid) to increase Se(IV) and Se(VI) solubility through sorption suppression.

Selenium speciation in phosphate mine soils and evaluation of a sequential extraction procedure using XAFS.

Selenium is a trace element found in western US soils, where ingestion of Se-accumulating plants has resulted in livestock fatalities. Therefore, a reliable understanding of Se speciation and bioavailability is critical for effective mitigation. Sequential extraction procedures (SEP) are often employed to examine Se phases and speciation in contaminated soils but may be limited by experimental conditions. We examined the validity of a SEP using X-ray absorption spectroscopy (XAS) for both whole and a sequence of extracted soils. The sequence included removal of soluble, PO4-extractable, carbonate, amorphous Fe-oxide, crystalline Fe-oxide, organic, and residual Se forms. For whole soils, XANES analyses indicated Se(0) and Se(-II) predominated, with lower amounts of Se(IV) present, related to carbonates and Fe-oxides. Oxidized Se species were more elevated and residual/elemental Se was lower than previous SEP results from ICP-AES suggested. For soils from the SEP sequence, XANES results indicated only partial recovery of carbonate, Fe-oxide and organic Se. This suggests Se was incompletely removed during designated extractions, possibly due to lack of mineral solubilization or reagent specificity. Selenium fractions associated with Fe-oxides were reduced in amount or removed after using hydroxylamine HCl for most soils examined. XANES results indicate partial dissolution of solid-phases may occur during extraction processes. This study demonstrates why precautions should be taken to improve the validity of SEPs. Mineralogical and chemical characterizations should be completed prior to SEP implementation to identify extractable phases or mineral components that may influence extraction effectiveness. Sequential extraction procedures can be appropriately tailored for reliable quantification of speciation in contaminated soils.

An instrument design for non-contact detection of biomolecules and minerals on Mars using fluorescence.

We discuss fluorescence as a method to detect polycyclic aromatic hydrocarbons and other organic molecules, as well as minerals on the surface of Mars. We present an instrument design that is adapted from the ChemCam instrument which is currently on the Mars Science Lander Rover Curiosity and thus most of the primary components are currently flight qualified for Mars surface operations, significantly reducing development costs. The major change compared to ChemCam is the frequency multipliers of the 1064 nm laser to wavelengths suitable for fluorescence excitation (266 nm, 355 nm, and 532 nm). We present fluorescence spectrum for a variety of organics and minerals relevant to the surface of Mars. Preliminary results show minerals already known on Mars, such as perchlorate, fluoresce strongest when excited by 355 nm. Also we demonstrate that polycyclic aromatic hydrocarbons, such as those present in Martian meteorites, are highly fluorescent at wavelengths in the ultraviolet (266 nm, 355 nm), but not as much in the visible (532 nm). We conclude that fluorescence can be an important method for Mars applications and standoff detection of organics and minerals. The instrument approach described in this paper builds on existing hardware and offers high scientific return for minimal cost for future missions.

Abiotic transformation of high explosives by freshly precipitated iron minerals in aqueous FeII solutions.

Zerovalent iron barriers have become a viable treatment for field-scale cleanup of various ground water contaminants. While contact with the iron surface is important for contaminant destruction, the interstitial pore water within and near the iron barrier will be laden with aqueous, adsorbed and precipitated Fe(II) phases. These freshly precipitated iron minerals could play an important role in transforming high explosives (HE). Our objective was to determine the transformation of RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine), HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine), and TNT (2,4,6-trinitrotoluene) by freshly precipitated iron Fe(II)/Fe(III) minerals. This was accomplished by quantifying the effects of initial Fe(II) concentration, pH, and the presence of aquifer solids (Fe(III) phases) on HE transformation rates. Results showed that at pH 8.2, freshly precipitated iron minerals transformed RDX, HMX, and TNT with reaction rates increasing with increasing Fe(II) concentrations. RDX and HMX transformations in these solutions also increased with increasing pH (5.8-8.55). By contrast, TNT transformation was not influenced by pH (6.85-8.55) except at pH values <6.35. Transformations observed via LC/MS included a variety of nitroso products (RDX, HMX) and amino degradation products (TNT). XRD analysis identified green rust and magnetite as the dominant iron solid phases that precipitated from the aqueous Fe(II) during HE treatment under anaerobic conditions. Geochemical modeling also predicted Fe(II) activity would likely be controlled by green rust and magnetite. These results illustrate the important role freshly precipitated Fe(II)/Fe(III) minerals in aqueous Fe(II) solutions play in the transformation of high explosives.

Oxidation of pentachlorophenol in manganese oxide suspensions under controlled Eh and pH environments.

Abiotic oxidation of pentachlorophenol (PCP) by manganese(IV) oxide (MnO2) was examined in orderto understand the physiochemical environment(s) where PCP oxidation occurs. An Eh-pH potentiostat was used to simulate natural groundwater environments where MnO2 (0.025 g L(-1)) and PCP (0.020 g L(-1)) suspensions were incubated from Eh -300 to 300 mV and pH 4.5 to 7.0. The pH-Eh region where maximum PCP sorption occurred corresponded to the same region where the greatest concentrations of soluble Mn(II) where measured (Eh > -100 mV and pH <5.0). Reduced Mn species [Mn(II,III)] released by reductive dissolution were readsorbed and restricted further abiotic oxidation of PCP by the MnO2 surface. A greater transformation of PCP to primarily tetrachloro-1,4-benzoquinone (p-chloranil) and smaller amounts of lesser chlorinated phenols occurred under increasing pH and Eh conditions.

Adsorption of arsenate and arsenite on ferrihydrite in the presence and absence of dissolved organic carbon.

The adsorption of As(V) and As(III) on synthetic two-line ferrihydrite in the presence and absence of a peat humic acid (HAp), Suwannee River fulvic acid (FA), or citric acid (CA) was investigated. Previous work with goethite has demonstrated the ability of dissolved organic carbon (DOC) to decrease As(V) and As(III) adsorption. The results obtained demonstrate that As(V) adsorption on ferrihydrite was decreased only in the presence of CA. Arsenate decreased the adsorption of all organic acids except HAp. Both FA and CA reduced As(III) adsorption on ferrihydrite, while HAp had no effect. Fulvic and citric acid adsorption on ferrihydrite was decreased in the presence of As(III); however, FA and CA adsorption increased at lower pH, which was consistent with decreased As(III) adsorption. Peat humic acid did not decrease As(III) adsorption, and we believe that the adsorption process of HAp and As(III) and As(V) on ferrihydrite are independent of each other. Previously, we observed that As(V) adsorption on goethite decreased in the presence of HAp > FA > CA, while As(III) adsorption on goethite was decreased similarly to that on ferrihydrite in the presence of CA > FA approximately HAp, yet As(III) adsorption on ferrihydrite was greater than on goethite. The observed differences between this study and the earlier study on goethite are believed to be an intricate function of ferrihydrite's surface characteristics, which affect the mechanisms of adsorption and hence the affinity of organic acids such as HAp, FA, and CA for the ferrihydrite surface. As such, the adsorption of DOCs to ferrihydrite are assumed to be less favorable and to occur with a fewer number of ligands, resulting in lower surface coverage of weaker bond strength.