Trace Element Mobility during Corg-Enhanced Denitrification in Two Different Aquifers

Nitrate (NO3−)-polluted groundwater treatment by enhanced denitrification is becoming increasingly important due to rising NO3− concentrations and decreasing degradation capacities in aquifers. Besides evaluating the efficacy of substrates added to trigger denitrification, secondary reactions must be closely monitored. Biodenitrification by applied organic carbon (Corg) can lead to considerable changes in redox potential (Eh) and pH, two decisive parameters for trace element mobility. In this study, two geologically and hydrogeochemically different groundwater catchments important for drinking water production were investigated and compared. Sediments were analyzed for trace elements as well as sulfur (S) and carbon (C) contents. Ongoing hydrogeochemical reactions were evaluated with depth-specific isotope characterization, and the potential for trace element mobilization by Corg addition was determined in column experiments. Results for enhanced denitrification showed up to 3.8 times lower reaction rates with respect to comparable studies, probably due to incomplete formation of the necessary denitrifying bacteria. Concentrations of trace elements such as nickel (Ni) must also be considered when evaluating enhanced denitrification, as these can negatively affect microorganisms. Added ethanol led to Ni concentrations dropping from 0.013 mg/L to below the detection limit. Thus, Corg addition may not only induce denitrification, but also lead to the immobilization of previously released trace elements.

Geochemical Characterization and Trace-Element Mobility Assessment for Metallic Mine Reclamation in Soils Affected by Mine Activities in the Iberian Pyrite Belt

The geochemical characterization of the mine deposits and soils in metal mining areas is essential in order to develop an effective mine reclamation strategy. The determination of total potentially toxic element (PTE) content, together with the application of chemical extraction procedures, can give insight into the behavior of contaminants after the application of different mine reclamation solutions, as well as identify the areas where urgent action is needed. This work presents a practical application to the evaluation of the pollution potential of trace elements in soils affected by mining activities, to be used in metallic mine reclamation. The PTE behavior was assessed by single extractions in order to simulate four environmental conditions: PTE mobility under rainfall conditions, acid mine drainage, reducing conditions, and plant uptake. The spatial distribution of contaminants in the study area was evaluated by determination of PTE total content in soil samples. Trace elements with high natural mobility, such as Zn, appeared concentrated at water and sediment discharge areas, while As, Pb, and Cu contents were higher near the mine wastes. The results obtained after the extractions suggested that the highest PTE content was extracted in the complexing–reducing medium, due to the dissolution of secondary sulfates and Fe3+ oxyhydroxides and the subsequent release of PTEs associated with those mineral phases. Reclamation strategies applied in the study area should promote efficient water drainage, infiltration, and subsuperficial water circulation in order to maintain oxidant conditions in the soil. The methodology applied in this study may constitute a valuable tool to define the geochemical constraints in metal mining areas, as well as help to develop appropriate mine reclamation solutions.

Comparison of trace element mobility from MSWI ash before and after plasma vitrification

A common perception of plasma arc treatment systems for municipal solid waste incineration ash is that the resulting vitrified slag is inert from an environmental perspective. Research was conducted to examine this hypothesis and to assess whether reduced pollutant release results from pollutant depletion during the process of the ash with plasma, or encapsulation in the glassy vitrified matrix. The concentrations of four discrete municipal solid waste incineration ash samples before and after plasma arc vitrification in a bench-scale unit were compared. Slag and untreated ash samples were leached using several standardized approaches and mobility among the four metals of interest (e.g. As, Cd, Pb and Sb) varied across samples, but was generally high (as high as 100% for Cd). Comparison across methods did not indicate substantial encapsulation in the vitrified slag, which suggests that reduced pollutant release from plasma arc vitrified slag is due to pollutant depletion by volatilization, not encapsulation. This has significant implications for the management of air pollution control residues from waste-to-energy facilities using plasma arc vitrification.

Nanoscale evidence of metamorphism – insights from natural and experimentally-treated zircon

<p>Nanoscale analyses of zircon have demonstrated that trace elements, including Pb, can be mobilized to discrete sites in radiation damaged zircon. Although several mechanisms for trace element mobility and segregation in zircon have been proposed, most of this work has been conducted on zircon grains with complex geologic histories, making it difficult to directly determine the mechanisms driving trace element mobility and segregation in zircon. To test among the existing hypotheses for mechanisms driving trace element mobility and segregation, we analyzed both untreated and experimentally heated (1450°C for 24h) Archean zircon using atom probe tomography and transmission electron microscopy (TEM). The sample has a simple, well-characterized thermal history, with no significant thermal events since original crystallization. Despite a high calculated radiation dose (>4 x 10<sup>18</sup> a/g), the untreated zircon does not contain anomalous nanoscale features. In contrast, the experimentally heated zircon contains abundant clusters of Y, Mg, Al, Pb + Yb that range from 5 nm to 25 nm in diameter with toroidal polyhedral morphologies. The <sup>207</sup>Pb/<sup>206</sup>Pb measured from Pb atoms located within these features is consistent with present-day segregation, thus confirming that these nanoscale features were produced by experimental heating in the laboratory. TEM analysis determined that the clusters are dislocation loops, and that cluster morphology is therefore crystallographically controlled. The largest loops are located in {100} and contain high concentrations of Mg and Al.</p><p>These experimentally induced, trace-element-enriched clusters are similar in size, morphology, composition, and crystallographic orientation to clusters observed in zircon affected by natural geologic processes (cf. Valley et al., 2015; Peterman et al., 2016). Although the calculated radiation doses for all analyzed grains are high, comparison of the nanoscale features indicates no apparent correlation between the radiation dose and the density or distribution of clusters. We also observe that trace-element-enriched clusters are conspicuously absent from zircon grains that lack younger igneous or metamorphic rims. These findings suggest that the pressure-temperature-time (P-T-t) history and the dT/dt significantly impact both the nanoscale redistribution of trace elements and the density of these features within zircon. Systematic evaluation of the composition and distribution of these features provides a framework for understanding the nanoscale record of metamorphism.</p><p> </p><p>References:</p><p>Peterman, E.M., Reddy, S.M, Saxey, D.W., Snoeyenbos, D.R., Rickard, W.D.A., Fougerouse, D., and Kylander-Clark, A.R.C. (2016) Nanogeochronology of discordant zircon measured by atom probe microscopy of Pb-enriched dislocation loops. Science Advances, 2, e:1601218.</p><p>Valley, J.W., Reinhard, D.A., Cavosie, A.J., Ushikubo, T., Lawrence, D.F., Larson, D.J., Kelly, T.F., Snoeyenbos, DR., and Strickland, A. (2015) Nano-and micro-geochronology in Hadean and Archean zircons by atom-probe tomography and SIMS: New tools for old minerals. American Mineralogist, 100, 1355-1377.</p>

Dislocation nucleation at nanoscale fluid inclusions: Direct observation from atom probe tomography data of naturally deformed pyrite

<p>In recent years, increasing developments in microscopy and microanalysis have allowed for the direct observation of nanoscale crystalline defects (i.e. dislocations). These defects are particularly important in naturally deformed materials yet this avenue of research remains understudied within the Earth Sciences. Dislocations can now be documented through the use of new and innovative structural and chemical analytical techniques such as electron channeling contrast imaging (ECCI), transmission electron microscopy, and atom probe tomography (APT). The presence and migration of dislocations in crystalline materials, including their role in trace element mobility, play a vital function in the way these materials respond to an applied stress. However, the mechanisms by which dislocations nucleate in minerals remain poorly understood. Prevailing models for dislocation nucleation include generation by Frank-Read sources, stress localization at crack-tips, atomic segregation, and free surface nucleation by critical stress-gradient criterion. Based on recent APT data from naturally-deformed pyrite, combined with electron backscatter diffraction (EBSD) mapping and ECC imaging, we propose a new nucleation mechanism where dislocations are generated by the local stress field in the vicinity of fluid inclusions. The investigated sample consists of a polycrystalline pyrite aggregate within a black shale host rock that has witnessed a peak temperature of 300°C. The combined EBSD and ECCI results reveal crystal plasticity in the form of lattice misorientation up to 8.5° and low-angle grain boundary development. APT data reveals nanoscale fluid inclusions enriched in As, O (H<sub>2</sub>O), Na and K as well as As- and Co-rich dislocations linked by fluid inclusions. This new model is the first documentation with APT methods of fluid inclusions (voids) in minerals, nanoscale features that are commonly misinterpreted as element clusters or chemically-enriched crystal-defects. The combined data has significant trans-disciplinary implications to the geosciences (structural geology, geochemistry, economic geology, geochronology), the material sciences (metals, ceramics, polymers), and analytical microscopy. Within geochronology voids and dislocations such as these in dated minerals may host elements or isotopes that negatively affect their age. Within ore deposit geology, voids in precious metal-hosting minerals may act as the necessary traps to structurally prevent the metals (gold, silver, copper) from migrating or diffusing out of the host mineral. In material sciences, the presence of such crystalline features can either limit or enhance the performance of engineering materials. Thus, performing APT analysis on crystalline material can help us better understand and predict their physical properties.</p>

Phytostabilization of Polluted Military Soil Supported by Bioaugmentation with PGP-Trace Element Tolerant Bacteria Isolated from Helianthus petiolaris

Lead (Pb) and cadmium (Cd) are major environmental pollutants, and the accumulation of these elements in soils and plants is of great concern in agricultural production due to their toxic effects on crop growth. Also, these elements can enter into the food chain and severely affect human and animal health. Bioaugmentation with plant growth-promoting bacteria (PGPB) can contribute to an environmentally friendly and effective remediation approach by improving plant survival and promoting element phytostabilization or extraction under such harsh conditions. We isolated and characterised Pb and Cd-tolerant root-associated bacteria from Helianthus petiolaris growing on a Pb/Cd polluted soil in order to compose inoculants that can promote plant growth and also ameliorate the phytostabilization or phytoextraction efficiency. One hundred and five trace element-tolerant rhizospheric and endophytic bacterial strains belonging to eight different genera were isolated from the aromatic plant species Helianthus petiolaris. Most of the strains showed multiple PGP-capabilities, ability to immobilise trace elements on their cell wall, and promotion of seed germination. Bacillus paramycoides ST9, Bacillus wiedmannii ST29, Bacillus proteolyticus ST89, Brevibacterium frigoritolerans ST30, Cellulosimicrobium cellulans ST54 and Methylobacterium sp. ST85 were selected to perform bioaugmentation assays in greenhouse microcosms. After 2 months, seedlings of sunflower (H. annuus) grown on polluted soil and inoculated with B. proteolyticus ST89 produced 40% more biomass compared to the non-inoculated control plants and accumulated 20 % less Pb and 40% less Cd in the aboveground plant parts. In contrast, B. paramycoides ST9 increased the bioaccumulation factor (BAF) of Pb three times and of Cd six times without inhibiting plant growth. Our results indicate that, depending on the strain, bioaugmentation with specific beneficial bacteria can improve plant growth and either reduce trace element mobility or enhance plant trace element uptake.