B13C-1377 INVITED
Reach Scale Studies of Trace Metal Transport in Mountain Streams: Do Wetlands Act as a Sink or a Source?
In the Rocky Mountains, there are many watersheds containing streams impacted by acid rock and acid mine drainage. These streams typically have high concentrations of dissolved metals and copious amounts of iron and aluminum oxides deposited on the streambed. Remediation of these contaminated streams is challenging because of the remote locations of the abandoned mines and the large amounts of metal oxides potentially generated in treatment of the drainage. Wetlands have the potential to attenuate trace metal transport. We examined retention of metals in an iron-oxide rich wetland near Leadville, CO, which had been receiving mine drainage for almost a century. We found that in the summer, the wetland did have a net retentive effect for most metals studied, limiting input into the adjacent stream, whereas, the wetland was a net source of metals during the winter. Furthermore, during spring snowmelt, the wetland did not retain the large pulse of zinc and iron leached by melting snow on the surrounding tailings piles. In another stream system receiving acid rock drainage, we found that riparian wetlands become sources of metals to the stream under drought conditions, which further exacerbated the metal enrichment associated with lesser snowmelt dilution.
B13C-1378
A Thermodynamic Study of Heavy Metal Adsorption at the Biofilm/Mineral Interface: Comparison Between Thermodynamic Model Results and In-situ Measurements
Fe- and Al-(oxyhydr)oxides are among the most reactive mineral surfaces in water-rock systems. Bacteria are effective adsorbents of metal ions and can form colonies on the surfaces of minerals. The resulting biofilm coatings may create local microenvironments that could change significantly trace metal ion sorption compared to biofilm-free mineral surfaces. In this study, ATR-FTIR spectroscopy were used to identify the different types of metal binding sites in Shewanella oneidensis strain MR-1 (wild type) as well as the nature of the functional groups involved in metal complexation. Adsorption affinities of these sites for Pb(II) and Zn(II) as well as site densities were determined by fitting metal adsorption data as a function of pH and ionic strength using a constant capacitance model in the FITEQL computer code. Electrokinetic techniques were used to determine the biofilm influence on the overall mineral surface charge. This influence was quantified by determining the in-situ partitioning of Pb(II) between S. oneidensis MR-1 biofilms and highly polished and oriented single crystal surfaces of α-alumina (1-102) and hematite (0001) using the long-period X-ray standing wave-fluorescent yield (XSW-FY) method. ATR-FTIR spectra of S. oneidensis strain MR-1 revealed the presence of carboxyl, amide, and phosphate groups, as well as carbohydrate moieties. Electrophoretic mobility measurements of S. oneidensis MR-1 cell surfaces indicate that the bacterial surfaces become negatively charged at pH ~ 3.5, suggesting a high Pb(II) and Zn(II) adsorption capacity of the biofilm. Changes in ionic strengths from 1M to 0.01M NaNO3 had no effect on the interactions of the metal ions with the bacteria, suggesting that the physical structure of the cell wall does not change over this range of electrolyte concentrations. The thermodynamic stability of metal complexes on S. oneidensis and the binding sites concentrations for these metal complexes have been determined by FITEQL modelling and these results will be compared to the binding affinities of a-alumina and hematite surfaces alone in order to be able to predict the biofilm influence on mineral surface reactivity for Pb(II) adsorption. In-situ XSW-FY measurements on S. oneidensis biofilm/α-Al2O3 (1-102) and α- Fe2O3 (0001) interfaces indicate that Pb(II) is located at the mineral surface for low Pb(II) concentrations (< 10e-5 M) and is increasingly partitioned into the biofilm at higher concentrations (> 10e-5 M). This partitioning at the biofilm/mineral interface will be compared to the previously determined thermodynamic model and the differences will be discussed in order to define the nature of the local microenvironments at the biofilm/mineral interface.
B13C-1379
Parameters Controlling the Partitioning of Trace Metals at the Shewanella oneidensis MR-1 Biofilm/Mineral/Water Interface: Long Period X-ray Standing Wave and XAFS Study
Microbial biofilms are common in natural and man-made environments and are often present as coatings on mineral surfaces in soils and aquatic systems. Compared to bare mineral surfaces, biofilms can induce significant changes in surface charges and sorption capacities for metal(loid) ions. However, the effects of biofilm coatings on mineral reactivity and metal cycling are still poorly understood at a molecular level due to the complex nature of these systems and the lack of appropriate tools to accurately probe such interfaces. In this study, we applied long-period X-ray standing wave-florescence yield (XSW-FY) spectroscopy to measure in-situ the partitioning of trace elements between S. oneidensis MR-1 biofilms and highly polished single crystal surfaces of alumina (1-102) and hematite (0001) as a function of several external parameters, including trace element concentration, pH, and time, we also studied competitive effects of different cations and anions by exposing the biofilm/mineral interface to different trace elements simultaneously. In addition, grazing incidence X- ray adsorption fine structure (GI-XAFS) spectroscopic measurements at specific x-ray incidence angles were conducted to probe ion speciation and local coordination environment at the mineral surface and in the biofilm. Long-period XSW-FY measurements on Pb(II) partitioning at S. oneidensis biofilm-coated alumina (1-102) and hematite (0001) surfaces under aerobic conditions indicate that Pb(II) is preferentially adsorbed on the mineral surface at low concentrations (10-7 to 10-6 M ) at pH 6.0 and is increasingly partitioned into the biofilm at higher concentrations (10-6 to 10-5 M). This finding indicates that S. oneidensis biofilm coatings do not block reactive sites on hematite and alumina. Decreasing solution pH from 6.0 to 4.0 for biofilm coated alumina (1-102) sample exposed to 10-6 M Pb(II) showed a shift of Pb(II) partitioning from interface to biofilm due to electrostatic effects. Significant changes in Pb(II) XSW-FY profiles at three different exposure times (30 minutes, 3 hours, and 1 day) on fresh samples in each case suggest that Pb(II) partitioning at the biofilm/mineral/water interface is diffusion limited. No apparent competitive effects were observed for Pb(II) and Zn(II). In addition, a variety of cations such as Ca(II), K, and Cu(II) were detected in biofilm-coated mineral samples, and each element exhibited a unique partitioning behavior. Results of Pb L3-edge GI-XAFS analysis of Pb(II)/S. oneidensis biofilm/hematite samples showed that carboxyl groups are responsible for Pb(II) complexation in the biofilm after 3 hours at pH 6.0. No evidence of biomineralization was observed under our experimental conditions. These studies provide new insights about the factors controlling trace element partitioning and speciation at complex microbe-mineral interfaces and an improved understanding of the nature of microenvironments created by microbial biofilms.
B13C-1380
Zinc and Arsenic Immobilization and Magnetite Formation Upon Maghemite Reduction by Shewanella putrefaciens ATCC 8071
Dissimilatory reduction of ferric iron oxides is recognized as an important component of the iron biogeochemical cycle, causing the dissolution of iron oxide minerals and the possible formation of Fe(II)-bearing minerals such as magnetite, green rusts, siderite, etc. These mineralogical transformations affect the mobility of surface- associated toxic metal(loid)s, which may be released into solution, adsorbed, or incorporated into newly formed minerals. Maghemite (γ-Fe2O3) is an iron oxide mineral that is found in certain tropical soils and as isolated deposits in more temperate regions. In these settings, maghemite may play an important role in the biogeochemical cycling of iron and of surface-associated trace metal(loids). However, the reduction of maghemite by iron-respiring bacteria, the impact of reductive dissolution on the release of associated contaminants, and the nature of biogenic Fe(II)-containing reaction products are not well documented. In the present study, we incubated samples of pure maghemite and As(V)- and Zn-adsorbed maghemite with an iron reducing bacterium, Shewanella putrefaciens strain ATCC 8071, in a batch system under anoxic conditions. As a result of Fe(III) bioreduction, all mineral suspensions turned from brown to black during the first hour of incubation, indicating the onset of magnetite formation. The presence of this mineral was confirmed by transmission Mössbauer spectroscopy at room temperature, which showed the formation of an almost stoichiometric magnetite. High-resolution transmission electron microscopy images indicate that the parent maghemite and the biogenic magnetite particles are octahedral in shape and of similar size (5 to 20 nm). The presence of 50 mg/L adsorbed Zn(II) did not affect the initial rate of iron reduction with respect to the Zn-free maghemite sample (0.62 mM Fe(II)/h and 0.66 mM Fe(II)/h, respectively). However, adsorption of 50 and 100 mg/L As(V) on maghemite decreased the initial iron reduction rate to 0.35 mM Fe(II)/h and 0.16 mM Fe(II)/h, respectively. Results show that before inoculation, the proportion of As(V) adsorbed on the maghemite surface is significantly higher (90%) than Zn(II) (50%) after a two-day equilibration period. During bioreduction, the remaining soluble zinc and arsenic content decreases progressively as a function of time, possibly as a result of coprecipitation or adsorption reactions on the newly formed biogenic magnetite. Overall, this study shows that magnetite formation induced by the bioreduction of maghemite is particularly effective for the removal of zinc and arsenic from solution.
B13C-1381
Sulfide-driven arsenic solubilization from arsenopyrite and pyritic black shale
Groundwater in the Newark Basin, located primarily in northern New Jersey and Eastern Pennsylvania, locally has arsenic levels in excess of 10 ug L-1 in up to 30% of the water supply wells sampled. One possible source of this arsenic is pyrite in black shale that weathers in ground-water recharge areas. As part of a larger study of microbial arsenic mobilization and transformation in the Newark Basin, we examined the role of sulfide in arsenic mobilization from arsenopyrite, pyritic black shale, and arsenic-rich pyrite. Based on preliminary observations of biologically mediated mobilization of arsenic from Newark Basin black shale under hypoxic (N2 atmosphere) conditions, we hypothesized that sulfide generated by biological activity drives arsenic release from arsenic-rich pyrite. To examine this hypothesis, we conducted a series of sulfide-arsenide exchange experiments with arsenopyrite, pyritic black shale, and arsenic-rich pyrite under oxic (21% O2), hypoxic (1-2% O2, N2), and anoxic (5% H2, 95%N2) conditions. Results show that sulfide (1 mM initial concentration) drives arsenic solubilization from all three solids under hypoxic and oxic, but not anoxic conditions. XANES results show that arsenic in pyrite from Newark Basin black shale has the same oxidation state (-1) as arsenic in arsenopyrite, supporting a proposed sulfide-arsenide exchange mechanism in which sulfide replaces arsenic in arsenopyrite and arsenide is oxidized to arsenite. Since the proposed reaction requires an oxidant in addition to sulfide, it would occur only under conditions of redox disequilibrium in which sulfide and an oxidant transiently co-exist.
B13C-1382
Enhancing Potentially Plant-Available Lead Concentrations in Contaminated Residential Soils Using a Biodegradable Chelating Agent
Chelation of heavy metals is an important factor in enhancing metal solubility and, hence, metal availability to plants to promote phytoremediation. In the present study, we compared the effects of application of a biodegradable chelating agent, namely, ethylenediaminedisuccinic acid (EDDS) on enhancing plant available form of lead (Pb) in Pb-based paint contaminated residential soils compared to that of a more commonly used, but non-biodegradable chelate, i.e., ethylenediaminetetraacetic acid (EDTA). Development of a successful phytoremediation model for metals such as Pb depends on a thorough understanding of the physical and chemical properties of the soil, along with the optimization of a chelate treatment to mobilize Pb from `unavailable' pools to potentially plant available fraction. In this context, we set out to perform batch incubation experiments to investigate the effectiveness of the two aforementioned chelates in enhancing plant available Pb at four different concentrations (0, 5, 10 and 15 mM/kg soil) and three treatment durations (0, 10 and 30 days). We selected 12 contaminated residential soils from two major metropolitan areas (San Antonio, TX and Baltimore, MD) with varying soil physico-chemical properties – the soils from San Antonio were primarily alkaline and those from Baltimore were typically acidic. Total soil Pb concentrations ranged between 256 mg/kg and 4,182 mg/kg. Our results show that both chelates increased the solubility of Pb, otherwise occluded in the complex soil matrix. For both EDTA and EDDS, the exchangeable concentrations of soil Pb also increased with increase in chelate concentration and incubation time. The most effective treatment was 15 mM chelate kg-1 soil incubated for 30 days, which caused many fold increase in potentially plant available Pb (a combination of the soluble and exchangeable fractions) relative to the unamended controls. Step wise multiple linear regression analysis using chelate-extractable Pb and soil properties showed that plant available Pb fraction could be assessed from the two inter-related soil parameters: soil organic matter and soil pH. Although EDTA was more effective in Pb solubilization than EDDS, the rapid kinetics of the Pb-EDTA complexation process and the prolonged persistence of EDTA in soils pose a potential groundwater contamination problem via metal leaching. In contrast to EDTA, EDDS addition caused relatively slow release of Pb from the soil matrix. The biodegradable nature (and short half life) of EDDS in soils makes it a promising chelating agent for use as soil amendment to enhance Pb solubilization and hence, potential plant uptake.
B13C-1383
Role of bacteria on the long term behaviour of waste confinement matrixes
Bacteria are often presented as source of problems because they are suspected to accelerate the alteration of materials. Numerous works are based on patterns of holes and channels attributed to bacterial activity while no laboratory experiments clearly support these conclusions. A review of this important question will be presented. If microorganisms like Acidi Thiobacillus thiooxydans is able to metabolize strong acid by sulphur oxidation and contribute to accelerate the degradation of rocks and minerals, others one like Pseudomonas aeruginosa may produce protective exopolysaccharides (EPSs). These EPS may protect materials and also trap potentially toxic elements. To model the long term behaviour of waste confinme,nt matrixes, the effect of bacteria cannot be describe only on the basis of morphological patterns. One have to measure real rate which is only possible with specific growth media permitting to analyze weak concentration variations by ICP-MS (tracers)
B13C-1384
Potassium Solubilization in Fungal Degradation of Aluminosilicate Minerals
Potassium is an essential soil nutrient that performs a multitude of important biological functions to maintain plant growth and health. However, plants cannot directly use mineralic potassium. Only those that are released by weathering or dissolved in soil water are available for plants' nutrient uptake. On the other hand, microorganisms and related biological activities often play critical roles in mineral weathering and hence participate heavily in the geochemical cycles of nutrient elements. Here, we study the microbial release of potassium from K-bearing minerals orthoclase and illite. A strain of thermophilic fungus A. fumigatus was cultured with a mixture of the minerals to determine if microbe-mineral interactions enhance the solubilization of mineralic potassium. Experiments were carried in two settings, one with the mineral grains and the fungal cells in direct contact, and the other employing a membrane (pore size 0.22 um) to separate the two. Measurements over a period of 30 days showed that, irrespective of the experimental setup, the concentration of free K in the culture was drastically higher than those in any of the control experiments where no living organism was present. Moreover, the occurrence of mineral-cell physical contact enhanced potassium release by an additional factor of 3 to 4 in comparison to the separation experiments. For contact experiments, Electron Probe Microanalysis revealed the formation of mycelium-mineral aggregates, and Atomic Force Microscopy imaging further indicated the possible ingestion of mineral particles by the fungus cells. Contrasting to what was observed and expected in control experiments, the potassium solubilization rate showed a positive dependence upon pH when fungi and minerals were mixed directly, and exhibited no correlations with solution acidity if cell-rock contact was restrained. These results appear to suggest that A. fumigatus promoted potassium release by means of at least three likely routes, one through the complexation of soluble organic ligands, another appealing to the immobile biopolymers such as the insoluble components of secretion, and the third related to the mechanical forces in association with the direct physical contact between cells and mineral particles.
B13C-1385
Phosphorous availability influences the dissolution of apatite by soil fungi
We conducted mineral dissolution experiments using fungi isolated from a grassland soil in northern California to determine the response of fungi to different levels of phosphorus availability and to identify pathways of apatite dissolution by fungal exudates. Fluorapatite dissolution experiments were performed either with fungi present or under abiotic conditions using cell-free liquid media conditioned by fungal growth at different phosphorus and calcium availabilities. Among biogeochemically active soil fungal isolates apatite dissolution was either active in response to phosphorus limiting growth conditions or passive as a result of mycelial growth. Zygomycete isolates in the order of Mucorales acidify their growth media substrate in the presence of phosphorus, mainly through production of oxalic acid. Cell-free exudates induced fluorapatite dissolution at a rate of 10 -0.9 ± 0.14 and 10 -1.2 ± 0.22 mmol P/m2/s. The Ascomycete isolate, in the family Trichocomaceae, induced fluorapatite dissolution at a rate of 10 - 1.1 ± 0.05 mmol P/m2/s by lowering the pH of the media under phosphorus-limited conditions, without producing significant amounts of low molecular weight organic acids (LMWOAs). Oxalate strongly etches fluorapatite along channels parallel to [001], forming needle like features, while exudates from Trichocomaceae induced surface rounding. We conclude that while LMWOAs are well-studied weathering agents these does not appear to be produced by fungi in response to phosphorus limiting growth conditions.