(NAS Colloquium) Geology, Mineralogy, and Human Welfare (1999) / Chapter Skim
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Mineral Surfaces and Bioavailabilty of Heavy Metals: A Molecular-scale Perspective
Mineral Surfaces and Bioavailabilty of Heavy Metals: A Molecular-scale Perspective
Pages 3388-3395
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From page 3388... ...
spectroscopy to determine the molecular-level speciation of these elements at concentrations of 50 to several thousand ppm in the contaminated environmental samples as well as in synthetic sorption samples. Our XAFS studies of As and Pb in the mine tailings show that up to 50% of these contaminants in the samples studied may be present as adsorbed species on mineral surfaces, which makes them potentially more bioavailable than when present in sparingly soluble solid phases.
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From page 3389... ...
When present in crystalline solids such as galena or pyromorphite, lead should be less soluble, less mobile, and less bioavailable than when present as sorbed species on mineral surfaces, where it may be relatively easily removed into solution by a reduction in pH from neutral to acidic, such as might occur when a lead-contaminated soil sample is ingested by an organism (see, e.g., ref.
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of surface complexes or precipitates that form when these metals or metalloids partition from aqueous solution onto the mineral surface, a process generally referred to as sorption when the details of the adsorption mechanism are not fully known. We also present similar spectroscopic studies on the speciation of these heavy metals/ metalloids in several natural multiphase systems, including Secontaminated soils and sediments and As- and Pb-contaminated mining wastes.
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From page 3391... ...
Sorption on mineral surfaces is an important process that can bind and sequester heavy metals and other aqueous contaminant ions. Sorption can dramatically reduce the mobility of contaminants in groundwater and, in the case of redox-sensitive elements, result in their transformation into a less (or more)
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From page 3392... ...
62~. EXAFS spectra of crystalline model compounds are shown in the center panel, and linear least-squares fits of EXAFS spectra of two mine tailings samples by using the model compound spectra are shown in the side panels Bar plots of the abundance of different lead species are shown in the bottom panels on each side of the figure.
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From page 3393... ...
The XAFS spectra of crystalline model compounds were collected in transmission mode under ambient conditions. In none of these experiments was evidence found for oxidation or reduction of the sorbate ion during or after XAFS data collection, as indicated by the lack of energy shifts in the edge positions, extended x-ray absorption fine structure (EXAFS)
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From page 3394... ...
In contaminated soils and mine tailings, for example, they can occur in primary minerals, secondary minerals formed by weathering of primary m~nerals in situ, solid precipitates formed by reactions of contaminant ions in groundwater with other aqueous ions, and adsorbed species. Although it is relatively easy to determine the types of solid phases present in a contaminated soil or mine tailings sample and the concentration levels of heavy metals and metalloids they contain by using a combination of x-ray diffraction and analytical methods, it is considerably more difficult to assess the importance of adsorbed heavy metal/metalloid species, particularly at very low surface coverages.
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From page 3395... ...
(1995) in Mineral Surfaces, Vaughan, D
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