RUI: Quantitative XRD-Sequential Extraction Analysis of Trace Metal Speciation in Ultramafic-Derived Soils and Waters

Middlebury College, Middlebury VT

Investigators

Abstract

0126018 Ryan In order to understand anthropogenic alterations to the trace metal budget of natural systems, it is first important to understand the mineralogical and chemical features of such systems. One type of system enriched in trace metals is the natural environment found in association with ultramafic rocks. Previous studies have documented elevated concentrations of trace metals such as Cr, Ni and Co in soils, plants and waters associated with ultramafic rocks-- in some cases, at levels considered to be detrimental to ecological and human health. What these studies have not done is quantitatively analyze speciation of trace metal-bearing phases, which is a crucial component of understanding mobility. We will apply a recently developed quantitative XRD method in conjunction with sequential chemical extractions (SCE) to assess trace metal partitioning. SCE is often used to sequentially dissolve minerals and organic matter in soil. However, it is only when combined with advanced powder XRD techniques that sequential extractions can be used to accurately determine mineral composition, structure, and abundance in the soil. This type of data is especially important because it provides information on (1) the solubility of solid phases in the soil, and (2) the influence of mineral composition and structure on elemental concentration, speciation and mobility. In addition to XRD and SCE (using ICP-AES), C-N analyses will be performed to aid in determinations of soil organic matter content, transmitted light and SEM analyses will be used to assess primary-secondary mineral spatial relationships, and laser diffraction will be used to assess particle size of various soils and mineral phases. Field analyses will include measurements of Eh, pH, Cr and Ni. The intent of this project is to characterize the speciation and concentration of naturally-occurring trace metals in surficial aspects of the hydrosphere, including ultramafic rock (fresh and chemically altered), soil (inorganic solids, organic matter, soil water), surface water, and stream sediment. In order to assess the main controls on weathering and release of trace metals from these rocks, we will sample and analyze rock, soil and water specimens from a wide range of weathering regimes, including (1) the cool temperate, recently glaciated northern Appalachians (Vermont), (2) cool, temperate, unglaciated Cornwall, England, (3) semiarid and subhumid northern California, and (4) humid tropical Costa Rica. Results of this comparative analysis will be applied to a predictive model of trace metal speciation in a wide range of environments.