Mineral surfaces and bioavailability of heavy metals: a molecular-scale perspective

Abstract

There is a continual influx of heavy metal contaminants and pollutants into the biosphere from both natural and anthropogenic sources. A complex variety of abiotic and biotic processes affects their speciation and distribution, including adsorption onto and desorption from mineral surfaces, incorporation in precipitates or coprecipitates, release through the dissolution of minerals, and interactions with plants and microbes. Some of these processes can effectively isolate heavy metals from the biosphere, whereas others cause their release or transformation to different species that may be more (or less) bioavailable and/or toxic to organisms. Here we focus on abiotic adsorption and precipitation or coprecipitation processes involving the common heavy metal contaminant lead and the metalloids arsenic and selenium in mine tailings and contaminated soils. We have used extremely intense x-rays from synchrotron sources and a structure-sensitive method known as x-ray absorption fine structure (XAFS) 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. Our XAFS studies of Se(VI) sorption on Fe2+-containing sulfates show that this element undergoes redox reactions that transform it into less bioavailable and less toxic species. This type of information on molecular-level speciation of heavy metal and metalloid contaminants in various environmental settings is needed to prioritize remediation efforts and to assess their potential hazard to humans and other organisms.

Figure 1

Schematic illustration of a variety of molecular environmental science processes affecting contaminant elements in soils and groundwater.

Figure 2

Blood lead levels in humans in several major lead mining districts (modified after ref. 12).

Figure 3

As K-edge EXAFS data and fluorescent x-ray images for As-contaminated mine tailings from the Mother Lode District of California (derived from data reported in ref. 59). Fits to the EXAFS data are shown in red and molecular models of the As(V) environments in the model compounds used in the fits are also shown.

Figure 4

Pb L_III-edge EXAFS data and fluorescent x-ray images for Pb-contaminated mine tailings from Leadville, CO (derived from data reported in ref. 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.