P.M. Huang

Biogeochemistry of soil cadmium and the impact on terrestrial food chain contamination

ABSTRACT Cadmium has been recognized to be a highly toxic element, but it was not until recently that concern has been expressed about the possible effects on human health of long-term exposure to low concentrations of this element. The discovery that Cd pollution from a basic metal mining operation could cause serious illness and possibly death has led to public anxiety as well as medical interest. There are many reviews on the chemistry, biochemistry, and biology of soil cadmium. However, to assess the impact of Cd pollution on food chain contamination and ecosystem health, major biogeochemical pathways, particularly those in the rhizosphere, have to be elucidated, and gaps in our knowledge must be identified for future research planning. This chapter addresses the biogeochemistry of soil Cd and the impact of Cd pollution on terrestrial food chain contamination through the rhizosphere. Knowledge of the total content of Cd does not imply comprehensive knowledge of its chemical behavior; rather, it is the chemical speciation of Cd that influences Cds chemical reactivity, mobility, bioavailability, and toxicity in the ecosystem. Therefore, it is essential to investigate the chemical speciation of Cd in soils and sediments, and especially in the rhizosphere, which is the bottleneck of terrestrial food chain contamination. The fractionation of metal–organic complex-bound Cd species is a recent innovation in sequential extraction schemes. The metal–organic complex-bound Cd is the most common among the particulate-bound Cd species of surface soils in temperate and tropical regions. Cadmium present as metal–organic complex-bound species is especially enriched in the rhizosphere soils after application of phosphate fertilizer. The importance of metal–organic complex-bound Cd fractions in assessing phytoavailability of soil Cd has been demonstrated, and thus merits attention.

CARBONATE-INDUCED STRUCTURAL PERTURBATION OF Al HYDROXIDES

The chemistry of Al transformation has been well documented, though little is known about the mechanisms of structural perturbation of Al precipitates by carbonates at a molecular level. The purpose of the present study was to investigate the structural perturbation of Al precipitates formed under the influence of carbonates. Initial carbonate/Al molar ratios (MRs) used were 0, 0.1, and 0.5 after aging for 32 days, then the samples were analyzed by X-ray absorption near edge structure spectroscopy (XANES), X-ray diffraction (XRD), Fourier-transform infrared absorption spectroscopy (FTIR), and chemical analysis. The XRD data were in accord with the FTIR results, which revealed that as the carbonate/Al MR was increased from 0 to 0.1, carbonate preferentially retarded the formation of gibbsite and had relatively little effect on the formation of bayerite. As the carbonate/Al MR was increased to 0.5, however, the crystallization of both gibbsite and bayerite was completely inhibited. The impact of carbonate on the nature of Al precipitates was also evident in the increase of adsorbed water and inorganic C contents with increasing carbonate/Al MR. The Al K- and L- edge XANES data provide the first evidence illustrating the change in the coordination number of Al from 6-fold to mixed 6- and 4-fold coordination in the structural network of short-range ordered (SRO) Al precipitates formed under the increasing perturbation of carbonate. The fluorescence yield spectra of the O K-edge show that the intensity of the peak at 534.5 eV assigned to σ* transitions of Al-O and O-H bonding decreased with increasing carbonate/Al MR. The XANES data, along with the evidence from XRD, FTIR, and chemical analysis showed clearly that carbonate caused the alteration of the coordination nature of the Al-O bonding through perturbation of the atomic bonding and structural configuration of Al hydroxides by complexation with Al in the SRO network of Al precipitates. The surface reactivity of an Al-O bond is related to its covalency and coordination geometry. The present findings were, therefore, of fundamental significance in understanding the low-temperature geochemistry of Al and its impacts on the transformation, transport, and fate of nutrients and pollutants in the ecosystem.