Synchrotron analyses of the chemical nature of isotopically exchangeable (potentially phytoavailable) soil cadmium

摘要

It is widely accepted that the chemical form of an element in soil which is potentially phytoavailable (L or E value) is surface-adsorbed and exchangeable with the soil solution. Nevertheless, it is often claimed that the isotopically exchangeable concentration of an element defies chemical definition as it varies as a function of contact time between the isotope tracer and soil. Therefore, this study used X-ray absorption spectroscopy (XAS) and linear combination fitting (LCF) techniques to compare the speciation of Cd in particle size fractions of a polluted calcareous soil (and its changes to acidification) to the soil's isotopically exchangeable Cd concentration. Despite relatively low Cd concentrations, reasonable Cd K-edge EXAFS spectra were obtained in the separated soil particle size fractions to 7.4mg Cd/kg. The Cd K-edge EXAFS spectra of the<25m particle size fractions were analogous to samples to which low concentrations of Cd (<50mg Cd/kg) had been added, suggesting that Cd is primarily in an exchangeable form in these particle sizes. In addition, acidification of this soil (pH 5.8) did not affect the speciation of Cd in the <25m particle size fractions. A simple comparison of Cd concentrations in the particle size fractions<25m to the total isotopically exchangeable Cd concentrations measured in this soil, at pH 7.5 (it's natural pH) and pH 5.8, revealed no significant difference between these values (t-test, p<0.05). These results, therefore, support the original contention of McAuliffe et al. (1947) that the concentration of surface-adsorbed element that is exchangeable with the soil solution is isotopically exchangeable and, hence, potentially phytoavailable.