Speciation and bioavailability of trace metals and phosphorus in soil systems

Accurate knowledge of trace metal speciation is important for many reasons. First, it greatly affects bioavailability and hence uptake/toxicity in organisms. Second, the speciation determines the mobility of the trace metal in the environment. Our mission is to combine state-of-the-art characterization methods (such as synchrotron-based X-ray methods) with geochemical modelling and toxicity studies to arrive at improved understanding of trace metal behavior in soils. Vanadium will be taken as an example of a trace metal for which speciation is key to understanding both the chemical reactivity and biological uptake. Our V K-edge XANES studies show that vanadium added to soils is recovered mostly as vanadate(V) except in acid organic soils in which vanadyl(IV) predominates. The solubility of vanadate(V) therefore turns out to be of major importance for bioavailability, as shown by toxicity studies for barley. Another example is provided concerning how ZVI stabilization affects the speciation and solubility of arsenic and copper.

For phosphorus in agricultural soils, the speciation is rather complex and includes a variety of inorganic P forms as well as organic P. The use of P K-edge XANES spectroscopy has provided us with a tool that allows assessments of how the speciation changes in response to long-term fertilization as well as with depth within a soil profile. From these studies it is clear that adsorbed Al and Fe phases play a crucial role in determining the P dynamics over a time scale from years to decades; however, in some cases Ca-P phases may also form after heavy fertilization, at least temporarily. Moreover these studies show the dramatic changes that occur in P speciation during soil development, from a predominance of apatite to Al and Fe-associated P as well as organic P.

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invited by the group of Environmental Geochemistry

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