Metal speciation in soils: the importance of the reactive pool and of aging processes

Metal chemistry and bioavailability in soils is strongly dependent upon metal reactivity. Research has shown how factors such as the pH, organic matter and mineral oxide contents of soils control the partitioning of metals between the solid and solution phases on timescales of minutes to hours. However, complementary research on longer term metal binding to soil clearly shows that on timescales ranging from days to years, partitioning to the soil solids gradually increases as sorbed metal is transformed into ‘unreactive’ forms, due to processes collectively termed aging. This has important consequences for metal retention in soils and for the bioavailability and toxicity of metals in field–contaminated soils as opposed to soils spiked for testing in the laboratory. The aging of a number of metals, including copper, nickel, copper, zinc, cadmium, molybdenum and silver, has been studied over ranges of soil types and timescales up to 2–3 years. These studies generally suggest that soil pH is a major control on the rate and extent of aging. Measurements of solid phase metal speciation using EXAFS or related methods have provided valuable insights into the solid phase speciation. Models of metal aging have been developed and can be incorporated into dynamic models to study how aging is predicted to influence metal cycling in soils over time periods of decades and longer. Key questions for further research include developing further understanding of aging mechanisms, and better understanding of the potential reversibility of aging in response to changes in future soil conditions.