Colloidal metals: a confounding factor in bioavailability assessment?

Abstract

Heavy metals and metalloids are strongly partitioned to soil organic matter and inorganic materials such as clays and Fe and Al oxy-hydroxides. A fraction of these soil constituents is mobile in the soil in the form of colloids. As a consequence, metals adsorbed to, or occluded in, colloids may be readily mobile in the environment. Furthermore, inadvertent analysis of colloidally associated metals could confound the assessment of bioavailability when this is extrapolated from chemical analyses of soil solution or metal extractability. In this study we investigated whether colloidal metals are in rapid equilibrium with soil solution. A procedure to measure the non-exchangeable fraction of colloidal metals was developed using a modified isotopic dilution technique. This procedure allows quantification of the fraction of colloidal metals that is not isotopically exchangeable and therefore non bioavailable. Three soils contaminated by heavy metals (Cd, Cu, Zn) and treated with different soil amendments to reduce metal mobility and toxicity were used. The results indicated that treatment with some amendments led to an increase in the amount of colloidal metal. The results also demonstrated that a significant proportion (up to 20%) of colloidal metals in the < 0.2 m fraction existed in a non-bioavailable form.