A radio-isotopic dilution technique for functional characterisation of the associations between inorganic contaminants and water-dispersible naturally occurring soil colloids

Abstract

Environmental context The fate and behaviour of inorganic contaminants are dominated by soluble complex formation and interactions with naturally occurring colloids. Although the importance of these interactions has long been debated, our understanding of the mobility and bioavailability of contaminant–colloid associations has been hampered by the limitations of common operationally defined analytical techniques. The method developed in this study facilitates a step forward from operationally defined characterisation of the association between contaminants and colloids to a functional characterisation in terms of their exchangeability and potential bioavailability. Abstract Despite evidence that the fate and behaviour of inorganic contaminants are influenced by their interactions with water-dispersible naturally occurring soil colloids, our understanding of the mobility and bioavailability of contaminant–colloid associations has been hampered by the limitations of common operationally defined analytical techniques. In this paper, an isotopic dilution method was developed to quantify the isotopically exchangeable and non-exchangeable forms of zinc and phosphorus in filtered soil-water extracts. In addition, the effect of filter size on the determination of Zn and P exchangeability was investigated. The results showed that the isotopically non-exchangeable Zn and P in filtered soil-water extracts respectively ranged between 5 and 60% and 10 and 50% and was associated with water-dispersible colloids. Filter pore size had a significant effect on Zn and P exchangeability. Whereas the <0.1-µm filtrates contained isotopically exchangeable Zn and P fractions equal to the total Zn and P concentrations (i.e. 100% isotopically exchangeable Zn and P), the filtrates obtained from larger filter sizes (0.22, 0.45 and 0.7µm) contained increasing proportions of non-exchangeable Zn and P.