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

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-mu m 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 0.7 mu m) contained increasing proportions of non-exchangeable Zn and P.