Speciation in metal contaminated soils as revealed by an ion exchange resin membrane fractionation procedure

Abstract

Ion exchangers have proven to be a useful tool in the study of metal speciation in aquatic environments, but have seen little application in the study of metal behavior in soil environments. The labile metal species in polluted soils were evaluated by equilibrating soil suspensions with ion exchange resin membranes of different types at pH values ranging from 3 to 9. The total soluble metal content of cadmium (Cd), chromium (Cr), nickel (Ni), and lead (Pb) contaminated Western Canadian soils was subdivided into (i) low‐pH labile, (ii) weak‐acid labile, (iii) weak‐base labile, (iv) high‐pH labile, and (v) non‐adsorbable forms using cation and anion exchange membranes. Soil suspension is mixed overnight with different types of resin membranes and the cations transferred from the soil are subsequently eluted from the membranes using 1N HCl. The HCl extract is then analyzed for Cd, Cr, Ni, and Pb. The aqueous phase remaining in contact with the soil residue is considered the amount of released non‐labile, non‐adsorbable species. The low‐pH labile fraction constituted the largest proportion of the added metal in poorly buffered (sandy) soils. Weak‐acid and base labile fractions were typically highest in highly buffered soils. Clearly, metal contaminated soils most likely to cause environmental damage are sandy textured soils subject to acidification, although the production of chelating substances by roots and microorganisms may also mobilize considerable quantities of metal in soils of high clay content.