Lead, antimony and arsenic in dissolved and colloidal fractions from an amended shooting-range soil as characterised by multi-stage tangential ultrafiltration and centrifugation

Abstract

Environmental context The size of soil colloids is – among other characteristics – crucial for the mobility of associated contaminants. We analysed the effect of liming on the size of colloids mobilised from strongly contaminated shooting-range soils using multi-stage tangential ultrafiltration (MTUF) for the size fractionation of dispersed soil colloids. Our results indicate the high analytical potential of MTUF and show that liming induces the aggregation of colloids, thereby decreasing the mobilisation of colloid-bound Sb and As, but increasing colloidal Pb. Abstract The size and composition of colloids are important factors controlling their relevance as carriers of metal(loid)s in soils. Liming, which is often used to reduce the effect of heavy metal contamination in soil, can alter concentrations and characteristics of colloids in soil suspension. In batch studies, we compared the influence of changing pH and cation valency on the size distribution and composition of dispersed colloids and on the concentrations of Pb, As and Sb associated with colloids and in solution following the addition of Ca(OH)2 and KOH to soil samples from a contaminated-shooting range site. Multi-stage tangential ultrafiltration (MTUF) and centrifugation were used for the size fractionation of colloids in aqueous suspension. An increase in soil pH resulted in an increase in colloid-associated Pb, with much higher concentrations in the KOH than in the Ca(OH)2 treated samples. In contrast colloid-associated Sb and As increased only in the KOH treated samples. Addition of the monovalent K-ion induced the dispersion of small (~9–220 nm) organo(-mineral) colloids, whereas the divalent Ca-ion suppressed their dispersion and led to the formation of larger colloids (220–1200 nm). Whereas centrifugation underestimated contaminants (i.e. Pb) associated with organic colloids (density <2.6 g cm–3) MTUF gave a distorted distribution of inorganic colloids (i.e. needle-shaped sesquioxides).