New insights into solubility control mechanisms and the role of particle- and colloid-facilitated transport of metals in contaminated soils

Particle- and colloid-mediated transport of metals can be an important leaching pathway in contaminated soils, making it necessary to include this process in risk assessments of contaminant transport. In this thesis, mechanisms involved in transport of particulate, colloidal and truly dissolved lead, chromium, zinc, arsenic and antimony were studied in an irrigation experiment performed on intact soil columns from four historically contaminated soils. Speciation of lead and chromium in bulk soil, particles and colloids was studied using extended X-ray absorption fine structure (EXAFS) spectroscopy and geochemical modelling. The ability of three standardised leaching tests (a percolation test and two batch tests using deionised water or calcium chloride) to describe leaching from intact soil columns was also investigated, using size-based elemental fractionation. The results from the irrigation experiment suggested that the tendency for metal(loid)s to be transported with particles and colloids followed the order: lead > chromium > zinc > arsenic > antimony. There were large differences between the soils as regards particulate and colloidal leaching of lead and chromium, whereas the differences between irrigation intensities were minor. Thus, particle- and colloid-mediated metal transport was mainly governed by soil properties. Interactions between lead and iron in particles and colloids were confirmed by EXAFS and geochemical modelling. In contrast, lead in bulk soil was mainly bound to organic matter in two soils and to aluminium hydroxide in one soil. In one soil, lead probably occurred mainly as mimetite (Pb5(AsO4)3Cl) in the bulk soil and in the particulate and colloidal phases. Using EXAFS and geochemical modelling, chromium in particles and colloids, and in the bulk soil, was identified as a dimeric chromium(III) complex with organic matter. A percolation test at a liquid-to-solid ratio of 10 proved useful for conservatively categorising soils into high-risk soils with respect to mobilisation of particulate and colloidal metal(loid)s. In batch tests, using calcium chloride instead of deionised water enabled better prediction of the truly dissolved fraction for elements prone to colloidal mobilisation, such as lead. These novel findings can be used to improve the description of transport processes in risk assessments, resulting in more accurate evaluations of total metal transport and metal solubility in historically contaminated soils