Using DMT and AF4-HR-ICP-MS to characterize trace metal speciation in soil water extracts

Diffuse contamination of soils with trace metals like Cu and Zn is an ubiquitous problem all over the world. Trace metals in 0.45 µm-filtered soil solution samples can be present in the free form or they can be associated with inorganic ligands, DOM, and inorganic colloids. Inorganic colloids like clay and Fe-(hydr)oxide nanoparticles can pass the 0.45 µmfilter, but their concentration, composition, and effect on trace metal speciation is largely unknown. For ecological risk assessment of contaminated soils, total trace metal concentrations in soil solution are often measured. However, uptake of trace metals by plants and soil organisms is not necessarily related to their total dissolved concentrations, but generally correlates best with their free concentrations. Hence, the speciation of trace metals in soil solution needs to be known to understand their bioavailability in soils. During the presentation, we will focus on the Donnan Membrane Technique (DMT) and Asymmetric Flow Field-Flow Fractionation (AF4), which can be used to measure the trace metal speciation in soil solution. The DMT allows for the measurement of free trace metal concentrations in solution [1]. Using a chemical equilibrium model in combination with the NICA-Donnan model, the complexation of trace metals with inorganic ligands and DOM can be predicted [2]. However, the role of inorganic colloids in soil solution in the binding of trace metals remains poorly understood when using this approach. An emerging analytical technique with a high potential to analyse the colloidal trace metal fraction in 0.45 soil µm-filtered solution samples is AF4. AF4 is a chromatographic-like technique for size fractionation of colloids, and allows for online multi-element detection in combination with HR-ICP-MS. We will briefly describe the analytical developments in the DMT and present examples of the application of both DMT and AF4-HR-ICP-MS to measure specific trace metal forms in soil solution extracts to demonstrate (i) the complementary character of both analytical techniques and (ii) their potential to obtain a full speciation of total dissolved trace metals