Tracking the mechanisms of natural assimilation of metal contaminations at the micro-scale: a pilot study

How reactive is a new metal contamination compared to the same metal already present in a soil? After a contamination event, upon hydrological fluctuations and associated redox variations, natural processes of redistribution and mineral transformation impact the speciation of the allochthonous metal input, thus affecting its (bio)availability and toxicity. This may lead to homogenization of the total metal pool of the soil with respect to the autochthonous metal, or to a shift in the total metal reactivity and associated risk. Because the terms “redistribution” or “natural attenuation” do not properly catch the meaning of this concept, the new term “natural assimilation” is proposed, that encompasses both the spatial redistribution and the parallel change in speciation of an allochthonous metal input in a soil that already contains a given amount of the same metal (autochthonous). Although such processes have been relatively well studied at the bulk soil scale, little is known about how the new metal input behaves compared to the preexisting soil metal pool at the microscopic scale, mainly because of technical difficulties in distinguishing autochthonous and allochthonous metal phases at that micro-scale.

Here, we are developing a methodology to allow such a distinction, by using isotope tracers to differentiate two metal pools in the soil, and combining spatially resolved analysis of the metal source proportions (isotope ratios mapped by laser ablation-inductively coupled plasma-mass spectrometry, LA-ICP-MS) and of the metal speciation (by synchrotron-based micro-X-ray absorption spectroscopy). In this pilot study, we performed laboratory incubations of natural soils already rich in iron (Fe) and zinc (Zn), in which isotopically labelled Zn-ferrihydrite (enriched in ⁵⁷Fe and ⁶⁸Zn) was amended. We then exposed the soil microcosms to wetting-drying cycles over several months in order to trigger redox fluctuations and subsequent Fe and Zn redistribution and transformation. Our preliminary ⁵⁷Fe Mössbauer spectroscopy data shows significant oscillations of the Fe oxidation state along with redox cycles. In parallel, we observe mineral transformation and reductive dissolution of the Zn-ferrihydrite within a few weeks along with Fe and Zn release into the soil solution. Thin sections of the reacted soils will first be mapped by micro-XANES at the Fe and Zn K-edges to determine the Fe and Zn speciation with a spatial resolution, and subsequently by LA-ICP-TOF-MS to determine the Fe and Zn source distribution.

This project is expected to create a proof-of-concept applicable to many environmental systems and experimental setups, opening new avenues of research on the fate of metallic elements in soils and sediments, with a focus on the differential reactivity of several sources of the same metal.