Sorption and distribution of Zn in a sludge-amended soil: influence of the soil clay mineralogy

International audiencePurpose Conventional pedological survey generally assesses soil metal pollution by comparing total metal concentrations in soil to geochemical baselines in parent rock. This global approach overlooks the sorption capacities of the clay minerals which form at micrometric scale in weathering microsystems and are essential for the behaviour of metals in soil. Therefore, our aim was to investigate the impact of these weathering microsystems and their clay mineralogy upon Zn sorption and distribution throughout a sludge-contaminated soil. Materials and methods Two soil profiles with the same diorite parent-rock were sampled: (i) an amended soil profile (AS) that received for ten years wet sewage sludges heavily loaded with Zn and (ii) a control soil profile (CS) free of sludge spreading. Soil samples were 2 carefully collected as undisturbed blocks using plastic core samplers to prevent sample metal contamination. Each sample was further divided into two subsamples: the first was devoted to bulk chemical analyses, mineral grains separation and X-ray diffraction (XRD) analyses, whereas the second part was used for thin sections preparation and electron probe microanalyses (EPMA). Results and discussion Zn in the control soil is inherited from the weathering parent-rock and its concentration is always lower than in the amended soil where Zn is supplied at the surface by sludge. Each rock-forming mineral weathers into specific clay minerals: amphibole into saponite and montmorillonite and plagioclase into montmorillonite and kaolinite. Each clay mineral, with its specific sorption capacity, controls the Zn distribution within the soil: the smectites produced by the amphiboles have high sorption capacity and favor Zn retention in the upper horizons of the soil; the kaolinites produced by the plagioclases have lower sorption capacity, do not retain Zn in the surface horizons, and allow it to migrate to deeper horizons where it is sorbed onto the montmorillonites. Conclusions The clay minerals appear to be important soil components controlling the mobility of Zn in the contaminated soils. The micrometric mineralogical approach proves to be relevant to describe the importance of the clay mineral species in the Zn fixation at the solid/solution interface. When applied to a wider range of heavy metals and clay minerals, it could be a useful improvement in the surface complexation modeling used to explain metal cation sorption in soils