Impacts of weathering and clay mineralogy on heavy metals sorption in sludge-amended soils

To date, a soil is classified contaminated when metal concentrations in its bulk horizons exceed baseline values taken as higher limits for non-contaminated soils. Such a pedological approach underestimates the influence of weathering microsystems which operate at micrometer scale in soil horizons and play a major role in the sorption of metals onto their specific newly formed clay minerals. This paper investigates the influence of these weathering microsystems upon the zinc (Zn), lead (Pb) and cadmium (Cd) behavior in a sludge‐amended soil. First, the mineralogical study reveals that each rock-forming mineral weathers into specific clay minerals: amphibole gives way to smectites (saponites and montmorillonites) and plagioclase produces montmorillonites followed by kaolinite/smectite mixed layers and kaolinite. Second, each clay mineral, with its specific sorption capacity, appears to control the distribution of the heavy metals within the soil: the smectites produced by the amphiboles have high sorption capacity and favor the retention of the metals in the upper horizons of the soil; the kaolinites produced by the plagioclases have low sorption capacity and do not retain the metals in the surface horizons, which allows them to migrate to deeper horizons where they are sorbed onto the montmorillonites.