Amendment of soils with metal-rich sludge: Potential water toxicity due to metal release via over-time slaking

Industrial metal-rich sludge can improve soil properties, but it is potentially toxic to soils and adjacent aquatic systems. The soil-sludge-water interactions influence metals bioavailability over time, a phenomenon mostly regulated by the still debatable "sludge physical protection" or "sludge delayed release" hypotheses. The present study aimed to investigate: (1) whether sludge increases soil aggregate stability against slaking, (2) which hypothesis mostly regulates metal release from soils to water and (3) the ecotoxicity of the metals released during soil slaking for aquatic organisms. Under a realistic field scale, soils amended with an industrial sludge or spiked with equivalent metal solutions (of Cr, Cu, Ni, Zn) were collected over three months to test soil aggregate stability, the ecotoxicity of the slaking water and metal contents in soil and water. The "sludge physical protection" was verified for all metals, though for Cu the "sludge delayed release" hypothesis appears plausible after three months. Soil amendment with sludge did not lead to effects on the growth of the microalga Raphidocelis subcapitata, contrarily to the observed for the metal-spiked soil. Criteria regulating soils sludge-amendment management should thus include doses not hazardous to biota, and not only metal threshold levels.