Reduction and Heavy Metal Stabilization of River Sediment by Hydrothermal Treatment

In this paper, hydrothermal treatment was used to treat river sediment. The physicochemical properties, chemical speciation, bioavailability, ecotoxicity, and leaching toxicity of heavy metals (Zn, Cu, Cr, Cd, Ni, Pb) in sediment under different hydrothermal treatment conditions were studied, and stabilization mechanisms of heavy metals were explained. The results showed that hydrothermal temperature and time had a huge impact on sediment reduction and heavy metal stabilization. The best reduction and stabilization of sediments were achieved at a hydrothermal temperature of 260°C and a hydrothermal period of 3 h. The hydrothermal treatment caused the sediment to undergo dehydration, dihydroxylation, decarboxylation, deamination, and aromatization with a gradual decrease in its H/C ratio, O/C ratio, and pH; a gradual increase in porosity and surface area; and a decrease in particle size. This facilitates the dehydration, reduction, and stabilization of the sediment. The bioavailable components of heavy metals in sediment were dramatically reduced following hydrothermal treatment, but the fraction of stable components rose significantly. This resulted in a marked decrease in the environmental bioavailability, ecotoxicity, and leaching toxicity of the heavy metals. Therefore, hydrothermal treatment might be an effective ex situ repairment way for reducing, stabilizing, and making river sediment harmless.