Sequential solidification/stabilization and thermal process under vacuum for the treatment of mercury in sediments

Purpose Millions of cubic meters of sediments are dredged every year in the world. About 10–20%onweight basis of this material is contaminated by organic and/or inorganic pollutants. This work presents the laboratory tests performed to study a system for the remediation and reuse of mercurycontaminated sediments. The treatment is based on a cementbased granulation step (solidification/stabilization (S/S)), followed by a thermal process under vacuum during which volatile and semi-volatile compounds are removed. The experiments focused on: (1) cement hydration reactions; (2) pollutant removal efficiencies; and (3) leaching behavior, in relation to temperature and duration of the thermal process. Mercury speciation was also investigated. Materials and methods Dredged at the marine harbor of Augusta (SR, Italy), the sediments used in the experiments were highly polluted by mercury (200 mgkg−1dry weight (d.w.)). The recipe applied in the S/S step was based on the particle size distribution of the resulting granulates. An indirectly–heated batch system operated under vacuum (2.6±1.3 103 Pa) at: (1) 150°C for 16 h; (2) 200°C for 6 h; (3) 250°C for 4 h; or (4) 280°C for 4 h. X-ray diffraction spectroscopy and scanning electron microscopy were used to study cement hydration reactions. Total mercury concentration and leaching tests were carried out to assess the effects of the different treatment conditions. Results and discussion The best results were obtained by treating the granulate at 250°C for 4 h. Under these conditions, mercury final concentration was 49 mgkg−1 d.w., resulting in a removal efficiency of 63% referred to the granular material before thermal treatment, and 75% referred to the sediment. The concentrations measured in the leachate were compatible with the Italian requirements for reuse, with some exceptions (pH, chlorides, nickel, copper, and COD) ascribed to the specific nature of the sediment. Mercury speciation analyses pointed out changes after both the treatment steps. Conclusions The final granulates accomplish most Italian requirements for reuse, even though an improvement in the S/S step or an additional washing step would help for the exceptions mentioned above. Different reuse options in civil engineering (e.g., filling material, road material, concrete aggregates, etc.) will be evaluated also taking into consideration the mechanical properties. Further studies will be carried out to assess the long-term leaching behavior and leaching under different pH conditions.