Stabilization of heavy metal contaminated marine sediments with red mud and apatite composite

Chemical stabilization (or immobilization) is an in situ remediation method that uses amendments to reduce heavy metals availability in contaminated sediments. In this study, stabilization of heavy metals (Pb, Cd, Zn, and Ni) in contaminated marine sediment was conducted using three amendments (i.e., red mud, apatite, and a red mud/apatite composite) made up of industrial by-product and natural mineral. Red mud, apatite, and their composite were prepared by natural drying and grinding for use in the experiments. Three proportions of immobilizing additives were added to each reactor: 5 wt% red mud, 5 wt% apatite, and 2.5 wt% red mud + 2.5 wt% apatite. The amendments were applied in a wet-curing stabilization process for 4 months. A sequential extraction procedure, a toxicity characteristic leaching procedure (TCLP), and a potential ecological risk index (PERI) were used to evaluate remediation performance, and comparison of the results was used to evaluate stabilization effect. The experimental data indicated that the sediment was polluted by heavy metals, especially Ni and Zn. Based on the results of sequential extraction, the sum of bioavailable fractions (exchangeable and carbonates) and potentially bioavailable fractions (oxides and organics) for Pb, Cd, Zn, and Ni were 92.1, 95.4, 84.3, and 72.0 % of the total concentrations in raw sediment, respectively. After the stabilization treatment with red mud, the mobility of the heavy metals was reduced. The sum of bioavailable fractions and potentially bioavailable fractions of Pb, Cd, Zn, and Ni in sediment decreased to 80.2, 88.6, 81.5, and 47.3 %, respectively. TCLP and ecological risk assessment were conducted to evaluate the mobility and bioavailability of heavy metals in the sediments before and after treatment. The mobility and bioavailability of heavy metals were also reduced compared to initial concentrations by stabilization treatment with the amendments. Results demonstrated that red mud, apatite, and red mud/apatite composite treatments were useful in reducing the harmful effects of heavy metals in contaminated sediments. In particular, red mud showed the best effect in controlling the bioavailability and toxicity of heavy metals in contaminated marine sediment and thus could be employed for sediment remediation.