Stabilizing Of Metal-contaminated Sediment By Phosphatation And Sintering

With the world industrilization developing, sediment pollution has become a world environmental problem for the content of pollutants (such as heavy metals) in sediment are higher than in water environment, so the safe treatment or diposition of sediment is commonly paid much attention.The technology of phosphation and sintering is a useful method for metal-contaminated sediment such as the sediments in Xiawan harbor. In this paper, the influences of sintering temperature, sintering time, phosphoric acid dosage and leachate pH were investigated by evaluating the heavy metal leachability from the treated sediments. The leachability was tested with the standard Toxicity Characteristic Leaching Procedure (TCLP), and results showed that at the optimal conditions (phosphate acid dosage of 0.17 mol·kg-1, sintering temperature of 700℃, and sintering time of 2h), the concentrations of Cd, Zn and Pb in leachates from the sediment treated by Novosol process were much lower than the limits in the national standard (GB 5085.3-2007, Identification standards for hazardous wastes-Identification for extraction toxicity). With pH decrease of the raw sediment and leachate, the leaching content of heavy metals increased largely. The heavy metals of treated sidiments only partially came out in soft-acidic condition for phosphate deposits or mineral can immobilize in large pH range, and the leaching efficiency of Zn and Cd was significantly influenced by leachate pH, while Pb leaching was hardly influence by leachate pH, because Pb was transformed into very stable mineral phases.At the same time, impact of sitering on heavy metals leaching was investigated by evaluating the heavy metal leachability from the treated sediments. Results indicated the leachability of heavy metals (Cd, Pb, Zn) was decreased after sitering (sitering at 700℃for 2h), and the leaching concentraiton of Cu was increased.Otherwise, we sudied the effect of the phosphation process to immobilise the heavy metals (Cd, Pb, Zn and Cu) using NaH₂PO₄ and Na5P3O10. The leaching content of heavy metals were all reduced by adding phosphation reagents ( Na₅P₃O₁₀ and NaH₂PO₄). Leaching results (HJ/T 300-2007, acetum buffer solution method) showed that Na₅P₃O₁₀ was a better choice as a phosphation reagent to immobilize heavy metals, for the concentrations of heavy metals (Cd, Pb and Zn) from the sediment treated by Na₅P₃O₁₀ were lower than by NaH₂PO₄ when their dosages were 0.83mol·kg-1(calculated by P). The process of phosphation and sintering using Na₅P₃O₁₀ as phosphation reagent (Na₅P₃O₁₀ dosage of 1.1 mol·kg-1, sintering temperature of 700℃, and sintering time of 2h) was investigated by evaluating the heavy metal leachability from the treated sediments. Leaching result (HJ/T 300-2007) showed that the concentrations of Cd, Pb, Zn and Cu in leachates from the treated sediment were 0.11mg·L-1,0.22mg·L-1,2.69mg·L-1 and 1.39 mg·L-1, respectively, and can be lower than the limits in the national standard (GB 16889-2008, Identification standards for living wastes-Identification for extraction toxicity). So Na₅P₃O₁₀ is a good substitute for H₃PO₄ as a phosphation reagent and can avoid the disadvantage of sediment acidification by adding H₃PO₄.