Evolution processes of trace metal speciation in leachates with different ages from Laogang Refuse Landfill, Shanghai

Trace metals in leachates are forced to be removed with the implement of the latest Chinese standards of landfill pollution control (GB-16889, 2008), and understanding the speciation distribution processes is important for the development of the leachate treatment process. In this work, evolution processes of trace metal speciation were investigated according to the size fractions, hydrophobic/hydrophilic fractions, and geochemical species with the disposal time. The main trace metals, that is, Pb, Ni, Cr Cu, and Zn, were around 0.04–0.52 mg Pb/L, 0.24–1.22 mg Ni/L, 0.12–0.86 mg Cr/L, 0–0.28 mg Cu/L, and 0.58–1.98 mg Zn/L in leachates with the disposal ages of 2–12 years. Pb was found to predominate in suspended solids, with the percentage of 46–100%, and the dominant fraction of Ni and Zn was in dissolved fractions, with the percentage of 28–83 and 34–67%, respectively, while Cr and Cu scattered between suspended solid and dissolved fractions. The predominated geochemical species of Pb, Ni, Zn, and Cu was Me dissolved organic matter and Me CO3, while Cr was dominated as hydroxide complexations. For the hydrophobic distribution in leachates, the main part of Pb, Ni, Cr, and Cu was presented as hydrophobic fractions, with the percentage of >50%, while Zn predominated in the hydrophilic fractions. The evolution processes of trace metals are dependent on both the metals’ nature and the landfill conditions greatly, and the corresponding treatment process should be changed with the landfill ages.