On-Site Solidification/Stabilization of Cd, Zn, and Pb Co-Contaminated Soil Using Cement: Field Trial at Dongdagou Ditch, Northwest China

Dongdagou Ditch in Baiyin City was one of the key demonstration areas for remediation of heavy metal contamination in China. Solidification/stabilization (S/S) is an appropriate technology that has positive impact on both leaching and strength performance of the riverbed site. This field trial was designed to investigate the long-term S/S effects and mechanism of ordinary Portland cement (OPC) on Cd, Zn, and Pb co-contaminated soil in Dongdagou Ditch, and to evaluate the influence of groundwater and low temperature. Leaching tests showed that 8 wt.%-OPC could effectively stabilize Cd, Zn, and Pb by 99.9%, 99.4%, and 67.9%, respectively, higher compared with 5 wt.%-OPC treatment, and had long-term stability. The bearing capacities of the OPC-treated soil reached 0.75-0.92 Mpa and met the USEPA (United States Environmental Protection Agency) standard for landfill disposal. Cd, Zn, and Pb were chemically stabilized and physically encapsulated by OPC simultaneously, the unstable acid-extractable Cd and Zn in soil were transformed into residual fraction after OPC treatment, and insoluble sulfides such as PbS and ZnS were specially formed due to high groundwater table of the site. In this field trial, groundwater showed negative effects on both leaching and strength properties of the OPC-treated soil due to anaerobic conditions and structural damage of S/S products. The 5 wt.%-OPC S/S effects on Cd and Zn decreased to 72.3% and 63.4%, respectively, and the soil strength reduced by 0.08-0.11 MPa below groundwater table. On-site S/S remediation with OPC was insensitive to low temperature, so that it was suitable for application in the cold Northwest China.