Highly effective immobilization of Pb and Cd in severely contaminated soils by environment-compatible, mercapto-functionalized reactive nanosilica

Soils contaminated with Pb and Cd greatly threaten the health of humans. With a view to decrease the environmental risk of Pb and Cd in the soil, we design and prepare a type of immobilization material possessing high stability and high immobilization efficiency towards Pb and Cd. Nanosilica which has good compatibility with soils is chosen as the matrix and functional groups (mercapto) are loaded on the nanosilica surface via the covalently bonding between 3-mercaptopropyl-trimethoxysilane (denoted as MPTS) and nanosilica. The as-prepared MPTS/nano-silica is characterized, and its immobilization efficiency towards Pb and Cd in contaminated soil as well as the stability of the immobilized Pb and Cd under acidic condition is evaluated. Results demonstrate that the as-prepared nanosilica has spherical or spheroidal shape and exhibits a diameter of 20–30 nm. In the meantime, MPTS/nano-silica selectively reacts with Pb and Cd to reduce the content of bioavailable-Pb and bioavailable-Cd from 1193.54 mg/kg and 12.46 mg/kg to 10.34 mg/kg and 0.22 mg/kg, respectively, providing an immobilization rate of 99.12% and 98.23% for Pb and Cd. The results of chemical species analysis of Pb/Cd indicate that the nanosilica can transform most Pb/Cd from unstable species into more stable species. More importantly, MPTS/nano-silica exhibits low impact on soil and the Pb- and Cd-species immobilized by MPTS/nano-silica exhibit greatly increased acid resistance, which could be significant for the long-term remediation of the heavy-metal-contaminated soils.