Cotransport of heavy metals and SiO2 particles at different temperatures by seepage

Abstract

The cotransport of heavy metals (HMs; i.e., Pb2+ and Cd2+) and SiO2 suspended particles (SPs) in a porous medium (quartz sand) at different temperatures was investigated by one-dimensional laboratory column experiments. Considering the significant differences in particle size and dielectric property between HMs and SPs, a theoretical model describing their cotransport was developed in this study. The results show that temperature and Darcy velocity have a negligible effect on the transport of individual HMs and that the recovery ratio of Cd2+ is higher than that of Pb2+, which can be attributed to the lower adsorption of Cd2+ on the solid matrix. The presence of SPs can facilitate the transport of HMs due to the strong adsorption of positively charged HMs on negatively charged SPs. However, this effect tends to decrease with the size of the SiO2 particles. An increase in temperature (e.g., T = 20 → 60 °C) results in a decrease in the recovery ratio of HMs because more HMs adsorbed on SPs can be deposited at high temperatures. The adsorption of HMs on SPs can also change the dielectric properties of the SPs, thus causing a reduction in the repulsion between the SPs and the solid matrix and a consequent decrease in the recovery ratio of SPs. In addition, the results show that the developed model fits well with the experimental results.