The seepage transport of heavy metal Pb²⁺ through sand column in the presence of silicon powders

The coupled transport of heavy metals with suspended particles has been a topic of growing interest. The main purpose of this study is to experimentally investigate the seepage transport of heavy metal Pb²⁺ in the presence of silicon powders (SPs) through a sand column under different seepage velocities (v = 0.087–0.260 cm/s), injection Pb²⁺ concentrations (C_P = 0–800 μg/ml) and SP sizes (D₅₀ = 2.8–25.5 μm), which were likely to be encountered in practical engineering. The sand column was installed in a cylindrical chamber of 300 mm in length and 80 mm in internal diameter. The results clearly show that the increase in acidity results in a reduction of the repulsive interactions between SPs and the matrix, and consequently a decrease in the peak values in breakthrough curves (BTCs), especially for larger-sized SPs. The peak values and recovery rate of Pb²⁺ are obviously increased and an earlier breakthrough can be observed, due to the higher capacity of SPs with negative charge to adsorb heavy metal pollutants such as Pb²⁺ with positive charge. The adsorption of Pb²⁺ on SPs can reduce the repulsive forces between SPs and the matrix, thus resulting in the increase of the deposition possibility of SPs and the decrease of peak value and recovery rate.