Silica colloids as non-carriers facilitate Pb2+ transport in saturated porous media under a weak adsorption condition: effects of Pb2+ concentrations

Transport of environmental pollutants in groundwater systems can be greatly influenced by colloids. In this study, the cotransport of Pb 2+ and silica (SiO 2 ) colloids at different Pb 2+ concentrations was systematically investigated by batch adsorption and saturated sand column experiments. Results showed that SiO 2 colloids had low adsorption capacity for Pb 2+ (less than 1% of the input) compared with sands. In saturated porous media, SiO 2 colloids showed a high mobility; however, with the increase of Pb 2+ concentration in the sand column, the mobility of SiO 2 colloids gradually decreased. Notably, SiO 2 colloids could facilitate Pb 2+ transport, although they did not serve as effective carriers of Pb 2+ . Under the condition of low Pb 2+ concentration, SiO 2 colloids promoted the Pb 2+ transport mainly through the way of “transport channel,” while changing the porosity of the medium and masking medium adsorption sites were the main mechanisms of SiO 2 colloid–facilitated Pb 2+ transport under the condition of high Pb 2+ concentration. The discovery of this non-adsorption effect of colloids would improve our understanding of colloid-facilitated Pb 2+ transport in saturated porous media, which provided new insights into the role of colloids, especially colloids with weak Pb 2+ adsorption capacity, in Pb 2+ occurrence and transport in soil-groundwater systems.