Silica colloids as non-carriers facilitate Pb2+ transport in saturated porous media under a weak adsorption condition: effects of Pb2+ concentrations
2020
Dai, Chaomeng | Zhou, Hui | You, Xueji | Duan, Yanping | Tu, Yaojen | Liu, Shuguang | Zhou, Feng | Hon, Leong Kah
Transport of environmental pollutants in groundwater systems can be greatly influenced by colloids. In this study, the cotransport of Pb²⁺ and silica (SiO₂) colloids at different Pb²⁺ concentrations was systematically investigated by batch adsorption and saturated sand column experiments. Results showed that SiO₂ colloids had low adsorption capacity for Pb²⁺ (less than 1% of the input) compared with sands. In saturated porous media, SiO₂ colloids showed a high mobility; however, with the increase of Pb²⁺ concentration in the sand column, the mobility of SiO₂ colloids gradually decreased. Notably, SiO₂ colloids could facilitate Pb²⁺ transport, although they did not serve as effective carriers of Pb²⁺. Under the condition of low Pb²⁺ concentration, SiO₂ colloids promoted the Pb²⁺ 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 SiO₂ colloid–facilitated Pb²⁺ transport under the condition of high Pb²⁺ concentration. The discovery of this non-adsorption effect of colloids would improve our understanding of colloid-facilitated Pb²⁺ transport in saturated porous media, which provided new insights into the role of colloids, especially colloids with weak Pb²⁺ adsorption capacity, in Pb²⁺ occurrence and transport in soil-groundwater systems.
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