Size fractionation of montmorillonite colloids through saturated porous media and their heterogeneous contribution to the transport of Pb2+

The size fractionation of colloids is an important process while they migrate through porous media. To date, the information about the contribution of the size fractionation of clay colloids on heavy metal mobility during their co-transport process is limited. Herein, taking montmorillonite as a typical clay mineral, the size fractionation characteristics (> 2.0 μm, 1.2–2.0 μm, 0.45–1.2 μm, 0.1–0.45 μm, and < 0.1 μm) of colloidal montmorillonite particles after passing through saturated sand and their different contribution to Pb ²⁺ transport were investigated. The results indicated that the extent of Pb ²⁺-mobilizing ability of colloids at pH 7.0 was higher than that at pH 5.0, ascribed to more Pb ²⁺ adsorbed to the colloids and greater mobility of colloids at higher pH values. Generally, the contribution of colloid size fractions on Pb ²⁺ mobility followed the order of (> 2.0 μm) > (< 0.1 μm) > 0.45–1.2 μm > 0.1–0.45 μm ≈ 1.2–2.0 μm, which depended on the colloid size distribution in the effluents (i.e., the larger proportion of fractions exhibited greater contribution to the enhancement of Pb ²⁺ mobility in this work). However, the relative contaminant-mobilizing abilities of different colloid size fractions (obtained by normalizing the fraction-facilitated Pb ²⁺ breakthrough with the respective fraction breakthrough) increased with the decrease in colloid size, which stemmed from the relatively higher mobility and greater metal-binding capacities of smaller size fractions. Additionally, the differences in the relative contaminant-mobilizing abilities of different fractions decreased with decreasing sand grain sizes (20–40 mesh (0.425–0.85 mm), 40–60 mesh (0.25–0.425 mm), and 60–80 mesh (0.178–0.25 mm)), which was related to the different mobility of the colloid size fractions. In summary, these findings indicate that size fractionation of natural colloids plays a critical role in heavy metal mobility and retention in groundwater systems.