Transport behavior of variable charge soil particle size fractions and their influence on cadmium transport in saturated porous media

Abstract

The co-transport of contaminants by soil colloids can generate substantial environmental risk in the subsurface, and this behavior is greatly affected by the particle size of soil colloids. A variable charge bulk soil was separated into four soil particle size fractions (i.e., 10–1, 1–0.45, 0.45–0.2 and < 0.2 μm) to investigate their transport behavior and the influence of the obtained soil particles on the co-transport of cadmium (Cd) in saturated sand column with different pH and ionic strength. As pH increases and ionic strength decreases, the mobility of soil particles all increased because of the increase of the electrostatic repulsion. Straining and aggregation were the important retention mechanisms for soil particles with low pH and high ionic strength. Soil colloids (<1 μm) presented a stronger mobility than that of large mobile soil particles (10–1 μm). For various soil colloid size fractions, the mobility of coarse and medium soil colloids (1–0.45 and 0.45–0.2 μm) were stronger than fine soil colloids (<0.2 μm) at pH 6.0 and 8.0, whereas a stronger mobility of fine soil colloids (<0.2 μm) was observed at high ionic strength (0.10 mol L−1, NaCl). Except at pH 4.0, Cd mobility was obviously inhibited in the presence of soil particles, but generally increased with decreasing pH and increasing ionic strength. At high pH and low ionic strength, the amount of Cd associated with soil colloids was greatly enhanced, indicating a potential environmental risk from co-transport. Under most chemical conditions, the Cd recoveries were lower in the presence of fine soil colloids (<0.2 μm) than that in the presence of coarse and medium soil colloids (1–0.45 and 0.45–0.2 μm). Nevertheless, higher Cc/Ct values and accumulation mass of Cd adsorbed by fine soil colloids (<0.2 μm) in the effluent were found, which implied a greater potential environmental risk resulting from the co-transport of Cd by fine soil colloids (<0.2 μm).