Colloid-facilitated and dissolved cadmium transport through layered soil columns with subsurface cracks

摘要

Cadmium is among the most toxic heavy metals in soil, but its mobility in contaminated soils is not fully understood. The objectives of this study were (i) to investigate relative contribution of colloid- facilitated and dissolved Cd transport to the total flux in contaminated soils with subsurface cracks, and (ii) to identify key processes controlling the downward transport of Cd from contaminated soils. Simulated rainfall experiments were conducted using single-layer columns repacked with topsoil spiked with 3 or 30 mg·kg<'-1> of Cd, and dual-layer columns composed of the repacked Cd-spiked topsoil and undisturbed subsurface soil with visible cracks. Distilled water or 5 mM CaCl<,2> was applied to the columns at a rainfall intensity of 7mm·h<'-1> for 5h on 6 consecutive days. The colloid concentration in the column effluent not only showed a quick response to the concentrations of coexisting ions, but was also affected by the remobilization of once-deposited particles, probably due to shearing force exerted by the water flow. Infiltration of CaCl<,2> completely suppressed the discharge of colloidal particles and colloid-associated Cd while significantly enhancing dissolved Cd transport. Although Cd transport was predominantly in the dissolved form, colloid-associated transport accounted for 30%-80% of the total transport when the ionic strength was lowered by the infiltration of distilled water. The addition of Ca (OH)<,2> to the soil substantially reduced the dissolved Cd concentration, while the colloid particle transport was not appreciably affected. The colloid-associated Cd can be the main form of total Cd transport in limed soils with subsurface cracks.