Transport of Colloids and Colloid-facilitated Cadmium in a Sri Lankan Agricultural Soil

Abstract

Naturally occurring colloids, particles of diameter < 2 μm, are ubiquitous in geo environments and can potentially facilitate transport of numerous contaminants in soil via colloid-facilitated transport (CFT). Colloid mobilization, transport and CFT in various geo-media are highly sensitive to physico-chemical perturbations. This study investigated colloid, and colloid facilitated cadmium transport in saturated porous media with a series of column experiments. Soil colloids were extracted from soils belonging to two areas affected by chronic kidney disease of unknown etiology (CKDu) in Sri Lanka. Colloid breakthrough curves were obtained from the column studies under different flow rates (0.50, 1.60, and 4.00 ± 0.05 cm3/s) and ionic strengths (0.01, 0.05, and 0.10 M NaCl). The CFT was studied using Cd(II) as a model contaminant together with colloidal suspension under selected scenarios for high colloidal deposition. Elevated colloid concentrations were observed in highly CKDu affected area. The experimental results were numerically simulated on an advection-diffusion/dispersion modelling framework coupled with first-order attachment, detachment and straining parameters inversely estimated using Hydrus 1D software. Experimental and simulated colloid breakthrough curves showed a good agreement and recognized colloid attachment as the key retention mechanism. Both colloids and CFT of Cd(II) showed pronounced deposition under low flow rates and high ionic strengths.