Transport of colloids and colloid-facilitated heavy metals in
agricultural soils: Could it be a potential causative factor for the
chronic kidney disease with unknown etiology (CKDu) in Sri Lanka?
Abstract
Naturally-occurring colloids, particles of diameter < 10μm,
are ubiquitous in geo-environments and can potentially facilitate
transport of numerous contaminants in soil including heavy metals,
pesticides, pathogens etc. via Colloid-Facilitated Transport (CFT). The
CFT of contaminants to groundwater is still an underrepresented
transport domain and may lead to significant environmental and health
problems related to groundwater contamination. Colloid mobilization,
transport and CFT in various geomedia are highly sensitive to
physico-chemical perturbations. This study investigated colloid
transport and colloid-facilitated heavy metal transport in saturated
porous media with a series of column experiments using soil colloids
extracted from two areas affected by Chronic Kidney Disease of Unknown
Etiology (CKDu) in North Central Province of Sri Lanka. Colloid
breakthrough curves were obtained from the column studies to observe the
colloid transport under different flow rates (0.5±0.05, 1.65±0.05,
4.10±0.05 cm3/s) and ionic strengths (NaCl - 0.01 M, 0.05 M, 0.1 M). The
CFT was studied using Cadmium (Cd(II)) as a model contaminant together
with colloidal suspension under selected scenarios for high colloid
deposition. Elevated colloid concentrations were observed in high CKDu
affected area compared to the low endemic 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 mechanism for
colloid immobilization in selected soil. Both colloids and CFT of Cd(II)
showed pronounced deposition under low flow rate and high ionic
strength.