Colloid-Mediated Transport and the Fate of Contaminants in Soils

The design and implementation of effective schemes for the remediation of contaminated soils require a clear understanding of the processes controlling the sorption and desorption of the contaminants. This context entails developments described in experimental, modeling, and large-scale studies on the impact of colloids on contaminant transport in saturated and unsaturated porous media: Stable colloidal particles can travel long distances in subsurface environments and carry particle-reactive contaminants with them. This chapter analyzes the movements of natural colloids in subsurface environments, which can intensify pollutant transport much more than predicted by the conventional advection-dispersion solute transport equations, mainly due to their association with mobile colloids. Soil characteristics are associated with the presence of the colloid and the fate and transport of contaminants. The behavior of colloids in solids and the colloid-associated contaminant transport are largely examined. Nanoparticles facilitating the transport of contaminants are considered a subgroup of natural colloids. An inventory of the most important classes of contaminants in soil (inorganic, organic, radionuclides, microorganisms) is discussed. The complex phenomena in porous media involving several basic processes, such as the release of colloidal fines, dispersion stabilization, migration, and entrapment/plugging at the pore constrictions and adsorption at the solid/liquid interface, are explored. The colloid-facilitated transport of contaminants in the subsurface is treated as a potential cause of the unexpected appearance of extremely low-solubility contaminants far from sources, because of the similarity between the chemical composition of colloidal particles and the porous media. Contaminants with high affinity to the solid phase can bind to colloidal particles, and that can act as a vehicle for transport. Although a saturated porous medium can be idealized as consisting of three phases—an aqueous phase, a carrier phase, and a stationary solid matrix phase—when colloids are present in an unsaturated porous medium, the system representation should include an extra phase (i.e. the air). The most important factor in unsaturated porous media is the presence of the air–water interface. The effect of colloid-facilitated transport becomes significant when the desorption rate of contaminants from colloids is relatively slow. Colloid mass transfer mechanisms among aqueous, solid matrix, and air phases, and contaminant mass transfer between aqueous and colloid phases are represented by kinetic expressions. Contaminant transport is interpreted by means of several models considered for predicting colloid-facilitated migration in soils. The analysis in this chapter also focuses on the occurrence of anthropogenic/engineered colloids/nanoparticles in soil and the risks generated by their presence in the environment and for human health.