Colloid‐Facilitated Solute Transport in Variably Saturated Porous Media: Numerical Model and Experimental Verification

Abstract

Strongly sorbing chemicals (e.g., heavy metals, radionuclides, pharmaceuticals, and explosives) in porous media are associated predominantly with the solid phase, which is commonly assumed to be stationary. However, recent field‐ and laboratory‐scale observations have shown that in the presence of mobile colloidal particles (e.g., microbes, humic substances, clays, and metal oxides), colloids can act as pollutant carriers and thus provide a rapid transport pathway for strongly sorbing contaminants. To address this problem, we developed a one‐dimensional numerical model based on the HYDRUS‐1D software package that incorporates mechanisms associated with colloid and colloid‐facilitated solute transport in variably saturated porous media. The model accounts for transient variably saturated water flow, and for both colloid and solute movement due to advection, diffusion, and dispersion, as well as for solute movement facilitated by colloid transport. The colloid transport module additionally considers the processes of attachment/detachment to/from the solid phase and/or the air–water interface, straining, and/or size exclusion. Various blocking and depth dependent functions can be used to modify the attachment and straining coefficients. The solute transport module uses the concept of two‐site sorption to describe nonequilibrium adsorption–desorption reactions to the solid phase. The module further assumes that contaminants can be sorbed onto surfaces of both deposited and mobile colloids, fully accounting for the dynamics of colloid movement between different phases. Application of the model is demonstrated using selected experimental data from published saturated column experiments, conducted to investigate the transport of Cd in the presence of Bacillus subtilis spores in alluvial gravel aquifer media. Numerical results simulating bacteria transport, as well as the bacteria‐facilitated Cd transport, are compared with experimental results. A sensitivity analysis of the model to various parameters is also presented.

Blocking effect of colloids on arsenate adsorption during co-transport through saturated sand columns

“…To investigate the fate of colloid-associated pollutants, various three-phase transport models (Simunek et al, 2006), including the nonreactive colloids with the solid phase model, the first-order kinetic attachment of colloids model, the irreversible nonlinear kinetic attachment of colloids model, and the Langmuir kinetics attachment of colloids model, have been developed to delineate the pollutant transport in the presence of colloids (Grolimund and Borkovec, 2005;Grolimund et al, 1996;Saiers, 2002;Saiers and Hornberger, 1996;Simunek et al, 2006;Zou and Zheng, 2013). A model, only considering the colloid as a carrier, simulated kaolinite colloid-facilitated 137 Cs transport, but calculated data greatly deviated from the observed data obtained in experiments (Saiers and Hornberger, 1996).…”

Remediation of colloid-facilitated contaminant transport in saturated porous media treated by nanoparticles

“…. Therefore, according to the reported results, the motion of suspended particles and subsequent colloid-facilitated contaminant transport could provide an additional pathway for the transport of inorganic contaminants (Morales 2011;Simunek et al 2006;Sun et al 2010).…”

“…Groundwater contamination due to the spread of various chemical compounds, such as heavy metals, poisonous materials, and agricultural and industrial wastes, has been widely reported in the literature (Esmaeili Bidhendi et al 2010;Lee and Jones 1993;Nouri et al 2008;Simunek et al 2006). The major mechanisms controlling the transport of contaminants may differ in the vadose and saturated zones (Sen and Khilar 2006).…”

Transport of oxytetracycline, chlortetracycline, and ivermectin in surface runoff from irrigated pasture

“…[31] When soil erosion occurs, pharmaceuticals can potentially be transported with the soil colloids in addition to water based transport thus increasing the total pharmaceutical loss. [33][34][35] However, colloid-associated transport depends on the chemistry of the specific compound, the soil type, and irrigation conditions. While studied pharmaceuticals were demonstrated to sorb strongly with particulate matter, their colloid-associated transport is limited by the absolute amount of colloids that can be carried with the water flow.…”