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Title: Equilibrium and kinetic models for colloid release under transient solution chemistry conditions

item Bradford, Scott
item TORKZABAN, SAEED - Commonwealth Scientific And Industrial Research Organisation (CSIRO)
item LEIJ, FEIKE - California State University
item SIMUNEK, JIRI - University Of California

Submitted to: Journal of Contaminant Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/8/2014
Publication Date: 4/17/2015
Citation: Bradford, S.A., Torkzaban, S., Leij, F., Simunek, J. 2015. Equilibrium and kinetic models for colloid release under transient solution chemistry conditions. Journal of Contaminant Hydrology. 181:141-152.doi: 10.1016/j.jconhyd.2015.04.003.

Interpretive Summary: Most colloid (including microorganisms) transport studies have been conducted under steady-state water flow and solution chemistry conditions. Results typically suggest limited mobility of colloids in soil. This paper demonstrates the sensitivity of colloid transport to changes in solution chemistry (ionic strength, pH, and cation type) conditions that frequently occur in the subsurface. A mathematical modeling framework is presented that relates colloid release to changes in the solid-water interfacial area that contributes to retention. The developed models were subsequently utilized to simulate microbial release and transport over a wide range of conditions. Results from this study will be of interest to scientists, engineers, regulators, and public health officials that are concerned with the release of colloids and associated contaminants into water supplies.

Technical Abstract: We present continuum models to describe colloid release in the subsurface during transient physicochemical conditions. Our modeling approach relates the amount of colloid release to changes in the fraction of the solid surface area that contributes to retention. Equilibrium, kinetic, equilibrium and kinetic, and two-site kinetic models were developed to describe various rates of colloid release. These models were subsequently applied to experimental colloid release datasets to investigate the influence of variations in ionic strength (IS), pH, cation exchange, colloid size, and water velocity on release. Various combinations of equilibrium and/or kinetic release models were needed to describe the experimental data depending on the transient conditions and colloid type. Release of Escherichia coli D21g was promoted by a decrease in solution IS and an increase in pH, similar to expected trends for a reduction in the secondary minimum and nanoscale chemical heterogeneity. The retention and release of 20 nm carboxyl modified latex nanoparticles (NPs) were demonstrated to be more sensitive to the presence of Ca2+ than D21g. Specifically, retention of NPs was greater than D21g in the presence of 2 mM CaCl2 solution, and release of NPs only occurred after exchange of Ca2+ by Na+ and then a reduction in the solution IS. These findings highlight the limitations of conventional interaction energy calculations to describe colloid retention and release, and point to the need to consider other interactions (e.g., Born, steric, and/or hydration forces) and/or nanoscale heterogeneity. Temporal changes in the water velocity did not have a large influence on the release of D21g for the examined conditions. This insensitivity was likely due to factors that reduce the applied hydrodynamic torque and/or increase the resisting adhesive torque; e.g., macroscopic roughness and grain–grain contacts. Our analysis and models improve our understanding and ability to describe the amounts and rates of colloid release and indicate that episodic colloid transport is expected under transient physicochemical conditions.