Although numerous studies have been conducted to discern colloid transport and stability
processes, the mechanistic understanding of how dissolved organic matter (DOM) affects
colloid fate in unsaturated soils (i.e., the vadose zone) remains unclear. This study
aims to bridge the gap between the physicochemical responses of colloid complexes
and porous media interfaces to solution chemistry, and the effect these changes have
on colloid transport and fate. Measurements of adsorbed layer thickness, density,
and charge of DOM-colloid complexes and transport experiments with tandem internal
process visualization were conducted for key constituents of DOM, humic (HA) and fulvic
acids (FA), at acidic, neutral and basic pH and two CaCl(2) concentrations. Polymeric
characteristics reveal that, of the two tested DOM constituents, only HA electrosterically
stabilizes colloids. This stabilization is highly dependent on solution pH which controls
DOM polymer adsorption affinity, and on the presence of Ca(+2) which promotes charge
neutralization and inter-particle bridging. Transport experiments indicate that HA
improved colloid transport significantly, while FA only marginally affected transport
despite having a large effect on particle charge. A transport model with deposition
and pore-exclusion parameters fit experimental breakthrough curves well. Trends in
deposition coefficients are correlated to the changes in colloid surface potential
for bare colloids, but must include adsorbed layer thickness and density for sterically
stabilized colloids. Additionally, internal process observations with bright field
microscopy reveal that, under optimal conditions for retention, experiments with FA
or no DOM promoted colloid retention at solid-water interfaces, while experiments
with HA enhanced colloid retention at air-water interfaces, presumably due to partitioning
of HA at the air-water interface and/or increased hydrophobic characteristics of HA-colloid
complexes.