Enhanced transport of ferrihydrite colloid by chain-shaped humic acid colloid in saturated porous media.

Both humic acid and colloid particle size effectively regulate colloid transport. However, little is known about effect of particle size and configuration of humic acid colloid (HAcolloid) on enhanced-transport of ferrihydrite colloid (FHcolloid) in porous media. Co-transport of HAcolloid and FHcolloid at different pH was systematically investigated by monitoring breakthrough curves (BTCs) in saturated sand columns. The colloid transport model and the (X)DLVO theory were used to reveal the mechanism of HAcolloid-enhanced FHcolloid transport in the columns. Results showed that HAcolloid enhanced FHcolloid transport in neutral and alkaline conditions. In neutral conditions, small HAcolloid (F-HAcolloid) with chain-shaped structure enhanced FHcolloid transport more prominently than pristine granular HAcolloid. The chain-shaped F-HAcolloid caused osmotic repulsion and elastic-steric repulsion between colloids and sand, leading to enhanced transport. However, the granular HAcolloid readily occurred as deposition due to attachment and straining, which decreased the enhanced transport of FHcolloid. In alkaline conditions, both HAcolloid and F-HAcolloid were chain-shaped, with longer chains of HAcolloid than F-HAcolloid. Ferrihydrite colloid transport was enhanced by HAcolloid more significantly than F-HAcolloid due to stronger repulsion between mixed HAcolloid-FHcolloid and sand. It suggested that regulation of particle size and morphology of HAcolloid would enhance FHcolloid transport and further help in understanding FHcolloid-facilitated contaminants transport in porous media.