Transport of colloids in unsaturated porous media: Explaining large‐scale behavior based on pore‐scale mechanisms

Abstract

[1] We conducted column-scale experiments to study the transport of colloids (latex particles and bacteriophage MS2) under water-unsaturated conditions. The objective was to draw connections between observations at the pore scale and the results obtained from column-scale experiments. The same system had been previously operated under saturated conditions to determine colloid collision efficiency. Breakthrough of colloids was first evaluated under unsaturated but steady water content conditions, with constant trickling flow. After monitoring the steady breakthrough of the colloids, the column was flushed with water at higher flow rate to increase the water content up to a saturated condition. Colloid breakthrough was monitored during the entire experiment, as water content increased. Colloid removal increases significantly with decreasing initial water saturation, reflecting retention at the air-water interface and straining in thin-water films. Colloid breakthrough occurs earlier than a conservative tracer even under unsaturated conditions, although the colloid concentrations are much lower than the tracer. After flushing at similar flow rates, there is increased colloid retention under unsaturated conditions even as the system approaches water saturation, indicating that additional removal is occurring, due possibly to the formation of colloidal clusters. These results can be explained to a great extent by pore-scale observations of retention and remobilization mechanisms.

A review of visualization techniques of biocolloid transport processes at the pore scale under saturated and unsaturated conditions

“…Transient flow, generated by rainfall and snowmelt events interspersed between dry periods or due to artificial aquifer recharge or other anthropogenic actions, can promote very rapid biocolloid mobilization (e.g., [34]). Under transient conditions it has been observed that the movement of biocolloids is affected by the movement of air bubbles and AWI (e.g., [46,45,71]). Sirivithayapakorn and Keller, Auset et al [122,5] observed in micromodels how infiltration events can mobilize the AWI, thicken the water films where colloids are immobilized, dissolve some of the gas phase and mobilize air bubbles (Fig.…”

“…Colloid remobilization appears to be a strong function of particle size [71]. Although intermittent filtration provides significant pathogen removal capacity, it is important to take into consideration the potential for biocolloid remobilization over time.…”

Gravity-driven transport of three engineered nanomaterials in unsaturated soils and their effects on soil pH and nutrient release

“…3). Since the soils were unsaturated and the flow rate did not fully saturate the soils, breakthrough of the suspension water was observed at the bottom of the columns, likely through the movement of thin water films (Keller and Sirivithayapakorn, 2004). In potting soil breakthrough occurred during ENM application or within 5 min, while in grassland and agricultural soils it occurred within 1e2 h due to their greater density.…”

Transport of Nanomaterials in Unsaturated Porous Media

“…Particle size has also been found to be important in determining the straining sites (air-water-solid contact line or thin water film) (47). In general, film straining is considered reversible, which means that the strained colloids can be quickly remobilized by the increase of water content (and the corresponding increase in film thickness), caused by transient water flow (46,52,53) or resaturation (43).…”