Iron Oxide Colloid Mobility As Affected By Various Surface and Flow Conditions.

The mobility of iron oxide colloids in soil and groundwater is of interest due to their capacity to act as “shuttles” for transport of adsorbed contaminants. The aim of this study is to investigate transport and retention of iron oxide colloids under complex, soil-typical conditions and to determine if effective chemical surface parameters may be used as a tool to predict general tendencies for goethite colloid mobility.

For that, goethite (α-FeOOH, average diameter 500 nm) colloids were percolated through quartz sand flow columns. The impact of several chemical and physical parameters on colloid mobility was determined: Concentration of dissolved organic matter (DOM) and CaCl₂as well as changes in flow regime including flow interruption and partial saturation. Additionally, quartz sand surface modifications with goethite coatings, organic matter coatings, and hydrophobication via dichlorodimethylsilane treatment were conducted. By means of contact angle and zeta potential measurements of the applied materials, both classic DLVO and Lewis acid/base parameter-extended DLVO interaction energies between colloids and solid matrix were estimated.

Experimental results elucidate that high goethite colloid mobility was distinctly restricted to a narrow set of conditions: Presence of DOM, low ionic strength, sufficient flow velocity and constant flow, full saturation, and quartz sand devoid of goethite coatings. Calculations based on effective chemical surface parameters showed that (i) primary and secondary energy minima yielded by the classic DLVO approach are sufficient to predict general trends of goethite colloid mobility, and (ii) extended DLVO calculations revealed strong surface-near (~5 nm) interaction energies due to Lewis acid-base interactions that do not influence goethite colloid transport. These short-range interaction energies may have been weakened by surface roughness of sand grains and colloids.