Iron Oxide Colloid Mobility as Affected by Solid Matrix Wetting Properties

Abstract

Core Ideas Decreasing quartz sand wettability leads to attractive hydrophobic interactions with colloids. In flow columns, transport of Fe oxide colloids is not affected by sand matrix wettability. The short‐range hydrophobic attraction is probably counteracted by colloid surface roughness. The influence of porous media wettability on the mobility of colloids is mostly unknown. In the present work, organic‐matter‐coated goethite (OMCG) colloids were percolated through three saturated soil materials differing in wettability: untreated quartz sand and two variants of hydrophobized sand. For each type of sand, three ionic strength levels were applied. Derjaguin–Landau–Verwey–Overbeek (DLVO) and Lewis acid–base extended DLVO (XDLVO) interaction energy profiles were calculated according to contact angles and zeta potentials. Flow column results elucidated that decreasing sand wettability had no relevant effect on OMCG colloid mobility. In contrast, colloid retention increased with ionic strength in each type of sand packing. Classic DLVO interactions could predict trends in colloid retention by the respective characteristics of energy barriers and secondary minima. The extension with Lewis acid–base interactions in the XDLVO approach led to the prediction of significant short‐range (∼2 nm) attractive interaction energies between colloids and hydrophobized sand, which were not reflected by colloid breakthrough behavior. This was probably due to substantial energy barriers calculated for larger distances (∼27 to ∼75 nm, depending on ionic strength) between the solid matrix and colloids. It is concluded that the distinct surface roughness of sand grains and colloids probably weakened the strength of the short‐range attractive interactions, because larger amounts of surface area were still outside the effective distance for the short‐range interactions predicted by XDLVO. Regarding colloidal mobility, we concluded for our saturated porous systems that near‐surface attractive XDLVO interaction energies between OMCG colloids and hyrophobized sand did not significantly affect colloid mobility.