Water repellency enhances the deposition of negatively charged hydrophilic colloids in a water-saturated sand matrix

The effect of grain water repellency on transport and deposition of hydrophilic colloids was studied by analyzing the breakthrough behavior of carboxylate-modified microspheres in water-saturated wettable and hydrophobic sand columns at different ionic strengths. Interaction free energies calculated from zeta (ζ)-potential and contact angle data were used to explain the specific colloid breakthrough behavior. Experimental breakthrough data could be well described with the finite-element code HYDRUS-1D using a one kinetic site model with attachment and detachment kinetics. Higher colloid deposition rates found for the hydrophobic sand could primarily be explained by its small electron-donor component of surface free energy (γs− = 1.6 × 10−2 mJ m−2, compared to γs− = 64.1 mJ m−2 for the wettable sand), leading to strongly attractive acid–base interactions at separation distances < 5 nm. Increasing ionic strength reduced the repulsive electrostatic interactions and generally increased colloid deposition with the effect being more pronounced in the hydrophobic sand. It can be concluded that grain water repellency tends to increase the deposition of negatively charged hydrophilic colloids, which can be ascribed to specific acid–base interactions. However, our results further revealed that the calculated interaction free energy profiles should be considered only as an approximation showing general trends because surface chemical heterogeneity as detected by atomic forces microscopy impeded the determination of the actual interaction energy conditions, resulting in an overestimation of electrostatic repulsion.