Aggregation and transport behavior of goethite colloids as affected by dissolved organic matter and pH: Electrostatic vs. hydrophilic interactions

Abstract

Wettable surfaces in aqueous media experience hydrophilic repulsion forces, which have been thoroughly examined for interactions between biocolloids. However, there is no systematic assessment yet on the effects of hydrophilic repulsion on aggregation and transport behavior of soil mineral colloids. The aim of this study was to fill that gap in knowledge, with an emphasis on whether hydrophilic or electrostatic forces govern the respective interactions. For that, the interactions of hydrophilic goethite colloids were examined by batch aggregation experiments and colloid breakthrough experiments using columns filled with hydrophilic quartz sand. To induce electrostatic interactions ranging from attractive to repulsive, colloid zeta potential was modified via changes in dissolved organic matter concentration and pH. Classic DLVO and the extended DLVO (XDLVO) theory that includes hydrophilic interaction energies were applied using electrophoretic mobility measurements and sessile drop contact angle data. Results showed that for each case in which hydrophilic repulsion was predicted to be stronger than electrostatic attraction, the latter always controlled the colloid aggregation and transport behavior in the experiment. The likely reason was that the irregular shape and surface roughness of the colloids, as shown by SEM images, rendered the short-range hydrophilic repulsion ineffective, so that the more long-range electrostatic interactions predominated. We conclude that in contrast to biocolloids, hydrophilic repulsion did not have a relevant effect on the interactions between soil mineral colloids. Hence, our study clearly shows that for the aggregation and mobility of soil mineral colloids, soil parameters affecting their surface charge are more important than parameters affecting their wettability.