Effects of organic matter coatings on the mobility of goethite colloids in model sand and undisturbed soil

Abstract

SummaryThe mobility of goethite (OMCG) colloids coated with organic matter was studied in goethite‐coated quartz sand and in undisturbed subsoil rich in natural coatings of iron oxide. Classic Derjaguin–Landau–Verwey–Overbeek (DLVO) interactions and DLVO extended by Lewis acid–base parameters (XDLVO) were estimated between colloids and goethite‐coated sand from zeta potentials and sessile drop contact angles. The OMCG colloids were retained completely in goethite‐coated sand, whereas preconditioning of the solid matrix with dissolved organic matter (DOM) enabled subsequent colloid transport. Zeta potential values showed that goethite‐coated sand was modified strongly by DOM preconditioning. The DLVO and XDLVO interactions were estimated for the pairs of OMCG colloids and for (i) quartz, (ii) goethite and (iii) organic matter‐coated goethite. Both DLVO approaches were able to predict general trends of colloid breakthrough. An exception was the XDLVO interaction for combination (iii), which showed short‐distance (<1.3 nm) repulsive interactions that were inconsistent with breakthrough behaviour. Analogous to OMCG colloid transport in the sand, DOM preconditioning was also the prerequisite for OMCG colloid transport in undisturbed subsoil. Total colloid breakthrough in the soil was less than in the sand, probably because of straining in pore throats and retention at air–water interfaces. We concluded that the DLVO and XDLVO approaches were in general capable of predicting trends of OMCG colloid mobility in goethite‐coated sand. We concluded further that the basic principles of OMCG colloid transport were transferrable from the simplified model system to the more complex natural system despite considerable differences in surface properties, pore structure and water content.Highlights How will organic matter (OM) coatings on the solid matrix affect mobility of iron oxide colloids? Effect of OM on mobility was assessed in simple model soil systems and in undisturbed soil. OM coatings modify properties of the solid matrix surface and are a prerequisite for colloid transport. Balances between OM transport, coating formation and decomposition control colloid mobility.