Humic acid adsorption and its role in colloidal-scale aggregation determined with the zeta potential, surface free energy and the extended-DLVO theory

The effect of colloidal forces involved in the adsorption of commercial humic acids (HAs) and particle cohesion was studied in the soil of an organic olive grove with the extended Derjaguin, Landau, Verwey and Overbeek (extended-DLVO) theory. Total interaction energy was determined from the zeta potential (zeta) and surface free energy, measured under different experimental conditions [natural and hydrogen peroxide (H2O2) organic matter-free mineral surfaces]. The soil was clayey, dominated by illite and vermiculite. It showed electron-donor behaviour, with negatively charged surfaces and zeta potential < 0 mV. Decreasing mV in the zeta potential, , curves and electron-donor component, gamma(-), when adding HA to natural surfaces showed effective HA adsorption, but only when soil organic matter had not been removed previously. Isotherms confirmed adsorption by natural soil (> 2.5 mg C g(-1)). Because the isotherms showed no relation with temperature, adsorption would be better attributed to weak physical interactions. On natural surfaces with HA, soil particle attraction forces increased slightly (approximate to 50 kT) through decreasing soil wettability. However, this effect on total surface energy was overcome largely by increasing electrostatic repulsive energy caused by the adsorption of negatively charged HA (> 300 kT). The DLVO-extended model showed that natural surfaces without H2O2 treatment or added HA seem to be the most favourable state for colloidal aggregate stability. We recommend some caution about the type and quality of organic matter added to increase organic carbon in soil.