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

Abstract

SummaryThe 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 extendedDerjaguin,Landau,Verwey andOverbeek (extended‐DLVO) theory. Total interaction energy was determined from the zeta potential (ζ) 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. DecreasingmVin the zeta potential,ζ, curves and electron‐donor component,γ–, when addingHAto natural surfaces showed effectiveHAadsorption, 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 withHA, soil particle attraction forces increased slightly (≈50kT) 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 chargedHA(> 300 kT). TheDLVO‐extended model showed that natural surfaces withoutH2O2treatment or addedHAseem 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.HighlightsWe applied the extendedDLVOmodel to study humic acid adsorption and its effects on soil structure.Adsorption on soil surfaces with their natural organic matter was mostly by weak physical forces.Adsorption increased the total particle interaction energy through an increase in electrostatic repulsion.Adsorption of some types of organic matter might decrease the colloidal stability of aggregates.