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Summary 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 (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. Decreasing mV in the zeta potential, ζ, curves and electron‐donor component, γ–, 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 (≈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. Highlights We applied the extended DLVO model 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.

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Landslides on the flanks of stratovolcanoes can significantly modify the structure of the volcano. Macroscopic factors that determine the stability of volcanic deposits are well understood, but the microscopic interactions between particles and their impact on deposit cohesion remain poorly understood. Deposit cohesion is related to the energy of interaction between particles, and its calculation depends on the surficial properties of the eruptive materials. The purpose of this study was to perform a preliminary comparative analysis of the surficial properties of volcanic materials from various tectonic settings, including electrical (zeta potential) and thermodynamic (surface free energy) components and to calculate the total interaction energy between particles under different environmental conditions. We analyzed eruptive materials from six active volcanoes (El Hierro, Pico Do Fogo, Vulcano, Stromboli, Mt. Etna, and Deception Island). The results show that deposit cohesion varies among volcanoes, and changes drastically with the pH of the medium. Among the volcanic systems investigated, El Hierro (pH = 3) has the most cohesive materials, while Mt. Etna (pH = 8) has the least cohesive materials. Our results suggest that microscopic electrical and thermodynamic properties play a role in the stability of volcanic deposits, and confirm the need for a greater research focus in this area.

Relationship between macroscopic and microscopic behavior of soils is a difficult issue, especially when dealing with complex properties such as structure or carbon sequestration, but it is necessary for a suitable understanding of the agricultural soil quality. In this work, we used the extended-DLVO theory to compute total energy of interaction between particles (kT), of three soils over granodiorites. This parameter allows predicting the tendency to aggregate formation at the colloidal scale, being the basis of soil structure. Furthermore, we characterized the mechanism and adsorption capacity of humic molecules on mineral surfaces, and its influence in the interaction energy, by means of the adsorption isotherms. The aim was to compare first the effects of organic farming on conventionally managed soils and, second, to compare these with a non-cultivated forest soil under Mediterranean climate. When total energies are negative (particle attraction), or positive (particle repulsion) but near 0 kT (&lt;100 kT), then particle flocculation occurs and the structure at colloidal scale can be developed. Total energy was less in the forest sample and greater in the soil of conventional groves, with intermediate values in organic farming soil. This indicates a tendency toward particle flocculation and more stability of the structure at colloidal scale in less disturbed soils, agreeing with other soil physical properties such as the total porosity (50, 41 and 37% in forest, organic and conventional plots, respectively) or the aggregate stability index (0.94, 0.73 and 0.66, respectively), which followed the same trend. Of the three components of the total energy of interaction, the acid-base force was a key factor. This component, related with the electron-donor component of surface free energy, γ−, yielded a strong attractive force (−150 kT at 3 nm) when calcium solutions were analyzed for the forest soil. This indicates a clear hydrophobic character of this sample. Because the mineralogical composition of the samples is quite similar, hydrophobicity should be attributed to the organic carbon content of the forest soil, which is much higher than that of the cultivated ones (12.03 vs. 1.44% and 0.88% in organic and conventional farms, respectively), proving to be an essential element for the development of the structure at the colloidal level.