Effects of humic acid release from sepiolite on the interfacial and rheological properties of alkaline dispersions

Humic acid (HA) is a major extractable and soluble component of soil organic matter that can play an important role in the stability of environmental colloidal dispersions. This study suggests that HA content of sepiolite is one of the essential reasons for non-Newtonian behaviour of sepiolite dispersions in alkaline media. Releases of HA from sepiolite matrix dramatically changed their rheological properties (thixotropy, apparent viscosity and yield stress), and interestingly, the level of sepiolite surface hydrophobicity. Alkaline environments also increased the release of Mg²⁺ ions from the sepiolite matrix and, thus, caused to a stable complex formation with HA macromolecules. This colloidal solid phase was directly observed and imaged by atomic force microscopy (AFM) which revealed a spherical shape of complexes between Mg²⁺ ions and HA in the size range of 18-63±8nm at pH11.5.Alkaline environments also led to significant changes in the surface properties of sepiolite; after HA release, acid/base components of the surface free energy of sepiolite were determined as γ_S^-/γ_S⁺=1.49 and defined as having a highly hydrophobic character like natural hydrophobic minerals, e.g. talc. The major soil organic matter component, HA, was also found to be responsible for the changes in the sepiolite surface hydrophilicity.