Interaction between humic acid and montmorillonite in the presence of calcium ions I. Interfacial and aqueous phase equilibria: Adsorption and complexation

The organic matter content and ionic composition of soil solutions have a dominant effect on soil structure, forming gradually an aggregation of mineral particles, in which clay fractions play a determining role. The colloidal interaction of particles, including repulsion and attraction, changes with their inherent surface properties and the interfacial layer composition around them, resulting in chemical and physical equilibria between solid and aqueous phases. These interactions and their relationship were studied in model systems containing soil constituents having a definite role in the formation of soil structure and are organized into two parts, here the interfacial processes as part I, with the particle network formation in the second part. The adsorption of humic acid (HA) on montmorillonite was studied under conditions relevant to the environment at pH 6.5 and 0.0014 M NaCl. The effect of Ca loading from 0.0013 to 0.0055 M was the focus of the work. Both HA and Ca2+ adsorption were measured. To reveal molecular insight into the interactions between HA, Ca2+ and montmorillonite, the HA concentration was related to the molar amount of its acidic functional groups. Adsorptive fractionation of HA was characterized using spectrophotometric parameters (SUVA254, E3/E4 and E4/E6). The fourfold increase in Ca loading resulted in more than ten times greater HA adsorption, but only three times greater Ca2+ adsorption, during which the distribution between the solid and liquid phases changed considerably. The dominant processes were preferential Na+/Ca2+ ion exchange and complexation by HA both in the aqueous phase and on montmorillonite (through Ca bridges on the faces and directly on Al–OH sites at the edges), causing adsorptive fractionation of organic matter and leaving the small, less humified HA fraction in the aqueous phase.