Experimental determination of the colloidal stability of Fe(III)-montmorillonite: Effects of organic matter, ionic strength and pH conditions

Abstract

Aggregation and disaggregation of particle colloids are one of the most important surface-driven phenomena encountered in the aquatic and terrestrial environments and a key factor controlling a number of important environmental processes. This study investigates the effects of pH, ionic strength, and humic acid concentration on the stability behavior of Fe(III)-montmorillonite, a natural colloid commonly present in natural waters. Time-resolved dynamic light scattering was used to monitor the increase of the aggregate size over time in the aggregation kinetics experiments. Aggregation rate, stability ratio, and CCC (critical coagulation concentration) were calculated to quantify the experimental results, and the DLVO theory was employed to explain the observed behaviors. The effect of humic acid on the colloidal electrosteric stability was also investigated. This study demonstrates that low pH and high ionic strength may destabilize Fe(III)-montmorillonite suspensions, while increasing humic acid concentrations has the opposite effect, stabilizing the suspension at any pH. Comparing the steric and electrostatic effects on the Fe-M stabilization, both have an important influence for all pH levels studied, although steric is more pronounced at low pH and high ionic strength. DLVO energy predictions support the experimental results. The obtained results contribute to the understanding of the behavior of colloidal particles in saline or freshwater natural environments as well as the role of humic acid in the mobility of contaminants associated with natural colloids.