Forces of interactions between iron and aluminum silicates: Effect of water chemistry and polymer coatings

Atomic force microscopy-based force spectroscopy (AFM) was employed to investigate the forces of interaction between aluminum silicates (mica and a synthetic aluminum–silicate) and iron particles, both bare and coated with carboxymethyl cellulose (CMC) polymer. Experiments were conducted in water and salt solutions (100 mM NaCl and 100 mM CaCl2) at pH 5.5, in water at pH 4 and 8, and in 10 mg/l humic acid solutions. In addition, humic acid sorption onto the synthetic aluminum–silicate was probed with a quartz crystal microbalance with dissipation monitoring (QCM-D). Interactions between bare iron particles and aluminum silicate were attractive except at pH 8 and in the presence of humic acids in which case forces upon approach were repulsive. Interactions between bare iron and mica were similar, except that repulsive forces upon approach were measured in 100 mM NaCl solutions, possibly due to increased hydration of mica compared to aluminum silicate. Interactions between CMC coated iron particles and aluminum–silicates were either repulsive or at most weakly attractive, likely due to repulsive electro-steric forces associated with the CMC. QCM-D results indicated that humic acids adsorbed to aluminum silicate, producing electro-steric repulsion to coated and uncoated iron. AFM data were successfully modeled using extended DLVO theory and a modified Ohshima’s model. This modeling provided insights into the contributions of various processes to the measured interaction forces, highlighting the importance of van der Waals and hydration forces.