Effect of salinity on adsorption of sodium hexametaphosphate and hydrophobically-modified polyacrylamide flocculant on kaolinite Al-OH surface

Solution salinity is critical for the inhibition effect of sodium hexametaphosphate on different mineral surfaces, further affecting the separation of minerals in aqueous solution. In this work, the effect of K+ and Ca2+ on adsorption of sodium hexametaphosphate (SHMP) and hydrophobically modified polyacrylamide, poly(AM-NaAA-C(16)DMAAC) (HMPAM), on alumina surface (i.e., kaolinite Al-OH surface) were investigated using quartz crystal microbalance with dissipation (QCM-D). The deposition layer on alumina surface with the effect of adding 1 mM SHMP in 10 mM of CaCl2 solution were examined by glancing angle X-ray diffraction (GAXRD), scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS). The 100 mM KCl solution promoted the adsorption of both HMPAM and SHMP on alumina surface, which would be mainly caused by the molecular configuration change. The more flexible and coiled configuration of HMPAM and SHMP in 100 mM KCl solution could be helpful for increasing the adsorption sites on alumina surface. Moreover, the intermolecular hydrophobic interaction between the alkane chains of HMPAM would lead to the multilayer adsorption of HMPAM instead of adsorbing directly onto alumina surface. Alternatively, 1 mM and 10 mM CaCl2 solutions lead to a lower adsorption amount of HMPAM on alumina surface, which is most likely caused by the suppressed electrostatic attraction and condensed conformation of HMPAM. The adsorption layer of SHMP formed on alumina surface could prevent the adsorption of HMPAM through different mechanisms in KCl and CaCl2 solutions at different concentrations. Interestingly, scattered micro-spheres were found to adsorb on alumina surface with 1 mM SHMP in 10 mM CaCl2 solution, mainly in forms of calcium hydrogen phosphate hydrate. Our work provides fundamental insights into the effect of adding dispersant (e.g., SHMP) and polymer flocculant (e.g., HMPAM) during selective flocculation of kaolinite, with helpful guidance for clay-involved mineral processing.