Effect of pH on adsorption of anionic surfactants on limestone: Experimental study and surface complexation modeling

Abstract We investigate surfactant adsorption on a model carbonate rock over a wide range of pH and surfactant-to-solid ratios, by both an experimental and a theoretical approach, to obtain a quantitative understanding of how mineral constituents affect the adsorption equilibrium and dynamics. A combination of bulk mineralogy, elemental composition analysis, dissolution behavior, and water ionic composition data were used to characterize the samples’ surface heterogeneity. The adsorption of anionic surfactants is 2.4 mg/m 2 at pH of ∼8 and decreases approximately linearly with pH values above 9. To constrain and compare the relative adsorption affinity and the likely modes of attachment on mineral constituents as pH changes, we performed surface complexation calculations using a two-surface multisite diffuse layer model. We propose the formation of two surface species on both the major (i.e., calcite) and trace (i.e., the oxide-like sites on the edges of clay platelets) minerals: a monodentate inner-sphere complex and a weak or hydrogen bonding complex. Our modeling results suggest that charge-regulated inner-sphere complexation is the dominant adsorption mechanism on the calcite and oxide-like sites at low pH values regardless of the surface loading. We found weak or hydrogen bond adsorption to be significant on the calcite surface, and this became the dominant adsorption mode at pH ∼10. While the adsorption on calcite increases with surface loading, adsorption on the oxide-like sites remains independent of surface loading. These results suggest that surfactant adsorption can be comparable on the abundant low-surface-area calcite and trace high-surface-area oxide-like sites.