Adsorption of silica nanoparticles onto calcite: Equilibrium, kinetic, thermodynamic and DLVO analysis

Recently, application of silica nanoparticles (SNP) for enhancing oil recovery during water flooding has been much attended. However, understanding how rock and nanoparticles (NP) interacts through adsorption onto the carbonate reservoir rocks is not well discussed. In this work, adsorption behavior of SNP onto the calcite had been characterized, through kinetic, equilibrium, thermodynamics and electrokinetic studies as well as interaction energy analysis by DLVO theory. Also, field emission scanning electron microscopy (FESEM) was utilized to visualize the adsorption process. It had been found that kinetic behavior of SNP–calcite system followed the pseudo-second order model. Equilibrium study revealed that Langmuir model yielded a better fit which implied that monolayer coverage of SNP onto the calcite surfaces was more probable. Analysis of kinetic data indicated that the intraparticle diffusion mechanism was not the sole rate-controlling mechanism and the boundary layer diffusion affected the adsorption to some extent. Thermodynamic study showed that physical/electrostatic adsorption was expected to be the prevailing mechanism for the adsorption with a spontaneous and endothermic nature. Also, adsorption process was enhanced by lowering the pH and increasing ionic strength of the solutions. These findings were argued based on the equilibrium reactions of the SNP and calcite in water and DLVO theory. Results revealed that repulsive DLVO interaction energies predominated from which an unfavorable attachment condition was deduced. In such situation, ionic strength of solution controlled the extent of the SNP–calcite interaction. Reduction in the height of energy barrier and the formation of a secondary minimum were responsible for enhancing the adsorption process at higher ionic strengths. Furthermore, FESEM-observations depicted a spherical morphology for the adsorbed SNP that uniformly distributed on the calcite surface.