Adsorption of cations on nanofiltration membrane: Separation mechanism, isotherm confirmation and thermodynamic analysis

In this research, the adsorption mechanism of the saturated brine cations (Fe2+, Mg2+ and Ca2+) on the negatively charged PVD membrane was investigated. The separation mechanism of the cations was cake deposition on the membrane surface due to adsorption. This was justified on the basis of Hermia blocking laws and surface cake analysis. Three equilibrium adsorption isotherms, Langmuir, Freundlich and Redlich–Peterson were fitted to the quasi-equilibrium adsorption data which were calculated using experimental flux and rejection. The Freundlich and Redlich–Peterson isotherms were found to well represent the measured sorption data on the basis of the acceptable coefficients of determination (r2). An increase in temperature and pressure enhances the adsorption characteristics of the cations. However there are some limitations for pressure increment. For all cations, the significant thermodynamic parameters such as ΔG°, ΔH° and ΔS° were calculated, compared and explained on the basis of charge density and hydration radius. The results indicate that the adsorption of cations on the nanofiltration membrane is endothermic and spontaneous due to positive ΔH° and negative ΔG°. The optimum temperature and pressure for sorption of the cations on PVD membrane were determined as 40 °C and 10 bar, respectively.