Thermodynamically comparison of Na⁺ and Ca²⁺ adsorption onto PVD and NF45 membranes

To better understand adsorptive fouling mechanism of nanofiltration membranes, adsorption of Na⁺ and Ca²⁺ ions from their binary solutions onto PVD and NF45 nanofiltration membranes was investigated. Cross-flow batch concentration procedure was applied to generate equilibrium adsorption data. Three equilibrium models such as Freundlich, Langmuir and Redlich-Peterson were employed to describe the adsorption of the cations onto the membranes. Heterogeneous nature of the adsorption was verified by the best conformity of the Freundlich isotherm with the equilibrium data. Thermodynamic analysis showed that for both ions adsorption was endothermic (ΔH°>0), spontaneous (ΔG°<0) and entropy driven (ΔS°>0). Decreasing ΔG° values with temperature suggested that adsorptive fouling can be limited by reducing temperature. Thermodynamic parameters exhibited that adsorption of Ca²⁺ compared with Na⁺ was more favorable and entropy inducer at the solid/liquid interface. Moreover, NF45 compared with PVD membrane showed superior capacity for adsorption of the cations due to the presence of more capable functional groups on its surface. The major parts of the membranes involved in adsorption was determined as their surface and pore walls. However, adsorption on pore walls was less probable during adsorption from CaSO₄ solution. Also, adsorption of the cations from mixed solutions (NaCl and CaSO₄) at total concentration levels of 100 and 250mg/l was examined. The minimum of total cation adsorption on the membranes was observed at Na⁺ to Ca²⁺ 30:70 weight ratio. The result suggested that adsorption from binary solutions was affected not only by ionic strength of the solution but also by potential barriers against ions diffusion toward membrane surfaces i.e. mass transfer hindrances.