Nanofiltration of bivalent nickel cations — model parameter determination and process simulation

Nanofiltration is a pressure-driven membrane separation method that is also used for selective removal of salts from liquid streams. The separation by nanofiltration is based on both sieving and charge effects and operated typically at relatively low pressure. Although several mathematical models for nanofiltration are already known, the modeling of mass transfer in complex aqueous solutions remains still under discussion. In this work, the filtration of different single model solutions containing mono- and bivalent cations like Na+ and Ni2+ is studied with several membranes. First, the effect of solution pH on the rejection of the cations and on the permeate flux is investigated. Furthermore, the influence of pH on membrane characteristics due to change of the membrane charge is shown. The rejection of cations increases at pH values below the isoelectric point, in consequence of repelling interactions between cations and membrane which is positively charged at low pH. According to this finding, the optimal separation condition in terms of pH value for the purpose of recovery of Ni2+ is ascertained for two membranes. From these experiments as well as from those with glycerol solution, three model parameters (pore radius, membrane charge and pore dielectric constant) are determined. These parameters are sufficient for simulating the process on the basis of a linearized transport model suggested by Bowen. The validity of this simplified approach is examined. The finally received model is applied for simulating a case study of a process integrated nickel recovery.