Separation Potential Characterization and Its Role in Selecting Nanofiltration Membranes for the Removal of Inorganic Ions

Water desalination with nanofiltration (NF) and reverse osmosis (RO) membranes presents an excellent solution to meet the ever–increasing water demand. NF membranes can achieve higher permeability at lower operating pressures and hence are energetically favorable compared to RO membranes. As such the overall motivation of this study was to improve the fundamental understanding of separation potentials for NF membranes and provide sound guidance for the selection of NF membranes for particular applications. The first objective of this study was to explain the importance of different separation potentials for the rejection of inorganic ions by two most commonly used active layers of NF membranes – polyamide (PA) and poly(piperazineamide) (PP). Effective pore size measurements, zeta potential and crossflow ion rejection were used to establish that both Donnan (charge) and steric exclusion are important for ion rejection with PP membranes and that steric exclusion was the dominant mechanism for PA membranes. Specific studies with barium and strontium ions confirmed the dominance of steric exclusion for PA membranes. Experimental studies were conducted to unravel the impact of chemical cleaning on physicochemical characteristics and performance of PA and PP membranes. In general, chemical cleaning with HCl and NaOH increased membrane permeability and decreased ion rejection due to the increase in effective pore sizes and changes in zeta potential but PP membranes were affected significantly more than PA membranes. The second objective of this study was to evaluate the use of NF membranes for treatment of abandoned mine drainage (AMD). Laboratory–scale optimization followed by pilot–scale testing demonstrated that polymeric NF membranes could achieve >98% removal of total dissolved solids without significant decrease in permeate flux. Polymeric NF membrane achieved higher permeability and ion rejection than ceramic NF membrane. This study demonstrated that AMD can be treated with polymeric NF membranes to recover high quality permeate and highlighted the need for improving ceramic NF membranes. The results obtained in this study provide new insights into NF separation mechanisms and their use for the treatment of AMD and contribute to further improvements in current membrane technologies to provide solutions for significant environmental problems and meeting the ever–increasing demand for clean water