The removal of inorganic contaminants using nanofiltration and reverse osmosis

Abstract

Improved methods of providing safe drinking water are essential in an era in which demand for water is increasing but surface water supplies remain scarce. Desalination of brackish groundwater via membrane filtration with nanofiltration and reverse osmosis (NF/RO) offers a solution to this problem. As such, the overall motivation of this study was to improve mechanistic understanding of NF/RO. The first main aim was to evaluate the performance of a renewable energy membrane system previously tested with real groundwater and varying energy conditions. Given sufficient solar availability, the system reliably removed salts and inorganic contaminants, although solute retention varied with energy (and consequently pressure and flow) and pH, depending on dominant retention mechanisms. The second main aim was to assess the specific impact of pH on inorganic contaminant removal in a bench-scale filtration system. The speciation of boron, fluoride and nitrate was linked with ion retention as a function of pH, with results suggesting that there may be important mechanisms such as ion dehydration controlling transport in NF/RO, which would explain the high retention of fluoride when compared to nitrate. The third main aim was to determine the importance of ion hydration in determining transport using molecular dynamics simulations of monovalent anions transporting through an idealized pore. Simulations demonstrated that energy barriers of transport were strongly dependent on ion properties and pore size and were directly attributable to dehydration. The final aim was to experimentally verify molecular dynamics simulations by quantifying energy barriers for ion transport in NF membranes. Experimentally-determined energy barriers were also solute and membrane-specific, with fluoride having a higher barrier than other solutes. Comparison of results with expected dehydration trends and molecular dynamics corroborated that energy barriers in nanofiltration may be due to dehydration. The results obtained in this thesis provide new insight into NF/RO transport mechanisms, which may contribute to improvements in current technologies and predictive models.