Membrane desalination is a popular technology for water desalination, reclamation, and purification. However, the cost and environmental harm associated with managing the concentrated byproducts requires product water recovery to be maximized. The principle phenomena limiting the recovery membrane desalination processes is scaling by sparingly soluble salts. In full-scale membrane desalination processes, water is continually removed from the feed stream while salt ions are retained. This leads to an increasing retentate salt concentration along the length of the treatment train. The ratio of retentate salt concentration to feed salt concentration is known as the concentration factor (CF), and the CF at which the least soluble mineral salt forms a scale is the “limiting CF.” Concentration polarization causes sparingly soluble salts to exceed their solubility at the membrane-water interface before the limiting CF is exceeded in the bulk. Further dewatering can lead to crystallization in the bulk solution or at the membrane-solution interface. Both surface scale formation and precipitated crystal deposition cause degradation of water permeability, product quality, and membrane lifespan. At the same time, advances in nanoscale science and engineering suggest that many of current problems involving water quality could be resolved or greatly ameliorated using nanoparticles which have much larger surface area than the bulk particles, and which can be functionalized with various chemical groups to increases their affinity toward target compounds. They all must get together the majority of the following characteristics: water solubility, appropriate size or molecular weight, high affinity to target metal ions, negligible affinity for non–target compounds, possibility of regeneration, chemical and mechanical stability, low toxicity, and low cost.

A thorough literature review and preliminary experimental results suggest that the selective removal of mineral cations from brackish waters can prevent scale formation and enhance product water recovery up to 90% without the need to adjust feed water pH or add antiscalants. Also, natural and synthetic materials show high metal-binding capacity and selectivity, especially for Ca2+, Ba2+, and Sr2+, which are the main scaling metal ion in most waters targeted for desalination. The separation of metal-polymer complexes, for example, can be achieved by low-pressure ultrafiltration. Our objective is to provide a highly selective calcium-binding material that is efficiently separated from water by low-pressure membrane filtration, and which, can be effectively removed by rapid permeate backpulsing prior to gel layer formation. Such a process would economically and efficiently remove the principle scaling metal ion in alternative waters, and enable high product water recovery in downstream membrane desalination.

Preliminary bench scale membrane desalination experiments were performed on agricultural drainage water samples collected from the Alamo River in Imperial Valley, California, which contains high concentrations of calcium, sodium, magnesium, potassium, bicarbonate, chloride, and sulfate as well as trace amounts of barium, strontium, and nitrate. Model solutions representing Alamo River water (ARW) were also used to conduct well controlled desalination experiments in which various mineral ions were selectively removed from the model ARW. Commercial reverse osmosis (RO) and nanofiltration (NF) desalination membranes were used in all experiments. The feed, retentate, and permeate concentrations of major ions was determined in real-time by ion selective electrodes. Membrane surfaces were imaged by SEM and EDS analysis to confirm elemental composition of scale layers formed and the limiting scaling salts. Various nanomaterals (natural and synthetic) such as colloidal matter (humic and alginic acid), as well as a number of synthetic chelating agents, metal-binding polymers, and ion-exchange nanoparticles (synthesized in our laboratory) were characterized to verify scaling metal ion binding strength, capacity, and regeneration. Additional experiments with the various metal-binding agents dosed into the ARW feed were performed to determine their efficacy at inhibiting mineral scale formation and enhancing overall water recovery. Although our current focus is on selective removal of scaling mineral cations, in the future, this approach could be tailored to enable selective removal of other target metal ions, toxic metals, or emerging contaminants.