Replenishing Mg(II) to desalinated water by seawater nanofiltration followed by magnetic separation of Mg(OH)_2(s)Fe₃O₄ particles

Magnesium (Mg²⁺) appears at high concentration in seawater and seawater-reverse-osmosis brines. In contrast, desalinated water is almost completely depleted of Mg²⁺, a mineral perceived essential for human health and agricultural irrigation. The paper introduces a cost-effective method to enrich desalinated water with an almost pure Mg(II) solution, originating from seawater. The method uses seawater nanofiltration (or nanofiltration of seawater-reverse-osmosis brine) to produce brine characterized by high Mg²⁺ concentration, accompanied by relatively low B, Cl⁻, and Na⁺ concentrations. Subsequently, Mg(II) is separated from the produced nanofiltration brine by precipitating Mg(OH)_2(s) and adsorbing it onto the surface of micro-magnetite particles. Finally, the solid slurry (Fe₃O₄ + Mg(OH)₂) is magnetically separated from the brine and the Mg(OH)_2(s) is re-dissolved into the desalinated water in a separate reactor. Application of the method results in a relatively pure Mg(II) addition to the desalinated water product. For example, for Mg(II) addition of 10 mg/L (as recommended by the World Health Organization), the following negligible concentrations of unwanted species are added to the water (in mg/L units): Na⁺: 0.04, Cl⁻: 0.18, Ca²⁺: 0.05, and B: 0.0094. The cost of adding 10 mg Mg/L was estimated at 0.76 cent$/m³ of desalinated water, i.e. competitive with previously suggested processes.