Highly-selective separation of divalent ions from seawater and seawater RO retentate

A membrane-based process is presented for separating divalent ions (namely Mg2+, Ca2+ and SO42−) from seawater in a highly selective fashion. The main goal is to selectively and cost-effectively separate Mg2+ from seawater with the intention of either dosing it into desalinated water in the desalination post-treatment stage or generating a Mg2+/Ca2+/SO42− solution that can be used for, e.g., precipitating struvite from high-strength wastewaters prior to anaerobic-digestion, with minimal addition of detrimental Cl−/Na+ ions. The process comprises three steps: First, seawater undergoes a high-recovery nanofiltration (NF) step with an “open” NF membrane aimed at reducing the molar ratio between divalent-cations and SO42− in the NF retentate. The retentate of this step is then subjected to further ∼65% recovery (conventional) NF step aimed at increasing the Mg2+ concentration in the retentate, which is thereafter subjected to a diananofiltration step aimed at reducing the monovalent ion concentrations while maintaining high Mg2+/SO42− concentrations in the product solution. The hypothesis (which was fully substantiated in the work) was that the reduction of the molar ratio between total hardness and SO42− in the retentate of the 1st NF step would result in a lower Cl− to Mg2+ concentration ratio in the product solution (retentate) of the NF-NF-DiaNF process sequence. Results are presented for a variety of operational conditions using both seawater and seawater reverse osmosis brine as raw solutions. The cost of separating one kg of Mg2+ from seawater using the method is significantly lower than the equivalent cost of the chemical MgSO4 in all the presented scenarios. However, reducing the Cl− concentration in the product solution by ∼62% more than doubled the cost. For example, dosage of 20 mg Mg2+/l to desalinated water was estimated at $0.006/m3 and $0.017/m3 for addition of 32 and 12 mg Cl−/l along with the Mg2+, respectively.