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.