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

Samuel C.N. Tang, Liat Birnhack, Paz Nativ, Ori Lahav*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

20 Scopus citations


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.

Original languageEnglish
Pages (from-to)460-468
Number of pages9
JournalSeparation and Purification Technology
StatePublished - 24 Mar 2017
Externally publishedYes


  • Desalination post treatment
  • DiaNanofiltration
  • Mg
  • Nanofiltration
  • Seawater


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