Selective separation of seawater Mg²⁺ ions for use in downstream water treatment processes

A nanofiltration-based method is presented for selectively separating soluble Mg(II) species from seawater, with the aim of using the Mg-loaded brine for either enriching desalinated water with magnesium ions or for enhancing precipitation of struvite from wastewater steams. Two 2.4' commercial NF membranes were tested under varying operational conditions. The membrane that was chosen for further investigation (DS-5 DL, Osmonics) showed lower Cl:Mg and Na:Mg concentration ratios in the brine, and improved performance (with respect to the investigated process) at high recovery values. Since the addition of antiscalants was perceived detrimental to the downstream uses of the brine, the aquatic chemistry program PHREEQC was used to simulate the critical (highest) recovery values at which no CaSO₄ would precipitate, assuming two concentration polarization factors. To prevent CaCO₃ precipitation at the critical recovery values a theoretical calculation was performed (PHREEQC) to determine the required strong acid dosages to the raw seawater. Using the DS-5 DL membrane at 64% recovery, the attained Mg(II) concentration in the brine was 3500mg/l. Therefore, for attaining 12.15mg Mg/l of desalinated water the brine should be dosed to the water at a 1:288 ratio, resulting in additional concentrations of 32.5mg SO₄^-2/l, 89.3mg Cl^-/l, 39.4mg Na⁺/l, 3.3mg Ca⁺²/l, and 0.01mg B/l. The overall cost of the proposed process was estimated at 0.00098$/m³ product water, i.e. approximately five times lower than two assessed alternative processes and more than one order of magnitude cheaper than implementing direct dissolution of chemicals, using either MgCl₂ or MgSO₄.