A method of selectively separating ions, including: (a) providing a reverse osmosis (RO) separation arrangement having an RO membrane; (b) introducing a feed brine to the RO membrane, the brine containing multivalent cations, alkali cations and halide anions, a total concentration of the alkali cations and halide anions being at least 4%, by weight of the feed brine; (c) applying a sub-osmotic pressure to the feed brine, to drive a first portion of the brine through the RO membrane to produce a permeate solution, a remainder of the feed brine being rejected by the RO membrane and being discharged as a reject solution; the sub-osmotic pressure being applied so as to preferentially distribute the multivalent cations to the reject solution, with respect to the permeate solution; the sub-osmotic pressure being less than 0.9 times a measured or theoretical osmotic pressure of the feed brine.