Coupling effects of feed solution pH and ionic strength on the rejection of boron by NF/RO membranes

The coupling effects of solution pH and ionic strength on boron rejection by nanofiltration (NF) and reverse osmosis (RO) membranes were investigated. Two NF membranes (namely NF270 and NF90) and three RO membranes (namely BW30, SW30 and UTC80) were used to provide a full spectrum of NF/RO membranes. The rejection of boron by all five membranes was pH-dependent. The dependency of boron rejection on the feed solution pH became much more substantial as the nominal salt (sodium or calcium) rejection value of the membrane decreased. At pH 11, boron rejections by the NF90 and the NF270 membranes were only 10% and 30% lower than those by the other three RO membranes, respectively. On the other hand, the permeabilities of the two NF membranes investigated here were 3–11 times higher than those of the RO membranes. The reported data suggest a possibility of using NF membranes for the second pass in seawater desalination applications to avoid over-demineralisation of the final product water. The reported results also reveal an intricate interplay among the feed solution pH, ionic strength and their effects on the rejection of boron by NF/RO membranes. At pH 10, which is immediately above the intrinsic pKa value (9.23) of boric acid, as the feed solution ionic strength increased up to 42.5mM, a considerable increase in boron rejection by both the NF270 and the BW30 membranes was observed. This phenomenon could be attributed to the reduction in the apparent pKa of boric acid as the ionic strength increased, which possibly resulted in the observed increase in boron rejection at pH 10. Results reported here suggest that the rejection of boron in the second pass could be further optimised by increasing the feed solution pH and allowing for a marginally higher salt passage in the first pass.