Effect of pH and ionic strength on boron removal by RO membranes

While removal efficiency of most components in water by RO membranes is very satisfactory, the removal of some trace elements such as boron by RO membranes, especially those with long service life, is relatively low. The interplay between pH and ionic strength is believed to be the key to understand the boron removal by RO membranes. Boron, with pK_a of 9.25, in water at low concentration is normally in the form of boric acid (B(OH)₃) at neutral pH and it is dissociated into negatively charged borate (B(OH)₄^-) form at high pH. Correspondingly, more than 99% boron removal by RO membranes can be achieved at high pH. However, it usually drops to 40-80% at neutral pH. It has been reported in the literature that higher ionic strength is theoretically suggested to be the cause of lower pK_a value. This means we can expect better boron removal at lower pH with higher ionic strength (salinity). However, the result in this study revealed that boron removal at higher salinity was lower than that at lower salinity and same pH. Boron removals at pH 10 for CPA2 membrane were found to be 81% and 71% at 500 mg/L and 15,000 mg/L NaCl, respectively. At pH 9, boron removals were 61% and 45% at 500 mg/L and 15,000 mg/L NaCl, respectively.