Boron is an important micronutrient for plants, animals and humans, although the range between deficiency and excess is narrow. The use of desalinated water and treated wastewater for irrigation may result in excess boron. In aqueous environments (i.e. neutral pH) boron is mainly present as boric acid, which is mostly undissociated and therefore only partially rejected by desalination membranes. Boric acid/borate reacts with neutral polyolic compounds, generating anionic complexes. This work reports on the complexation of boron with mannitol integrated with membrane desalination. The separation of the complex was studied in a wide range of conditions by nanofiltration (NF), and partially by reverse osmosis (RO) membranes. NMR analysis and chemical equilibrium modeling system (Mineql+) were applied to provide a better understanding of experimentally observed rejection patterns. The addition of mannitol in excess formed mainly a 2,2-di-borate ester and some monoborate esters as function of the pH, improving boron rejection by NF up to 90% at pH 9. Mineql+ calculations indicate that reactants concentration has a strong influence on the ionized boron species and therefore on the rejection of boron. Sea water-RO membranes having a much higher basal rejection for boric acid, rejected almost 97% mannitol-complexated boron at pH of 9.
- Boron removal
- Reverse osmosis