Transport model for boric acid, monoborate and borate complexes across thin-film composite reverse osmosis membrane Original Research Article

The transport mechanism of boric acid, monoborate and borate complexes with polyols across thin-film composite reverse osmosis (RO) membrane was investigated. The polyols applied were d-mannitol, sodium d-gluconate and N-methyl d-glucamine. A mathematical model for boric acid dissociation and complex formation equilibrium was combined with the Keedem–Katchalsky or Spiegler–Keedem model for mass transport in RO membranes. The mass transport coefficients: permeabilities and reflection coefficients for individual species were estimated. The proposed model was shown to predict boron flux with sufficient accuracy. The estimated mass transport coefficients were found to strongly depend on the species type. The highest permeability and the lowest reflection coefficient were observed for boric acid. Borate complexes with polyols were characterised by the lowest permeabilities and the highest reflection coefficients. The results of numerical calculations with the proposed model allowed for the assessment of the effect of process parameters on the effectiveness of boron rejection. The effect of retentate pH on boron rejection and boron flux is discussed. It is shown that complex formation reaction decreases the retentate pH required to achieve a given rejection of boron. The effects of permeate flux, retentate boron content and alcohol/boron molar ratio are also presented.