A mechanistic study on boron rejection by sea water reverse osmosis membranes

Boron is a vital element for organism growth, but excessive exposure can cause detrimental effects to plants, animals, and possibly humans. However, it has been challenging for many of the existing sea water reverse osmosis (SWRO) membrane plants to remove boron and meet the current World Health Organization Guidelines for Drinking Water Quality. The objective of this study was to evaluate the effect of key operating parameters such as pH and temperature on boron rejection and develop a corresponding mechanistic predictive model. Bench-scale cross-flow filtration experiments were performed to estimate the rejection of boron by six commercial SWRO membranes. The rejection of boron appeared to follow a mechanism which is different from those of other ionic solutes and could not be readily correlated with their rejections. An irreversible thermodynamic model coupled with film theory was applied to quantitatively analyze the experimental observations. The model accurately predicted the boron rejection performances of the SWRO membranes at different operating conditions. The model was further modified to account for the boric acid speciation by pH and temperature dependence of the model parameters. The model developed herein will constitute fundamental for performance prediction and design of SWRO processes.