The removal of boron from model solutions and seawater using reverse osmosis membranes

The main objective of this work was to investigate and compare the removals of boron from model solutions (boron as single solute in distilled and deionized water) and seawater using two commercial high rejection SWRO membranes. The impacts of dissolved solids in seawater and pH on boron rejection and permeate flux were determined. A lab-scale cross-flow flat-sheet configuration test unit was used for all RO experiments. Much higher boron rejections were obtained in model solutions than seawater at a pH of 8.2, while boron rejections >98% were consistently achieved at pH 10.5 by both membranes in model solutions and seawater. Consistent for both pH levels, lower permeate flux values were observed in seawater than model solutions at constant conditions due to much higher osmotic pressures in seawater. Increasing pH from 8.2 to 10.5 decreased the flux values in seawater at constant membrane pressure, mainly due to membrane fouling and enhanced scale formation by Mg and Ca compounds. As the pressure was increased from 41.3 to 55.2 bar higher boron rejections were observed in seawater. High salinity levels in seawater reduced the rejection of boron by SWRO membranes compared to rejection of boron as single solute. On the other hand, by enhancing boron rejections due to a shift towards charged boron species, increasing seawater pH to 10.5 compensated the negative impacts of dissolved solids in the seawater on boron rejections. However, such advantages of increase in pH occurred at the expense of reduced permeate flux.

Experiments with seawater and single solute model solutions were conducted at different pH values to investigate the impacts of dissolved solids in seawater on boron rejection and permeate flux. Two commercial high rejection SWRO membranes were used. For both membranes, much higher boron rejections were obtained in model solutions than seawater at a pH of 8.2. On the other hand, boron rejections >98% were consistently achieved at pH 10.5 by both membranes in model solutions and seawater, indicating that the positive impacts of elevated pH levels on boron rejections balanced the negative impacts of dissolved solids in seawater. In other words, the negative effects of concentration polarization which increase the passage of boron through membranes are compensated by converting neutral boron species to ionized species through increase in pH. For both membranes, permeate boron concentrations less than 0.1 mg/L were easily achieved at pH 10.5 both in model solutions and seawater. Much lower permeate flux values were observed in seawater than model solutions at constant conditions due to much higher osmotic pressures in seawater. This trend was consistent for both pH levels. However, increasing pH from 8.2 to 10.5 decreased the flux values in seawater for both membranes at constant membrane pressure, mainly due to membrane fouling and enhanced scale formation by Mg and Ca compounds at higher pH values. As the pressure was increased from 41.3 to 55.2 bar higher boron rejections were observed in seawater. The results from single solute model solution and seawater experiments overall indicated that high salinity levels in seawater affect the permeation of boron in SWRO membranes in addition to reducing flux values. This negative impact can be balanced by elevated pH levels, but at the expense of reduced permeate flux.