Doctor of Philosophy
School of Civil, Mining, and Environmental Engineering
Tu, Kha Le, Boron Removal by Nanofiltration and Reverse Osmosis Membranes, Doctor of Philosophy thesis, School of Civil, Mining, and Environmental Engineering, University of Wollongong, 2014. https://ro.uow.edu.au/theses/4299
This dissertation aims to enrich the literature on boron rejection by nanofiltration (NF) and reverse osmosis (RO) membranes. A novel method to improve boron rejection by NF/RO membranes, which is based on the complexation reactions between boron and poly-alcohols (polyols), was investigated. The impacts of chemical cleaning and chemical preservation on the boron rejection efficiency of RO membranes were examined. In addition, the dissertation assessed the feasibility of utilising boron rejection data as a surrogate for the rejection of Nnitrosodimethylamine (NDMA) – an emerging pollutant which attracts major concerns in contemporary water reclamation applications. Filtration experiments were conducted using a laboratory-scale cross-flow membrane filtration system. Experimental results were elaborated based on the recognised transport mechanisms of boron through NF/RO membranes and also on the characterised membrane surface properties.
In the presence of polyols, significant boron rejection improvement was obtained and the extent of the impact was directly related to the stability constant of the boron– polyol complex. Polyols could complex with boron in either the boric acid or borate anion forms; however the complexation between polyol and boric acid appeared to be incomplete. With and without the presence of polyols, boron rejection was strongly pH dependent. The increase in boron rejection due to polyol addition was higher for the NF membrane compared to the RO membrane. A boron:polyol molar ratio of 1:1 appeared to be adequate. The presence of polyols did not cause any observable membrane fouling. Results reported here suggest that the addition of polyols could allow NF membranes to be effectively used for boron removal.
Experimental results of this study showed that chemical cleaning can significantly change the hydrophobicity and water permeability of the RO membrane; however, its impacts on the rejections of boron and sodium were marginal. Although the presence of surfactant or chelating agent could cause decreases in the rejections, solution pH was found to be the key factor responsible for the loss of membrane separation and surface properties. The impacts of solution pH on the water permeability could be reversed by applying a subsequent cleaning with the opposite pH condition. Nevertheless, the impacts of solution pH on boron and sodium rejections were irreversible in most cases.
Chemical preservation could change the membrane surface properties, and consequently water permeability and solute rejection efficiency of the membrane were negatively impacted. The impacts of preservation on boron rejection and sodium rejection were similar in magnitude and more significant than those on water permeability. The results indicated that the impact of chemical preservation on the membrane depends on both the preserving chemicals used and the solution pH value. More importantly, the undesirable impacts of chemical preservation could be minimised by appropriate selection of the preservatives and by preserving the membrane in a reducing condition. A near-neutral pH (i.e., pH 7) is necessary to avoid any significantly negative impacts on membrane performance due to chemical preservation using either formaldehyde or sodium metabisulfite. The study results suggested that the previously recommended minimum pH value of 3 of the preservative solution may be inadequate.
A strong linear correlation (R2 = 0.95) between the rejections of boron and Nnitrosodimethylamine (NDMA) by six different reverse osmosis (RO) membranes was obtained, suggesting that boron can be used as a surrogate for NDMA rejection. This proposal is based on the premise that the rejection of both boric acid and NDMA is governed by steric hindrance and that they have similar molecular dimensions. The concept proposed here is shown to be valid at pH 8 or below where boron exists as the neutral boric acid species and NDMA is also a neutral solute. Observed changes in the rejections of these two species, as a function of permeate fluxes and feed solution temperatures, were also almost identical. Boron rejection increased from 21 to 79% and the correlation coefficient of the linear regression between boron and NDMA rejections was 0.99 as the permeate flux increased from 5 to 60 L.m-2.h-1. Similarly, a linear correlation between boron and NDMA rejections was observed as the feed solution temperature increased from 10 to 40 °C. This linear correlation was also validated in a tertiary treated effluent matrix.