Modeling pH variation in reverse osmosis

The pH of bottled water commercially available in Australia and its implications for oral health

With a higher pH level and being unlikely to erode the tooth, bottled water has been considered a safe alternative to acidic beverages. However, recent studies have reported some bottled water products in different countries to be acidic. The present paper aimed to examine the pH values of 42 bottled waters commercially available in Australia, using a pH meter and probe, and classify their risks to cause erosive tooth wear in comparison with the critical pH of enamel and dentine. Of the 42 bottled water samples collected, 81.0 and 73.8% were considered erosive to tooth dentine and enamel, respectively. Flavoured waters were the most acidic, followed by sparkling waters, spring waters, artesian waters, mineral waters, and alkaline waters. All sparkling waters and flavoured waters showed an erosive risk to the enamel and dentine. A portion of spring waters and artesian waters was also acidic enough to erode tooth structures. The findings of this work were of concern given the risk of sustaining erosive tooth wear from consuming bottled waters. Health promotion strategies including public awareness and education on oral health consequences related to the consumption of bottled water are needed. Future epidemiological and in vivo investigations are also warranted.

Ion partitioning and permeation in charged low-T* membranes

Understanding ion transport in membrane materials is key to engineering and development of desalination and water purification technologies as well as electro-membrane applications. To date, modeling of ion transport has mainly relied on mean-field approaches, originally intended for weak inter-ionic interactions, i.e., high reduced temperature T*. This condition is violated in many membranes, which could explain disagreement between predicted trends and experiments. The paper highlights observed discrepancies and develops a new approach based on the concept of ion association, more adequate in the low-T^⁎ limit. The new model addresses ion binding and mobility consistently within the same physical picture, applied to different types of single and mixed salts. The resulting relations show a significantly weaker connection between ion partitioning and permeability than the standard ones. Estimates using primitive model (PM) of ions in a homogeneous dielectric suggest that non-PM mechanisms, originating from the molecular structure of the ion-solvating environment, might enhance ion association in membranes. PM analysis also predicts that ion solvation and association must be rigidly related, yet non-PM effects may decouple these phenomena and allow a crossover to non-trivial regimes consistent with experiments and simulations. Despite the crude nature of the presented approach and some questions remaining open, it appears to explain most available experimental data and presents a step towards predictive modeling of ion-selective membrane separations in water-, environment- and energy-related applications.

Selectivity and polarization in water channel membranes: lessons learned from polymeric membranes and CNTs

The aspects of ion exclusion and concentration polarization are highlighted as critical for achieving high selectivity in an artificial water channel.

Theory of pH changes in water desalination by capacitive deionization

In electrochemical water desalination, a large difference in pH can develop between feed and effluent water. These pH changes can affect the long-term stability of membranes and electrodes. Often Faradaic reactions are implicated to explain these pH changes. However, quantitative theory has not been developed yet to underpin these considerations. We develop a theory for electrochemical water desalination which includes not only Faradaic reactions but also the fact that all ions in the water have different mobilities (diffusion coefficients). We quantify the latter effect by microscopic physics-based modeling of pH changes in Membrane Capacitive Deionization (MCDI), a water desalination technology employing porous carbon electrodes and ion-exchange membranes. We derive a dynamic model and include the following phenomena: I) different mobilities of various ions, combined with acid-base equilibrium reactions; II) chemical surface charge groups in the micropores of the porous carbon electrodes, where electrical double layers are formed; and III) Faradaic reactions in the micropores. The theory predicts small pH changes during desalination cycles in MCDI if we only consider phenomena I) and II), but predicts that these pH changes can be much stronger if we consider phenomenon III) as well, which is in line with earlier statements in the literature on the relevance of Faradaic reactions to explain pH fluctuations.

When Salt-Rejecting Polymers Meet Protons: An Electrochemical Impedance Spectroscopy Investigation

Polymeric membranes are widely used for salt removal, but mechanism of ion permeation is still insufficiently understood. Here we analyze ion transport in polymers relevant to desalination, dense aromatic polyamide Nomex and cellulose acetate (CA), using impedance spectroscopy, focusing on the effects of the salt type, concentration and pH. The results highlight the role of proton uptake in ion permeation. For Nomex the exceptionally high affinity to proton results in a power-low scaling of conductivity with salt concentrations with an unusual exponent 1/2. The results for CA suggest dominance of pore transport, with pore charge increasing with decreasing pH, which contradicts previous view of CA as a weakly acidic polymer and points to proton uptake as possible pore-charging mechanism. The observed effects may have far-reaching consequences in desalination, as even at neutral pH they may both enhance and suppress salt permeation and affect pH changes.

Reducing the specific energy consumption of 1st-pass SWRO by application of high-flux membranes fed with high-pH, decarbonated seawater

A new operational approach is presented, which has the potential to substantially cut down on the energy and cost demand associated with seawater reverse osmosis (SWRO) desalination, without changing the currently-installed infrastructure. The approach comprises acidification/decarbonation of the feed seawater followed by high-pH single RO pass using high-flux membranes. Since the limitation imposed by CaCO3(s) precipitation is overcome, the recovery ratio can be significantly increased. This work presents a new operational concept aimed at maximizing the benefits that can be obtained from new low-energy RO membranes available on the market. Results obtained from operating a pilot RO system revealed that following an acidification and decarbonation step, recovery ratio of 56% could be practically attained, along with effluent TDS and boron concentrations of 375 and 0.3 mg/l, respectively (feed water pH was adjusted to pH9.53 following the decarbonation step). The specific energy consumption (SEC) of this operation was calculated to be 5%-10% lower than the SEC typically associated with "conventional" SWRO operation. Two further scenarios were theoretically considered, under which the limiting operational parameter became Mg(OH)2(s) and BaSO4(s) precipitation. It was concluded that despite the fact that higher recovery ratios could be obtained, the high pressure required in these scenarios made them less appealing from both the SEC and cost standpoints. The normalized cost of the suggested approach was found to be ∼$0.07 ± 0.02/m(3) cheaper than the currently-practiced SWRO approach for obtaining product water characterized by TDS < 500 and B < 0.5 mg/l.