Multi-component ion equilibria and transport in ion-exchange membranes.

At the interface between an ion-exchange membrane and a multi-electrolyte solution, charged species redistribute themselves to minimize the free energy of the system. In this paper, we explore the Donnan equilibrium of membranes with quaternary electrolyte (Na+/Mg2+/K+/Ca2+/Cl-) solutions, experimentally. The data was used to calculate the ion activity coefficients for six commercial cation-exchange membranes (CEMs). After setting one of the activity coefficients to an arbitrary value, we used the remaining (N-1) activity coefficients as fitting parameters to describe the equilibrium concentrations of (N) ionic species with a mean relative error of 3 %. At increasing solution ionic strengths, the equivalent ion fractions of monovalent counter-ions inside the membrane increased at the expense of the multivalent ones in alignment with the Donnan equilibrium theory. The fitted activity coefficients were employed in a transport model that simulated a Donnan dialysis experiment involving all four cations simultaneously. The arbitrary value assigned to one activity coefficient affects the calculated Donnan potential at the membrane interface. Nevertheless, this arbitrary value does not affect the prediction of the ion concentrations inside the membrane and consequently does not affect the modeled ion fluxes.

Salt and Water Transport in Reverse Osmosis Membranes: Beyond the Solution-Diffusion Model.

Understanding the salt-water separation mechanisms of reverse osmosis (RO) membranes is critical for the further development and optimization of RO technology. The solution-diffusion (SD) model is widely used to describe water and salt transport in RO, but it does not describe the intricate transport mechanisms of water molecules and ions through the membrane. In this study, we develop an ion transport model for RO, referred to as the solution-friction model, by rigorously considering the mechanisms of partitioning and the interactions among water, salt ions, and the membrane. Ion transport through the membrane is described by the extended Nernst-Planck equation, with the consideration of frictions between the species (i.e., ion, water, and membrane matrix). Water flow through the membrane is governed by the hydraulic pressure gradient and the friction between the water and membrane matrix as well as the friction between water and ions. The model is validated using experimental measurements of salt rejection and permeate water flux in a lab-scale, cross-flow RO setup. We then investigate the effects of feed salt concentration and hydraulic pressure on salt permeability, demonstrating strong dependence of salt permeability on feed salt concentration and applied pressure, starkly disparate from the SD model. Lastly, we develop a framework to analyze the pressure drop distribution across the membrane, demonstrating that cross-membrane transport dominates the overall pressure drop in RO, in marked contrast to the SD model that assumes no pressure drop across the membrane.

Electrochemical removal of amphoteric ions.

Several harmful or valuable ionic species present in seawater, brackish water, and wastewater are amphoteric, weak acids or weak bases, and, thus, their properties depend on local water pH. Effective removal of these species can be challenging for conventional membrane technologies, necessitating chemical dosing of the feedwater to adjust pH. A prominent example is boron, which is considered toxic in high concentrations and often requires additional membrane passes to remove during seawater desalination. Capacitive deionization (CDI) is an emerging membraneless technique for water treatment and desalination, based on electrosorption of salt ions into charging microporous electrodes. CDI cells show strong internally generated pH variations during operation, and, thus, CDI can potentially remove pH-dependent species without chemical dosing. However, development of this technique is inhibited by the complexities inherent to the coupling of pH dynamics and ion properties in a charging CDI cell. Here, we present a theoretical framework predicting the electrosorption of pH-dependent species in flow-through electrode CDI cells. We demonstrate that such a model enables insight into factors affecting species electrosorption and conclude that important design rules for such systems are highly counterintuitive. For example, we show both theoretically and experimentally that for boron removal, the anode should be placed upstream and the cathode downstream, an electrode order that runs counter to the accepted wisdom in the CDI field. Overall, we show that to achieve target separations relying on coupled, complex phenomena, such as in the removal of amphoteric species, a theoretical CDI model is essential.

Unravelling pH Changes in Electrochemical Desalination with Capacitive Deionization.

Membrane capacitive deionization (MCDI) is a water desalination technology employing porous electrodes and ion-exchange membranes. The electrodes are cyclically charged to adsorb ions and discharged to desorb ions. During MCDI operation, a difference in pH between feed and effluent water is observed, changing over time, which can cause the precipitation of hardness ions and consequently affect the long-term stability of electrodes and membranes. These changes can be attributed to different phenomena, which can be divided into two distinct categories: Faradaic and non-Faradaic. In the present work, we show that during long-term operation, as the electrodes age over time, the magnitude and direction of pH changes shift. We studied these changes for two different feed water solutions: a NaCl solution and a tap water solution. Whereas we observe a pH decrease during the regeneration with a NaCl solution, we observe an increase during regeneration with tap water, potentially resulting in the precipitation of hardness ions. We compare our experimental findings with theory and conclude that with aged electrodes, non-Faradaic processes are the prominent cause of pH changes. Furthermore, we find that for desalination with tap water, the adsorption and desorption of HCO3-and CO32- ions affect the pH changes.

WaterROUTE: A model for cost optimization of industrial water supply networks when using water resources with varying salinity.

Water users can reduce their impact on scarce freshwater resources by using more abundant regional brackish or saline groundwater resources. Decentralized water supply networks (WSN) can connect these regional groundwater resources with water users. Here, we present WaterROUTE (Water Route Optimization Utility Tool & Evaluation), a model which optimizes water supply network configurations based on infrastructure investment costs while considering the water quality (salinity) requirements of the user. We present an example simulation in which we determine the optimal WSN for different values of the maximum allowed salinity at the demand location while supplying 2.5 million m3 year-1 with regional groundwater. The example simulation is based on data from Zeeuws-Vlaanderen, the Netherlands. The optimal WSN configurations for the years 2030, 2045 and 2110 are generated based on the simulated salinity of the regional groundwater resources. The simulation results show that small changes in the maximum salinity at the demand location have significant effects on the WSN configuration and therefore on regional planning. For the example simulation, the WSN costs can differ by up to 68% based on the required salinity at the demand site. WaterROUTE can be used to design water supply networks which incorporate alternative water supply sources such as local brackish groundwater (this study), effluent, or rainwater.

Modification of Cation-Exchange Membranes with Polyelectrolyte Multilayers to Tune Ion Selectivity in Capacitive Deionization.

Capacitive deionization (CDI) is a desalination technique that can be applied for the separation of target ions from water streams. For instance, mono- and divalent cation selectivities were studied by other research groups in the context of water softening. Another focus is on removing Na+ from recirculated irrigation water (IW) in greenhouses, aiming to maintain nutrients. This is important as an excess of Na+ has toxic effects on plant growth by decreasing the uptake of other nutrients. In this study, we investigated the selective separation of sodium (Na+) and magnesium (Mg2+) in MCDI using a polyelectrolyte multilayer (PEM) on a standard grade cation-exchange membrane (Neosepta, CMX). Alternating layers of poly(allylamine hydrochloride) (PAH) and poly(styrene sulfonate) (PSS) were coated on a CMX membrane (CMX-PEM) using the layer-by-layer (LbL) technique. The layer formation was examined with X-ray photoelectron spectroscopy (XPS) and static water contact angle measurements (SWA) for each layer. For each membrane, i.e., the CMX-PEM membrane, CMX membrane, and for a special-grade cation-exchange membrane (Neosepta, CIMS), the Na+/Mg2+ selectivity was investigated by performing MCDI experiments, and selectivity values of 2.8 ± 0.2, 0.5 ± 0.04, and 0.4 ± 0.1 were found, respectively, over up to 40 cycles. These selectivity values indicate flexible switching from a Mg2+-selective membrane to a Na+-selective membrane by straightforward modification with a PEM. We anticipate that our modular functionalization method may facilitate the further development of ion-selective membranes and electrodes.

Diffusion of hydrophilic organic micropollutants in granular activated carbon with different pore sizes.

Hydrophilic organic micropollutants are commonly detected in source water used for drinking water production. Effective technologies to remove these micropollutants from water include adsorption onto granular activated carbon in fixed-bed filters. The rate-determining step in adsorption using activated carbon is usually the adsorbate diffusion inside the porous adsorbent. The presence of mesopores can facilitate diffusion, resulting in higher adsorption rates. We used two different types of granular activated carbon, with and without mesopores, to study the adsorption rate of hydrophilic micropollutants. Furthermore, equilibrium studies were performed to determine the affinity of the selected micropollutants for the activated carbons. A pore diffusion model was applied to the kinetic data to obtain pore diffusion coefficients. We observed that the adsorption rate is influenced by the molecular size of the micropollutant as well as the granular activated carbon pore size.

Exceptional Water Desalination Performance with Anion-Selective Electrodes.

Capacitive deionization (CDI) typically uses one porous carbon electrode that is cation adsorbing and one that is anion adsorbing. In 2016, Smith and Dmello proposed an innovative CDI cell design based on two cation-selective electrodes and a single anion-selective membrane, and thereafter this design was experimentally validated by various authors. In this design, anions pass through the membrane once, and desalinated water is continuously produced. In the present work, this idea is extended, and it is experimentally shown that also a choice for anion-selective electrodes, in combination with a cation-selective membrane, leads to a functional cell design that continuously desalinates water. Anion-selective electrodes are obtained by chemical modification of the carbon electrode with (3-aminopropyl)triethoxysilane. After chemical modification, the activated carbon electrode shows a substantial reduction of the total pore volume and Brunauer-Emmett-Teller (BET) surface area, but nevertheless maintains excellent CDI performance, which is for the first time that a low-porosity carbon electrode is demonstrated as a promising material for CDI.