Exploring the Function of Ion-Exchange Membrane in Membrane Capacitive Deionization via a Fully Coupled Two-Dimensional Process Model

Abstract

In the arid west, the freshwater supply of many communities is limited, leading to increased interest in tapping brackish water resources. Although reverse osmosis is the most common technology to upgrade saline waters, there is also interest in developing and improving alternative technologies. Here we focus on membrane capacitive deionization (MCDI), which has attracted broad attention as a portable and energy-efficient desalination technology. In this study, a fully coupled two-dimensional MCDI process model capable of capturing transient ion transport and adsorption behaviors was developed to explore the function of the ion-exchange membrane (IEM) and detect MCDI influencing factors via sensitivity analysis. The IEM enhanced desalination by improving the counter-ions’ flux and increased adsorption in electrodes by encouraging retention of ions in electrode macropores. An optimized cycle time was proposed with maximal salt removal efficiency. The usage of the IEM, high applied voltage, and low flow rate were discovered to enhance this maximal salt removal efficiency. IEM properties including water uptake volume fraction, membrane thickness, and fixed charge density had a marginal impact on cycle time and salt removal efficiency within certain limits, while increasing cell length and electrode thickness and decreasing channel thickness and dispersivity significantly improved overall performance.

Carbon‐Based Capacitive Deionization Electrodes: Development Techniques and its Influence on Electrode Properties

“…This is advantageous in controlled, long term operations, and can increase the longevity of the electrode. Hence, MCDI is an excellent method to achieve better desalination capacity, efficiency, and stability than conventional CDI [29–30] …”

Theoretical Analysis of Constant Voltage Mode Membrane Capacitive Deionization for Water Softening

“…Our model is based on a single-pass CV mode MCDI with flow direction in parallel with the electrodes [ 32 ]. Common components in brackish waters, including the divalent ions typically responsible for hardness, calcium and magnesium, their reported hydrated radii [ 36 ] and diffusion coefficients [ 37 ] are displayed in Table 1 .…”

“…A number of process models have been developed for depicting dynamic ion transport and adsorption in (M)CDI but all to-date have been limited to treating ions as point charges [ 20 , 27 , 28 , 29 , 30 , 31 , 32 ]. Yet treating ions as point charges is unable to capture excluded ion volume effects [ 33 ], limiting the ability to simulate ion adsorption and selectivity in multicomponent solutions containing ions with different hydration radii.…”

“…However, hardness removal efficiency is a more important performance metric in water softening. CV mode MCDI was observed to reach high salt removal efficiency by reversing electrode polarity at maximum salt removal efficiency (termed here as “cut-off” mode) [ 32 ]. In this work, water softening performance of a CV mode MCDI is theoretically explored by investigating the selectivity and removal efficiency of hardness ions and the energy behaviors.…”

“…In this work, water softening performance of a CV mode MCDI is theoretically explored by investigating the selectivity and removal efficiency of hardness ions and the energy behaviors. The objectives of this work are to 1) extend our previously built MCDI process model [ 32 ] to incorporate excluded ion volume effects and compare the respective simulation results to those achieved with the original model, 2) compare selectivity and removal efficiency of hardness ion under varying operating duration, 3) explore the trade-offs between selectivity and removal efficiency of hardness ions in a cut-off CV mode MCDI and 4) analyze the cell performance, energy consumption and feasibility of applying MCDI to soften waters of various purposes, including industrial cooling tower blowdown water and domestic tap water.…”