(307a) Effects of Temporal Parameters of Pulsed Electric Field Operation on Desalination Performance and Water Dissociation in Electrodialysis

Climate change, as well as the increased agricultural, municipal, and industrial consumption are exacerbating the ongoing pressure on limited freshwater supplies, leading to a global water scarcity crisis. Development of energy-efficient desalination processes for treatment of saline water is quintessential in reaching a sustainable water supply in the future. Among the existing desalination techniques, electrochemical membrane processes provide the opportunity for tunable and selective water treatment which allows for the adjustment of the treated water quality in accordance with the properties of interest for each specific application. Such a fit-for-purpose treatment approach can significantly improve the economics of desalination and its feasibility. Electrodialysis (ED) is an electrochemical membrane desalination process which utilizes an electric field to drive ions through cation and anion exchange membranes (AEMs and CEMs), alternately placed between two electrodes. The final products of ED are diluate and concentrate streams with, respectively, lower and higher salinity than the feed water. In an ED process, the higher transport number of counterions inside IEMs relative to the bulk solution leads to formation of concentration gradients in the adjacent boundary layers (BLs). Ionic concentration in the BL of the diluate channels decreases, while increasing in the BL of concentrate compartments. This phenomenon is known as “concentration polarization” which can intensify water dissociation in diluate channels and leads to formation of over-saturated regions in the concentrate channels. Water dissociation raises the hydroxide salt precipitation while over-saturated solutions indicate higher propensity for precipitation of sparingly soluble carbonate and sulfate salts. To improve the desalination performance, increase water recovery, and enhance the membrane lifespan, it is essential to mitigate organic and inorganic precipitation which lead to membrane fouling and scaling.

Pulsed electric field operation (PEF) is a low-cost approach for mitigation of membrane fouling and scaling through suppressing the concentration polarizations in the channels. In PEF-ED, the pulsing periods (t_on) with imposed electric field are followed by pausing lapses (t_off) with zero input current. The relaxation time provided during the pausing periods allows for the concentration in the boundary layers to return back to their bulk values, removing the concentration polarization formed during the pulse lapses. The performance of PEF-ED is controlled by adjusting pulsing frequency, duty cycle, and potential. Effectiveness of PEF operation for scale mitigation purposes relies on appropriate selection of pulsing potential and temporal parameters. Changes in pulsing conditions can affect the desalination rate, energy consumption of the process, and the pH variations in the system. In the literature, controversial results have been reported with respect to the pH variations in PEF-ED. While a number of studies have reported lower or identical pH changes in PEF-ED compared to the conventional ED (CED), several other investigations have determined intensified pH variations with PEF-ED. Minimizing water dissociation and pH changes are especially important for avoiding hydroxide salt precipitation and membrane scaling that are formed in basic pH. To reduce water splitting, ED is traditionally operated below the limiting potential at which the concentration of ions at the membrane-solution interface approaches zero. Existing literature does not address the effects of pulsing parameters on limiting potential which serves as the upper bound for the pulsing potential. Developing a systematic framework for selection of pulsing parameters that could improve the desalination rate while minimizing pH changes is an essential step, enabling the application of this approach for various water compositions.

We develop a 1-D transient model for PEF-ED by utilizing the fundamental equations applied to electrochemical systems including the Nernst-Planck, Navier-Stokes, and electroneutrality assumption. We further design and construct a bench scale PEF-ED set-up which facilitates the measurement and monitoring of voltage, current, conductivity, pH, and pressure drops in the system. Our theoretical and experimental analyses determine that limiting potential is greater in PEF-ED compared to that in CED for the same feed water salinity. Greater limiting potential achieved in PEF-ED allows for imposing higher pulsing potential which further improves the desalination rate. The higher limiting voltage results in minimal pH changes for a wider range of the imposed potentials, further reducing the possibility of hydroxide salt precipitation. The limiting conditions, however, depend on temporal parameters of pulsing and can vary at various t_on/t_off ratios. Our results illustrate the effects of PEF operation on pH changes and clarify the controversy in the literature. Our theoretical model and experimental procedure provide a systematic approach for tuning the pulsing parameters to improve the desalination performance in PEF-ED and mitigate water dissociation to avoid hydroxide salts formations.