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

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Exploring the Function of Ion-Exchange Membrane in Membrane Capacitive Deionization Via a Fully Coupled Two-Dimensional Process Model processes Article Exploring the Function of Ion-Exchange Membrane in Membrane Capacitive Deionization via a Fully Coupled Two-Dimensional Process Model Xin Zhang 1 and Danny Reible 1,2,* 1 Department of Chemical Engineering, Texas Tech University, Lubbock, TX 79409-3121, USA; [email protected] 2 Department of Civil, Environmental, and Construction Engineering, Texas Tech University, Lubbock, TX 79409-1023, USA * Correspondence: [email protected] Received: 13 September 2020; Accepted: 16 October 2020; Published: 19 October 2020 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. Keywords: brackish water desalination; membrane capacitive deionization (MCDI); ion-exchange membrane (IEM); ion transport and adsorption; hydraulic dispersion; non-ideal IEM; cycle time; salt removal efficiency 1. Introduction Freshwater is essential in our daily life with diverse demands for drinking water, agricultural irrigation, and industrial water. Demand for freshwater, combined with the potential for supply disruptions from climate change, has exacerbated freshwater scarcity [1]. Alternative technologies for providing freshwater are increasingly sought including desalinating saline water due to abundant seawater and brackish groundwater resources [2–4]. Although thermal distillation and reverse osmosis are the most popular desalination techniques [4], capacitive deionization (CDI) exhibits potential advantages including tunable effluent concentration, selective ion removal capability, high and flexible water recovery, simple pretreatment procedures, and reduced fouling and scaling problems, particularly when treating low salinity brackish water [5,6]. CDI may be particularly appropriate for a low volume of water desalination [7]. Developments such as flow-electrode CDI, increased productivity and continuous electrosorption have expanded its applicability [8,9]. Processes 2020, 8, 1312; doi:10.3390/pr8101312 www.mdpi.com/journal/processes Processes 2020, 8, 1312 2 of 18 Processes 2020, 8, x FOR PEER REVIEW 2 of 18 MembraneMembrane capacitive capacitive deionization deionization (MCDI) (MCDI) is is a amodification modification of of conventional conventional C CDIDI that that has has an an ionion-exchange-exchange membrane membrane (IEM) (IEM) on on electrodes electrodes [ [66,10,10]].. Schematic Schematic graphs graphs of of MCDI MCDI depicting depicting both both desalinationdesalination and and regeneration regeneration processes processes are are shown shown in in Figure Figure 11.. A A c cation-exchangeation-exchange membrane membrane (CEM) (CEM) andand an an anion anion-exchange-exchange membrane membrane (AEM) (AEM) are are inserted inserted betwe betweenen a spacer a spacer-filled-filled channel channel and and a pair a pair of porousof porous electrodes. electrodes. During During desalination, desalination, ions ions are arecollected collected on onthe the oppositely oppositely charged charged electrode. electrode. DuringDuring regeneration, regeneration, the the captured captured ions ions are are repelled repelled back back into into the the channel, channel, generating generating a aconcentrate concentratedd stream.stream. The The IEM IEM he helpslps to to slow slow co co-ions’-ions’ migration, migration, which which refer referss to to the the ions ions with with the the same same charge charge as as thethe fixed fixed charge charge on on the the IEM. IEM. This This maintains maintains the the majority majority of of the the co co-ions-ions inside inside the the electrode electrode during during desalinationdesalination and and slows slows the the co co-ions’-ions’ penetration penetration from from the the channel channel solution solution into into the the electrode electrode during during regenerationregeneration [11 [11].]. (a) (b) FigureFigure 1. 1. SchematicSchematic diagrams diagrams of of MCDI, MCDI, (a ()a )desalination desalination process, process, (b (b) )regeneration regeneration process. process. LeeLee et et al. al. [ 1122] firstfirst proposedproposed MCDI MCDI and and achieved achieved a highera higher salt salt removal removal rate rate compared compared to CDI to whenCDI whendesalinating desalinating power power plant plant wastewater. wastewater. Advantages Advantages of MCDI of MCDI compared compared to conventional to conventional CDI include CDI inhigherclude salthigher removal salt e ffiremovalciency [ 13efficiency–16], higher [13 current–16], ehigherfficiency current [14,15, 17efficiency,18], faster [ desalination14,15,17,18], ratefaster [16], desalinationand lower energy rate [16 consumption], and lower [18energy,19]. Theconsumption feasibility [ of18,19 energy]. The recovery feasibility in MCDI of energy further recovery decreases in MCDIthe net further energy decreases consumption the net [20 energy,21]. Propertiesconsumption of the[20,21 electrode]. Properties [22] andof the the electrode IEM [23 ],[22 feed] and water the IEMquality [23] [,10 feed], and water operating quality conditions [10], and [19operating,24,25] including conditions operating [19,24,25 mode,] including applied operating voltage/ current,mode, appliedflow rate, voltage/current, water recovery, flow and adsorptionrate, water duration recovery directly, and controladsorption cell performance, duration directly such ascontrol the number cell performance,of ions removed, such the as waterthe number quality of of ions the removed desalinated, the stream, water quality and the of energy the desalinated efficiency ofstream, MCDI. and the energyBuilding efficiency a comprehensive of MCDI. and accurate MCDI process model is essential to better understand the mechanismsBuilding a and comprehensive analyze the key and influences accurate MCDI on cell process performance. model is An essential MCDI processto better model understand should thecapture mechanisms ion transport and analyze and adsorption the key influences dynamics on in cell the performance. electrode, IEM, An and MCDI the process channel. model A model should ofa captureporous ion electrode transport has and been adsorption proposed treatingdynamics electrode in the electrode, macropores IEM as, and an ionthe transportchannel. A pathway, model of and a porouselectrode electrode micropores has been as adsorption proposed sites treating [11,26 electrode]. The small macropores size of micropores as an ion transport suggests thatpathway, the electric and electrodedouble layer micropores (EDL) in as these adsorption pores overlap, sites [ creating11,26]. The a nearly small uniform size of potential micropores distribution suggests throughout that the electricmuch ofdouble the micropores layer (EDL) [26 in,27 these]. Ion pores electrosorption overlap, creating behavior a nearly in micropores uniform haspotential been simulated distribution via throuthe Gouy–Chapman–Sternghout much of the micropores (GCS) model [26,27 [28]., 29Ion], classicalelectrosorption Donnan behavior theory [30 in], micropores and modified has Donnan been simulatedtheory [11 via,26, 31the–34 Gouy]. The–Chapman modified– DonnanStern (GCS) theory model expands [28,29 classical], classical Donnan Donnan theory theory by introducing [30], and modifieda Stern layerDonnan between theory the [11,26,31 micropore–34]. surface The m andodified diff useDonnan layer theory and considering expands classical non-electrostatic Donnan theoryattractions by introducing [35] from the a Stern micropore layer surfacebetween towards the micropore the approaching surface and ions diffuse [32]. The layer macroscopic and considering porous nonelectrode-electrostatic (MPE) attractions model [36, 37[35]] approximates from the micropore microscopic surface pores towards as volume the approaching averaged adsorption ions [32]. The sites. macroscopicThe MPE theory porous avoids electrode dealing (MPE) with themode complicatedl [36,37] morphologyapproximates of microscopic the porous electrodepores as andvolume treats averagedthe sub-grid adsorption scale behavior sites. The of MPE the micropores theory avoids as andealing adsorptive with the sink complicated term in macroscopic morphology transport of the porousequations electrode [31,36 ,and38,39 treats]. The the Nernst–Planck sub-grid scale (NP) behavior equation
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