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Fluoride Removal from Brackish Groundwaters by Constant Current Capacitive Deionization (CDI) Wangwang Tang, Peter Kovalsky, Baichuan Cao, Di He, and T. David Waite Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b03307 • Publication Date (Web): 08 Sep 2016 Downloaded from http://pubs.acs.org on September 12, 2016
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Fluoride Removal from Brackish Groundwaters by Constant
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Current Capacitive Deionization (CDI)
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Wangwang Tang†, Peter Kovalsky†, Baichuan Cao ‡, Di He†, T. David Waite†*
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†
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NSW 2052, Australia
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‡
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Beijing Jiaotong University, Beijing 100044, P. R. China
School of Civil and Environmental Engineering, University of New South Wales, Sydney,
Department of Municipal and Environmental Engineering, School of Civil Engineering,
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Email addresses:
[email protected] (Wangwang Tang);
[email protected] 11
(Peter Kovalsky);
[email protected] (Baichuan Cao);
[email protected] (Di He);
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[email protected] (T. David Waite)
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Environmental Science & Technology
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(Revised and submitted, September 2016)
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_____________________________________________
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*Corresponding Author: Professor Trevor David Waite, School of Civil and Environmental
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Engineering, University of New South Wales, Sydney, NSW 2052, Australia; Phone: +61-2-
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9385-5060, E-mail:
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ABSTRACT
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Charging capacitive deionization (CDI) at constant voltage (CV) produces an effluent stream
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in which ion concentrations vary with time. Compared to CV, charging CDI at constant
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current (CC) has several advantages, particularly a stable and adjustable effluent ion
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concentration. In this work, the feasibility of removing fluoride from brackish groundwaters
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by single-pass constant-current (SPCC) CDI in both zero-volt and reverse-current desorption
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modes was investigated and a model developed to describe the selective electrosorption of
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fluoride and chloride. It was found that chloride is preferentially removed from the bulk
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solution during charging. Both experimental and theoretical results are presented showing
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effects of operating parameters, including adsorption/desorption current, pump flow rate and
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fluoride/chloride feed concentrations, on the effluent fluoride concentration, average fluoride
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adsorption rate and water recovery. Effects of design parameters are also discussed using the
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validated model. Finally, we describe a possible CDI assembly in which, under appropriate
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conditions, fluoride water quality targets can be met. The model developed here adequately
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describes the experimental results obtained and shows how change in the selected system
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design and operating conditions may impact treated water quality.
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KEYWORDS
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Capacitive Deionization, Constant Current, Fluoride Removal, Low-salinity Groundwater,
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Ion Selectivity
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INTRODUCTION
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Capacitive deionization (CDI) is an emerging and fast-growing electrochemical
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technology which usually employs porous carbon electrodes to remove dissolved and charged
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species from aqueous solutions during a charging step, followed by ion release to regenerate
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the electrodes during a discharging step.1,
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techniques such as reverse osmosis, electrodialysis and distillation, CDI offers the advantages
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of low-pressure operation, low operating and maintenance costs, high energy efficiency and
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environmental friendliness, primarily for water with a low to moderate salt content.1-4 Recent
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research interests in the field of CDI include synthesis of new electrode materials (e.g.,
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controllable surface functionalities),4-6 design of novel CDI architectures (e.g., flow-electrode
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CDI)7 and investigation of effects of Faradaic reactions (e.g., hydrogen peroxide generation).8
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In addition to the removal of major ions from waters, removal of other charged
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species including cations such as copper and zinc, oxyanions such as arsenic and a range of
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anions including nitrate and fluoride may be required.2, 9-11 The presence of fluoride (F−) in
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groundwaters, as a consequence of both anthropogenic and natural processes, is causing
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increasing concern worldwide12,
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consumption of the fluoride-contaminated groundwater with excessive concentrations of F−
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(>1.5 mg L−1) will result in permanent bone and joint deformations, and dental or skeletal
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fluorosis, although an F− level of 0.5−1.5 mg L−1 has beneficial effects on human teeth and
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bones, especially for young children.14, 15
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As an alternative to other water treatment
and is the focus of the work described here. Human
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Removal of single electrolytes (such as NaCl and KCl) using CDI has been widely
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studied and models that reliably describe ion transport within such electrochemical devices
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have been developed. Hemmatifar et al.16 formulated and solved the first two-dimensional
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model for capturing ion adsorption/desorption dynamics in a CDI cell under constant
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charging voltage. Jande and Kim17-19 developed models for both CDI batch-mode operation
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and single-pass operation under constant charging voltage or current and examined the effects
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of various input parameters on ion adsorption and desorption. Porada et al.1 successfully
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proposed a simplified dynamic CDI transport model for batch-mode operation based on the
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modified Donnan model and was able to reasonably describe the variation in NaCl
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concentration over time in the recycle vessel. Kim et al.20 ably described observed dynamic
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ion transfer and charge storage by including the improved modified Donnan model in the
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CDI porous electrode transport theory. Due to the similarities of NaF and NaCl, these models
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can be easily extended to the description of F− removal. However, very few studies have been
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conducted, either theoretically or experimentally, to investigate F− electrosorption in the
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presence of other electrolytes such as NaCl as is typical in environmental scenarios. In fact,
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previous studies have indicated that ion selective removal is possible in CDI even for ions
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with the same valence and very similar hydrated radius,21-23 not to mention those with
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different charge and hydrated radius.24, 25
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In our previous work, we investigated the removal of F− in the presence of NaCl by
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batch-mode CDI21 and single-pass CDI22, in both cases using constant charging voltage, and
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developed appropriate models to describe the dynamic outlet concentration of both F− and
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Cl−. Nevertheless, constant charging voltage may not be the most practical operational mode
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since the effluent ion concentrations vary greatly in time. To obtain purified water with
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relatively constant ion concentrations below a specified limit (say, an F− level of 0.5−1.5 mg
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L−1), it is more advantageous to operate in constant current (CC) mode rather than constant
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voltage (CV) mode in which a constant charging voltage is applied across the electrodes.
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In this study, we investigate both the adsorption and desorption behavior of F− and Cl−
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by single-pass constant-current (SPCC) CDI in both zero-volt desorption (ZVD) and reverse-
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current desorption (RCD) modes. Also, a much more realistic CDI design was used in this
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work rather than the single pair and a model was developed which enables prediction of the
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ion selective transport in a system much closer to reality than that in our previous work.22
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Effects of various parameters including operating and design parameters on three important
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performance indicators, namely, the effluent ion concentration, the average fluoride
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adsorption rate (AFAR) and water recovery (WR) are discussed with theoretical calculations
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compared to experimental data. Finally, possible combinations of CDI modules able to
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achieve a specific F− target (0.5−1.5 mg L−1) are presented and limitations of CDI systems
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operated in the SPCC mode analyzed.
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MATERIALS AND METHODS
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CDI Module. The CDI module used in this study (AQUA EWP, USA) and the
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schematic diagram of the inner structure of the CDI module are displayed in Figures S1 and
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S2, respectively. The CDI module contains a stack of N = 100 cells in parallel. One cell
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consists of two graphite sheets as current collectors which are alternatingly positively and
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negatively biased, and two porous carbon electrodes composed of powdered activated carbon
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and a polymer binder (polytetrafluoroethylene). The electrode is 10 cm × 10 cm in area and
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100 µm in thickness with a BET surface area of 1068 m2 g−1. Each carbon electrode pair is
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separated by a 200 µm thick non-conductive nylon cloth to prevent electrical short circuit and
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to act as a spacer channel. The water is pumped into the module through an opening located
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in one of the four corners and flows out from the opening in the middle.
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Experimental Methods. The schematic diagram of the experimental setup used in
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SPCC CDI tests is shown in Figure S3. The system consists of a feed vessel, a diaphragm
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pump (SEAFLO, China), a flowmeter (SCINTEX, Australia), a CDI module, a DC power
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supply (WEP, Yihua Electronic Equipment Co., Ltd, China), a digital electrical conductivity
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(EC) meter (F-54, HORIBA, Japan) and an effluent vessel. The cell voltage of the CDI
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module, measured by digital multimeter (Jaycar Electronics, Australia), should be 0 V prior
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to each test. The module was fully flushed using the tested feed solution until the effluent
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conductivity equaled the influent conductivity. During adsorption, a constant electrical
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current was applied to the module until the cell voltage reached the final charging voltage of
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1.6 V. At this moment we switched from adsorption step to desorption step. During
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desorption, we either short-circuited the module (ZVD mode) or applied a constant reverse
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electrical current until the cell voltage dropped back to 0 V (RCD mode). Preliminary tests
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indicated that the initial cycle did not differ appreciably from the steady periodic behavior of
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the system (Figure S4), thus, only the first cycle was examined in the studies described here.
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Analytical grade sodium chloride (NaCl) and sodium fluoride (NaF) were used for the
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preparation of feeding solutions. The methods used to determine the effluent ion
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concentrations as well as the experimental and theoretical specific ion j electrosorption per
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unit mass of electrodes have been described previously.22
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MODEL DERIVATION
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To quantitatively describe the effluent concentration of F− and Cl− within a full cycle,
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as a function of various geometrical and experimental parameters for SPCC CDI in both ZVD
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and RCD modes, a plug flow model was developed based on our previous work.21,
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output of a single charge/discharge cycle was determined by numerically solving the
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equations describing key processes operating in the SPCC CDI system as outlined below. As
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shown in Figure 1, the spacer channel of a CDI cell is divided into M (M = 100) sequential
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sub-cells to describe the ongoing ion transport in the flow direction. In each sub-cell, the
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solution is considered to be ideally stirred, thus leading to a uniform concentration of the
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specific ions (F−, Cl− and Na+).20, 26 The porous CDI carbon electrodes are considered to
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consist of macropores where ions migrate and micropores where electric double layers
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(EDLs) are largely formed and ions are primarily adsorbed.1 In the macropores, the ion
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concentration gradient is neglected and the ion concentration assumed to be equal to that in
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the spacer channel,27 an approximation which may not be particularly accurate when using a
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full porous electrode transport model.16,
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model, valid in the limit of strongly overlapped EDLs, is used to represent the EDLs’
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structure which is characterized by a relatively constant value of diffuse layer potential and
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ion concentration (non-varying with pore position).1, 31 Moreover, it is assumed that ions are
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not adsorbed onto the carbon surface within electrodes until a non-zero charging voltage is
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applied.
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28-30
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The
In the micropores, a modified Donnan (mD)
Based on these assumptions, in the i-th sub-cell, the concentration of ion j in the micropores (,, ) (mM) is given by ,, = , · exp (− ∙ ∆, )
(1)
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where , is the concentration of ion j (mM) in the macropores and spacer channel, is the
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ion charge number (+1 for the cation and −1 for the anion) and ∆, is the dimensionless 8 ACS Paragon Plus Environment
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Donnan potential (positive for the anode and negative for the cathode). Meanwhile, in the
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macropores and spacer channel, local ion electro-neutrality is maintained, i.e.,
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∑ · , = 0
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The micropore volumetric ion charge density, , (mM), is expressed as
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, = ∑ · ,, = −2 · , · sinh ∆,
(2)
(3)
The micropore charge density , relates to the dimensionless Stern potential, ∆!", (positive for the anode and negative for the cathode), according to , ∙ # = −∆!", ∙ $!" ∙ %&
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(4)
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where F is the Faraday’s constant (C mol−1), $!" is the volumetric Stern capacity of the cell (F
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m−3), and VT is the thermal voltage (V). , in the anode is also related to the ion current
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density ' (mol m−2 s−1) by
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*
() ∙ *+ , ∙ , = −'
(5)
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where () is the thickness of single electrode (m), , is the electrode microporosity, ' is
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defined to be positive during adsorption and to be negative during desorption. Based on Eq.
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(5), assuming zero charge is maintained at the electrode surface at t=0, we can account for
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charge accumulation at each moment in time and each sub-cell, ,,+ , according to ,,+ =
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,,+-*+ − ',+-*+ ∙ .//(() ∙ , ). Meanwhile, the ion current density ' is given by
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' = '12, + '4, − ', = 512 · 12, + 54 · 4, + 5 · , · (-7 )66 ∙ ∆"8,
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where ', is the flux of ion j (mol m−2 s−1), as given by an approximation of the Nernst-Planck
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equation, ' = −5 (. /.9 + ./.9) , under the assumptions of gradient-less
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concentration profile and a linearized potential profile. Dj is the effective diffusion coefficient
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of ion j (m2 s−1), ()66 describes the total effective ion transport resistance of the spacer and
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both electrodes (m).27 ∆"8, is the dimensionless voltage drop driving ion transport and is
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related to the charging voltage, %:;8
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(7)
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where %:;8
