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Selective removal of lead ions through capacitive deionization: Role of ion-exchange membrane
Accepted Manuscript Selective Removal of Lead Ions through Capacitive Deionization: Role of IonExchange Membrane Qianqian Dong, Xiaoru Guo, Xingkang Huang, Lianjun Liu, Rebecca Tallon, Bruce Taylor, Junhong Chen PII: DOI: Reference:
S1385-8947(18)32170-3 https://doi.org/10.1016/j.cej.2018.10.208 CEJ 20271
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
18 July 2018 8 October 2018 28 October 2018
Please cite this article as: Q. Dong, X. Guo, X. Huang, L. Liu, R. Tallon, B. Taylor, J. Chen, Selective Removal of Lead Ions through Capacitive Deionization: Role of Ion-Exchange Membrane, Chemical Engineering Journal (2018), doi: https://doi.org/10.1016/j.cej.2018.10.208
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Selective Removal of Lead Ions through Capacitive Deionization: Role of Ion-Exchange
Membrane Qianqian Dong,a,† Xiaoru Guo,a,† Xingkang Huang,a,* Lianjun Liu,b Rebecca Tallon,b Bruce Taylor,c and Junhong Chena,* a
Department of Mechanical Engineering, University of Wisconsin-Milwaukee, 3200 North
Cramer Street, Milwaukee, WI, 53211, USA. b
A.O. Smith Corporation, Corporate Technology Center, 12100 W Park Place, Milwaukee, WI,
53224, USA c
KX Technologies LLC, 55 Railroad Avenue, West Haven, Connecticut 06516 USA
Abstract
Capacitive deionization (CDI) is a rising technology as a low-energy-consumption and low-cost
option for water purifications and treatments; however, selective removal of heavy metal ions
*
Corresponding authors.
E-mail addresses: [email protected] (X. Huang), [email protected] (J. Chen). † These authors contributed equally. 1
has been rarely reported. In this study, the impacts of ion-exchange membranes on the selective removal of lead ions (Pb2+) by CDI are investigated with a single-pass model. Both
cation-exchange membrane (CEM) and anion-exchange membrane (AEM) demonstrate improved Pb2+ removal efficiencies. The presence of CEM retards discharge of adsorbed Pb2+, which eliminates the selectivity of removing Pb2+ against other cations (such as Ca2+ and Mg2+);
in contrast, without the CEM, the CDI cell with only the AEM (called AEM-CDI) exhibits enhanced discharge efficiency and retains the selectivity of removing Pb2+ against Ca2+ and Mg2+. More importantly, the AEM-CDI presents improved Pb2+ discharge efficiency when applying
inverted voltages, which is not possible in membrane-free CDI because desorbed ions can be
re-adsorbed to the counter electrode. In addition, surface modification for porous carbon with functional groups to enhance the affinity between Pb2+ and electrode surface is expected to further improve the selectivity of removing Pb2+ against other cations, thereby showing very
promising potential in purification of drinking water.
Keywords: capacitive deionization; ion-exchange membrane; lead removal; selectivity; single
pass.
2
1. Introduction
Capacitive deionization (CDI) is an emerging energy-efficient technique for water treatment to
remove salt[1, 2], heavy metal ions[3], and hardness[4] from water. By applying voltage,
double-layer capacitance can be built on electrodes in CDI devices, which attracts, accumulates,
and removes ions. Various CDI architectures such as flow-through and flow-between with
ion-exchange membrane variations have been explored to remove ions in water,[5] in which
flow-between model is widely used due to its simple configuration and its suitability for studying
material performance.[6-8] Meanwhile, different water feeding strategies such as batch mode and
single-pass mode have been investigated; the batch mode requires cycling water in a tank for
multiple cycles to achieve high removal performance, while a single-pass flow allows water to
pass the system.[9] Compared with batch modes, the single-pass one is less investigated but
closer to practical application due to its continuous feeding of constant ion concentrations.[10,
11]
3
Ion-exchange membranes have been introduced to CDI to improve ion removal efficiency, which
is called as membrane CDI (MCDI)[12, 13]. In the MCDI, membrane traps co-ions into
intraparticle pores and thus increases accumulation of counter-ions in macropores[14]. In
addition, an inverted voltage is allowed to be applied to the MCDI cell upon discharging because
the ion exchange membranes prevents the desorbed ions from being re-adsorbed to the counter
electrode, thereby enhancing electrode regeneration efficiency[15].
Despite the above-mentioned benefits of ion-exchange membranes, current membranes are typically designed for small ions (such as H+ and Na+) in desalination process and it is rarely reported on lead (Pb2+) removal. As a neurotoxin, Pb2+ exposes health risk to people and is
frequently detected in drinking water[16, 17]. The total lead concentration allowance in the
drinking water in US is 15 ppb according to US Environmental Protection Agency (EPA)[18];
and the World Health Organization (WHO) has set the limit to 10 ppb, as well as many countries[19]. However, the aging pipelines hike the Pb2+ concentration for most municipals at
4
the household end. Currently many water treatment technologies such as reverse osmosis are
widely studied and applied to address this issue; but the high maintenance cost and high energy
consumption limits the feasibility for low-cost daily use.
CDI has great potentials in removing heavy metal ions such as Pb[3, 20], Cr[3, 21], Cu[22, 23],
Cd[24], and As[25]. When an electrical field is produced between parallel porous carbon
electrodes, metal ions can be transported from contaminated water to the micropore structures,
where the electrical double layers (EDLs) form[26]. When the electrical field is revoked or
reversed, the adsorbed ions are discharged into the effluent so that the active materials are regenerated for the next cycle[27]. However, many beneficial ions, such as Ca2+ and Mg2+, have a higher concentration than Pb2+ and compete with Pb2+ upon ion removal during CDI processes. Therefore, it is necessary to investigate selective removal of Pb2+ against other cations (e.g., Ca2+ and Mg2+) with CDI devices. At the same time, the selectivity of ion removal is loosely defined
and rarely studied. It is necessary to compare the removal rate at the same competing condition
and in this way the ion selectivity can be directly compared.
5
In this study, the feasibility of selectively removing Pb2+ by CDI and MCDI was investigated
with the single-pass model at a constant feed concentration, which has not been reported. In
MCDI, cation and anion-exchange membranes were individually or jointly added to the electrode surface to fully understand the impacts of ion-exchange membranes on Pb2+ removal. In addition,
a widely used commercial activated carbon was employed as the electrode material to reveal basic removal behavior of Pb2+ against Ca2+ and Mg2+.
2. Experimental
2.1 Materials
Lead(II) nitrate, calcium chloride dehydrates, and magnesium chloride hexahydrate were used as Pb2+, calcium (Ca2+), and magnesium (Mg2+) source in this study. Nitric acid (67%) was used as
digester for ion concentration tests. Polyvinylidene fluoride (PVDF) and N-methyl-2-pyrrolidone
(NMP) were purchased from Sigma-Aldrich and used to prepare coating slurries. Ultrapure water
(Millipore, The U.S.A.) was used to prepare feeding solution. Activated carbon (YP50F, Sanwa)
6
with a surface area of ~1694 m2 g-1 was used as the active material, and the porosity is shown in
supporting information (Figure S1). The surface area and pore size measurements were carried
out by N2 adsorption/desorption on a Micromeritics ASAP 2020. X-ray photoelectron
spectroscopy (XPS) spectra of samples were obtained using a PerkinElmer PHI 5440 ESCA spectrometer with monochromatic Mg Kα radiation as the X-ray source.
To study the ion-exchange membrane behaviors, the cation-exchange membrane (CEM,
FKS-PET-130, physical and chemical data shown in Table S2) and anion-exchange membrane
(AEM, FAS-PET-130, physical and chemical data shown in Table S2) were purchased from
FUMATECH BWT GmbH. Both membranes are evaluated for common desalination studies and
the physical and chemical data are evaluated in forms of Na/K ions vs. H form. The effects of both membranes are essential for the discussion of Pb2+ removal.
2.2 Electrode preparation
Electrodes were prepared using a doctor blade method. Activated carbon was mixed with carbon
7
black and polyvinylidene fluoride (PVDF) binder (72:8:20, by weight) in
N-methyl-2-pyrrolidone (NMP) to form a homogeneous slurry. The resulting slurry was coated onto a graphite foil, dried under vacuum at 80 C overnight, and cut to 4.24.2 square inches to
obtain single-side electrodes. The mass loading of activated carbon on the single-side electrode was approximately 2.2 mg cm-2. To prepare double-side electrodes, the carbon slurry was coated
on the backside of the single-side electrode before cutting. The electrode morphology was
evaluated through scanning electron microscopy, performed on a Hitachi S-4800, and shown in
supporting information (Figure S2).
2.3 CDI experiments
A single-pass model was utilized throughout this study. As shown in Figure 1a, feeding water with 1 ppm Pb2+, 1 ppm Ca2+, and 1 ppm Mg2+ was pumped to the CDI cell from a reservoir by a peristaltic pump at a flow rate of 23 mL min-1. The concentration of three cations were controlled
in the same fashion to study the selectivity of electrode materials. Note that the flow rate of 23 mL min-1 is a measured value when the flow rate was set as 25 mL min-1 by the pump. The
8
voltages applied to the CDI cell were controlled by a CHI 670E electrochemical workstation.
Figure 1b exhibits a two-layer-electrode CDI cell, in which the cell was organized by
sandwiching a double-side electrode to two single-side coated electrodes. The electrodes were
separated by non-conductive flow meshes in CDI cells, and ion-exchange membranes were
added for MCDI cells as needed. The cell allows a radial flux through the CDI cell from bottom
center to four corners of the cathode and finally out of the cell from top center. Similarly,
four-layer-electrode and eight-layer-electrode cells were assembled with 3 and 7 layers of
double-side electrodes, respectively. Note that silicone gaskets were used on each electrode layer
to seal the CDI.
Figure 1. (a) Photo of the prototype CDI device with a peristaltic pump and (b) illustration of a two-layer-electrode 9
CDI cell, in which water enters from the center of the bottom, passes through an anode and anion-exchange membrane (AEM), flows by a mesh, and then passes a CEM, a double-sided cathode, and another CEM through four pores at the corners; after a mesh again, the water flow comes out of a CEM, an anode and a plate from the top center.
Each test started with a 5-min wetting to saturate the pure chemical adsorption of electrode
materials and exclude air, and followed by two cycles of 60-min charging and 10-min
discharging. The charging voltage was controlled at -1.2 V and the discharging voltage was applied per test requirements. Note that to remove Pb2+, the charge voltages for the working
electrodes were set to be negative by the electrochemical potentiostat.
2.4 Analysis of adsorption and desorption capacity
To study the concentration change of each metal ion, the concentration of the effluent water was
measured by inductively-coupled plasma-optical emission spectroscopy (ICP-OES) according to
the US EPA standard[28]. The removal efficiency of each cation was calculated through
Equation 1:
10
where
represents the concentration at the given sampling time, and
is the initial
concentration. The overall removal rate of cations was calculated through Equation 2, and
is
the charging duration that was typically 60 minutes in this study:
The desorption ability was determined by the ratio of each accumulated discharged cations (such as Pb2+, Ca2+, and Mg2+) to corresponding accumulated adsorbed cations based on the Equation
3,
where
is the volume of discharge effluent, and
is the concentration of discharged cation.
Selective removal of Pb2+ was indicated by the relative removal coefficient calculated based on
Equation 4,
where
and
indicate the removal efficiency of Pb2+ and the other species (e.g., Ca2+ and
Mg2+), respectively.
11
3. Results and discussion 3.1 Baseline performance of CDI for Pb2+ removal
A baseline was set up to compare the performances of CDI and MCDI cells. In the baseline, two
layers of electrodes without ion-exchange membranes were applied. The discharge voltage was
set at 0 V to prevent ions from adsorbing to the counter electrode during discharge process.
Figure 2 shows ion concentrations in the effluent and the removal efficiencies along with time.
Error bars are reported for concentration measurements and removal efficiencies were calculated
at the median of measured concentrations. The sharp drop in wetting process indicates the immediate adsorption of Pb2+ by chemical adsorption potential. The chemical adsorption was
considered saturated after 5 min, with the ion concentrations in effluent recovered.
At the beginning of charging process, a significant decrease of 0.7 ppm of Pb2+ concentration was observed together with Ca2+ and Mg2+, due to the electric potential. The removal efficiency of Pb2+ reached 80% at the beginning of charging process and decreased gradually after 10 minutes. Significant selective removal of Pb2+ against Ca2+ and Mg2+ was observed after 5-min
12
charging; for example, the Pb2+ removal efficiency retained approximately 40%, while the removal rates of Ca2+ and Mg2+ fell below 20% after 1-h charging (Figure 2a). The baseline
results were reproduced as shown in Figure S3. The very reproducible results are mainly ascribed
to the uniform activated carbon electrode sheet (Figure S2a), produced by a coating machine with a doctor blade. The high removal efficiency of Pb2+ can be explained by the hydroxyl and
carboxyl groups on the surface of the activated carbon. [29] A similar trend was observed in the 2nd cycle as shown in Figure 2b, which represents the cycling response of as-designed CDI system. Note that the initial point of the 2nd cycle gave high Pb2+, Ca2+ and Mg2+ concentrations, which is due to a quick release of residual adsorbed ions by reversed electric field polarization.[30] This also indicated the incomplete discharge in the 1st
cycle.
13
Figure 2. Ion concentrations and removal efficiencies of Pb2+, Ca2+, and Mg2+ by CDI baseline in (a) the first cycle (5-min wetting, 1-h charging and 10-min discharging) and (b) the second cycle (directly following the first cycle with 1-h charging and 10-min discharging) during charging and discharging. Note that concentrations at the third point during discharge processes were the average concentrations of all the discharge water except that taken for the first two discharge samples. The total collected amount for each ion was calculated based on the concentrations and the collected solution volumes of the first (10 mL), the second points (10 mL), and all the other solution for the third point (210 mL).
According to Equation 2, the average removal rate of Pb2+ is about 47.3% for the first cycle and 46.6% for the second cycle, corresponding to total adsorbed Pb2+ of 0.7 and 0.66 mg, respectively. Upon discharging, approximately 37% and 59.3% of the adsorbed Pb2+ were released for the first and the second cycle, respectively. The incomplete Pb2+ discharge can be explained by the strong bonding between Pb2+ and oxygen-containing groups (such as carboxyl)
14
on the surface of the activated carbon.[31, 32] XPS analysis was conducted for the AC,
indicating the oxygen content on the surface of the activated carbon is approximately 18 at.% in
the forms of C-O, C=O, and O-C=O (Figure S4). Upon discharging, the Pb2+ desorbed from the upstream electrode might be re-adsorbed by the downstream electrode. This assumption is supported by two facts: (1) the Pb2+ discharge rate increased to 59.3% at the 2nd cycle from 37% at the first cycle; (2) the difficult discharge of Pb2+
in the absence of ion-exchange membrane was worse in the case of a longer flow path; for example, in a four-layer-electrode cell, the discharge rate of the 1st cycle dropped to 1% (Figure
S5).
Despite the relatively low Pb2+ discharge rate, the baseline performance of the CDI cell in the absence of ion-exchange membranes demonstrates the possibility of selective removal of Pb2+
from drinking water.
3.2 Discharge deficiency of cation-exchange membrane
15
Enhanced desalination performance using CEM has been reported in the literature.[33] Thus,
ion-exchange membranes (including CEM and AEM) were expected to improve electrode recovery and enhance ion removal efficiency of Pb2+ in our investigation of selective removal of Pb2+. As shown in Figure 3a, when CEM and AEM were applied (denoted CEM-AEM-CDI), the average removal efficiencies of Pb2+, Ca2+, and Mg2+ were improved significantly over 60-min
charging. Compared with the removal efficiency of 47.3% in the baseline case, the number
increased to 84.6% with the help of CEM and AEM. On the other hand, the effluent
concentrations of the three cations were observed lower than 1 ppm upon discharging. In other words, no Pb2+, Ca2+, and Mg2+ were released during discharge at 0 V; instead, some of these
cations were still being removed.
16
Figure 3. Removal efficiency of (a) the 1st cycle (5-min wetting, 1-h charging and 10-min discharging) and (b) the 2nd cycle (directly following the first cycle with 1-h charging and 10-min discharging) for Pb2+, Ca2+, Mg2+ by CEM-AEM-CDI (in the presence of CEM and AEM) with 1 ppm solution and a discharge voltage of 0 V. Comparison of discharge performance of Pb2+ between CDI cells in the baseline, with CEM, and with CEM plus AEM at various discharge voltages (c) for the 1st cycle and (d) for the 2nd cycle.
The high removal rate and zero discharge of Pb2+ were also observed in the second cycle (Figure 3b). Considering the possible strong bonding between Pb2+ and function groups (such as
carboxyl) on the surface of activated carbon, increase of the discharge voltages was expected to enhance the discharge rate of Pb2+; however, as indicated by the positive removal efficiencies 17
upon discharging (Figure 3c, d, and Figure S6), Pb2+ was still adsorbed onto the electrode, rather than being released into solution at the discharge voltages of 0.6 or 1.2 V. Note that Ca2+ showed a higher removal efficiency than Mg2+ in Figure 3 because Mg2+ processes a stable inner
hydration shell, thereby leading to a water exchange rate five orders of magnitude slower than that for Ca2+[34, 35].
To find the reasons for the deficient discharge, we took the AEM out of the MCDI cell while leaving the CEM to work alone. Similar to the case of the CEM-AEM-MCDI, no Pb2+ could be
discharged in the CEM-CDI, as shown in Figure 3c and d. Thus, CEM is likely responsible for the inability to discharge the Pb2+.
To further confirm the assumption of CEM accounting for the ineffective discharge, a saturation test was carried out with influent of 10 ppm of Pb2+, Ca2+ and Mg2+ for 2.5 h in the presence of the CEM. As shown in Figure S7, the average removal rate of Pb2+ in charging process quenched
to 65%, which indicates the loss of capability; however, the discharge remained ineffective for
18
Pb2+ with the effluent concentration close to that of influent.
Based on the above experiments, the mechanism for the inability to discharge the Pb2+ is
illustrated in Figure 4. Protons as the counter ion in CEM, have higher mobility than other
cations in the system, and are preferentially adsorbed and desorbed from charged surface[36]. Upon 5-min wetting, the concentrations of Pb2+, Ca2+, and Mg2+ decreased much more
significantly in the presence of CEM (Figure 3a) than those in the baseline (Figure 2a). This
means that fast ion exchange occurred between the cations and protons during the wetting process (Figure 4a). When applying a voltage, protons were transferred to electrode while Pb2+ moved into the CEM (Figure 4b). During discharge, protons instead of Pb2+ were released in the
cation-exchange membranes because of the higher mobility of the protons (Figure 4c).
To validate this hypothesis, pH values were tracked in additional tests in the presence of CEM or
CEM with AEM in standard test environment, as shown in Figure S8. The pH value of the
influent was approximately 5.7 due to the hydrolysis of lead ions. During the wetting process, the
19
pH value decreased to 3.8, indicating the release of protons because of the ion-exchange on the
CEM. Upon 60-min charge, despite slight pH value difference with and without AEM, the
effluents in both cases exhibited lower pH value than the influent, suggesting the CDI process
accompanied by the ion exchange process. When the cell was discharged, the pH value dropped
to approximately 3, indicating the release of the adsorbed protons from the electrode.
Figure 4. Schematic illustration of ion exchange between Pb2+ and H+ to explain the ineffectiveness of Pb2+ discharge.
As additional evidence, current responses during CDI processes in the presence of CEMs
decreased significantly, compared with that in the baseline (Figure S9). The reason is a part of the Pb2+ was participating ion exchange (i.e., a chemical process) instead of involving charge
transfer to the electrode (an electrochemical process). Current response is related to the actual 20
concentrations of cations participating in the electrochemical process. The ion-exchange of Pb2+ with protons reduced the amount of the Pb2+ involved in the electrochemical process, thereby
decreasing the current response.
In addition, during the discharge process, higher discharging currents were observed at the beginning because of the easier release of protons than Pb2+. As a conclusion, the presence of CEMs led to high Pb2+ removal efficiency but hindered Pb2+ from discharging, which leads to inability of regeneration. Also, note that the CDI cell did not exhibit selective removal of Pb2+ against Ca2+ and Mg2+ in the presence of CEM, compared with in the baseline (Figure 3). Thus, CEM is an unnecessary part in this study aimed to investigate selective removal of Pb2+.
3.3 Enhanced discharge of anion-exchange membrane CDI (AEM-CDI). The unsuccessful discharge of Pb2+ in the presence of CEM encouraged us to take it out of the
MCDI; the MCDI working with AEM alone is called AEM-CDI for convenience. As shown in Figure 5a, with -1.2 V charging voltage, Pb2+ removal efficiency was improved to 58.2% for the
21
AEM-CDI from 47.3% for the baseline in the 1st cycle. Upon discharging at 0 V, 47.1% of the adsorbed Pb2+ was released, significantly higher than that in the baseline (37.0%). When cycling the system, average Pb2+ removal for AEM-CDI maintained at 51.5% (vs. 46.6% for baseline) and 61.5% of adsorbed Pb2+ was discharged (vs.59.3% for baseline). The performance
enhancement of AEM-CDI is from the improved ion separation by the AEM. Note that during the initial charging state, the removal rate of Ca2+ was higher than that of Mg2+ due to the high hydration of Mg2+ (Figure 2); in contrast, at the later stage, the removal efficiency of Ca2+ was lower than Mg2+. As a result, the average removal efficiency of Ca2+ was slightly lower than that of Mg2+ (23.7% vs. 33.1%). The reason for the relatively lower removal efficiency of Ca2+ compared with Mg2+ is unclear at this stage and is still under investigation.
Among the three mass transport approaches (convection, diffusion, and migration), the
convection is enhanced due to the flowing solution in CDI cells. In the absence of ion-exchange
membranes, the enhanced convection could flush part of the accumulated ions on the electrode
surfaces into the flowing solution. This issue can be relieved with the help of ion-exchange
22
membranes because ions are accumulated on the electrode surfaces instead of the membrane
surfaces, which avoids the concentrated ions that are flushed by the flowing solution. The
reduced loss of the accumulated ions thereby improves the removal efficiency. During
discharging, the ion-exchange membranes can help to block cations (or anions) from transporting
to anode (or cathode), thereby improving the discharge efficiency.
Another advantage of AEM-CDI is to realize further improved Pb2+ discharge efficiency by
applying a reversed voltage, which is not possible in membrane-free CDI because a reversed
voltage leads the released ions to be attracted to the counter electrode. As indicated in Figure 5c
and d, the discharge rates improved to 78.8% and 85.0% for the first and the second cycle,
respectively, at 1.2 V discharge voltage.
23
Figure 5. Performance of AEM-CDI at the discharge voltage of 0 V in (a) the 1st cycle and (b) the 2nd cycle, compared with those at the discharge voltage of 1.2 V in (c) the 1st cycle and (d) the 2nd cycle. The first cycle has 5-min wetting, 1-h charging and 10-min discharging. The second cycle directly follows the first cycle with 1-h charging and 10-min discharging.
In addition, compared with no selectivity of removing Pb2+ in the presence of CEM (Figure 3a and b), AEM-CDI showed good selective removal of Pb2+ against Ca2+ and Mg2+ (Figure 5).
Thus, this very encouraging results urged us to further investigate the effects discharge voltage
and flow path length on the CDI performance. As summarized in Table 1, when the electrode 24
layer number increased from 2 to 4 and 8, the flow path length increased, resulting in the increased Pb2+ removal rates. For example, a removal rate of 58% for a 2-layer-electrode
AEM-CDI improved to 72% and 82% for 4-layer-electrde and 8-layer-electrode AEM-CDI cells,
respectively, during the first cycle.
Table 1. Charging and discharging efficiency of three AEM-CDI modules at different voltages. Pb2+
The discharge efficiency of AEM-CDI improved by enhancing the discharge voltages. For
example, in the 2-layer-electrode CDI, the discharge rates are 47%, 76%, and 78% at discharge
voltages of 0, 0.6 and 1.2 V, respectively, during the first cycle. The effect of flow path length on
the discharge rate is apparently complicated because a lower discharge efficiency was observed 25
with a longer flow path; e.g., at 0.6 V, the discharge efficiencies for the 2-layer-electrode and the
4-layer-electrode AEM-CDI cell were 76% and 47%, respectively. The possible reason is that an AEM-CDI with more electrode layers has a higher Pb2+ removal rate, but lower adsorbed Pb2+
per unit electrode at the same charge duration because double of the electrode layers did not
result in a double removal efficiency. In other words, less electrode layers in the AEM-CDI cell suggests adsorption of Pb2+ closer to saturation compared with a case with more electrode layers.
Electrochemical adsorption allows multiple layers (Helmholtz layer and Gouy-Chapman layer) of ions to be adsorbed on electrode surface.[37] In the inner layer (Helmholtz layer) Pb2+
possesses the relatively strong bonding with the functional groups (e.g., carboxyl) on the activated carbon surface; in contrast, the Pb2+ adsorbed in the outer layer (Gouy-Chapman layer)
is much easier to detach upon discharging. Thus, the AEM-CDI with fewer electrode layers had more Pb2+ adsorbed in the outer layers, thereby exhibiting higher discharge rates.
When competing with Ca2+ and Mg2+ at the same concentrations in AEM-CDI, selective removal of Pb2+ can be achieved as mentioned above. By converting concentration to selectivity through
26
Equation 4, an overall higher selectivity of removal of Pb2+ over Ca2+ and Mg2+ (Figure 6). This
is likely related to surface functional groups, as reported in the literature.[38, 39] Two trends can
be observed from Figure 6. First, lower selectivity was observed when longer flow path or more
electrodes were applied. More electrodes suggest a higher ion adsorption capacity, resulting in
more competing cations adsorbed at the given duration. According to Equation 4, an increased
removal efficiency of competing cation (
) can lead to a lower removal selectivity. It is
expected that with enough electrode layers, the removal efficiencies of Pb2+, Ca2+, and Mg2+ are
all 100%, resulting in a selectivity of 1; in other words, no selectivity can be observed in such a
case. The second trend can be observed in Figure 6 is that the removal selectivity of Pb2+ was improved in the 2nd cycle for different CDI modules, compared with that in the first cycle. This can be explained by the mechanism of selective removal of Pb2+ against Ca2+ and Mg2+. Ca2+ and Mg2+ possess higher mobility than Pb2+, and should be more easily adsorbed onto electrodes than Pb2+; however, the relatively strong affinity between Pb2+ and the functional groups (such as carboxyl) [31, 32] allows Pb2+ to replace the Ca2+ and Mg2+ earlier arriving on the electrode.
27
After initial saturation of the material in the 1st cycle, the replacement process increased the release of Ca2+ and Mg2+. The replacement process also suggests the potential enhancement of selective removal of Pb2+ against other cations by surface modification of activated carbon with
functional groups with stronger affinity, which is currently under investigation. Note that adsorbed Na+ was observed being replaced by Ca2+, [29] in which the mechanism may be different with our replacement of Ca2+/Mg2+ by Pb2+, though.
Note also that the removal
selectivity of Pb2+ against Ca2+ and Mg2+ at the second cycle for the 4-layer-electrode cell was
lower than that for the 8-layer-electrode cell; the reason is uncertain at this stage and needs
further investigation.
Figure 6. Selectivity on the Pb2+ removal among different AEM-CDI modules.
28
In the end, to further validate the selectivity of the used activated carbon, we tested the AEM-CDI module in more aggressive competition environment of Pb2+, Ca2+, and Mg2+ concentration at 1, 30, and 10 ppm, respectively, to simulate Pb2+ contaminated drinking water system. As shown in Figure S10, the Ca2+ and Mg2+ saturate quickly within 10 min; in contrast, at the end of 1-h charging process, Pb2+ can still maintain ~24% removal. The average removal efficiencies of Ca2+ and Mg2+ over the 1-h charging were 6.3% and 8.4%, respectively, much lower than that of Pb2+ (36%). This result also verifies our discussion on ion replacement
behavior on the electrode. It is important to mention that due to the ion replacement, the longer
the charging time is, the higher selectivity of the unit shows.
4. Conclusion
The impacts of AEM and CEM ion-exchange membranes on the CDI performance were investigated to achieve selective removal of Pb2+ competing with Ca2+ and Mg2+ under the same conditions, using activated carbon electrodes. Although CEM improves the Pb2+ removal efficiency, no Pb2+ is discharged because Pb2+ is trapped in the CEM while releasing protons.
29
Meanwhile, no selective removal of Pb2+ over other cations (e.g., Ca2+ and Mg2+) can be
achieved in the presence of CEM, which suggests the CEM is not suitable in aim to selectively remove Pb2+. In contrast, AEM-CDI was able to discharge Pb2+, and achieved higher Pb2+
removal efficiency and discharge efficiency, compared with the baseline CDI (membrane-free). More importantly, the AEM-CDI allows to selectively remove Pb2+ against other cations (such as Ca2+ and Mg2+), which does not occur in the presence of CEM. The mechanism of selective removal of Pb2+ against Ca2+ and Mg2+ is believed to be a replacement process, in which Ca2+ and Mg2+ possess higher mobility than Pb2+, and are more easily adsorbed onto electrodes than Pb2+ at the initial stage; however, the relatively strong affinity between Pb2+ and the functional groups (such as carboxyl) allows Pb2+ to replace the Ca2+ and Mg2+ earlier arriving on the
electrode. This mechanism suggests that the selectivity of AEM-CDI system can be improved by
modifying carbon materials with functional groups. Therefore, it is very promising to construct a
high-efficiency and low-cost CDI system to purify drinking water by selectively removing heavy
metal ions. This approach can be developed into an economical alternative for in-house or
municipal water treatment.
30
Acknowledgments
This project was supported by National Science Foundation Industry/University Cooperative
Research Center on Water Equipment & Policy located at University of Wisconsin-Milwaukee
(IIP-1540032) and Marquette University (IIP-1540010). The authors acknowledge Jayme R.
Kolarik (Pentair), Brennon Garthwait (Veolia Research & Innovation), Malcolm Kahn (Marmon
Water, Inc.), and Dawn Marsh (Baker Manufacturing Company, LLC) for their helpful advices
and discussions.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the online version.
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39
Figure Captions:
Figure 7. (a) Photo of prototype of CDI device with a peristaltic pump and (b) illustration of a two-layer-electrode CDI cell, in which water enters from the center of the bottom, passes through an anode and anion-exchange membrane (AEM), flows by a mesh, and then passes a CEM, a double-sided cathode, and another CEM through four pores at the corners; after a mesh again, the water flow comes out of a CEM, an anode and a plate from the top center. Figure 8. Ion concentrations and removal efficiencies of Pb2+, Ca2+, and Mg2+ by CDI baseline in (a) the first cycle (5-min wetting, 1-h charging and 10-min discharging) and (b) the second cycle (directly following the first cycle with 1-h charging and 10-min discharging) during charging and discharging. Note that concentrations at the third point during discharge processes were the average concentrations of all the discharge water except that taken for the first two discharge samples. The total collected amount for each ion was calculated based on the concentrations and the collected solution volumes of the first (10 mL), the second points (10 mL), and all the other solution for the third point (210 mL). Figure 9. Removal efficiency of (a) the 1st cycle (5-min wetting, 1-h charging and 10-min discharging) and (b) the 2nd cycle (directly following the first cycle with 1-h charging and 10-min discharging) for Pb2+, Ca2+, Mg2+ by CEM-AEM-CDI (in the presence of CEM and AEM) with 1 ppm solution and a discharge voltage of 0 V. Comparison of discharge performance of Pb2+ between CDI cells in the baseline, with CEM, and with CEM plus AEM at various discharge 40
voltages (c) for the 1st cycle and (d) the 2nd cycle. Figure 10. Schematic illustration of ion exchange between Pb2+ and H+ to explain the ineffectiveness of Pb2+ discharge. Figure 11. Performance of AEM-CDI at the discharge voltage of 0 V in (a) the 1st cycle and (b) the 2nd cycle, compared with those at the discharge voltage of 1.2 V in (c) the 1st cycle and (d) the 2nd cycle. The first cycle has 5-min wetting, 1-h charging and 10-min discharging. The second cycle directly follows the first cycle with 1-h charging and 10-min discharging. Figure 12. Selectivity on the Pb2+ removal among different AEM-CDI modules.
41
Highlights:
1. Selective removal of lead ions against calcium and magnesium ions is demonstrated.
2. CEM leads to a high lead removal rate but inability to discharge lead ions.
3. AEM-CDI shows improved selective removal of lead and enhanced lead discharge.
42
S1385-8947(18)32170-3 https://doi.org/10.1016/j.cej.2018.10.208 CEJ 20271
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
18 July 2018 8 October 2018 28 October 2018
Please cite this article as: Q. Dong, X. Guo, X. Huang, L. Liu, R. Tallon, B. Taylor, J. Chen, Selective Removal of Lead Ions through Capacitive Deionization: Role of Ion-Exchange Membrane, Chemical Engineering Journal (2018), doi: https://doi.org/10.1016/j.cej.2018.10.208
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Selective Removal of Lead Ions through Capacitive Deionization: Role of Ion-Exchange
Membrane Qianqian Dong,a,† Xiaoru Guo,a,† Xingkang Huang,a,* Lianjun Liu,b Rebecca Tallon,b Bruce Taylor,c and Junhong Chena,* a
Department of Mechanical Engineering, University of Wisconsin-Milwaukee, 3200 North
Cramer Street, Milwaukee, WI, 53211, USA. b
A.O. Smith Corporation, Corporate Technology Center, 12100 W Park Place, Milwaukee, WI,
53224, USA c
KX Technologies LLC, 55 Railroad Avenue, West Haven, Connecticut 06516 USA
Abstract
Capacitive deionization (CDI) is a rising technology as a low-energy-consumption and low-cost
option for water purifications and treatments; however, selective removal of heavy metal ions
*
Corresponding authors.
E-mail addresses: [email protected] (X. Huang), [email protected] (J. Chen). † These authors contributed equally. 1
has been rarely reported. In this study, the impacts of ion-exchange membranes on the selective removal of lead ions (Pb2+) by CDI are investigated with a single-pass model. Both
cation-exchange membrane (CEM) and anion-exchange membrane (AEM) demonstrate improved Pb2+ removal efficiencies. The presence of CEM retards discharge of adsorbed Pb2+, which eliminates the selectivity of removing Pb2+ against other cations (such as Ca2+ and Mg2+);
in contrast, without the CEM, the CDI cell with only the AEM (called AEM-CDI) exhibits enhanced discharge efficiency and retains the selectivity of removing Pb2+ against Ca2+ and Mg2+. More importantly, the AEM-CDI presents improved Pb2+ discharge efficiency when applying
inverted voltages, which is not possible in membrane-free CDI because desorbed ions can be
re-adsorbed to the counter electrode. In addition, surface modification for porous carbon with functional groups to enhance the affinity between Pb2+ and electrode surface is expected to further improve the selectivity of removing Pb2+ against other cations, thereby showing very
promising potential in purification of drinking water.
Keywords: capacitive deionization; ion-exchange membrane; lead removal; selectivity; single
pass.
2
1. Introduction
Capacitive deionization (CDI) is an emerging energy-efficient technique for water treatment to
remove salt[1, 2], heavy metal ions[3], and hardness[4] from water. By applying voltage,
double-layer capacitance can be built on electrodes in CDI devices, which attracts, accumulates,
and removes ions. Various CDI architectures such as flow-through and flow-between with
ion-exchange membrane variations have been explored to remove ions in water,[5] in which
flow-between model is widely used due to its simple configuration and its suitability for studying
material performance.[6-8] Meanwhile, different water feeding strategies such as batch mode and
single-pass mode have been investigated; the batch mode requires cycling water in a tank for
multiple cycles to achieve high removal performance, while a single-pass flow allows water to
pass the system.[9] Compared with batch modes, the single-pass one is less investigated but
closer to practical application due to its continuous feeding of constant ion concentrations.[10,
11]
3
Ion-exchange membranes have been introduced to CDI to improve ion removal efficiency, which
is called as membrane CDI (MCDI)[12, 13]. In the MCDI, membrane traps co-ions into
intraparticle pores and thus increases accumulation of counter-ions in macropores[14]. In
addition, an inverted voltage is allowed to be applied to the MCDI cell upon discharging because
the ion exchange membranes prevents the desorbed ions from being re-adsorbed to the counter
electrode, thereby enhancing electrode regeneration efficiency[15].
Despite the above-mentioned benefits of ion-exchange membranes, current membranes are typically designed for small ions (such as H+ and Na+) in desalination process and it is rarely reported on lead (Pb2+) removal. As a neurotoxin, Pb2+ exposes health risk to people and is
frequently detected in drinking water[16, 17]. The total lead concentration allowance in the
drinking water in US is 15 ppb according to US Environmental Protection Agency (EPA)[18];
and the World Health Organization (WHO) has set the limit to 10 ppb, as well as many countries[19]. However, the aging pipelines hike the Pb2+ concentration for most municipals at
4
the household end. Currently many water treatment technologies such as reverse osmosis are
widely studied and applied to address this issue; but the high maintenance cost and high energy
consumption limits the feasibility for low-cost daily use.
CDI has great potentials in removing heavy metal ions such as Pb[3, 20], Cr[3, 21], Cu[22, 23],
Cd[24], and As[25]. When an electrical field is produced between parallel porous carbon
electrodes, metal ions can be transported from contaminated water to the micropore structures,
where the electrical double layers (EDLs) form[26]. When the electrical field is revoked or
reversed, the adsorbed ions are discharged into the effluent so that the active materials are regenerated for the next cycle[27]. However, many beneficial ions, such as Ca2+ and Mg2+, have a higher concentration than Pb2+ and compete with Pb2+ upon ion removal during CDI processes. Therefore, it is necessary to investigate selective removal of Pb2+ against other cations (e.g., Ca2+ and Mg2+) with CDI devices. At the same time, the selectivity of ion removal is loosely defined
and rarely studied. It is necessary to compare the removal rate at the same competing condition
and in this way the ion selectivity can be directly compared.
5
In this study, the feasibility of selectively removing Pb2+ by CDI and MCDI was investigated
with the single-pass model at a constant feed concentration, which has not been reported. In
MCDI, cation and anion-exchange membranes were individually or jointly added to the electrode surface to fully understand the impacts of ion-exchange membranes on Pb2+ removal. In addition,
a widely used commercial activated carbon was employed as the electrode material to reveal basic removal behavior of Pb2+ against Ca2+ and Mg2+.
2. Experimental
2.1 Materials
Lead(II) nitrate, calcium chloride dehydrates, and magnesium chloride hexahydrate were used as Pb2+, calcium (Ca2+), and magnesium (Mg2+) source in this study. Nitric acid (67%) was used as
digester for ion concentration tests. Polyvinylidene fluoride (PVDF) and N-methyl-2-pyrrolidone
(NMP) were purchased from Sigma-Aldrich and used to prepare coating slurries. Ultrapure water
(Millipore, The U.S.A.) was used to prepare feeding solution. Activated carbon (YP50F, Sanwa)
6
with a surface area of ~1694 m2 g-1 was used as the active material, and the porosity is shown in
supporting information (Figure S1). The surface area and pore size measurements were carried
out by N2 adsorption/desorption on a Micromeritics ASAP 2020. X-ray photoelectron
spectroscopy (XPS) spectra of samples were obtained using a PerkinElmer PHI 5440 ESCA spectrometer with monochromatic Mg Kα radiation as the X-ray source.
To study the ion-exchange membrane behaviors, the cation-exchange membrane (CEM,
FKS-PET-130, physical and chemical data shown in Table S2) and anion-exchange membrane
(AEM, FAS-PET-130, physical and chemical data shown in Table S2) were purchased from
FUMATECH BWT GmbH. Both membranes are evaluated for common desalination studies and
the physical and chemical data are evaluated in forms of Na/K ions vs. H form. The effects of both membranes are essential for the discussion of Pb2+ removal.
2.2 Electrode preparation
Electrodes were prepared using a doctor blade method. Activated carbon was mixed with carbon
7
black and polyvinylidene fluoride (PVDF) binder (72:8:20, by weight) in
N-methyl-2-pyrrolidone (NMP) to form a homogeneous slurry. The resulting slurry was coated onto a graphite foil, dried under vacuum at 80 C overnight, and cut to 4.24.2 square inches to
obtain single-side electrodes. The mass loading of activated carbon on the single-side electrode was approximately 2.2 mg cm-2. To prepare double-side electrodes, the carbon slurry was coated
on the backside of the single-side electrode before cutting. The electrode morphology was
evaluated through scanning electron microscopy, performed on a Hitachi S-4800, and shown in
supporting information (Figure S2).
2.3 CDI experiments
A single-pass model was utilized throughout this study. As shown in Figure 1a, feeding water with 1 ppm Pb2+, 1 ppm Ca2+, and 1 ppm Mg2+ was pumped to the CDI cell from a reservoir by a peristaltic pump at a flow rate of 23 mL min-1. The concentration of three cations were controlled
in the same fashion to study the selectivity of electrode materials. Note that the flow rate of 23 mL min-1 is a measured value when the flow rate was set as 25 mL min-1 by the pump. The
8
voltages applied to the CDI cell were controlled by a CHI 670E electrochemical workstation.
Figure 1b exhibits a two-layer-electrode CDI cell, in which the cell was organized by
sandwiching a double-side electrode to two single-side coated electrodes. The electrodes were
separated by non-conductive flow meshes in CDI cells, and ion-exchange membranes were
added for MCDI cells as needed. The cell allows a radial flux through the CDI cell from bottom
center to four corners of the cathode and finally out of the cell from top center. Similarly,
four-layer-electrode and eight-layer-electrode cells were assembled with 3 and 7 layers of
double-side electrodes, respectively. Note that silicone gaskets were used on each electrode layer
to seal the CDI.
Figure 1. (a) Photo of the prototype CDI device with a peristaltic pump and (b) illustration of a two-layer-electrode 9
CDI cell, in which water enters from the center of the bottom, passes through an anode and anion-exchange membrane (AEM), flows by a mesh, and then passes a CEM, a double-sided cathode, and another CEM through four pores at the corners; after a mesh again, the water flow comes out of a CEM, an anode and a plate from the top center.
Each test started with a 5-min wetting to saturate the pure chemical adsorption of electrode
materials and exclude air, and followed by two cycles of 60-min charging and 10-min
discharging. The charging voltage was controlled at -1.2 V and the discharging voltage was applied per test requirements. Note that to remove Pb2+, the charge voltages for the working
electrodes were set to be negative by the electrochemical potentiostat.
2.4 Analysis of adsorption and desorption capacity
To study the concentration change of each metal ion, the concentration of the effluent water was
measured by inductively-coupled plasma-optical emission spectroscopy (ICP-OES) according to
the US EPA standard[28]. The removal efficiency of each cation was calculated through
Equation 1:
10
where
represents the concentration at the given sampling time, and
is the initial
concentration. The overall removal rate of cations was calculated through Equation 2, and
is
the charging duration that was typically 60 minutes in this study:
The desorption ability was determined by the ratio of each accumulated discharged cations (such as Pb2+, Ca2+, and Mg2+) to corresponding accumulated adsorbed cations based on the Equation
3,
where
is the volume of discharge effluent, and
is the concentration of discharged cation.
Selective removal of Pb2+ was indicated by the relative removal coefficient calculated based on
Equation 4,
where
and
indicate the removal efficiency of Pb2+ and the other species (e.g., Ca2+ and
Mg2+), respectively.
11
3. Results and discussion 3.1 Baseline performance of CDI for Pb2+ removal
A baseline was set up to compare the performances of CDI and MCDI cells. In the baseline, two
layers of electrodes without ion-exchange membranes were applied. The discharge voltage was
set at 0 V to prevent ions from adsorbing to the counter electrode during discharge process.
Figure 2 shows ion concentrations in the effluent and the removal efficiencies along with time.
Error bars are reported for concentration measurements and removal efficiencies were calculated
at the median of measured concentrations. The sharp drop in wetting process indicates the immediate adsorption of Pb2+ by chemical adsorption potential. The chemical adsorption was
considered saturated after 5 min, with the ion concentrations in effluent recovered.
At the beginning of charging process, a significant decrease of 0.7 ppm of Pb2+ concentration was observed together with Ca2+ and Mg2+, due to the electric potential. The removal efficiency of Pb2+ reached 80% at the beginning of charging process and decreased gradually after 10 minutes. Significant selective removal of Pb2+ against Ca2+ and Mg2+ was observed after 5-min
12
charging; for example, the Pb2+ removal efficiency retained approximately 40%, while the removal rates of Ca2+ and Mg2+ fell below 20% after 1-h charging (Figure 2a). The baseline
results were reproduced as shown in Figure S3. The very reproducible results are mainly ascribed
to the uniform activated carbon electrode sheet (Figure S2a), produced by a coating machine with a doctor blade. The high removal efficiency of Pb2+ can be explained by the hydroxyl and
carboxyl groups on the surface of the activated carbon. [29] A similar trend was observed in the 2nd cycle as shown in Figure 2b, which represents the cycling response of as-designed CDI system. Note that the initial point of the 2nd cycle gave high Pb2+, Ca2+ and Mg2+ concentrations, which is due to a quick release of residual adsorbed ions by reversed electric field polarization.[30] This also indicated the incomplete discharge in the 1st
cycle.
13
Figure 2. Ion concentrations and removal efficiencies of Pb2+, Ca2+, and Mg2+ by CDI baseline in (a) the first cycle (5-min wetting, 1-h charging and 10-min discharging) and (b) the second cycle (directly following the first cycle with 1-h charging and 10-min discharging) during charging and discharging. Note that concentrations at the third point during discharge processes were the average concentrations of all the discharge water except that taken for the first two discharge samples. The total collected amount for each ion was calculated based on the concentrations and the collected solution volumes of the first (10 mL), the second points (10 mL), and all the other solution for the third point (210 mL).
According to Equation 2, the average removal rate of Pb2+ is about 47.3% for the first cycle and 46.6% for the second cycle, corresponding to total adsorbed Pb2+ of 0.7 and 0.66 mg, respectively. Upon discharging, approximately 37% and 59.3% of the adsorbed Pb2+ were released for the first and the second cycle, respectively. The incomplete Pb2+ discharge can be explained by the strong bonding between Pb2+ and oxygen-containing groups (such as carboxyl)
14
on the surface of the activated carbon.[31, 32] XPS analysis was conducted for the AC,
indicating the oxygen content on the surface of the activated carbon is approximately 18 at.% in
the forms of C-O, C=O, and O-C=O (Figure S4). Upon discharging, the Pb2+ desorbed from the upstream electrode might be re-adsorbed by the downstream electrode. This assumption is supported by two facts: (1) the Pb2+ discharge rate increased to 59.3% at the 2nd cycle from 37% at the first cycle; (2) the difficult discharge of Pb2+
in the absence of ion-exchange membrane was worse in the case of a longer flow path; for example, in a four-layer-electrode cell, the discharge rate of the 1st cycle dropped to 1% (Figure
S5).
Despite the relatively low Pb2+ discharge rate, the baseline performance of the CDI cell in the absence of ion-exchange membranes demonstrates the possibility of selective removal of Pb2+
from drinking water.
3.2 Discharge deficiency of cation-exchange membrane
15
Enhanced desalination performance using CEM has been reported in the literature.[33] Thus,
ion-exchange membranes (including CEM and AEM) were expected to improve electrode recovery and enhance ion removal efficiency of Pb2+ in our investigation of selective removal of Pb2+. As shown in Figure 3a, when CEM and AEM were applied (denoted CEM-AEM-CDI), the average removal efficiencies of Pb2+, Ca2+, and Mg2+ were improved significantly over 60-min
charging. Compared with the removal efficiency of 47.3% in the baseline case, the number
increased to 84.6% with the help of CEM and AEM. On the other hand, the effluent
concentrations of the three cations were observed lower than 1 ppm upon discharging. In other words, no Pb2+, Ca2+, and Mg2+ were released during discharge at 0 V; instead, some of these
cations were still being removed.
16
Figure 3. Removal efficiency of (a) the 1st cycle (5-min wetting, 1-h charging and 10-min discharging) and (b) the 2nd cycle (directly following the first cycle with 1-h charging and 10-min discharging) for Pb2+, Ca2+, Mg2+ by CEM-AEM-CDI (in the presence of CEM and AEM) with 1 ppm solution and a discharge voltage of 0 V. Comparison of discharge performance of Pb2+ between CDI cells in the baseline, with CEM, and with CEM plus AEM at various discharge voltages (c) for the 1st cycle and (d) for the 2nd cycle.
The high removal rate and zero discharge of Pb2+ were also observed in the second cycle (Figure 3b). Considering the possible strong bonding between Pb2+ and function groups (such as
carboxyl) on the surface of activated carbon, increase of the discharge voltages was expected to enhance the discharge rate of Pb2+; however, as indicated by the positive removal efficiencies 17
upon discharging (Figure 3c, d, and Figure S6), Pb2+ was still adsorbed onto the electrode, rather than being released into solution at the discharge voltages of 0.6 or 1.2 V. Note that Ca2+ showed a higher removal efficiency than Mg2+ in Figure 3 because Mg2+ processes a stable inner
hydration shell, thereby leading to a water exchange rate five orders of magnitude slower than that for Ca2+[34, 35].
To find the reasons for the deficient discharge, we took the AEM out of the MCDI cell while leaving the CEM to work alone. Similar to the case of the CEM-AEM-MCDI, no Pb2+ could be
discharged in the CEM-CDI, as shown in Figure 3c and d. Thus, CEM is likely responsible for the inability to discharge the Pb2+.
To further confirm the assumption of CEM accounting for the ineffective discharge, a saturation test was carried out with influent of 10 ppm of Pb2+, Ca2+ and Mg2+ for 2.5 h in the presence of the CEM. As shown in Figure S7, the average removal rate of Pb2+ in charging process quenched
to 65%, which indicates the loss of capability; however, the discharge remained ineffective for
18
Pb2+ with the effluent concentration close to that of influent.
Based on the above experiments, the mechanism for the inability to discharge the Pb2+ is
illustrated in Figure 4. Protons as the counter ion in CEM, have higher mobility than other
cations in the system, and are preferentially adsorbed and desorbed from charged surface[36]. Upon 5-min wetting, the concentrations of Pb2+, Ca2+, and Mg2+ decreased much more
significantly in the presence of CEM (Figure 3a) than those in the baseline (Figure 2a). This
means that fast ion exchange occurred between the cations and protons during the wetting process (Figure 4a). When applying a voltage, protons were transferred to electrode while Pb2+ moved into the CEM (Figure 4b). During discharge, protons instead of Pb2+ were released in the
cation-exchange membranes because of the higher mobility of the protons (Figure 4c).
To validate this hypothesis, pH values were tracked in additional tests in the presence of CEM or
CEM with AEM in standard test environment, as shown in Figure S8. The pH value of the
influent was approximately 5.7 due to the hydrolysis of lead ions. During the wetting process, the
19
pH value decreased to 3.8, indicating the release of protons because of the ion-exchange on the
CEM. Upon 60-min charge, despite slight pH value difference with and without AEM, the
effluents in both cases exhibited lower pH value than the influent, suggesting the CDI process
accompanied by the ion exchange process. When the cell was discharged, the pH value dropped
to approximately 3, indicating the release of the adsorbed protons from the electrode.
Figure 4. Schematic illustration of ion exchange between Pb2+ and H+ to explain the ineffectiveness of Pb2+ discharge.
As additional evidence, current responses during CDI processes in the presence of CEMs
decreased significantly, compared with that in the baseline (Figure S9). The reason is a part of the Pb2+ was participating ion exchange (i.e., a chemical process) instead of involving charge
transfer to the electrode (an electrochemical process). Current response is related to the actual 20
concentrations of cations participating in the electrochemical process. The ion-exchange of Pb2+ with protons reduced the amount of the Pb2+ involved in the electrochemical process, thereby
decreasing the current response.
In addition, during the discharge process, higher discharging currents were observed at the beginning because of the easier release of protons than Pb2+. As a conclusion, the presence of CEMs led to high Pb2+ removal efficiency but hindered Pb2+ from discharging, which leads to inability of regeneration. Also, note that the CDI cell did not exhibit selective removal of Pb2+ against Ca2+ and Mg2+ in the presence of CEM, compared with in the baseline (Figure 3). Thus, CEM is an unnecessary part in this study aimed to investigate selective removal of Pb2+.
3.3 Enhanced discharge of anion-exchange membrane CDI (AEM-CDI). The unsuccessful discharge of Pb2+ in the presence of CEM encouraged us to take it out of the
MCDI; the MCDI working with AEM alone is called AEM-CDI for convenience. As shown in Figure 5a, with -1.2 V charging voltage, Pb2+ removal efficiency was improved to 58.2% for the
21
AEM-CDI from 47.3% for the baseline in the 1st cycle. Upon discharging at 0 V, 47.1% of the adsorbed Pb2+ was released, significantly higher than that in the baseline (37.0%). When cycling the system, average Pb2+ removal for AEM-CDI maintained at 51.5% (vs. 46.6% for baseline) and 61.5% of adsorbed Pb2+ was discharged (vs.59.3% for baseline). The performance
enhancement of AEM-CDI is from the improved ion separation by the AEM. Note that during the initial charging state, the removal rate of Ca2+ was higher than that of Mg2+ due to the high hydration of Mg2+ (Figure 2); in contrast, at the later stage, the removal efficiency of Ca2+ was lower than Mg2+. As a result, the average removal efficiency of Ca2+ was slightly lower than that of Mg2+ (23.7% vs. 33.1%). The reason for the relatively lower removal efficiency of Ca2+ compared with Mg2+ is unclear at this stage and is still under investigation.
Among the three mass transport approaches (convection, diffusion, and migration), the
convection is enhanced due to the flowing solution in CDI cells. In the absence of ion-exchange
membranes, the enhanced convection could flush part of the accumulated ions on the electrode
surfaces into the flowing solution. This issue can be relieved with the help of ion-exchange
22
membranes because ions are accumulated on the electrode surfaces instead of the membrane
surfaces, which avoids the concentrated ions that are flushed by the flowing solution. The
reduced loss of the accumulated ions thereby improves the removal efficiency. During
discharging, the ion-exchange membranes can help to block cations (or anions) from transporting
to anode (or cathode), thereby improving the discharge efficiency.
Another advantage of AEM-CDI is to realize further improved Pb2+ discharge efficiency by
applying a reversed voltage, which is not possible in membrane-free CDI because a reversed
voltage leads the released ions to be attracted to the counter electrode. As indicated in Figure 5c
and d, the discharge rates improved to 78.8% and 85.0% for the first and the second cycle,
respectively, at 1.2 V discharge voltage.
23
Figure 5. Performance of AEM-CDI at the discharge voltage of 0 V in (a) the 1st cycle and (b) the 2nd cycle, compared with those at the discharge voltage of 1.2 V in (c) the 1st cycle and (d) the 2nd cycle. The first cycle has 5-min wetting, 1-h charging and 10-min discharging. The second cycle directly follows the first cycle with 1-h charging and 10-min discharging.
In addition, compared with no selectivity of removing Pb2+ in the presence of CEM (Figure 3a and b), AEM-CDI showed good selective removal of Pb2+ against Ca2+ and Mg2+ (Figure 5).
Thus, this very encouraging results urged us to further investigate the effects discharge voltage
and flow path length on the CDI performance. As summarized in Table 1, when the electrode 24
layer number increased from 2 to 4 and 8, the flow path length increased, resulting in the increased Pb2+ removal rates. For example, a removal rate of 58% for a 2-layer-electrode
AEM-CDI improved to 72% and 82% for 4-layer-electrde and 8-layer-electrode AEM-CDI cells,
respectively, during the first cycle.
Table 1. Charging and discharging efficiency of three AEM-CDI modules at different voltages. Pb2+
The discharge efficiency of AEM-CDI improved by enhancing the discharge voltages. For
example, in the 2-layer-electrode CDI, the discharge rates are 47%, 76%, and 78% at discharge
voltages of 0, 0.6 and 1.2 V, respectively, during the first cycle. The effect of flow path length on
the discharge rate is apparently complicated because a lower discharge efficiency was observed 25
with a longer flow path; e.g., at 0.6 V, the discharge efficiencies for the 2-layer-electrode and the
4-layer-electrode AEM-CDI cell were 76% and 47%, respectively. The possible reason is that an AEM-CDI with more electrode layers has a higher Pb2+ removal rate, but lower adsorbed Pb2+
per unit electrode at the same charge duration because double of the electrode layers did not
result in a double removal efficiency. In other words, less electrode layers in the AEM-CDI cell suggests adsorption of Pb2+ closer to saturation compared with a case with more electrode layers.
Electrochemical adsorption allows multiple layers (Helmholtz layer and Gouy-Chapman layer) of ions to be adsorbed on electrode surface.[37] In the inner layer (Helmholtz layer) Pb2+
possesses the relatively strong bonding with the functional groups (e.g., carboxyl) on the activated carbon surface; in contrast, the Pb2+ adsorbed in the outer layer (Gouy-Chapman layer)
is much easier to detach upon discharging. Thus, the AEM-CDI with fewer electrode layers had more Pb2+ adsorbed in the outer layers, thereby exhibiting higher discharge rates.
When competing with Ca2+ and Mg2+ at the same concentrations in AEM-CDI, selective removal of Pb2+ can be achieved as mentioned above. By converting concentration to selectivity through
26
Equation 4, an overall higher selectivity of removal of Pb2+ over Ca2+ and Mg2+ (Figure 6). This
is likely related to surface functional groups, as reported in the literature.[38, 39] Two trends can
be observed from Figure 6. First, lower selectivity was observed when longer flow path or more
electrodes were applied. More electrodes suggest a higher ion adsorption capacity, resulting in
more competing cations adsorbed at the given duration. According to Equation 4, an increased
removal efficiency of competing cation (
) can lead to a lower removal selectivity. It is
expected that with enough electrode layers, the removal efficiencies of Pb2+, Ca2+, and Mg2+ are
all 100%, resulting in a selectivity of 1; in other words, no selectivity can be observed in such a
case. The second trend can be observed in Figure 6 is that the removal selectivity of Pb2+ was improved in the 2nd cycle for different CDI modules, compared with that in the first cycle. This can be explained by the mechanism of selective removal of Pb2+ against Ca2+ and Mg2+. Ca2+ and Mg2+ possess higher mobility than Pb2+, and should be more easily adsorbed onto electrodes than Pb2+; however, the relatively strong affinity between Pb2+ and the functional groups (such as carboxyl) [31, 32] allows Pb2+ to replace the Ca2+ and Mg2+ earlier arriving on the electrode.
27
After initial saturation of the material in the 1st cycle, the replacement process increased the release of Ca2+ and Mg2+. The replacement process also suggests the potential enhancement of selective removal of Pb2+ against other cations by surface modification of activated carbon with
functional groups with stronger affinity, which is currently under investigation. Note that adsorbed Na+ was observed being replaced by Ca2+, [29] in which the mechanism may be different with our replacement of Ca2+/Mg2+ by Pb2+, though.
Note also that the removal
selectivity of Pb2+ against Ca2+ and Mg2+ at the second cycle for the 4-layer-electrode cell was
lower than that for the 8-layer-electrode cell; the reason is uncertain at this stage and needs
further investigation.
Figure 6. Selectivity on the Pb2+ removal among different AEM-CDI modules.
28
In the end, to further validate the selectivity of the used activated carbon, we tested the AEM-CDI module in more aggressive competition environment of Pb2+, Ca2+, and Mg2+ concentration at 1, 30, and 10 ppm, respectively, to simulate Pb2+ contaminated drinking water system. As shown in Figure S10, the Ca2+ and Mg2+ saturate quickly within 10 min; in contrast, at the end of 1-h charging process, Pb2+ can still maintain ~24% removal. The average removal efficiencies of Ca2+ and Mg2+ over the 1-h charging were 6.3% and 8.4%, respectively, much lower than that of Pb2+ (36%). This result also verifies our discussion on ion replacement
behavior on the electrode. It is important to mention that due to the ion replacement, the longer
the charging time is, the higher selectivity of the unit shows.
4. Conclusion
The impacts of AEM and CEM ion-exchange membranes on the CDI performance were investigated to achieve selective removal of Pb2+ competing with Ca2+ and Mg2+ under the same conditions, using activated carbon electrodes. Although CEM improves the Pb2+ removal efficiency, no Pb2+ is discharged because Pb2+ is trapped in the CEM while releasing protons.
29
Meanwhile, no selective removal of Pb2+ over other cations (e.g., Ca2+ and Mg2+) can be
achieved in the presence of CEM, which suggests the CEM is not suitable in aim to selectively remove Pb2+. In contrast, AEM-CDI was able to discharge Pb2+, and achieved higher Pb2+
removal efficiency and discharge efficiency, compared with the baseline CDI (membrane-free). More importantly, the AEM-CDI allows to selectively remove Pb2+ against other cations (such as Ca2+ and Mg2+), which does not occur in the presence of CEM. The mechanism of selective removal of Pb2+ against Ca2+ and Mg2+ is believed to be a replacement process, in which Ca2+ and Mg2+ possess higher mobility than Pb2+, and are more easily adsorbed onto electrodes than Pb2+ at the initial stage; however, the relatively strong affinity between Pb2+ and the functional groups (such as carboxyl) allows Pb2+ to replace the Ca2+ and Mg2+ earlier arriving on the
electrode. This mechanism suggests that the selectivity of AEM-CDI system can be improved by
modifying carbon materials with functional groups. Therefore, it is very promising to construct a
high-efficiency and low-cost CDI system to purify drinking water by selectively removing heavy
metal ions. This approach can be developed into an economical alternative for in-house or
municipal water treatment.
30
Acknowledgments
This project was supported by National Science Foundation Industry/University Cooperative
Research Center on Water Equipment & Policy located at University of Wisconsin-Milwaukee
(IIP-1540032) and Marquette University (IIP-1540010). The authors acknowledge Jayme R.
Kolarik (Pentair), Brennon Garthwait (Veolia Research & Innovation), Malcolm Kahn (Marmon
Water, Inc.), and Dawn Marsh (Baker Manufacturing Company, LLC) for their helpful advices
and discussions.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the online version.
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Figure Captions:
Figure 7. (a) Photo of prototype of CDI device with a peristaltic pump and (b) illustration of a two-layer-electrode CDI cell, in which water enters from the center of the bottom, passes through an anode and anion-exchange membrane (AEM), flows by a mesh, and then passes a CEM, a double-sided cathode, and another CEM through four pores at the corners; after a mesh again, the water flow comes out of a CEM, an anode and a plate from the top center. Figure 8. Ion concentrations and removal efficiencies of Pb2+, Ca2+, and Mg2+ by CDI baseline in (a) the first cycle (5-min wetting, 1-h charging and 10-min discharging) and (b) the second cycle (directly following the first cycle with 1-h charging and 10-min discharging) during charging and discharging. Note that concentrations at the third point during discharge processes were the average concentrations of all the discharge water except that taken for the first two discharge samples. The total collected amount for each ion was calculated based on the concentrations and the collected solution volumes of the first (10 mL), the second points (10 mL), and all the other solution for the third point (210 mL). Figure 9. Removal efficiency of (a) the 1st cycle (5-min wetting, 1-h charging and 10-min discharging) and (b) the 2nd cycle (directly following the first cycle with 1-h charging and 10-min discharging) for Pb2+, Ca2+, Mg2+ by CEM-AEM-CDI (in the presence of CEM and AEM) with 1 ppm solution and a discharge voltage of 0 V. Comparison of discharge performance of Pb2+ between CDI cells in the baseline, with CEM, and with CEM plus AEM at various discharge 40
voltages (c) for the 1st cycle and (d) the 2nd cycle. Figure 10. Schematic illustration of ion exchange between Pb2+ and H+ to explain the ineffectiveness of Pb2+ discharge. Figure 11. Performance of AEM-CDI at the discharge voltage of 0 V in (a) the 1st cycle and (b) the 2nd cycle, compared with those at the discharge voltage of 1.2 V in (c) the 1st cycle and (d) the 2nd cycle. The first cycle has 5-min wetting, 1-h charging and 10-min discharging. The second cycle directly follows the first cycle with 1-h charging and 10-min discharging. Figure 12. Selectivity on the Pb2+ removal among different AEM-CDI modules.
41
Highlights:
1. Selective removal of lead ions against calcium and magnesium ions is demonstrated.
2. CEM leads to a high lead removal rate but inability to discharge lead ions.
3. AEM-CDI shows improved selective removal of lead and enhanced lead discharge.
42