Accepted Manuscript Title: Individual and Competitive Removal of Heavy Metals Using Capacitive Deionization Author: Zhe Huang Lu Lu Zhenxiao Cai Zhiyong Jason Ren PII: DOI: Reference:
S0304-3894(15)30119-9 http://dx.doi.org/doi:10.1016/j.jhazmat.2015.09.064 HAZMAT 17137
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
14-4-2015 24-9-2015 28-9-2015
Please cite this article as: Zhe Huang, Lu Lu, Zhenxiao Cai, Zhiyong Jason Ren, Individual and Competitive Removal of Heavy Metals Using Capacitive Deionization, Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2015.09.064 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.
Individual and Competitive Removal of Heavy Metals Using Capacitive Deionization
Zhe Huanga, Lu Lua, Zhenxiao Caib, Zhiyong Jason Rena* [email protected]
Department of Civil, Environmental, and Architectural Engineering, University of Colorado Boulder, Boulder, CO 80309, USA b
Access Business Group LLC, 7575 Fulton Street East, Ada, MI, 49301 *Corresponding author. Tel.: (303) 492-4137; fax: (303) 492-7217.
Highlights - Capactive deionization can effectively remove cadmium, lead, and chromium from water - The removal rates of the individual metal ions vary due to adsorption difference - The interplay between different metal ions when co-present affects ion removal
Abstract This study presents the viability and preference of capacitive deionization (CDI) for removing different heavy metal ions in various conditions. The removal performance and mechanisms of three ions cadmium (Cd2+), lead (Pb2+) and chromium (Cr3+) were investigated individually and as a mixture under different applied voltages and ion concentrations. It was found that CDI could effectively remove these metals, and the performance was positively correlated with the applied voltage. When 1.2 V was applied into solution containing 0.5 mM individual ions, the Cd2+, Pb2+, and Cr3+ removal was 32%, 43%, and 52%, respectively, and the electrosorption played a bigger role in Cd2+ removal than for the other two ions. Interestingly, while the removal of Pb2+ and Cr3+ remained at a similar level of 46% in the mixture of three ions, the Cd2+ removal significantly decreased to 14%. Similar patterns were observed when 0.05 mM was used to simulate natural contaminated water condition, but the removal efficiencies were much higher, with the removal of Pb2+, Cr3+, and Cd2+ increased to 81%, 78%, and 42%, respectively. The low valence charge and lack of physical sorption of Cd2+ were believed to be the reason for the removal behavior, and advanced microscopic analysis showed clear deposits of metal ions on the cathode surface after operation. Keywords: capacitive deionization; heavy metal; electrosorption; physical sorption.
1. Introduction Heavy metal pollution in water bodies poses great health and environmental concerns due to the toxicity, pervasiveness, and persistence of different metal species [1-5]. Most metals are non-biodegradable and mobile in aqueous systems, which, if not removed before consumption, tend to accumulate in living tissues and cause diseases and disorders. Current heavy metal removal methods for drinking water or wastewater treatment span the realm of physical, chemical, and biological methods, including chemical precipitation , ion exchange using resins or zeolites [7, 8], adsorption on carbon materials and biomass [9, 10], membrane filtration, as well as recent electrochemical processes such as electrodeposition, electrocoagulation, and bioelectrochemical systems [11-13]. These methods all have their own advantages and limits . For example, chemical precipitation can be cost effective and easy to operate when treating wastewater with high metal concentrations at high pH, but it is not efficient in treating low-strength metal laden water, especially in the presence of complexing agents, with the large amount of sludge being an additional long-term environmental concern . Ion exchange and membrane filtration are widely used due to their high efficiency and low concentrate volume, but there are lingering concerns over material revitalization due to fouling by metallic or organic matter . The energy consumption can be another challenge for some technologies such as membrane filtration and traditional electrochemical processes [16-18], especially for remote and developing communities that are lacking reliable and easy access to electricity.
Capacitive deionization (CDI) has been studied as a low-cost and easy-to-operate process for heavy metal removal and water purification [19, 20]. CDI is an electrosorption process that uses a low electrical field (0.6-2.0 VDC) to remove ions from solution by adsorbing them onto the electric double layer (EDL) of two porous electrodes [21-23]. The negatively charged cathode attracts cations including Calcium (Ca2+), Magnesium (Mg2+), and heavy metal ions such as Chromium (Cr3+), Lead (Pb2+), and Cadmium (Cd2+), while the positively charged anode adsorbs anions such as Nitrate (NO3-), Sulfate (SO42-), and Arsenate (AsO43-), resulting in water deionization and purification. CDI technology is energy efficient and cost competitive compared to membrane filtration process such as reverse osmosis (RO) at a low TDS concentration range (<10 g/L) , which can be especially beneficial for saline groundwater treatment for rural or remote communities. Unlike ion exchanges that often use hydrochloric acid or sulfuric acid for resin regeneration, CDI uses electrical regeneration through zeroing or reversing the cell voltage. Studies have shown that CDI electrodes can be regenerated effectively by short-circuiting , or applying reverser polarity , which enables easy secondary treatment of heavy metals and electrode materials reuse with minimal fouling and scaling problems .
Previous studies show that electrosorption processes can effectively remove individual metals such as chromium (Cr3+) [24, 25], copper (Cu2+) [18, 26], nickel (Ni2+) , and iron  from aqueous solution and recover valuable metals during simple regeneration, but few studies have reported the performance of CDI in removing multiple metal ions from the same solution. Since 5
it is common for multiple metal ions to co-exist in groundwater and wastewater streams [28, 29], it is important to understand and assess the removal behavior of these ions in CDI. This study investigated the viability and preference of CDI in removing cadmium (Cd2+), lead (Pb2+) and chromium (Cr3+) ions at low concentrations. Batch tests were conducted to identify the removal of individual ions under different applied voltages and metal concentrations, and further characterizations were conducted to understand the removal sequence and interactions when all three ions were present in the solution.
2.1 CDI electrode assembly
The CDI electrode assembly consisted of activated carbon cloth (ACC, FM70) electrodes, current collectors made of stainless steel mesh, plastic mesh separators, and titanium wires. The ACC electrodes were 1.0 inch by 1.0 inch squares and separated by four layers of non-conductive plastics mesh to prevent short-circuiting. A conductive stainless steel mesh was attached to the outside of each ACC electrode assembly as a current collector (Fig. 1) . The ACC was soaked in DI water at 60℃ for 5 minutes before assembling. Then, a DI rinse was applied until the conductivity in the rinsed water reached zero, to ensure no initial ion leakage from the ACC. The weight of each ACC electrode was 0.12 gram, and the distance between the two electrodes was 2.7 mm with an electrical resistivity of 52 Ohms/cm. Titanium wires attached 6
to the stainless steel meshes were connected to a direct current power supply (EXTECH 382200). The electrode assembly was immersed into the CDI reactor with solution containing different metal ions.
2.2 Individual and competitive removal experiment setup
To investigate the electrosorption of individual heavy metal ions on ACC electrodes, initial batch experiments were conducted with 60ml 0.5mM Cd(NO3)2, Pb(NO3)2, and Cr(NO3)3 (≥99%, SIGMA-ALDRICH), respectively, without any buffer solution. The initial pHs of the three solutions were 5.5, 5.0 and 3.8 for Cd(NO3)2, Pb(NO3)2 and Cr(NO3)3, respectively. For comparison, additional tests were conducted in pH controlled condition by adjusting the initial pH to 3.80 using 1M nitric acid. A range of voltages between 0V-1.2V, namely 0V, 0.6V, 1.0V and 1.2V, were then applied to the CDI electrode assembly to investigate the impacts of voltage on metal removal. In each experiment, the designated voltage was applied to pre-charge the CDI reactor for 10 minutes before deionization experiment to ensure a steady state was reached . The pH was measured at the beginning and end of each experiment to ensure the presence of free metals.
To characterize the competitive electrosorption of three metal ions, 60 ml solution containing a mixture of Pb(NO3)2, Cd(NO3)2 and Cr(NO3)3 was studied under the same applied voltages as in the individual removal experiments. Two different concentrations, 0.05 mM and 0.5 mM of each 7
ion,ion were tested to investigate the concentration effects and to compare with the individual adsorption results. The initial pH of the two mixture solutions were 4.43 and 3.68, respectively, and the ending pH of the solutions were 4.5-4.7 and 3.7-3.9, respectively.
2.3 Conductivity and ion concentration analysis A total of 12 reactors were operated, and for each testing, 20 ml solution was collected from each reactor every 10 minutes (from 10, 20, till 120min). In all experiments, the heavy metal removal reached to the maximum level within 120 minutes, which was defined as three same consecutive readings. Conductivity was continuously monitored at 5-minute intervals; and pH was measured at the beginning and end of each experiment. The initial conductivity was 125.5 us/cm, 143 us/cm, and 250 us/cm for the individual removal experiments, and 59.3us/cm (0.05mM) and 500us/cm (0.5mM) for the two mixtures. All the experiments were duplicated and results were averaged.
2.4 Removal rate calculations
The applied voltages were monitored using a digital multi-meter (AMPROBE, 15XP-B). Solution conductivities were measured by conductivity meter (HQ330d multi, HACH Co., USA). Heavy metal ion concentrations (Pb2+, Cd2+ and Cr3+) were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES). The pH of each sample was adjusted below 8
1.5 using nitric acid concentrate (trace metal grade) before ion analysis. The metal removal was determined as removal % = (C0-Ct)/C0 × 100%, where C0 and Ct (mM/L) are the initial and residual concentration, respectively. The slopes of the removal curves can be calculated as mM/min = (C0-Ct=100)/100, where C0 and Ct=100 (mM) are the initial and concentration after 100min operation, respectively.
Both anode and cathode were dissembled and dried overnight after CDI operation, and then surface structure was analyzed by scanning electron microscopy (SEM). The elemental composition change on the electrode surface was analyzed by energy-dispersive spectroscopy (EDS). Details of the SEM-EDS experimental protocol can be found in our previous studies [31, 32].
3. Results and discussion
3.1 Individual heavy metal removal
Fig. 2 shows the individual metal ion removal under different applied voltages measured through conductivity and concentration changes. With initial pH control, overall the CDI electrosorption improved the removal of all heavy metal ions as compared to pure physical adsorption, and the removal efficiency was positively correlated with the applied voltage without significant variation (Fig. 2A). For example, when an external voltage of 1.2 V was applied, the 9
average removal of Cd2+ approached 31.85 % (concentration), which was approximately 8 times the removal rate in the control reactor (0 V) with only physical adsorption (4.20 %) (Fig. 2B1). The removal efficiencies under 0.6V and 1.0V were 11.28 % and 27.78 %, respectively, which was also positively correlated with the applied electrical forces. In comparison, the physical adsorption of Pb2+ and Cr3+ played a more important role than it did for Cd2+. As shown in Fig. 2B2 and 2B3, the average concentration removal of Pb2+ was 29.59 % under 0 V control, and it improved to 42.62 % under 1.2 V. Similarly, the Cr3+ removal under 1.2 V was 52.48 %, which was 12.6% higher than it under 0 V (41.52 %). The pH values slightly increased from 3.8 to 3.9-4.1 after the tests. The removal performance was also evaluated without adjusting the initial pH, which is similar to contaminated groundwater condition. For the same adsorption tests conducted, higher removals for Pb2+ and Cd2+ were observed. On average, due to the higher pH (5.0 and 5.8) as compared with 3.8, the removal for Pb2+ and Cd2+ increased by 7-10% in both cases and the improved removal appeared to be more significant when the applied voltage was lower (Fig S1). The overall trend was in agreement with a previous study where a slight decrease was seen when pH was reduced from ca. 5 to 3 . The weaker physical adsorption of Cd2+ in this study is in agreement with previous adsorption studies using materials such as multiwalled carbon nanotubes , natural sepiolite  and coconut husk . It was proposed that physical uptake of metal ions is not a simple ion-exchange process but a process involving complex formation and surface adsorption 10
mechanisms. Cr3+ is strongly favorable due to its higher valence and affinity to displace protons from surface hydroxyls , while Cd2+ is less favorable due to the bigger hydrated radius  (Table 1). In addition, the pH condition affects the removal of different metal ions due to its current carrier status and its effects on OH− complex formation. Different metals have stability constants (pKa) whereby smaller pKa values indicate stronger metal ligands to OH- and lower OH- stability . The pKa1 of Cr3+ is the lowest (4.00) compared with Pb2+ (pKa1 = 7.71) and Cd2+ (pKa1 =10.08), indicating that the OH- complex concentrations were different in different conditions, but such concentrations were not directly monitored in this study.  Another interesting finding in the individual adsorption study is that the overall removal rate of Cd2+ is much slower than Pb2+ and Cr3+, which can be significant when considering competitive removal of all ions in one solution. As shown in the slopes of the removal curves in Fig. 2B under 1.2V, the removal of Cd2+ was a gradual process, with an average removal rate of 1.33 uM/min. In contrast, most removal of Cr3+ occurred within the first 50 mins, and the average removal rate was 2.6 uM/min. The average removal rate of Pb2+ was 1.76 uM /min, ranged in between Cd2+ and Cr3+. Such difference could be attributable to two reasons. First, Cd2+ removal and Pb2+ and Cr3+ removal were achieved through different mechanisms. Our results indicated that Cd2+ was removed primarily through the relatively slower electrosorption while Pb2+ and Cr3+ were primarily removed through the relatively faster physical adsorption. The rate difference between the slower electrosorption and the faster physical adsorption observed in this study agrees with the experiment results in previous studies . Secondly, 11
proton as a current carrier directly affects the kinetics of CDI, reflected by the pH changes, but the impact of pH in CDI has yet been fully characterized, which is an area worth exploring in future studies.
3.2 Competitive heavy metal removal
Since most contaminated water contains multiple metal ions, the competitive removal study characterized the removal relationships among the three metals under two different conditions. Fig 3 shows the removal of the three metal ions under 0.5 mM each, which is the same concentration used in the individual study. Similar positive correlation between applied voltage and removal performance was observed as increased voltage improved the overall removal presented as both conductivity and concentration changes. However, significant changes in the ion removal were observed compared to individual removal experiments. The removal of Pb2+ and Cr3+ only slightly decreased compared with the individual studies: when 1.2 V was applied in the mixed solution, the overall removal of Pb2+ and Cr3+ was 45.53 % and 46.22 %, respectively, which were similar to the individual ion study. In comparison, the removal of Cd2+ was largely inhibited in the presence of Pb2+ and Cr3+. When the applied voltage was 1.2 V, the removal of Cd2+ decreased from 31.85 % in individual removal experiments to 13.57 % in competitive removal experiments (Fig. 3 B1-B3). Similar decreases were also observed under 0.6 V and 1.0V, with the removal percentage of Cd2+ decreased by 5.03 % and 16.31 %, respectively. 12
Similar removal patterns were observed among the three metal ions when lower concentration (0.05mmol/L) was used to simulate natural contaminated water condition. However, the removal efficiency was much higher (Fig. 4). The removal of Pb2+, and Cr3+ increased to 72.41−80.82 % and 76.87−78.36 %, respectively, under applied voltages of 0.6−1.2 V, and even though similar inhibition was found for Cd2+, its removal still improved from 21.97 % to 41.87 %. This significant improvement across the ions is attributed to the higher ratio between CDI electrode surface area and the lower ion concentration, which demonstrate the CDI is able to remove low concentration ions effectively [20,21]. Several factors are hypothesized to be involved in the less preferential adsorption of Cd2+ in this study. First of all, since Cd2+ removal depends more largely on electrosorption as indicated in the individual ion study, the low rate of electrosorption may have caused the removal of Cd2+ to be outcompeted by other ions which rely more largely on the faster physical adsorption. Additionally, in electrosorption, hydraulic radius (Pb2+
3.3 Metal Adsorption Characterization
Fig. 5 shows the surface morphology of the anode (Cd-A, Pb-A, Cr-A) and cathode (Cd-C, Pb-C, Cr-C) after individual metal removal studies. While the anode surface kept smooth and 13
clear, the cathode surface can be seen accumulated with dotted metal crystals. EDS analysis (Fig 6) shows that significant amounts of cadmium, lead or chromium element was detected on the surface of each used cathode, confirming the deposition of the metals. No such metals were detected on the used anode surface and new electrodes.
4. Conclusions This study demonstrates that CDI can be an effective process to remove heavy metals from contaminated water. The removal performance was positively correlated with the applied voltage, which could increase two-fold in the case of chromium when the applied voltage was increased from 0.6V to 1.2V. In addition, CDI was most effective at chromium removal and least effective at cadmium removal in individual removal tests. A similar trend was seen in mixed removal tests, wherein chromium had higher removal percentage than lead and cadmium (in that order). However, in mixed removal tests, the adsorption of Cd2+ was inhibited by the presence of Pb2+ and Cr3+. Acknowledgements We appreciate the financial support from the Access Business Group International, LLC.
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Fig. 1. The schematic of CDI assembly for metal removal.
Fig. 2. (A) Removal correlation with applied voltages for the three metal ions. (B) Time-course removal of Cd2+ (B1), Pb2+ (B2), and Cr3+(B3) under different voltages in terms of ion concentration during the individual removal study (Initial pH=3.8).
Fig. 3. Overall conductivity (A) and individual metal concentration changes (B1-B3) (from 0.5 mM) in the competitive removal study. (A) Overall solution conductivity drops under different voltages. (B) Concentration change and percentage removal of Pb2+, Cr3+, and Cd2+ under different applied voltages.
. Fig. 4. Overall conductivity (A) and individual metal concentration changes (B1-B3) (from 0.05 22
mM) in the competitive removal study. (A) Overall solution conductivity drops under different voltages. (B) Concentration change and percentage removal of Pb2+, Cr3+, and Cd2+ under different applied voltages.
Fig. 5. ESEM images of the used anode (left) and cathode (right) surface after CDI operation. While anode surface remains clear, cathode surface was deposited with metal crystals.
Fig. 6. EDS spectrum of the used anode (left) and cathode (right) after CDI operation.
Tables Table 1 listed the charge size and hydraulic radius of each heavy metal ions.” Ions Charge Size Hydrated Radius(Å) 2+ Cd +2 4.26 2+ Pb +2 4.01 3+ Cr +3 4.61