Cadmium Removal from Aqueous Solution by a Deionization

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Cadmium Removal from Aqueous Solution by Deionization Supercapacitor with Birnessite Electrode Qichuan Peng, Lihu Liu, Yao Luo, Yashan Zhang, Wen-Feng Tan, Fan Liu, Steven L. Suib, and Guohong Qiu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12224 • Publication Date (Web): 30 Nov 2016 Downloaded from http://pubs.acs.org on December 6, 2016

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ACS Applied Materials & Interfaces

Cadmium Removal from Aqueous Solution by Deionization Supercapacitor with Birnessite Electrode Qichuan Peng,† Lihu Liu,† Yao Luo,† Yashan Zhang,‡ Wenfeng Tan,† Fan Liu,† Steven L. Suib,‡ Guohong Qiu*,† †

Key Laboratory of Arable Land Conservation (Middle and Lower Reaches of Yangtse River), Ministry

of Agriculture, College of Resources and Environment, Huazhong Agricultural University, Wuhan 430070, China ‡

Department of Chemistry, University of Connecticut, 55 North Eagleville Road, Storrs, Connecticut,

06269-3060, USA

ABSTRACT: Birnessite is widely used as an excellent adsorbent for heavy metal ions and as active electrode materials for supercapacitors. The occurrence of redox reactions of manganese oxides is usually accompanied by the intercalation-deintercalation of cations during the charge-discharge processes of supercapacitors. In this study, based on the charge-discharge principle of supercapacitor and excellent adsorption properties of birnessite, birnessite-based electrode was used to remove Cd2+ from aqueous solutions. The Cd2+ removal mechanism and the influences of birnessite loading and pH on the removal performance were investigated. The results showed that Cd2+ was adsorbed on the surfaces and interlayers of birnessite, and the maximum electrosorption capacity of birnessite for Cd2+ was about 900.7 mg g-1 (8.01 mmol g-1), which was significantly higher than the adsorption isotherm capacity of birnessite (125.8 mg g-1). The electrosorption specific capacity of birnessite for Cd2+ increased with an increase in initial Cd2+ concentration, and decreased with an increase in the loading of active birnessite. In the pH range of 3.0–6.0, the 1

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electrosorption capacity increased at first with an increase in pH and then reached equilibrium above pH 4.0. This work provides a new method for the highly efficient adsorption of Cd2+ from polluted wastewater. Keywords: Electrochemical removal; Deionization supercapacitor; Birnessite; Galvanostatic charge-discharge; Cd2+

1. INTRODUCTION In recent years, the shortage of clean drinking water sources has become a global issue mostly due to water pollution caused by intensive human activities. Heavy metals ions, as major components of water pollutants, accumulated in vivo and difficult to degrade, cause increasing risks to the ecological environment and human health.1,2 In particular, cadmium, released from industrial processes such as battery production, dyes and metallurgy,3 if enriched in human body, can cause skeletal deformities, chronic pulmonary problems, renal degradation, muscular cramps, diarrhea and erythrocyte destruction.4,5 The guideline value for the discharge of cadmium in water recommended by World Health Organization (WHO) is 0.005 mg L-1.5 Hence, it is highly necessary to strictly control the concentration of cadmium in discharged wastewater. Currently, the common methods for Cd2+ removal from water bodies include chemical precipitation, membrane filtration, ion-exchange, and adsorption etc.5,6 For instance, Ca(OH)2, which is used as a precipitant, can remove 99.67% of Cd2+ with an initial Cd2+ concentration of 150 mg L-1.7 Under high pressure conditions, polyamide membrane can reduce the concentration of Cd2+ from 200 mg L-1 to 2 mg L-1 by reverse osmosis technology.8 Synthetic zeolites can remove 100% of Cd2+ with an initial Cd2+ concentration of 100 mg L-1, but appropriate pretreatment 2


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systems such as the removal of suspended solids from the wastewater are required before ion exchange.9 The adsorption capacity of activated carbon cloth treated by HNO3 for Cd2+ is about 146 mg g-1 (1.30 mmol g-1).10 However, the application of most of these methods is limited due to their high cost, complex operation or secondary pollution. As a salt ion removal technology, capacitive deionization has attracted enormous attention in recent years due to the advantages of low energy consumption, simple operation and environmental friendliness.11–14 The extensively used electrode for deionization capacitor are carbon materials including porous carbon,13 grapheme,15 carbon aerogels,16 and nitrogen-doped carbon materials.17,18 Under a direct voltage, an electric double layer can be formed on the surface of the electrodes to store oppositely charged ions.14–20 This technique is also used to remove heavy metal ions. For example, the removal efficiencies could reach 81%, 78%, and 42% for 0.05 mmol L-1 Pb2+, Cr3+, and Cd2+, respectively, using active carbon electrode under a cell voltage of 1.2 V.21 MnO2/carbon fiber composite was prepared as an electrosorptive electrode with Cu2+ eletrodeposition capacity of 172.88 mg g-1 (2.72 mmol g-1).22 The removal efficiencies reached 67% and 58% for As(III) and As(V), respectively, using flexible carbon fabric supported magnetite multiwalled carbon nanotube nanocomposite by cyclic voltammetry.23 The principle of capacitive deionization is electrostatic adsorption, which is similar to that of electric double-layer capacitors. The energy storage of pseudocapacitors relies on redox reactions accompanied by the intercalation-deintercalation of alkali metal ions and H+ on electrode materials.24 As for electrochemical supercapacitors, besides double-layer capacitance, pseudocapacitance also contributes much to the specific capacitance during the charge-discharge processes.24 These processes can also facilitate the adsorption of other

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metal ions, such as heavy metal ions, on electrode surface, which has received rather limited research attention. In the field of electrochemical energy storage, some transition metal compounds, such as Fe3O4, MnO2, V2O5, SnO2, and MoS2, have been investigated as common pseudocapacitive electrode materials.25–27 Manganese oxide has attracted enormous attention due to its characteristics of high specific capacitance, abundant resources, low cost and environmental friendliness. Birnessite, the most common manganese oxide mineral, is composed of layers of edge-sharing MnO6 octahedra separated by layers of water molecules and alkali metal ions.28 A previous research has indicated that Cd2+ adsorbed on birnessite occupies sites the above and below octahedral vacancies at the birnessite interlayers, and the adsorption capacity of Cd2+ is 148 mg g-1.29 Manganese dioxide nanosheets were prepared from tetramethylammonium hydroxide, H2O2, and MnCl2·4H2O, and have superior adsorption capacity for Cd2+ (about 348.1 mg g-1).30 In this study, based on its characteristics of excellent capacitance and adsorption capacity for Cd2+, birnessite was used as a pseudocapacitive electrode materials to remove Cd2+ from aqueous solutions by the galvanostatic charge-discharge method. The electrochemical performance of birnessite was evaluated by cyclic voltammetry (CV). The crystal structure, morphology and relative contents of Mn with different valence states in birnessite after charge-discharge tests were characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM) and X-ray absorption spectroscopy (XPS), respectively. The mechanism of electrochemical sorption of Cd2+ was further explored and discussed. The influence of initial pH of electrolyte and the loading of electrode materials on the electrosorption capacity for Cd2+ was clarified. This work provides a new method for the efficient adsorption of Cd2+ from wastewater. 4


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2. MATERIALS AND METHODS 2.1. Preparation of Birnessite. Birnessite was prepared from the reduction of boiling solution of potassium permanganate by concentrated hydrochloric acid.31 A 300 mL solution of 0.667 mol L-1 potassium permanganate was stirred and heated at 100 oC, and then 45 mL 6 mol L-1 hydrochloric acid was added dropwise to the boiling solution with vigorous stirring at a speed of 0.7 mL min-1 with a constant flow pump. After 30 min reaction, the precipitate was aged for 12 h at 60 oC. The mineral was washed with deionized water until the conductivity of the filtrate was less than 20 µS cm-1, and subsequently washed three times with ethanol. The product was dried in an oven at 40 oC. The synthetic mineral was ground in an agate mortar and sieved with 100-mesh sieve.

2.2. Electrochemical Removal of Cd2+. Electrochemical experiments were carried out at room temperature in a three-electrode system, using a saturated calomel electrode as reference electrode, 1.5×3.0 cm2 carbon fabric as counter electrode. The volume of electrolyte was 30 mL, and 0.1 mol L-1 Na2SO4 was used as background electrolyte. The working electrodes were prepared by mixing 75 wt% of the as-prepared birnessite, 15 wt% of acetylene black (AB), and 10 wt% of polyvinylidene fluoride (PVDF) slurry. The mixtures of the as-prepared birnessite and AB were ground for 30 min, and then transferred to a 5 mL centrifuge tube in which PVDF with N-methyl-2-pyrrolidone (NMP) as dispersant was added. After ultrasonic dispersion, the slurry was uniformly coated on carbon fabric and dried in a vacuum drying oven at 40 oC for 12 h. The loading of electrode materials (birnessite, AB, and PVDF) was controlled to 5 mg. CdSO4 solutions with the initial concentrations of 0, 200, 400, 600, 800, 1000, 1400, 1800, and 2200 mg L-1 were 5



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respectively used as electrolytes and the pH of the electrolyte was adjusted to 6.0 with 0.1 mol L-1 H2SO4 or NaOH. 50 cycles of galvanostatic charge-discharge tests (battery testing system, Shenzhen Neware Electronic Ltd., China) were carried out in electrolytes with different initial concentrations of Cd2+ to study the electrosorption capacity of the as-prepared birnessite for Cd2+. To simply compare the Cd2+ ion adsorption rate, the 1, 4, 7, 10, 20, 30, 40, and 50 cycles of galvanostatic charge-discharge tests were conducted and the corresponding electrosorption capacities were recorded in 200 and 2200 mg L-1 Cd2+ electrolytes. The potential window was set as 0–0.9 V (vs SCE) and the current density was 0.1 A g-1. The electrosorption capacity was calculated using formula (1):

Qe =

(C0 − Ce )V m

(1)

where Qe (mg g-1) is the electrosorption capacity, C0 (mg L-1) is the initial concentration of Cd2+, Ce (mg L-1) is the concentration of Cd2+ after charge-discharge test, V (mL) is the volume of electrolyte and m (mg) is the mass of active birnessite on the electrode. Specific capacitance C (F g-1) of birnessite was calculated using formula (2):

C=

It m ∆V

(2)

where I (A) is the discharge current, t (s) is the discharge time, m (mg) is the mass of birnessite on the electrode and ∆V (V) is the potential window.

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In order to study the effect of loading of active birnessite on electrosorption capacity for Cd2+, 50 cycles of galvanostatic charge-discharge tests were carried out with different masses of electrode materials: 0, 5, 10, 15, and 20 mg in 200 mg L-1 Cd2+ solution with an initial pH of 6.0. The effects of initial pH values of electrolyte on electrosorption capacity for Cd2+ were also investigated. Galvanostatic charge-discharge tests of 50 cycles were carried out with 5 mg electrode materials in 200 mg L-1 Cd2+ solution with different values of initial pH. The initial pH values of electrolyte were respectively adjusted to 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, and 6.0 with 0.1 mol L-1 H2SO4 or NaOH. The above experiments were performed three times and the mean was recorded. The cyclic electrochemical adsorption experiments of birnessite electrode were conducted at pH 6.0. After 50 cycles of charge-discharge tests in the electrolytes containing different concentrations of Cd2+ (200 and 1000 mg L-1), the working electrodes were respectively transferred into 0.1 mol L-1 Na2SO4 electrolyte for 50 cycles of charge-discharge activation. After activation and regeneration, the corresponding birnessite electrode was then transferred into the mixed solutions of 0.1 mol L-1 Na2SO4 and Cd2+ with the same concentrations (200 and 1000 mg L-1) to test the electrochemical adsorption performance for the second time. To compare the electrochemical adsorption capacity, constant cell voltages deionization was also performed at 0, 0.3, 0.6, and 0.9 V in the electrolyte containing 200 mg L-1 Cd2+ for 12 h. Carbon fabric and the as-prepared birnessite were used as positive and negative electrodes, respectively. The potential of birnessite electrode was determined using a saturated calomel electrode during the electrochemical processes. In order to explore the adsorption sites of Cd2+ on birnessite, the sorption isotherms experiments for Cd2+ were performed. The synthetic birnessite was suspended in 0.1 mol L-1 Na2SO4 solution to 7



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obtain 5 g L-1 suspension. The suspension was equilibrated for a few days, during which the pH of the suspension was adjusted to 6.0±0.05 with 0.1 mol L-1 H2SO4 or NaOH. Cd2+ solutions were prepared using 15 mmol L-1 CdSO4 with 0.1 mol L-1 Na2SO4. Sorption isotherm experiments were performed by adding 0–8 mL Cd2+ solution to 5 mL mineral suspension in a series of 50 mL polypropylene bottles, respectively. The total volume of solution in each bottle was made up to 15 mL by adding 0.1 mol L-1 Na2SO4. The final suspensions were shaken at 25±1 oC and 200 r min-1. The pH of the suspensions was adjusted to 6.0±0.05 during the equilibration period. After 24 h of equilibration, the suspensions were filtered by 0.22-µm microporous membrane. The sorption isotherm experiment was performed three times and the mean was recorded. All of the above chemical reagents were of analytical grade and were purchased from China National Medicine Group Shanghai Chemical Reagent Company. All aqueous solutions at different concentrations were freshly prepared with ultrapure water (18 MΩ cm resistivity). The carbon fabric (HCP020) was supplied by Shanghai Hesen Electric Co., Ltd.

2.3. Characterization and Analysis. The crystal structure of the as-prepared birnessite was characterized by X-ray powder diffraction (XRD, Bruker D8 Advance diffractometer with Cu Kα) at a tube voltage of 40 kV, tube current of 40 mA, scanning rate of 1° min-1 and step intervals of 0.02°. Field emission scanning electron microscope (FESEM, Hitachi, SU8000) was used to observe the morphology of the product. The functional groups of synthetic birnessite were characterized by Fourier transform infrared spectroscopy (FTIR, Nicolet 8700). The average oxidation state of manganese in birnessite was determined by the oxalic acid-permanganate back-titration method.32 The BET specific surface area and pore size distribution of the as-prepared 8


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samples were measured with a fully automatic surface area analyzer (Micromeritics ASAP2020) using nitrogen isothermal adsorption-desorption method. About 0.2 g of sample was degassed at 110 °C for 12 h. X-ray photoelectron spectroscopy (XPS, Multilab 2000) with a monochromatized Mg Kα X-ray source (1253.6 eV) was used to analyze the relative contents of Mn with different valence states in the as-prepared birnessite, using C1s peak (284.6 eV) as charge referencing. The chemical composition of the product was determined by the following method: 0.01 g sample and 0.1 g hydroxylamine hydrochloride were accurately weighed, and after dissolving in deionized water, the volume of the solution was fixed to 250 mL with deionized water. The contents of Mn and K in the dilute solution were measured by atomic absorption spectroscopy (AAS, Varina AAS240FS) and flame photometer (Sherwood Model 410). The crystal water content of the sample was calculated by thermal gravimetric analysis (TG, Diamond TG, PerkinElmer Instruments) at a heating rate of 10

o

C min-1 under nitrogen atmosphere. Before and after 50 cycles of

charge-discharge tests and constant cell voltages deionization experiment, the concentrations of Cd2+ and released Mn2+ in liquid phase were measured by AAS. The cyclic voltammetry (CV) tests of the as-prepared birnessite in Cd2+ electrolytes were carried out on the electrochemical workstation (CHI660E, Shanghai Chenhua Instrument Co. Ltd., China) at a scan rate of 0.5 mV s-1 in a potential window of 0–0.9 V (vs. SCE). The influences of concentration, pH, and the mass of electrode materials on the electrochemical behaviors of Cd2+ were further investigated.

3. RESULTS

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3.1. Preparation and Characterization of Birnessite. Figure 1a shows the XRD pattern of the as-prepared product, and the six main peaks, which respectively occurred at 0.713, 0.355, 0.246, 0.177, 0.142, and 0.122 nm, corresponded to the (001), (002), (100), (004), (110), and (202) crystal planes of birnessite (JCPDS No. 86-0666). According to the d value of (001) crystal plane (d001), the average crystal size was 7.55 nm as calculated by the Scherrer formula.33 There was no other detectable peak in the XRD patterns, indicating that single-phase birnessite was obtained. The FESEM image of the synthetic birnessite indicated that uniform flower-like microspheres (with diameters of about 300 nm) composed of lamellar plates were formed (Figure 1b), which was consistent with the typical morphology of hexagonal birnessite.34 The product was further characterized by FTIR spectroscopy (Figure S1). The two bands at 446 and 518 cm-1 could be attributed to the Mn-O lattice vibrations of birnessite, and the dominant absorption peaks at 1645 and 3391 cm-1 were attributed to the stretching and bending vibrations of adsorbed water and crystalline water.35 The results of FTIR further showed that the synthesized product was single-phase birnessite. The manganese average oxidation state (AOS) was determined to be 3.79 by the oxalic acid-permanganate back-titration method. The total Mn and K contents of the synthetic birnessite were 53.72% and 9.92%, respectively. The content of crystal water in the product was 13.22% as measured by TGA (Figure S2). According to above analyses, the chemical composition of the birnessite was calculated to be K0.26MnO2.02·0.75H2O. Figure S3 shows the N2 adsorption-desorption isotherms and pore size distributions of the birnessite. As can be observed, the isotherms followed typical IUPAC type-V adsorption isotherm patterns with the presence of a H3-type hysteresis loop, indicating that the birnessite had mesoporous structures.36 The BET specific surface area and pore volume of the sample were 14.2 10


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m2 g-1 and 0.067 cm3 g-1, respectively. The average pore diameter was calculated to be 3.585 nm by the Barrett-Joyner-Halenda (BJH) method (Figure S3b).

3.2. Electrochemical Removal of Cd2+. Charge-discharge tests were carried out with different initial concentrations of Cd2+ in electrolyte ranging from 0 to 2200 mg L-1 with 5 mg electrode material. As shown in Figure 2a, when the concentration of Cd2+ was 0 mg L-1, the specific capacitance was kept at about 167.9 F g-1. This result indicated that the capacitive property of the birnessite was stable in 0.1 mol L-1 Na2SO4 solution, which was consistent with the results of previous studies.37,38 After Cd2+ was added to the electrolyte, the initial specific capacitance was obviously higher than that in 0.1 mol L-1 Na2SO4 solution and gradually increased with the increasing concentration of Cd2+. Meanwhile, with the increase of charge-discharge cycles, decay of specific capacitance was observed, and the higher the Cd2+ concentration was, the more significant the decay of specific capacitance would be. The results suggested that the electrochemical stability of birnessite decreased in the electrolyte containing Cd2+. The electrochemical behaviors of Cd2+ on the birnessite electrode surface were confirmed by CV. Figure 2b shows the CV curves with 5 mg electrode materials in different concentrations of Cd2+ at a scan rate of 0.5 mV s-1. When the concentration of Cd2+ was 0 mg L-1 in the electrolyte, a couple of symmetrical redox peaks were observed in the CV curves. The anodic current peak at 0.55 V corresponded to the oxidation of Mn(III) to Mn(IV) while the cathodic peak at 0.35 V corresponded to the reduction of Mn(IV) to Mn(III).39 In addition, a rectangular shape of the CV curve was observed, suggesting excellent reversibility and capacitance properties of the birnessite electrode in 0.1 mol L-1 Na2SO4 solution. When there was Cd2+ in the electrolyte, the closed area of CV curves 11



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increased with the increasing concentration of Cd2+, indicating that the intercalation of Cd2+ could increase the specific capacitance of birnessite, which was consistent with the results of galvanostatic charge-discharge tests (Figure 2a). However, the CV curves deviated from a regular rectangular shape in the Cd2+ electrolyte compared with single Na2SO4 electrolyte, and the degree of deviation increased with the increase of Cd2+ concentration. These results confirmed that the intercalation of Cd2+ in birnessite reduced the reversibility of the electrode. In order to determine the electrosorption capacity of birnessite for Cd2+, the concentration of Cd2+ in the bulk electrolyte was further measured. After 50 cycles of charge-discharge tests in the electrolyte with different initial concentrations of Cd2+, the electrosorption capacity of Cd2+ was measured as shown in Figure 3a. When the concentration of Cd2+ was 200 mg L-1, the electrosorption capacity of Cd2+ was 196.5 mg g-1. The electrosorption capacity increased at first with the increase of initial Cd2+ concentrations and then reached equilibrium in 1400 mg L-1 Cd2+. The maximum electrosorption capacity reached as high as 900.7 mg g-1 when Cd2+ concentration was increased to 2200 mg L-1. However, the removal efficiency decreased with the increase of Cd2+ concentration in the electrolyte (Figure S4). The removal efficiency reached the maximum of 12.28% with the initial concentration of Cd2+ at 200 mg L-1, and it decreased to 5.16% at the Cd2+ concentration of 2200 mg L-1. In the initial stage, the adsorption capacity increased with an increase in the cycles of charge-discharge. Low concentrations of Cd2+ facilitated the rapid electrochemical adsorption. For example, the electrosorption capacity reached equilibrium at 10 and 20 cycles for the electrolyte at the Cd2+ concentrations of 200 and 2200 mg L-1, respectively (Figure 3b). In order to analyze the effect of Cd2+ enrichment on the crystal structure of birnessite, the wet working electrode was characterized using XRD (Figure 4). The birnessite was not converted into 12


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other crystal phases. The d001 after charge-discharge tests gradually decreased with increasing Cd2+ concentration in the electrolyte, indicating a decrease of crystallinity and partial dissolution of birnessite. The d110 value of initial birnessite was 0.1419 nm and the values after charge-discharge tests in 200, 600, and 1000 mg L-1 Cd2+ solutions increased to 0.1428, 0.1424, and 0.1429 nm, respectively, suggesting that the Mn AOS of the birnessite decreased after charge-discharge tests.40 The chemical components of the pristine birnessite and birnessite-based electrodes after charge-discharge tests in 1000 mg L-1 Cd2+ electrolyte were further characterized by XPS. XPS broad scans of the initial birnessite sample showed that Mn, O, and K were present in the birnessite structure (Figure 5a). Cd was observed in the XPS broad scans and the Cd3d spectrum (Figure 5a, b) after charge-discharge tests, indicating the presence of Cd on the surface of electrode materials. In order to infer the relative contents of the three species Mn(II), Mn(III), and Mn(IV), the multiplet fitting of the Mn 2p3/2 peaks was conducted as shown in Figure 5c, d. After charge-discharge tests in 1000 mg L-1 Cd2+ electrolyte, the relative contents of Mn(II), Mn(III), and Mn(IV) varied from 6.62% to 4.17%, from 13.10% to 16.34%, and from 82.73% to 77.04%, respectively. The corresponding Mn AOS decreased from 3.78 to 3.70, which was consistent with the result of XRD (Figure 4). SEM was used to analyze the effect of Cd on the morphology of birnessite. As shown in Figure 6a, the flower-like microspheres of birnessite disappeared due to dissolution-recrystallization process during the charge-discharge tests in Na2SO4 solution.37 After charge-discharge tests in 200 mg L-1 Cd2+ electrolyte, the birnessite retained the morphology of flower-like microspheres and was partially dissolved (Figure 6b). However, the flower-like microspheres disappeared in 600 and 1000 mg L-1 Cd2+ electrolytes (Figure 6c, d), and the diameter of birnessite particles decreased with 13



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increasing the concentration of Cd2+, suggesting that the dissolution of the birnessite was increased. This was consistent with the results of XRD experiments (Figure 4). The cyclic electrochemical adsorption performance of the birnessite electrode was studied at pH 6.0. Compared with the capacitance performance of fresh birnessite electrode in 0.1 mol L-1 Na2SO4, no significant difference in specific capacitance was observed when the birnessite electrode was transferred from the electrolyte containing 200 mg L-1 Cd2+ to background electrolyte of 0.1 mol L-1 Na2SO4; however, when the initial concentration of Cd2+ was controlled at 1000 mg L-1, the specific capacitance of the birnessite electrode significantly decreased after transferring to 0.1 mol L-1 Na2SO4 solution (Figure 2a and Figure S5). After 50 cycles of charge-discharge activation and regeneration in 0.1 mol L-1 Na2SO4 solution, the release ratios of the electrochemically adsorbed Cd2+ ions were about 49.77% and 11.97% for the birnessite electrodes previously treated in 200 and 1000 mg L-1 Cd2+ solutions, respectively. When the activated birnessite electrodes were used to remove Cd2+ ions for the second time, the specific adsorption capacity reached 90.2 and 229.5 mg g-1 when the initial Cd2+ concentration was controlled at 200 and 1000 mg L-1, respectively.

3.3. Effect of Initial pH and Loading of Active Birnessite. The pH could affect the capacitance characteristics of birnessite41 and thereby affect the electrochemical removal of Cd2+. To investigate the effect of initial pH, 50 cycles of galvanostatic charge-discharge tests were carried out in 200 mg L-1 Cd2+ electrolyte with pH ranging from 3.0 to 6.0 using 5 mg electrode material. As shown in Figure S6a, the initial specific capacitance decreased with increasing pH, which was consistent with the results of CV (Figure S6b). Decay of specific capacitance was observed with increasing cycles and became more obvious with decreasing pH. As shown in Figure S7, the XRD patterns of the wet 14


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working electrode indicated that the birnessite was not converted into other crystal phases and the relative intensity of diffraction peak gradually decreased with decreasing pH, indicating that some of the birnessite was dissolved at low pH values. Figure 7a shows that the electrosorption capacity of Cd2+ was 54.78 mg g-1 at pH 3.0 and rose at first with increasing pH and then reached equilibrium at pH 4.0. After 50 cycles of charge-discharge tests, the release of Mn2+ decreased with the increase of pH (Figure S8), which agreed with the fact that high pH facilitated the chemical stability of birnessite (Figure S7). In order to study the effect of birnessite loading on electrosorption capacity of Cd2+, the loading of electrode materials was controlled at 0, 5, 10, 15, and 20 mg. As shown in Figure S9a, the specific capacitance of pure carbon fabric was about 0 F g-1, confirming that the carbon fabric almost had no electrochemical activity. The initial specific capacitance decreased with increasing loading of birnessite on the electrode, which was consistent with the results of CV (Figure S9b). The decay of specific capacitance was observed with increasing cycles and became more significant with decreasing loading. The XRD patterns (Figure S10) of the wet working electrode showed that the birnessite was not converted into other crystal phases. The electrosorption capacity of Cd2+ using 0 mg electrode material was 0 mg g-1, which was consistent with the result of galvanostatic charge-discharge tests (Figure S9a). Hence, the Cd2+ removal was not affected by carbon fabric. The electrosorption capacity of Cd2+ using 5 mg electrode material was 196.5 mg g-1 and decreased with increasing loading of electrode materials (Figure 7b).

4. DISCUSSION

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4.1. Electrochemical Removal Mechanism. In the charge-discharge process of electric double-layer capacitors, the charged ions in the electrolyte could move towards the oppositely charged electrodes and accumulate on the electrode surface due to electrostatic forces. This principle is used for desalination.11–20 The charge storage mechanism of pseudocapacitance for manganese oxides could be described as MnO2 + A+ + e- ⇌ MnO2A, where A+ is alkali metal ion or H+.24,39 During the charge-discharge processes of birnessite in Na2SO4 solution, the redox reaction between Mn(IV) and Mn(III) is accompanied by the intercalation-deintercalation of Na+ and H+, and this process is preferably reversible.37,39 In this work, the symmetrical rectangular shape of CV curve and the stable crystal structure (Figure 2b and Figure 4) also confirmed that the birnessite in Na2SO4 solution had good pseudocapacitive performance. In this work, the specific capacitance of birnessite increased with increasing Cd2+ concentration due to the increase of conductance and ion diffusion rate. However, the increase of Cd2+ concentration improved the specific capacitance of birnessite only at the initial stage, which was accompanied by the decrease of cyclic stability (Figure 2a), and the room for the increase of capacitance was limited, which could also be indicated by cyclic voltammograms (Figure 2b). The ions with large hydrated radius lead to the decrease in the reversibility of manganese oxide electrodes. In the charge-discharge process of batteries, incompletely reversible intercalation-deintercalation of Zn2+ and Mg2+ causes Mn2+ release and the decline of specific capacitance of birnessite and γ-MnO2.42,43 In this work, the (001) layer spacing of the as-prepared birnessite was 0.71 nm. When the electrolyte contained Cd2+, the large hydrated radius of Cd2+ (0.426 nm)44 led to poorly reversible intercalation-deintercalation. The Cd2+ adsorbed on electrode was incompletely deintercalated in the charge process, and after several cycles of charge-discharge, the Cd2+ was accumulated on the electrode. The adsorption capacity increased 16


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and then reached equilibrium with the increase of the cycles of charge-discharge, and high initial concentration facilitated the irreversible enrichment of Cd2+ on the birnessite electrode (Figure 3b). The Mn2+ release of MnO2 electrode also suggested the incompletely reversible electrochemical processes. During the discharge process, Mn(IV) is reduced to Mn(III), which can be disproportionated into Mn2+ ions and Mn(IV) oxides due to the Jahn-Teller effect.43 During the discharge process, approximately one-third of the total Mn in cryptomelane was released into ZnSO4 electrolyte, which caused a decrease of specific capacitance.43,45 Here, the content of released Mn2+ was shown in Figure S11 after charge-discharge tests with different initial concentrations of Cd2+. When there was Cd2+ in the electrolyte, the maximum Mn2+ release was 38.02% of the total Mn in the birnessite, which was significantly higher than the Mn2+ release in the absence of Cd2+ (about 3.40%). This result suggests that the Mn2+ released into the electrolyte is derived from the reduction of Mn(IV). The incomplete deintercalation of Cd2+ adsorbed on the electrode in the charge process results in the incomplete oxidation of Mn(III) to Mn(IV), and the unoxidized Mn(III) disproportionates into Mn(IV) oxide and Mn2+ ions, which are then released into the electrolyte. At the same time, due to the Mn2+ release, the content of active Mn in the electrode decreased, which caused the decay of specific capacitance of the birnessite. The release of Mn2+ increased with increasing Cd2+ concentration (Figure S11), leading to a decrease of Mn AOS and XRD diffraction peak (Figure 4) after charge-discharge. Besides the release of activated Mn, the undesorpted Cd2+ ions in the interlayer of birnessite also significantly decreased the adsorption capacity during the regeneration process. However, a high electrochemical adsorption capacity of 229.5 mg g-1 suggested that the birnessite electrodes could be potentially re-used for many times. 17



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When ordered mesoporous carbon/carbon nanotube composite electrodes were used to remove NaCl, the ions could be adsorbed onto the electrode surface or into the mesoporous by electrostatic forces.11 In sorption isotherms of Cd2+ on birnessite, the Cd2+ adsorbed onto birnessite was found to occupy the sites above and below MnO6 octahedral vacancies.29 Therefore, in the discharge process, Cd2+ moves towards the working electrode due to electrostatic forces. After reaching the electrode surface, a part of Cd2+ ions occupy MnO6 octahedral vacancies or are inserted into the interlayer and the other part are adsorbed on the electrode surface or into the mesoporous. Obviously, the mesoporous structure is favorable for the electrochemical adsorption of Cd2+ by birnessite. In order to further analyze the sorption site of Cd2+ on birnessite, the sorption isotherms of Cd2+ on synthetic birnessite were carried out. As shown in Figure 8, the adsorption capacity sharply increased at low concentrations and then the increase rate slowed down, and the sorption isotherm curve was conformed to L-type.30 The data were fitted by the Langmuir equation and the maximum adsorption capacity was 125.8 mg g-1 (1.12 mmol g-1), which is consistent with the results of previous studies.29 The maximum Mn2+ release was calculated to be 2.14% of the total Mn in the birnessite, and in addition, the crystal structure of birnessite was not obviously changed by the adsorption of Cd2+ as seen from the XRD patterns (Figure S12). Mn2+ in the solution was derived from the Mn(II) which was exchanged by Cd2+ from the sites above and below octahedral vacancies. The above analyses demonstrate that Cd2+ is mainly adsorbed on the electrode surface or into the mesoporous channel. Higher adsorption capacity of Cd2+ was obtained owing to the electrochemical processes. A series of Cd2+ adsorption capacities of different adsorbents were compared (Table S1). As for isothermal adsorption, the capacity for activated carbon derived from bagasse, synthetic zeolites, activated 18


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carbon cloth and manganese dioxide nanosheets reached as high as 49.07, 50.8, 146, 348.1 mg g-1, respectively.5,9,10,30 The inorganic adsorption capacity of as-obtained birnessite for Cd2+ was 125.8 mg g-1, which approached the previously reported value of 148 mg g-1.29 During the charge-discharge processes, dissolution-recrystallization decreased the particle size and increased the specific surface area of the birnessite electrode (Figure 6). Larger surface area and the incompletely reversible intercalation-deintercalation reaction remarkably enhanced the adsorption specific capacity during the charge-discharge processes. In this study, multi-cycle charge-discharge processes contributed to the larger surface area owing to the dissolution-recrystallization. Capacitive deionization was also conducted to compare the removal capacity under a constant voltage. When cell voltages were set at 0, 0.3, 0.6, and 0.9 V for 12 h, the adsorption capacities were 59.3, 64.9, 105.6, and 161.8 mg g-1, the contents of released Mn2+ were about 5.6, 4.8, 4.6, and 4.7 mg g-1, and the removal efficiencies were 3.71, 4.05, 6.60, and 10.11%, respectively (Figure S13). The potential of birnessite electrode was respectively determined to be 0.33, 0.30, 0.23, and 0.03 V (vs SCE) in the above systems (Figure S14). Electrostatic interaction was possibly the main mechanism for the removal of Cd2+, as indicated by the excellent chemical stability in crystal structure (Figure S15) and low release of Mn2+ during the electrochemical adsorption processes. The potential for the electrochemical intercalation of Cd2+ into birnessite was lower than 0.3 V, as shown in the cyclic voltammograms (Figure. 2b). In the present work, when the cell voltage of 0.9 V was applied, the potential of the birnessite electrode reached 0.03 V, which facilitated the intercalation of Cd2+ into birnessite with the highest removal capacity. These results further indicate that the pseudocapacitive property of birnessite contributies greatly to the higher electrochemical Cd2+adsorption capacity. 19



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4.2. Effect of Initial pH and Loading of Active Birnessite. In Na2SO4 electrolyte, the concentration of H3O+ had a more significant effect on the specific capacitance of birnessite than that of Na+.41 The mobility of H+ is considerably higher than that of Na+, which reduces the ion transfer resistance between electrolyte and electrode interface.46 In this experiment, a lower pH of the electrolyte usually resulted in higher initial specific capacitance. Decay of specific capacitance was observed with increasing cycles and became more obvious with decreasing pH (Figure S6a), which is mainly due to the dissolution of Mn and the consumption of H+. A previous study showed that MnO2 would be reduced to Mn2+ and dissolved during charge-discharge in low pH electrolytes.47 After 50 cycles of charge-discharge tests in 200 mg L-1 Cd2+ solution with different pH values, the XRD patterns and the release of Mn2+ also showed that the birnessite was partially dissolved (Figure S7, S8). The dissolution of Mn caused a decrease of active birnessite content on the electrode and the consumption of H+, which led to the increase of ion transfer resistance between electrolyte and the electrode interface. After charge-discharge tests in electrolyte with initial pH of 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, and 6.0, the pH values were changed to 3.51±0.05, 5.84±0.06, 6.54±0.07, 6.62±0.01, 6.84±0.04, 6.88±0.08, and 6.88±0.05, respectively. These results further suggest that the release of Mn2+ led to the consumption of H+ and thereby the decrease of specific capacitance. The release of Mn2+ also caused the reduction of active birnessite mass on the electrode and thus the decrease of electrosorption capacity of Cd2+. High concentration of H+ accelerates the disproportionation reaction and the release of Mn2+, resulting in the decrease of the crystallinity of the birnessite and the capacity fade of supercapacitor

20


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during the charge-discharge processes. Therefore, the effect of pH on the dissolution of birnessite should be attributed to the electrochemical processes rather than physical reaction. The change of pH will affect the surface charge of birnessite in inorganic adsorption processes. High pH will increase the negative charge on the surface of birnessite with a subsequent increase in the adsorption capacity for heavy metals. In this work, the adsorption of Cd2+ occurred on the birnessite electrode, and surface charge changed during the charge-discharge processes. The electrosorption capacity of Cd2+ was 54.8 mg g-1 at pH 3.0 and rose at first with increasing pH and then reached equilibrium at pH 4.0 (Figure 7a). There was no obvious effect of pH on the electrosorption capacity of Cd2+ at pH 4.0–6.0. That is to say, the decrease of the competitive adsorption of H+ at higher pH did not enhance the electrochemical adsorption capacity. These results further confirm that electrostatic adsorption possibly plays a minor role in Cd2+ electrochemical adsorption. The finding that pseudocapacitive property of birnessite contributes much to the electrochemical adsorption of Cd2+ could be further confirmed by other results. For example, the adsorption capacity increased with the increase in the cycles of charge-discharge due to the incompletely reversible intercalation-deintercalation reaction. The electrochemical stability significantly decreased with the increase of Cd2+ concentration in the electrolyte owing to the redox reaction accompanied by the release of active Mn from birnessite (Figure 2a). During the activation and regeneration processes, the release ratio of electrochemically adsorbed Cd2+ reached as high as 49.77%, and these Cd2+ ions should be from the surface and the interlayers of birnessite. In the charge-discharge process of MnO2-based supercapacitors, redox reactions occur on the electrode surface.41,48 In this work, the electrode thickness increased with increasing loading of electrode materials. During the charge-discharge process, the cations in the electrolyte could not 21



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easily reach the inside of electrode materials. Therefore, the specific capacitance and the electrosorption capacity of Cd2+ decreased with increasing the mass of birnessite on the electrode. In this work, though the electrosorption capacity of birnessite for Cd2+ could reach up to 900.7 mg g-1, the Mn2+ release was evaluated to be about 204.3 mg g-1. In future studies, we will improve the experimental devices and electrochemical methods to improve the stability of birnessite electrode and reduce the release amount of Mn2+. Moreover, this method can also be applied to remove other heavy metal ions from contaminated waters.

5. CONCLUSIONS In summary, birnessite could be used as electrode materials to electrochemically remove Cd2+ from aqueous solutions based on the principle of supercapacitors. In the charge-discharge processes accompanied by cation intercalation-deintercalation, Cd2+ moves towards the working electrode due to electrostatic forces and incompletely reversible insertion reaction. The adsorption capacity increased and then reached equilibrium with the increase in charge-discharge cycles. The maximum electrosorption capacity was evaluated to be about 900.7 mg g-1 (8.01 mmol g-1), which is significantly higher than the adsorption isotherm capacity (125.8 mg g-1). Upon reaching the electrode surface, a small amount of Cd2+ occupies MnO6 octahedral vacancies and most of the Cd2+ ions are adsorbed on the electrode surfaces or into the mesoporous channels of birnessite. In low pH electrolyte, the partial dissolution of the birnessite would result in the decrease of Cd2+ removal efficiency. The electrosorption capacity of Cd2+ increased with increasing pH and reached equilibrium in the pH range of 4.0–6.0. Excess birnessite on the electrode is not in full contact with the electrolyte, causing a decrease of Cd2+ electrosorption capacity. This work provides a new 22


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method for the efficient adsorption of high concentration Cd2+ and other heavy metal ions from wastewater.

ASSOCIATED CONTENTS Supporting Information This includes the conventional characterization of as-prepared birnessite, electrochemical properties for birnessite electrodes, XRD patterns of birnessite after charge-discharge test, and adsorption isotherm, released Mn2+ concentration and electrosorption capacity for Cd2+ after charge-discharge tests and capacitive deionization experiments under constant voltages. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Qiu GH, E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China (Grant No. 41571228, 41330852), the Fok Ying-Tong Education Foundation (Grant No. 141024), the Natural Science Foundation of Hubei Province of China (Grant No. 2014CFA016), and the Fundamental Research Funds for the Central Universities (Program No. 2662015JQ002) for financial support. Steven L. 23



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Suib thanks support of the US Department of Energy, Office of Basic Energy Sciences, Division of Chemical, Biological and Geological Sciences under grant DE-FG02-86ER13622.A000.

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and


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Figure Captions: Figure 1. XRD patterns (a) and SEM images (b) of the as-prepared birnessite. Figure 2. Cyclic capacitance (a) and cyclic voltammograms (b) for birnessite electrodes in electrolyte with different initial concentrations of Cd2+. Figure 3. Electrosorption capacities of birnessite electrodes for Cd2+ after 50 cycles of charge-discharge tests in electrolyte with different initial concentrations of Cd2+ (a) and in 0.1 mol L-1 Na2SO4 with Cd2+ concentrations of 200 and 2200 mg L-1 at different cycles in charge-discharge tests (b). Figure 4. XRD patterns of birnessite electrodes after 50 cycles of charge-discharge tests in electrolytes with different initial concentrations of Cd2+: (a) 0 mg L-1, (b) 200 mg L-1, (c) 600 mg L-1, (d) 1000 mg L-1, (e) 1400 mg L-1, (f) 1800 mg L-1, and (g) 2200 mg L-1. Figure 5. XPS broad scans (a) and Cd3d spectra (b) of pristine birnessite and electrodes after 50 cycles of charge-discharge tests in 1000 mg L-1 Cd2+ electrolyte, and Mn2p3/2 spectra of pristine birnessite (c) and electrodes (d) after 50 cycles of charge-discharge tests in 1000 mg L-1 Cd2+ electrolyte. Figure 6. SEM images of birnessite electrodes after 50 cycles of charge-discharge tests with different initial concentrations of Cd2+: (a) 0 mg L-1, (b) 200 mg L-1, (c) 600 mg L-1, and (d) 1000 mg L-1. Figure 7. Electrosorption Cd2+ capacity of birnessite electrodes after 50 cycles of charge-discharge tests in 200 mg L-1 Cd2+ solution with (a) different pH values and (b) masses of electrode materials. Figure 8. Isotherm for Cd2+ adsorption and released Mn2+ at 25 oC and pH 6.0.

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Figures a

b 200 nm

1.0 µm Figure 1. XRD patterns (a) and SEM images (b) of the as-prepared birnessite.

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a

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b

Figure 2. Cyclic capacitance (a) and cyclic voltammograms (b) for birnessite electrodes in electrolyte with different initial concentrations of Cd2+.

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a

b

Figure 3. Electrosorption capacities of birnessite electrodes for Cd2+ after 50 cycles of charge-discharge tests in electrolyte with different initial concentrations of Cd2+ (a) and in 0.1 mol L-1 Na2SO4 with Cd2+ concentrations of 200 and 2200 mg L-1 at different cycles in charge-discharge tests (b).

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g f e d c b a

Figure 4. XRD patterns of birnessite electrodes after 50 cycles of charge-discharge tests in electrolytes with different initial concentrations of Cd2+: (a) 0 mg L-1, (b) 200 mg L-1, (c) 600 mg L-1, (d) 1000 mg L-1, (e) 1400 mg L-1, (f) 1800 mg L-1 and (g) 2200 mg L-1.

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a

b

c

d

Figure 5. XPS broad scans (a) and Cd3d spectra (b) of pristine birnessite and electrodes after 50 cycles of charge-discharge tests in 1000 mg L-1 Cd2+ electrolyte, and Mn2p3/2 spectra of pristine birnessite (c) and electrodes (d) after 50 cycles of charge-discharge tests in 1000 mg L-1 Cd2+ electrolyte.

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b

a 100 nm

1.0 µm

1.0 µm

c

d 50 nm

50 nm

1.0 µm

1.0 µm

Figure 6. SEM images of birnessite electrodes after 50 cycles of charge-discharge tests with different initial concentrations of Cd2+: (a) 0 mg L-1, (b) 200 mg L-1, (c) 600 mg L-1, and (d) 1000 mg L-1.

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a

b

Figure 7. Electrosorption Cd2+ capacity of birnessite electrodes after 50 cycles of charge-discharge tests in 200 mg L-1 Cd2+ solution with (a) different pH values and (b) masses of electrode materials.

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Y=

1 1 + R 2 =0.9544 125.8 1.264Ce

Figure 8. Isotherm for Cd2+ adsorption and released Mn2+ at 25 oC and pH 6.0.

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ToC Graphic


Cadmium Removal from Aqueous Solution by a Deionization

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