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. 2020 Jan 6;5(2):1062–1067. doi: 10.1021/acsomega.9b03050

Electrochemical Adsorption of Cs+ Ions on H-Todorokite Nanorods

Hanping Zhang †, Jingyi Zhu ‡, Lei Ma §, Liping Kang †,*, Mancheng Hu †, Shuni Li †,*, Yu Chen ‡
PMCID: PMC6977079  PMID: 31984262

Abstract

graphic file with name ao9b03050_0006.jpg

In this work, manganese oxide with a 3 × 3 tunnel structure was synthesized. Mg-todorokite was treated with HNO3 to obtain H-todorokite, which was used as an electrode for adsorption of Cs+ from aqueous solution by an electrochemical reaction. H-todorokite was electrochemically reduced to lower oxidation state to enhance its negative charge and simultaneously increase the amount of cations because of the charge compensation. The Cs+ adsorption on H-todorokite was completed by a coupled electrochemical reaction (the redox reaction between Mn3+ and Mn4+) and an ion-exchange reaction between Cs+ and H+ ions. Cyclic voltammetry measurements at different pHs and Cs+ concentrations were performed. H-todorokite revealed high electrochemical adsorption capacity for Cs+ because of the high crystallinity and stability of the materials, which reached 6.0 mmol g–1 in 0.1 mol·L–1 Na2SO4 solution.

1. Introduction

Manganese dioxide (MnO2)-based materials are used widely as electrochemical supercapacitors because of the redox reaction between Mn(III) and Mn(IV).1−11 A manganese oxide (MnO2)-based supercapacitor has been paid great attention because of its large theoretical specific capacitance of 1300 F·g–1. For example, Mg-doped sodium birnessite-type MnO2 can be used as an active supercapacitor electrode material, which exhibits a stable capacitance of 145 F·g–1 over 1100 cycles.8 MnO2 was also used as an effective electrochemical adsorbent for heavy metal ions because of the charge–discharge reaction during the electrochemical process. For example, the adsorption capacities of birnessite for Ni2+, Zn2+, Cu2+, and Cd2+ can be greatly improved by constant-potential electrolysis compared to isothermal adsorption.12−15 The electrochemical adsorption capacity of Cd2+ on tunnel-structured manganese oxide cryptomelane could reach 192 mg·g–1, which was much higher than the initial 12 mg·g–1 adsorption capacity in conventional adsorption.13 This result indicated that the redox process made great contributions to adsorption of ions by manganese oxides. For the adsorption of Cs+, especially 137Cs+ from nuclear waste, different materials, such as Prussian Blue16−18 and its analogues,19 clays,20−22 and zeolites,23 have been used. In 1984, MnO2 was used for adsorption of microamounts of radiotracer 137Cs using a radiotracer technique by Hasany and Chaudhary.24 However, the removal of the adsorbed Cs+ on MnO2 is very difficult. Manganese octahedral molecular sieves with porous structure and one-dimensional tunnel structure have been found to be favorable in cation adsorption. Synthetic todorokite-type MnO2 with one-dimensional (3 × 3) tunnel structure was used for metal ion extraction–insertion reactions in the aqueous phase by Feng and co-worker.25 Cs+ adsorption using synthetic todorokite-type MnO2 by Sonoda26 showed that todorokite (Mg–H) and todorokite (Ca–H) can almost completely adsorb Cs+ (ca. 100%) from drinking water intentionally contaminated with 0.1 mmol·dm3 of CsCl with an adsorptive capacity of 0.50 mmol·g–1. The radiocesium retention onto birnessite and todorokite by Yu and co-workers27 indicates that the structural factors of Mn oxides significantly affect the retention capacity of radioactive Cs+. However, the traditional adsorption by manganese oxides was completed by the ion exchange between the balanced cations and Cs+. Thus, the lower adsorption capacity and full desorption are still difficult for these absorbents. Todorokite, the tunnel-structured manganese oxide with a 6.9 Å × 6.9 Å tunnel, was thought to be an adsorbent26,28 with suitable pore size for Cs+. Thus, in this work, Mg-todorokite was synthesized, and acid treatment-type H-todorokite was used for the electrochemical adsorption of Cs+. The micromorphology, structure, and chemical composition before and after electrochemical adsorption were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS).

2. Results and Discussion

2.1. XRD, SEM, and Energy-Dispersive X-ray Characterization of the Samples

Mg-Tod, H-Tod, and H-Tod after electrochemical reduction [labeled H-Tod(Re)] and after adsorption of Cs+ (Cs-Tod) are first examined by XRD and field-emission SEM equipped with an energy-dispersive X-ray (EDX) accessory.

The XRD patterns of the samples are plotted in Figures 1 and S1. The XRD patterns of H-Tod(Re) and Cs-Tod show the characteristic diffraction peaks of Tod together with the peak from carbon cloth. This explains that the structure of Tod is still stable after the electrochemical process and adsorption. The EDX results (Figure S2) show a Mg/Mn ratio of 0.15 in Mg-Tod and 0.06 in H-Tod, which suggests that only 60% of Mg2+ are extracted from Mg-Tod by the acid treatment. It indicates that Mg2+ exists both in the framework and tunnels of Mg-Tod.29 The decreased amount of Mg2+ is replaced by H+, which can be ion-exchanged by Cs+.

Figure 1.

Figure 1

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XRD patterns (A) and SEM images (B) of the samples.

SEM images show that Mg-Tod and H-Tod possess aggregated rodlike morphology (Figure 1). The morphology of H-Tod(Re) and Cs-Tod has almost not changed. The adsorption of Cs+ into H-Tod was characterized by EDX elemental analysis (Figure S2). A Cs/Mn ratio of 0.015 for Cs-Tod is achieved because of the charge compensation, as for other MnO2 electrodes.6

Thermogravimetric analyses were performed on a thermogravimetric analyzer under nitrogen atmosphere in a temperature range of 30–700 °C at a heating rate of 5 °C·min–1. The thermogravimetry curves of Mg-Tod and H-Tod are shown in Figure 2A. The weight loss from 30 to 200 °C is caused by the loss of physically adsorbed water and crystal lattice water. The weight loss between 200 and 600 °C corresponds to the transformation of Mn(IV) to Mn(III) with the release of oxygen. The residue at 700 °C is confirmed by XRD (Figure 2B). For H-Tod, the residue is Mn3O4. However, in the final products of Mg-Tod, there is Mn3O4 and manganese magnesium oxide (MgO)x(MnO)y because of the higher Mg content in Mg-Tod.

Figure 2.

Figure 2

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Thermogravimetry curves of the samples (A) and XRD patterns of the residue (B).

The surface chemical state and electronic structure of samples were determined by XPS. The survey XPS results of the samples are depicted in Figure S3. Mn, O, C, and Mg signals were observed for all the samples. For Cs-Tod, a Cs 3d signal can be observed, indicating that Cs+ is absorbed by the material (Figure S4). The Mn 2p, Mn 3s, and O 1s spectra often can be used to obtain the information related to the different oxidation states of Mn. The Mn 2p spectrum appears at binding energies of about 642 eV (Mn 2p3/2) and 653 eV (2p1/2) (Figure S5). Usually, the position of Mn 2p peaks does not use to evaluate the oxidation state of Mn because the position of the peaks does not change sensitively with the oxidation state of Mn.30−32 However, the identification of Mn3+ and Mn4+ can be achieved by the Mn 2p spectrum. Further analysis usually assigns the peaks around 643.5 and 654.8 eV as the Mn4+ species and the peaks around 642 and 653 eV as the Mn3+ species.32−35

The Mn 3s and O 1s (Figure 3) regions can give further information about the chemical state of Mn. The Mn 3s spectrum splits into two peaks, and the value of ΔE3s can be used to calculate the average oxidation state (AOS) of Mn.31,36 According to the equation VMn = 8.95–1.13 ΔE3s32,33 the AOS in Tod can be obtained as listed in Table 1. Based on the calculated ΔE3s energy difference (4.62–4.72 eV), the AOS of Mn is around 3.62–3.73, which indicates that the sample contains mainly Mn4+ with some Mn3+ contribution.

Figure 3.

Figure 3

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Mn 3s and O 1s XPS spectra.

Table 1. Experimental Binding Energies (eV) of O 1s and Mn 3s and AOS of Mn Obtained by the Measurement of O 1s and Mn 3s Splittinga.

sample O 1s/eV Mn 3s/eV AOS1 AOS2
Mg-Tod 531.2 529.9 88.83 84.14 3.69 3.65
H-Tod 531.3 530.0 89.18 84.56 3.76 3.73
H-Tod (Re) 531.4 529.9 89.11 84.39 3.61 3.62
Cs-Tod 531.4 529.9 89.08 84.44 3.72 3.71
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a

AOS1: calculated from O 1s spectrum. AOS2: calculated from Mn 3s spectrum.

The O 1s spectra usually reflect the types of surface oxygen in the materials. Based on the peak positions, three types of oxygen species can be identified. The low binding energy peak observed at 530 eV is often thought of lattice O because all manganese atoms are bonded to an oxygen atom, corresponding to MnOOH and MnO2 species. The peak at about 531 eV is assigned to the surface-adsorbed O, OH groups, and O vacancies. The high binding energy peak around 533 eV is usually identified as adsorbed water. Taking into account the area of the Mn–O–Mn and Mn–O–H contribution in the O 1s spectrum,6,37 the AOS of Mn can also be calculated according to the following equation

2.1. 1

The results are listed in Table 1. We can see that acid treatment of Mg-Tod to H-Tod causes a slight increase in the AOS of Mn, owing to the Mn(III) disproportional reaction to Mn(IV) and Mn(II) in the acid solution.25,28,39,40 The electrochemical reduction form H-Tod(Re) shows a slight decrease of Mn AOS, which indicates the reduction of Mn4+ to Mn3+. An increase of Mn AOS in Cs-Tod suggests that Cs+ intercalates into the tunnel of MnO2. The process of the ion-exchange reaction is similar to the H+ exchange from Mg-Tod to H-Tod, together with the disproportion of Mn(III) in the solution.

2.2. Electrochemical Adsorption of Cs+ by H-Tod(Re)

The electrochemical behavior of H-Tod electrode was investigated by cyclic voltammetry (CV) technique in a N2-saturated 0.1 mol·L–1 Na2SO4 aqueous electrolyte between −0.3 and 1.35 V versus a saturated calomel electrode (SCE) (Figures 4A and S6). One cathodic peak (from 0.5 to 0.7 V), one anodic peak (from 0.8 to 1.1 V), and one unobvious cathodic peak around 0.15 V can be found. The shape of cyclic voltammograms for H-Tod is different from the nearly symmetrical rectangular shape for carbon materials,41 which is characteristic of the electrochemical double-layer capacitive behavior. For MnO2-based materials, the energy stored in the capacitors is either capacitive (non-Faradaic process) or pseudocapacitive (Faradaic process). Based on the different process, the charge separation can take place at the electrode/electrolyte interface, or the redox reactions can occur in the electrode materials. The cyclic voltammograms in this work show both the rectangular shape and redox peak, which indicate the Faradaic reaction occurring both on the surface and in the bulk of the electrodes. The Faradaic process on the surface causes the surface adsorption of cations on MnO2 from the electrolyte, while the Faradaic reaction corresponding to the redox peak can be ascribed to the intercalation/deintercalation of cation between the electrode and the electrolyte accompanied by the electrochemical conversion between Mn(III) and Mn(VI). Also, the presence of the redox peak may be due to the higher degree of crystallization of the sample used in our experiments.8,42 The effect of scan rate on H-Tod in 0.1 mol L–1 Na2SO4 indicates that the cation and anodic peak all shift with different scan rates. It may further have an influence on the electrochemical intercalation/deintercalation of cation.1,42 The CV curves of H-Tod electrode during 1–1000 cycles (Figure 4B) still show clearly the peaks during reduction and oxidation. The slight shift of the peaks does not transform the H-Tod structure, which can be seen from the XRD pattern after 1000 cycles (Figure S7).

Figure 4.

Figure 4

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CV curves for H-Tod in 0.1 M Na2SO4 solution at different scan rates (A), different scan cycles at a scan rate of 0.1 V s–1 (B), different pHs at a scan rate of 0.1 V s–1 (C), and different amounts of Cs+ at a scan rate of 0.1 V s–1 (D).

H-Tod is used for the adsorption of Cs+ in our experiments because of the good performance on ion intercalation, as can be seen from CV curves when used as an electrode. The mechanism of the intercalation of Cs+ is due to the reduction of Mn(IV) to Mn(III)43 and the ionic exchange of Cs+ for charge compensation as described for other MnO2-based materials.7,9,11

2.2.

Because the reduction of Mn(IV) to Mn(III) is beneficial for the ion storage because of the charge compensation, the electrode with H-Tod was subjected to 5 min of electrochemical reduction at −0.3 V potential [labeled H-Tod (Re)] for Cs+ adsorption in the following experiment. To test the electrochemical stability of H-Tod, the reduction process is processed at −0.3 V potential for 12 h (Figure S8). In the whole process, the current change is very small, confirming their high activity and long-term stability. The sample after 12 h reduction tests is determined by XRD (Figure S9), which shows the characteristic peak of Tod. This is further evidence for the high stability of H-Tod used in the experiment.

The effect of pH on the electrochemical behavior is investigated by adjusting the pH of the electrolyte solution using H2SO4 or NaOH. The cyclic voltammograms are plotted in Figure 4C. It can be seen that in strong acid medium, the redox peaks on the CV curves are strong with the reduction potential shifting to a positive direction. This suggests that MnO2 is more easily reduced to a lower state according to Nernst equation. By increasing the pH, the typical shoulder peak can be observed. However, in strong basic medium (pH = 11 and 13), the cyclic voltammograms only show a rectangular shape, which implies that strong basic environment is not favorable for the intercalation/deintercalation of the cations from the electrolyte. That is, the charge storage mechanism varies with pH. Although the shape of cyclic voltammograms change at different pHs, the electrode is stable both in strong acid and basic medium. The XRD of the electrode after electrochemical at different pHs has no change in the process (Figure S10), indicating the stability of H-Tod with pH. Therefore, the material in this work can be used in a broad pH range of aqueous solution.

The CV curves of H-Tod(Re) with different amounts of Cs+ are plotted in Figure 4D. It can be seen that the potential separation between the two obvious peaks is lessened with the increase of Cs+ concentration, which favors the electrochemical intercalation/deintercalation of cations.42

The adsorption capacity of Cs+ is obtained by soaking H-Tod(Re) in the solution containing different Cs+ concentrations for 20 min. After that, the Cs+ in the solution is calculated by the difference before and after adsorption. The concentrations of Mg2+, Mn2+, and Cs+ were measured by HITACHI ZA3000 atomic absorption spectroscopy. With increasing Cs+ concentration in the Na2SO4 electrolyte, the adsorption capacity H-Tod(Re) first increases and then remains nearly constant (Figure 5A). The adsorption capacity of H-Tod(Re) with support of 0.002 g on the electrode is about 6.0 mmol·g–1. The XRD pattern of the sample after adsorption (Figure S11) shows nearly no change. The adsorption capacity of H-Tod(Re) with 1 g·L–1 of Cs+ in the Na2SO4 electrolyte at different pHs is plotted in Figure 5B. The adsorption capacity curve first decreases slightly and then increases slightly by increasing the solution pH. In acidic medium, H+ competes with Cs+ in the tunnel of H-Tod. However, with more OH– in the solution, it consumes H+ and thus facilitates the insertion of Cs+. Strong acid may promote the disproportionation of Mn(III) and contributes to the slight increase of the adsorption.

Figure 5.

Figure 5

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Cs+ adsorption capacities of the H-Tod (Re) electrodes (A) with different concentrations of Cs+ in the electrolyte and (B) at different pHs.

The content of Mg2+ and Mn2+ released during Cs+ adsorption is shown in Figure S12. It can be seen that the concentration of Mg2+ and Mn2+ is almost constant by increasing the Cs+ content in the electrolyte. That is, Mg2+ and Mn2+ (come from the disproportionation reaction of Mn3+) are released from the framework of H-Tod, which contributes to the insertion of Cs+ into the tunnel of the structure. However, long-time reduction will not be favorable for stabilizing the structure of H-Tod because of the disproportionation property of Mn(III). Therefore, the electrode should be regenerated after adsorption, which can be achieved from the electrochemical oxidation process.

However, when we conduct the isothermal adsorption, until about 48 h, the adsorption equilibrium can be achieved. The adsorption capacity is only about 2.1 mmol·g–1 (Figure S13), which is similar to the maximum value by todorokite in ref (27). The result shows that the adsorption capacity of H-Tod for Cs+ increased from 2.1 to 6.0 mmol g–1 after electrochemical reduction process. That is, the electrochemical reaction contributes a lot to the adsorption capacity. Therefore, the electrochemical process is effective for Cs+ adsorption from aqueous solution.

3. Conclusions

The Cs+ adsorption capacity can be greatly improved using H-Tod in 0.l mol·L–1 Na2SO4 solutions based on an electrochemical reduction method. When the electrochemical adsorption is completed at a potential of −0.3 V, the adsorption capacity of H-Tod(Re) for Cs+ after electrochemical process can reach 6.0 mmol·g–1, which is much larger than 2.1 mmol·g–1 isothermal adsorption with H-Tod. The reduction from Mn(IV) and Mn(III) in the structure contributes to the high adsorption capacity of Cs+, which is caused by the decrease of Mn AOS. H-Tod with higher thermal stability and suitable tunnel size contributes to the efficiency of Cs+ adsorption. With obvious advantages of high adsorption capacity, thermal and chemical stability, electrochemical adsorption of Cs+ by H-Tod thus holds great promise for practical application in enrichment and separation of Cs+ in aqueous solution.

4. Experimental Details

4.1. Sample Preparation

Mg-todorokite (Mg-Tod) was synthesized by mixing 0.2 g of Mg-buserite (wet solid) and 10 mL of deionized water at 160 °C for 2 days.25 Protonated todorokite (H-Tod) was formed by treating 1.0 g of Mg-Tod with 100 mL of 1 mol·L–1 HNO3 solutions for 2 days at room temperature.

4.2. Electrochemical Adsorption of Cs+

The electrochemical adsorption of Cs+ was conducted using a three-electrode system. The H-Tod (75 wt %), acetylene black (15 wt %), and polyvinylidene fluoride (10 wt %) were added into 1-methyl-2-pyrrolidinone. The mixture was ultrasonically dispersed to be used as an adsorbent on the working electrode. The electrochemical adsorption was processed by a CHI 660D electrochemical workstation. The potentials were referred to the SCE. Carbon fabric (1.5 × 2.5 cm2) with a 100 μL mixture (H-Tod 0.002 g) was used as the working electrode. SCE and carbon electrode were used as the reference and counter electrodes, respectively. Na2SO4 solution (50 mL 0.1 mol L–1) was used as the electrolyte. Cs2SO4 solid was added into the Na2SO4 solution to prepare the Cs+-containing solutions. The Cs+ concentration varied from 0.2 to 1.8 g·L–1. The adsorption capacity at different pHs was conducted in the above solutions under controlled pH. All the experiment was completed at room temperature. The isothermal adsorption of Cs+ by H-Tod was conducted at constant pH 7.0. The adsorption process and capacity were compared with those of electrochemical adsorption.

The Cs+ adsorption capacity of H-Tod was calculated according to the following equation

4.2. 2

Qs (mol·g–1) was the Cs+ adsorption capacity, c0 (mol·L–1) was the initial concentration of Cs+ in the reaction system, cs (mol·L–1) was the concentration of Cs+ in the solution after adsorption, Vs (L) was the volume of solution, and m (g) was the mass of manganese oxides on the electrode (or the materials used in the isothermal adsorption).

Acknowledgments

This research was sponsored by the National Natural Science Foundation of China (nos. U1607116 and 21571120) and the Youth Research Foundation from Qinghai University (no. 2017-QGY-1)

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.9b03050.

  • Synthesis and characterization of Na-birnessite and Mg-buserite, isothermal adsorption experiment and characterization of the samples, EDX and XPS (survey, Cs 3d, Mn 2p, and Mg 1s) of the samples, CV curves at −0.3 to 0.9 V, 12 h test of H-Tod, XRD patterns of the sample (after a 12 h stability test, after the 1000 cycles test, and after the CV test at different pHs and different concentrations of Cs+), and Mg2+ and Mn2+ released concentrations in the experiment (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao9b03050_si_001.pdf (874.5KB, pdf)

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