Effective Recycling Performance of Li+ Extraction from Spinel-Type

Article pubs.acs.org/IECR

Effective Recycling Performance of Li+ Extraction from Spinel-Type LiMn2O4 with Persulfate Jun-Sheng Yuan,*,† Heng-Bo Yin,‡ Zhi-Yong Ji,† and Hui-Ning Deng† †

Engineering Research Center of Seawater Utilization Technology of Ministry of Education and ‡School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin 300130, China ABSTRACT: To solve the problem of manganese dissolution, which damages spinel structure of LiMn2O4, Li+ extraction performances with persulfate at different temperatures were investigated by chemical analysis, and then XRD, FTIR, and XPS analyses were used to characterize the corresponding mechanism. Results showed that at high temperatures and after treatment with three kinds of persulfate, lithium ions were nearly extracted thoroughly with almost no manganese dissolution. Na2S2O8 was considered the most suitable for Li+ extraction. The lithium extraction/insertion was repeated several times, and the samples maintained considerably high lithium exchange capacity and fairly little manganese dissolution, indicating high cyclic stability and efficiency of the spinel structure. The extraction mechanism was based on the hydrolysis of persulfate, was accompanied by ion exchange and oxidation−reduction reactions, and involved radicals participating in the reactions. The sulfate radical generated by activated persulfate can effectively decrease manganese dissolution, making it possible for cyclic utilization of LiMn2O4.

1. INTRODUCTION Lithium and its compounds, known as the “21st Century Energy Metals”, are widely used in electronics, metallurgy, chemical, pharmaceutical, energy, and other fields. Researchers are concerned with deriving lithium from seawater and salt lake brine because of the gradual lack of lithium resources on land. Extracting lithium by traditional methods is difficult due to the fairly low lithium concentration in brine. Yet, the new typical absorbent, we call it ion-sieve, obtained by extracting lithium from the spinel-type lithium manganese oxide precursor has good selectivity and a high capacity of adsorption for Li+ in solution; thus, it is one of the most widely studied methods.1,2 Recently, various kinds of spinel-type oxides precursor were prepared, such as LiMnO2,3 LiMn2O4,4−6 Li1.27Mn1.73O4,7 Li1.6Mn1.6O4,8−10 LiZn0.5Mn1.5O4,11 and Li1.51Mn1.63O4,12 and many efforts have been made to study their properties. Among these oxides, LiMn2O4 has drawn great attention. However, even though ion-sieve prepared by extracting lithium from LiMn2O4 precursor has high lithium exchange capacity and selectivity, the effective exchange capacity is much lower than the theoretical one, and a great amount of manganese dissolution loss exists in the lithium extraction process using acid such as HCl. This key problem may gradually damage the spinel structure and severely limit the lasting efficiency and reuse of the absorbent. A series of views have been proposed to explain the phenomenon of manganese dissolution loss and the mechanism in the process of extraction and insertion.13−16 Many researchers focus on the preparation of new kinds of lithium manganese oxide precursor, expecting that the precursor will be more stable and the manganese dissolution loss during the lithium extraction process can significantly decline. Two main methods of precursor preparation are element doping17−19 and Li-rich lithium manganese oxides.20 The compound ion-sieve prepared by element doping can improve the cyclic stability of lithium manganese oxide with spinel structure,21 yet the introduction of some elements may reduce the adsorption capacity. Li-rich lithium manganese © 2014 American Chemical Society

oxides improve the stability of the lattice; meanwhile, excessive lithium may cause distortion of the structure and coexistence of a variety of impurity phases.22 Studies should not be limited to simply exploring new ways to prepare different lithium manganese oxides. It is of great practical significance to look for new kinds of eluent suitable for lithium extraction from LiMn2O4, which could ensure both a high extraction rate of lithium and low manganese dissolution loss, thus making it possible to recycle the materials. Ooi23 studied the extraction of Li+ by using a solution containing either an acid (HCl) or an oxidizing agent (Br2, K2S2O8) and found that the rate of Li+ was rapid in a HCl solution and the dissolution of Mn was observed markedly. In the case of oxidizing agents, the extraction of Li+ could proceed with a slight dissolution of Mn samples after the extraction maintained the spinel structure. Yet more analysis and explanation have not been given about the specific properties and performance of persulfate treatment. In the present work, lithium manganese oxides (LiMn2O4) were prepared by a high-temperature solid-state reaction method, which was simple and environmentally friendly. Three kinds of persulfateNa 2 S 2 O 8 , K 2 S 2 O 8 , and (NH4)2S2O8were used in the lithium extraction from LiMn2O4. Lithium extracting properties were specifically investigated, and cyclic performance and structure properties of lithium manganese oxides treated with persulfate were studied. More detailed studies have been proposed to explain the results. Received: Revised: Accepted: Published: 9889

March 14, 2014 May 18, 2014 May 20, 2014 May 20, 2014 dx.doi.org/10.1021/ie501098e | Ind. Eng. Chem. Res. 2014, 53, 9889−9896

Industrial & Engineering Chemistry Research

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Figure 1. Lithium extraction rate at different temperatures.

2. EXPERIMENTAL SECTION

3. RESULTS AND DISCUSSION Performance of LiMn2O4 Treated with Persulfate under Different Temperatures. The extraction performance of the three kinds of persulfate showed significant difference at different temperatures. Experimental results revealed that at low temperatures (20 and 40 °C), the rate of lithium extraction was quite low (lower than 10%) as precursor was treated by Na2S2O8 and K2S2O8, while the rate of lithium extraction with (NH4)2S2O8 was higher than 80% (Figure 1). However, at high temperatures (60 and 80 °C), the lithium ion was nearly extracted thoroughly, no matter what kind of persulfate was taken (Figure 1). The lithium extraction mainly occurred between 0.5 and 3.0 h during the elution process. Almost no more Li+ moved out from the solid phase after 10 h. Results in Figure 2 show that at low temperatures (20 and 40 °C), the rate of manganese dissolution was quite low as LiMn2O4 precursor was treated with Na2S2O8 and K2S2O8

Sample Preparation. LiMn2O4 precursor was prepared by heat-treating mixtures of powdered Li2CO3 and MnCO3 (Li/ Mn molar ratio was 0.5) in air in a muffle furnace. The mixture was heated to 800 °C at a program and maintained for 5 h.24 Li+ extraction reactions were investigated under different conditions. Experiment A: 0.2 mol/L Na2S2O8, K2S2O8, and (NH4)2S2O8 at 20, 40, 60, and 80 °C. Experiment B: mixing 0.2 mol/L Na2S2O8 and (NH4)2S2O8 with methanol at 80 °C. Experiment C: mixing the complex solution consisting of Na2S2O3, FeSO4·7H2O, and (NH4)2S2O8 at 20 °C. To obtain various kinds of lithium-extracted samples, 2.0 g of LiMn2O4 was immersed in the aforementioned solutions (240 cm3) and stirred for 12 h. The samples were filtered, washed with deionized water, and air-dried at 70 °C. The Li+ insertion reaction was investigated by immersing the lithium-extracted sample (treated by Na2S2O8 at 80 °C) in 0.025 mol/L (LiCl + LiOH), and the lithium-rich solution was replaced and filtered every 12 h, unless there was no change in the composition of the leaching solution. The extraction/insertion reactions were repeated several times to investigate the cyclic performance. Chemical Analysis. The lithium and manganese contents of the LiMn2O4 were determined by atomic absorption spectrometry (AAS, model AA-320, Shanghai Precision & Scientific Instrument Co., Ltd.) after the LiMn2O4 was dissolved in a mixture of HCl and H2O2. The available oxygen of each sample was determined by the standard oxalic acid method. The mean oxidation state of manganese (ZMn) was evaluated from the available oxygen value of the sample.25 Sample Characterization. Phase identification of different products after Li+ extraction reactions was recorded by X-ray powder diffraction (XRD) data on a Bruker D8 Focus diffractometer using Cu Kα radiation. Infrared data was measured with a Nexus 470 FT-IR spectrometer. X-ray photoelectron spectroscopy data was tested by the U.S. Edax Genesis 60S X-ray energy spectrometer.

Figure 2. Manganese dissolution rate at different temperatures. 9890

dx.doi.org/10.1021/ie501098e | Ind. Eng. Chem. Res. 2014, 53, 9889−9896


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persulfate solution was around 1.5. The pH value of all the three kinds of persulfate at 60 °C showed a clear downward trend compared with those at 20 °C. The pH of the ammonium persulfate solution (pH ≈ 1.0) was still lower than that of the other two persulfate solutions, which could be attributed to the dual hydrolysis property of ammonium persulfate:

soaking, while the manganese dissolution loss with (NH4)2S2O8 was higher than 25%, which may cause collapse of the layered structure.26 However, the result of manganese dissolution loss at high temperatures (60 and 80 °C) was satisfying, and almost no manganese dissolution loss (lower than 0.5%) was observed, no matter what kind of persulfate was used. We could find that the rate of Li+ extraction was closely related to the concentration of H+ as the following aspects: (1) Both the pH changes of the solution during the Li+ extraction process from LiMn2O4 using persulfate and pH changes of the blank experiments with only persulfate thermostatic at 60 °C were monitored (Figure 3). It was obvious that the pH value of

S2 O82 − + 2H 2O → 2HSO4 − + H 2O2

(1)

HSO4 − ↔ H+ + SO4 2 −

(2)

NH4 + + H 2O ↔ NH3·H 2O + H+

(3)

S2O82−

The hydrolysis function of may strongly occur, providing sufficient protons for ion exchange. We conclude that the high temperature can promote hydrolysis and improve the concentration of hydrogen ions in solution, which resulted in a high extraction rate of lithium-ion. We attribute manganese dissolution to the following two aspects: from the view of electronic arrangement, Mn(II) is d5, and orbital d can be half-filled by electrons in five layers, while Mn(IV) is d3, and electrons can be half full on a low-energy orbit t2g. Mn(III) is d4, and it is more unstable compared with Mn(II) and Mn(IV) and therefore prone to disproportionation: Mn 3 + →

1 1 Mn 2 + + Mn 4 + 2 2

(4)

On the macro level, the manganese dissolved in the form of soluble Mn2+ from the solid to the liquid phase. Trivalent manganese and tetravalent manganese each accounted for half of the total manganese in LiMn2O4 precursor. When the disproportionation reaction (eq 4) occurred, half of the trivalent manganese would be reduced to divalent state Mn2+ and then dissolved in the solution, causing the manganese dissolution loss. The other half was oxidized to tetravalent state Mn2+, which was still preserved in the solid skeleton. This meant that no more than half of the trivalent manganese, that is, a quarter of the total amount of manganese, would be lost ultimately. However, it was worth noting that the rate of manganese loss using (NH4)2S2O8 was higher than 25%, indicating another major cause of manganese dissolution loss other than the disproportionation reaction. The excessive manganese loss resulted from the strong acidity of (NH4)2S2O8 accompanied by H2O2 produced by the hydrolysis of S2O82−, shown in eqs 2−1. In this case, the mixture of H+/H2O2 formed in the solution led to strong dissolution of lithium manganese oxide:27

Figure 3. pH changes during the elution process and comparative blank experiment at 60 °C.

the blank experiment was lower than that of the corresponding extraction process, which provided strong evidence for the consumption of H+ and for Li+ extraction. (2) The pH changes of the solution during the Li+ extraction process were recorded at 20 and 60 °C (Figure 4). As seen in Figure 4, at 20 °C, the pH values of the sodium persulfate and potassium persulfate solutions were stable at pH 4.0 or a little higher than 4.0, while in stark contrast to the above, the pH of the ammonium

10H+ + H 2O2 + 2LiMn2O4 → 4Mn 2 + + 2Li+ + 6H 2O + 2O2

(5)

This reaction caused the excessive dissolution of manganese at low temperatures. However, with the decomposition of H2O2 at high temperatures, the H+/H2O2 system disintegrated, which would reduce dissolution of the solid phase. Further analysis for little manganese dissolution loss will be discussed later. From what has been discussed above, we can clearly find that at high temperatures, treatment of the spinel-type material LiMn2O4 with all the three kinds of persulfate showed excellent performance: the lithium ion was nearly extracted thoroughly, and at the same time, almost no manganese was lost due to dissolution. However, in spite of the same performance of the three kinds of persulfate at high temperature, in practical industrial

Figure 4. pH changes at different temperatures during the elution process. 9891



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than 10 cycles was relatively excellent and promising for industry application. XPS and Average Oxidation State Analysis. Figure 5 shows the O 1s spectra of the samples. The spectrum of LiMn2O4 precursor was the reference for the lithium-extracted samples. In the O 1s spectrum, both the LiMn2O4 precursor and lithium-extracted sample had a common peak at 529.9 eV BE, which was related to the oxygen in manganese oxide. In the lithium-extracted sample, the O 1s peak was resolved into two individual peaks, indicating two different kinds of chemical environment surrounding the oxygen.28 Besides the peak at 529.9 eV BE, another peak appeared at 531.5 eV BE, which was assigned to the oxygen of hydroxyl groups. This finding suggested the existence of hydrogen in the lithium-extracted sample and the form of hydroxyl by combining with oxygen atoms, and some points of view may support the occurrence of H+/Li+ exchange reaction by combination with the analysis of Mn 2p spectrum. Figure 6 shows Mn 2p spectra of the samples. Results showed that the Mn 2p spectrum of the lithium-extracted sample were nearly the same with LiMn2O4 precursor, indicating that the surface Mn ions of lithium-extracted sample remained in the same average oxidation state as LiMn2O4, which was 3.5. However, the lithium was extracted thoroughly, and the surface oxygen atoms were associated with protons forming hydroxyl groups; thus, we infer that the composition at the surface is HMn2O4. This phenomenon could be explained by the following reasons: protons adsorbed on the surface to form the hydroxyl groups first, and then excessive protons replaced lithium ions from surface to the interior. In the surface disproportionation mechanism:

production, the use of (NH4)2S2O8 will produce a great amount of ammonia, which is harmful to the environment and brings difficulties to the subsequent treatment process. Compared with those of K2S2O8, production cost of Na2S2O8 is much lower and the solubility of Na2S2O8 is much larger in water, so Na2S2O8 should be the most suitable persulfate applied to the Li+ extraction process. Cyclic Performance of LiMn2O4 Treated with Persulfate. Table 1 shows the cyclic performance of LiMn2O4 treated Table 1. Cyclic Performance of LiMn2O4 Treated with Persulfate Na2S2O8 (0.2 mol/L, 80 °C)

1st extraction 1st adsorption 2nd extraction 2nd adsorption 3rd extraction 3rd adsorption 10th extraction 10th adsorption

lithium exchange capacity (mg/g)

manganese dissolution rate (%)

36.46 29.61 30.09 29.49 30.18 29.44 19.18 19.02

0.05 0.01 0.06 0.02 0.06 0.03 0.05 0.04

with persulfate. Because it is an eluent with practical meaning applied to the extraction process of lithium manganese oxides, its cyclic performance should be taken into consideration, which means not only the theoretical significance but also practical value. From the cyclic results in Table 1, we see that even the extraction and adsorption reactions (repeated three times, the samples treated with Na2S2O8 at 80 °C) always maintained both considerably high lithium exchange capacity and fairly little manganese dissolution loss. While the cycles increased to 10, there was still no obvious change about manganese dissolution loss, even though lithium exchange capacity showed a certain degree of reduction, it maintained at a relatively high level. We conclude that during the extraction/ insertion process, the lithium manganese oxide spinel structure maintained high cyclic stability and efficiency, and compared with the commonly used eluent, the cyclic performance longer

4(Li)[Mn IIIMn IV ]O4 + 8H+ → 3(□)[Mn IV 2]O4 + 4Li+ + 2Mn 2 + + 4H 2O

(6)

The electrons that migrated from the interior of the particle reduce the surface Mn3+ ions to Mn2+ ions with simultaneous formation of water. Protons adsorbed on the surface of the particle, forming surface hydroxyl groups first, and excessive protons exchanged with the surface lithium ions before the Mn2+ dissolution. However, it is not so easy for the migration of H+ to the interior or the exchange with Li+ ions through the

Figure 5. XPS O 1s spectra of (a) sample treated with Na2S2O8 at 80 °C and (b) LiMn2O4. 9892



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Figure 6. XPS Mn 2p spectra of (b) LiMn2O4 (a) sample treated with Na2S2O8 at 80 °C.

Table 2. Compositions of Precursor and Lithium-Extracted Samples samples

ZMn

Li/Mn molar ratio

oxygen/cation

chemical composition

LMO HMO−Na2S2O8 (20 °C) HMO−K2S2O8 (20 °C) HMO−Na2S2O8 (80 °C) HMO−K2S2O8 (80 °C) HMO−(NH4)2S2O8 (80 °C)

3.492 3.505 3.502 3.644 3.648 3.747

0.500 0.445 0.445 0.020 0.021 0.020

1.33 1.34 1.34 1.49 1.48 1.59

LiMn2O4 Li0.89H0.10Mn2O4 Li0.89H0.10Mn2O4 Li0.04H0.64Mn2O4 Li0.04H0.66Mn2O4 Li0.04H0.47Mn2O4

three-dimensional arrays of the spinel lattice without the vacancies at the 16d octahedral sites. Therefore, the dissolution reaction will not take place except for excess protons adsorbed on the surface of LiMn2O4. From what we have discussed above, the preceding problem that the high rate of lithium extraction and manganese dissolution loss took place only at low pH conditions could be well explained at the same time. As shown in Table 2, compared with the LiMn2O4 precursor, the average oxidation state of manganese in the lithiumextracted samples did not show a significant change as the rates of both lithium extraction and manganese dissolution loss were low (treated with Na2S2O8 and K2S2O8 at 20 °C). The H content in the persulfate-treated samples was equal to the amount of lithium extracted from the precursor, indicating the occurrence of H+/Li+ exchange reaction. The manganese dissolution loss was so great that it was impossible and meaningless to analyze the composition of sample treated with (NH4)2S2O8 at 20 °C; therefore, the corresponding composition was not listed in the table. However, average oxidation state of manganese of the lithium-extracted samples increased with high rates of both lithium extraction and manganese dissolution treated with three kinds of persulfate at 80 °C, and H content was less than the equivalent amount of lithium extracted from the precursor. In theory, if only the H+/Li+ exchange reaction existed, the average valence of manganese should not have variation. Analogously, the average valence of manganese may reach 4.0 if only redox reaction occurs. The experimental result that the valence of manganese was between 3.5 and 4.0 implied the existence of both the ion exchange reaction and redox reaction at the same time. XRD Characterization and Analysis. The results in Figure 7 show that the diffraction peaks corresponding to the

Figure 7. XRD characterization of ion-sieve eluted at different temperatures. Samples treated with (a) Na2S2O8, (b) K2S2O8, and (c) (NH4)2S2O8 at 80 °C; samples treated with (d) Na2S2O8, (e) K2S2O8, and (f) (NH4)2S2O8 at 20 °C; and (g) LiMn2O4.

spinel structure remained after the Li+ extraction. This indicated that Li+ extraction progressed topotactically, maintaining the spinel structure. However, compared with LiMn2O4 precursor, the overall peaks of lithium-extracted products shifted to the high angle direction, which meant that the lattice constant became smaller. This was caused by H+/Li+ ion exchange and manganese oxidation−reduction reaction: those protons who had much smaller ion radii than Li+ replaced the 9893



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position of Li+ in the particle; part of Mn(III) was oxidized to Mn(IV), leading to the enhancement of O2− polarization in the crystal framework, thus, Mn−O bonded more closely and the bond length became shorter, causing a decrease in the crystal lattice parameter. The overall peaks of samples treated with Na2S2O8 and K2S2O8 at 20 °C (Figure 7d,e) did not change much compared to the lithium-extracted samples treated with (NH4)2S2O8 at 20 °C (Figure 7f) and Na2S2O8, K2S2O8, and (NH4)2S2O8 at 80 °C (Figure 7a−c). According to the rules of lithium extraction rate, the degree of angle deviation was closely related to lithium content in samples: the lower the concentration of lithium, the greater the peaks shifted to the high-angle direction. FTIR Characterization and Analysis. Figure 8 shows the FTIR spectra of the precursor and the lithium-extracted

Roman-active, while only two modes were IR-active. Thus, the corresponding vibration modes could be detected by IR; we called the larger stretching vibration mode ν1 and the other ν2. As for Mn, the value of ν1 was usually 1.85 times as large as ν2. The value of the stretching vibration frequency (ν1) was related to the oxidation state and atomic mass of the central atom. Normally, the value of ν1 increased with rising oxidation state. Because the oxidation state of Mn4+ was higher than that of Mn3+, the stretching vibration frequency (ν1) of trivalent manganese should be larger than that of tetravalent manganese. Therefore, we infer that the bands at 501.5 and 618.6 cm−1 could be assigned to Mn(III)−O stretching vibrations and Mn(IV)−O stretching vibrations, respectively. However, there was a particular peak at about 1124 cm−1 in the infrared spectrum of LiMn2O4 that did not exist in the two kinds of ionsieves soaked by persulfate, and the peak was related to lithium positioned on the 8a tetrahedral sites,29 which resulted from antisymmetric stretching vibration of Li−O in the spinel structure (LiO4 tetrahedron). The peak at about 1124 cm−1 disappeared with the lithium ion nearly extracted thoroughly from LiMn2O4. In the infrared spectra of the three substances, no Mn−O stretching vibrations attributed to other manganese valence appeared. Effect of Sulfate Radical (SO4•−) on Decreasing Manganese Dissolution Loss. The almost completely different performance of persulfate at different temperatures suggested some specific substances produced at high-temperature conditions may be important to the inhibition of manganese dissolution. Persulfate anion (S2O82−) is a thermodynamically strong, two-electron oxidant (E0(S2O82−/ 2SO42−) = +2.0 V), but direct reaction of persulfate with most reductants is slow. When appropriately activated, persulfate will decompose to sulfate radical (SO4•−), which is a strong oneelectron oxidant (E0(SO4•−/SO42−) = +2.43 V).30 The activation of heat resulting radicals is as shown in the following formula:

Figure 8. FTIR spectra of samples treated with (a) Na2S2O8, (b) (NH4)2S2O8, and (c) LiMn2O4 at 80 °C.

S2 O82 − → 2SO4•−

samples. There were two sharp peaks in the infrared spectra of both the LiMn2O4 precursor and the lithium-extracted sample. The relationship between vibration modes and infrared activity could be explained on the basis of the knowledge of group theory. The structure of LiMn2O4 was classified as Fd3m space group, and manganese ions occupied octahedral sites. As for manganese ions, the vibration mode was similar to XY6 type and had six modes in total. Among them, three modes were

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Furthermore, the form that trivalent and tetravalent manganese alternately arranged on the skeleton provides a possibility for the electron transfer. When the electron migrated from the interior of the particle to the surface, Mn(III) accepted it and turned into Mn2+, which caused the disproportionation of manganese and the dissolution of

Figure 9. Effect of additional agents on lithium extraction and manganese loss rate. 9894



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manganese, while the appearance of SO4•− provided an additional preferable choice for electron acceptor. To reveal the effect of sulfate radical, some agents (methanol and ferrous salt solution) were added during the persulfate soaking process (Figure 9). The alcohols containing α-H31 (e.g., methanol) can be used as a capture agent for SO4•−, contrasting the two experiments at 80 °C: by adding methanol during the elution process, the ionic sieves still maintained a high rate of lithium extraction, but the rate of manganese dissolution loss reached a considerably high level. Once the electron transferred from the interior to the surface, Mn(III) may accept it and turn into Mn(II), dissolved into the liquid phase. The sulfate radical (SO4•−) activated by heat would replace Mn(III) as the acceptor of electron, shown in the following: SO4•− + Mn 3 + → Mn 4 + + SO4 2 −

ammonia, which is harmful to the environment. Compared with those of K2S2O8, production cost of Na2S2O8 is low and the solubility of Na2S2O8 is much greater; therefore, Na2S2O8 should be the most suitable persulfate applied to the Li+ extraction process. The reaction mechanism was based on the hydrolysis of persulfate accompanied by the ion exchange and oxidation−reduction reactions and included the participation of radicals.

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Corresponding Author

*Tel: +86(22)60204598. Fax: +86(22)60204274. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the National Natural Science Foundation of China (20806019), the Natural Science Foundation of Hebei Province (B2009000024), and the Science and Technology Research and Development Program of Hebei Province (12276713D). The work is also supported by the Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT, IRT1059), and Tianjin Research Program of Application Foundation and Advanced Technology (12JCQNJC03300).

Trivalent manganese was oxidized to tetravalent manganese and remained in the solid skeleton; the disproportionation of manganese was effectively blocked. Once we added methanol to the solution as the capture agent, SO4•− could be promptly removed, resulting in more dissolution of manganese. The interpretation of the results at 20 °C was similar to the above: the persulfate was inactive at low temperature, and manganese dissolution caused by disproportionation was obvious. FeSO4 and Na2S2O3 could stimulate the production of sulfate radicals in the following ways: Fe2 + + S2 O82 − → Fe3 + + SO4•− + SO4 2 −

Fe3 + + S2 O32 − → Fe 2 + +

1 S4 O6 2 − 2

xFe2 + + S2 O32 − → Complex anion

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REFERENCES

(1) Sun, S. Y.; Song, X.; Zhang, Q. H.; Wang, J.; Yu, J. G. Lithium Extraction/Insertion Process on Cubic Li-Mn-O Precursors with Different Li/Mn Ratio and Morphology. Adsorption 2011, 17, 88. (2) Chung, K. S.; Lee, J. C.; Kim, E. J.; Lee, K. C.; Kim, Y. S.; Ooi, K. Recovery of Lithium from Seawater Using Nano-manganese Oxide Adsorbents Prepared by Gel Process. Mater. Sci. Forum 2004, 449, 277. (3) Myung, S. T.; Komaba, S.; Kumagai, N. Hydrothermal Synthesis and Electrochemical Behavior of Orthorhombic LiMnO2. Electrochim. Acta 2002, 47, 3287. (4) Liu, W.; Farrington, G. C.; Chaput, F.; Dunn, B. Synthesis and Electrochemical Studies of Spinel Phase LiMn2O4 Cathode Materials Prepared by the Pechini Process. J. Electrochem. Soc. 1996, 143, 879. (5) Zhang, Q. H.; Li, S. P.; Sun, S. Y.; Yin, X. S.; Yu, J. G. LiMn2O4 Spinel Direct Synthesis and Lithium Ion Selective Adsorption. Chem. Eng. Sci. 2010, 65, 169. (6) Koyanaka, H.; Matsubaya, O.; Koyanaka, Y.; Hatta, N. Quantitative Correlation Between Li Absorption and H Content in Manganese Oxide Spinel λ-MnO2. J. Electroanal. Chem. 2003, 559, 77. (7) Kopec, M.; Dygas, J. R.; Krok, F.; Mauger, A.; Gendron, F.; Jaszczak-Figiel, B.; Gagor, A.; Zaghib, K.; Julien, C. M. Heavy-Fermion Behavior and Electrochemistry of Li1.27Mn1.73O4. Chem. Mater. 2009, 21, 2525. (8) Chitrakar, R.; Kanoh, H.; Miyai, Y.; Ooi, K. A New Type of Manganese Oxide (MnO2·0.5H2O) Derived from Li1.6Mn1.6O4 and Its Lithium Ion-Sieve Properties. Chem. Mater. 2000, 12, 3151. (9) Ammundsen, B.; Jones, D. J.; Rozière, J.; Berg, H.; Tellgren, R.; Thomas, J. O. Ion Exchange in Manganese Dioxide Spinel: Proton, Deuteron, and Lithium Sites Determined from Neutron Powder Diffraction Data. Chem. Mater. 1998, 10, 1680. (10) Xiao, J. L.; Sun, S. Y.; Wang, J.; Li, P.; Yu, Y. J. Synthesis and Adsorption Properties of Li1.6Mn1.6O4 Spinel. Ind. Eng. Chem. Res. 2013, 52, 11967. (11) Feng, Q.; Kanoh, H.; Miyai, Y.; Ooi, K. Li+ Extraction/Insertion Reactions with LiZn0.5Mn1.5O4 Spinel in the Aqueous Phase. Chem. Mater. 1995, 7, 379. (12) Sato, K.; Poojary, D. M.; Clearfield, A.; Kohno, M.; Inoue, Y. The Surface Structure of the Proton-Exchanged Lithium Manganese

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S2 O82 − + Complex anion → SO4•− + SO4 2 − + Fe3 + + Residue

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The rate of manganese dissolution sharply decreased from 25.02% to 4.68%.

4. CONCLUSIONS The performance of persulfate showed significant differences with changes of temperature. At low temperatures, the rates of both the lithium extraction and manganese dissolution were quite low as the precursor was treated with Na2S2O8 and K2S2O8, while the eluting effect of (NH4)2S2O8 was the opposite, with high rates of both the lithium extraction and the manganese dissolution loss. The disproportionation of the Mn3+ and H+/H2O2 dissolution effects are the two main reasons for the dissolution of manganese at low temperature, while the SO4•− produced by activating persulfate can be another acceptor of an electron, thus effectively inhibiting the disproportionation of manganese. At high temperatures, the lithium ion was nearly extracted thoroughly with almost no manganese dissolution loss; as we repeated the lithium extraction/insertion several times, the samples treated with Na2S2O8 at 80 °C maintained both considerably high lithium exchange capacity and fairly little manganese dissolution, which indicated that the lithium manganese oxide spinel structure maintained high cyclic stability and efficiency, and persulfate could be well applied to the LiMn2O4 extraction process. However, in practical applications, (NH4)2S2O8 may produce 9895



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Effective Recycling Performance of Li+ Extraction from Spinel-Type

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