Adsorption and Desorption Behavior of Lithium Ion in Spherical PVC

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Adsorption and Desorption Behavior of Lithium Ion in Spherical PVC−MnO2 Ion Sieve Guoping Xiao, Kefeng Tong, Longsheng Zhou, Jiali Xiao, Shuying Sun,* Ping Li, and Jianguo Yu* National Engineering Research Center for Integrated Utilization of Salt Lake Resources, East China University of Science and Technology, Shanghai, China ABSTRACT: A spherical PVC−MnO2 ion sieve of 2.0−3.5 mm diameter was prepared by the antisolvent method using synthesized Li4Mn5O12 ultrafine powder as the precursor, poly(vinyl chloride) as the binder, and N-methyl-2-pyrrolidone as solvent. Batch experiments of the adsorption capacity (isotherm) and adsorption rate of Li+ on the spherical PVC−MnO2 ion sieve were studied. Spherical PVC−MnO2 had a high adsorption capacity for Li+, and the isotherm data were well fitted by the Langmuir model; the adsorption kinetics were well described by the Lagergren equation. Furthermore, a mathematical model was set up to calculate the film mass transfer coefficient (kf) and pore diffusivity (Dp) of the adsorbent. Continuous flow experiments for study of Li+ adsorption breakthrough and the subsequent desorption (elution) in a PVC−MnO2 packed column were carried out employing six feed solutions of various pH values and concentrations of Li+, Na+, K+, and Mg2+ for simulating brine samples of various salt lakes and/or seawaters. After the adsorption treatment to concentrate the Li+ on PVC−MnO2, the column was regenerated by 1.0 mol/L HCl which supplied H+ to accomplish elution of the adsorbed Li+ by ion exchange. The experimental results demonstrate that PVC−MnO2 had high selectivity for Li+ and that its adsorption of Li+ from the feed were little affected by Na+, K+, and Mg2+ also present in the feed solution. Spherical PVC−MnO2 is an attractive medium for large scale lithium extraction from brine or seawater.

1. INTRODUCTION The recovery of Li+ from brine and seawater has attracted great interest in recent years due to its wide application in rechargeable lithium batteries1,2 and other related fields. In China, there are many salt lakes containing large amounts of lithium, such as Qarhan salt lake, West Taijar salt lake in Qinghai Province, and Lop Nur salt lake in Xinjiang Province. Therefore, developing an efficient proprietary technology to recover lithium from brine is of great value. The adsorption method has been recognized as the potentially most costeffective3 and environmentally friendly technology for recovering Li from high Mg/Li feed streams. Li−Mn−O ternary oxides have been used to prepare ion sieves for lithium recovery from aqueous solutions including brine and seawater,4−7 with extremely high selectivity for Li+ and stability because the Li−Mn−O framework maintained the cubic spinel structure8,9 during the Li+ inserting and extracting process.10,11 A series of lithium manganese oxide adsorbents of various Li/Mn molar ratios, including LiMn 2 O 4 , 1 2 Li1.33Mn1.67O4,13,14 and Li1.6Mn1.6O415−17 have been prepared via a sol−gel method,18−21 solid-state reaction,22 and a hydrothermal process.5,7 Li4Mn5O12 ultrafine powder4,23 (20− 140 nm in diameter and 0.8−4.0 μm in length) prepared in our lab had a remarkable selectivity for Li+; it was synthesized via a combination of hydrothermal reaction and low-temperature solid-phase calcination process. Furthermore, Miyai24 prepared a granular lithium adsorbent with high adsorption capacity (2.57 mmol/g) using poly(vinyl chloride) (PVC) as the binder and dimethylformamide (DMF) as the solvent. However, DMF is a toxic solvent under restriction and it is not suitable for use on a large scale. Therefore, we improve this granulation process by using N-methyl-2-pyrrolidone (NMP) as the solvent instead of DMF. Also, we have recently developed a spherical-type © 2012 American Chemical Society

PVC−MnO2 (H type of PVC−Li4Mn5O12) composite material, 2.0−3.5 mm in diameter, using Li4Mn5O12 ultrafine powder as the ion-sieve precursor, poly(vinyl chloride) as the binder, and N-methyl-2-pyrrolidone (NMP) as the solvent. This composite material well maintains the adsorption property of the ultrafine powder, and its column can process at a relatively high feed flow without a large pressure drop. It is important to optimize the adsorption system for recovering Li+ from brine/seawater to study the adsorption thermodynamics, kinetics, and selectivity. However, not much work has focused on this aspect. Wang25 investigated the activation of Li−Mn−O powder in a buffer solution (pH 8.0) and found that its adsorption capacity data for Li+ were well fitted by the Langmuir model, and the uptake data were consistent with pseudo-second-order kinetics. There has been no study on the adsorption behaviors of the lithium ion sieve after forming, and no systematic study on the selectivity of the lithium ion sieve. In this research, the adsorption behavior of a spherical-type PVC−MnO2 composite ion sieve was investigated at various adsorption temperatures. The adsorption capacity (isotherms) and uptake rate experiments were carried out in batch experiments, and the selective adsorption of Li+ was investigated in a continuous flow fixed-bed column employing six feed solutions to simulate the various brine/seawater samples. The Langmuir adsorption model was used to fit the isotherm data, and the pseudo-first-order kinetics model Received: Revised: Accepted: Published: 10921

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(Lagergren equation) was used to describe the rate data. The thermodynamic parameters and other related isothermal and kinetic coefficients were calculated to study the Li+ uptake process. Also, the mathematical model was set up to calculate the film mass transfer coefficient (kf) and pore diffusivity (Dp) of the adsorbent. The effects of coexisting cations in feed solution, such as Na+, K+, and Mg2+, on the selectivity and adsorption capacity of the spherical PVC−MnO2 ion sieve were investigated by adsorption breakthrough and desorption (elution) experiments.

Table 1. Characteristic Parameters of Spherical PVC−MnO2 Ion Sieve specific surf. area (m2/g) average outer diam of sphere (m) average inner diam of sphere (m) pellet density (kg/m3) pellet porosity bulk density (kg/m3) PVC content in sphere (wt %) MnO2 content in sphere (wt %)

2. EXPERIMENTAL SECTION 2.1. Preparation of Spherical PVC−MnO2 Ion Sieve. The spherical PVC−MnO2 ion sieve was prepared by the antisolvent method, where the synthesized Li4Mn5O12 ultrafine powder was used as a MnO2 ion-sieve precursor. Li4Mn5O12 ultrafine powder (20−140 nm in diameter and 0.8−4.0 μm in length) was first synthesized via a combination of hydrothermal reaction and solid-phase calcinations as described in our published work.4,23 The spherical PVC−MnO2 ion sieve was then prepared in the following manner: 4.87 g of poly(vinyl chloride) (PVC) was dissolved in 66.0 mL of N-methyl-2-pyrrolidone (NMP) by stirring until the PVC dissolved completely; 20.0 g of Li4Mn5O12 ultrafine powder was added to the solution and mixed uniformly to form a slurry. Then the slurry was dripped into deionized water to form spheres quickly. Finally, the spheres were washed completely with deionized water, dried at 378.0 K for 12 h in static air, and treated with a hydrochloric acid solution (1.0 mol·L−1, molar ratio H+/Li+ = 2) to extract lithium, resulting in the spherical PVC−MnO2 ion sieve with diameters of 2.0−3.5 mm, as shown in Figure 1, and analyzed by scanning electron microscopy (JSM-6360LV, JEOL, Japan). Also, the characteristic parameters of the spherical PVC−MnO2 ion sieve are shown in Table 1. Industrial grade PVC (polymerization degree = 1000 ± 20) was purchased from Shanghai Nuotai Chemical Co., Ltd., and analytically pure NMP was purchased from Shanghai Chemical Co. 2.2. Batch Experiments of Li+ Adsorption on Spherical PVC−MnO2 Ion Sieve. Li+ adsorption equilibrium experi-

33.65 3.11 × 10−3 1.94 × 10−3 410.6 0.782 267.1 20.4 79.6

ments were conducted by stirring (150 rev·min−1) PVC− MnO2 samples (containing about 0.10 g of MnO2) in LiCl solutions of various initial Li+ concentrations (1.08, 2.31, 2.78, 3.09, 3.88, 5.83, 9.82, 29.6 mmol·L−1) for 72 h at four temperatures (304.5, 309.0, 313.8, and 324.4 K); a 0.1 mol·L−1 NH4Cl and 0.1 mol·L−1 NH3·H2O buffer solution with the NH4Cl/NH3·H2O molar ratio of 0.25 was employed to adjust the initial pH to 10.10. The metal ion concentrations of the supernatant solution were determined in situ by a Metrohm 861 advanced compact ion chromatograph (Metrohm Co. Ltd., Switzerland) with a Metrosep C4-100 column (1.7 mmol·L−1 HNO3 and 0.7 mmol·L−1 dipicolinic acid as eluent). The equilibrium adsorption capacity (Qe) was calculated according to eq 1, in which Qe is the amount of metal ions adsorbed per gram of MnO2 in spherical PVC−MnO2 at equilibrium (mmol·g−1), Ce is the concentration of metal ions at equilibrium (mol·L−1), C0 is the initial concentration of lithium ions (mol·L−1), V is the solution volume (mL), and W is the weight of MnO2 in spherical PVC−MnO2 adsorbent (g). Q e = (C0 − Ce)V /W

(1)

+

Li adsorption kinetics were investigated by stirring (150 rev·min−1) spherical PVC−MnO2 samples (containing about 0.10 g of MnO2) in 100 mL of LiCl solution (pH 10.10) with a uniform initial concentration of Li ions (10.00 mmol·L−1) at the same four temperatures. The uptake capacity (Qt) was calculated according to eq 2, in which Qt is the amount of metal ions adsorbed per gram of MnO2 contained in spherical PVC− MnO2 at time t (mmol·g−1), Ct is the concentration of lithium ions at t time (mol·L−1), C0 is the initial concentration of lithium ions (mol·L−1), V is the solution volume (mL), and W is the weight of the ion sieve in spherical PVC−MnO2 adsorbent (g). Q t = (C0 − Ct )V /W

(2) +

2.3. Continuous Flow Experiments of Li Adsorption and Desorption in PVC−MnO2 Column. The PVC−MnO2 packed column employed for the Li+ adsorption and desorption experiments had an inner diameter of 15 mm and a packed height of 150 mm; Figure 2 depicts the experimental setup for the study. The Li+ containing solution was pumped to feed the column from the bottom at a flow rate of 3.0 mL·min−1, equivalent to a liquid hourly space velocity (LHSV) of 6.79 h−1. After PVC−MnO2 in the column became fully saturated with Li+, as evidenced by its total breakthrough in the effluent, the column was washed with deionized water and then the adsorbed Li+ was desorbed (eluted) by 1.0 mol·L−1 hydrochloric acid solution at a flow rate of 1.0 mL·min−1, equivalent to a LHSV of 2.26 h−1. Six feed solutions of different pH values and/or concentrations of Li+, Na+, K+, and Mg2+ were prepared to simulate the

Figure 1. SEM photomicrographs of spherical PVC−MnO2 ion sieve: (A) whole particle, (B) profile image of the particle, (C) exterior particle surface, and (D) a section of the particle. 10922

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Figure 2. Schematic diagram of the PVC−MnO2 column for dynamic adsorption−desorption experiments.

brine samples of various salt lakes. For example, at Qarhan salt lake in China, the Li+ concentration is about 32.0 mmol·L−1 and the Mg2+ concentration is much higher at 50.0−960 mmol·L−1 with relative pH values. In some brines, the Na+ and K+ concentrations are also very high, up to 650 and 350 mmol·L−1, respectively. Table 2 presents the characteristics of the six feed solutions of the PVC−MnO2 column for the Li+ adsorption and desorption experiments.

Figure 3. Li+ adsorption isotherm data and Langmuir representations for spherical PVC−MnO2 ion sieve at four temperatures (pH 10.10).

Table 3. Langmuir Isotherms of PVC−MnO2’s Adsorption Capacity for Li+ at Four Temperatures (pH 10.10)

Table 2. Characteristics of the Feed Solutions of the PVC− MnO2 Column for Li+ Adsorption and Desorption Experiments feed

Li+ (mmol·L−1)

Na+ (mmol·L−1)

K+ (mmol·L−1)

Mg2+ (mmol·L−1)

pH value

1 2 3 4 5 6

32.0 32.0 32.0 32.0 32.0 32.0

− 650 − − − −

− 350 − − − −

− − − 50.0 − 960

10.10 10.10 9.19 9.19 8.06 8.01

temp (K)

Qm (mmol/g(MnO2))

KL (L·mmol−1)

R12

324.4 313.8 309.0 304.5

3.41 3.26 3.09 2.88

6.20 5.18 6.06 8.92

0.9999 0.9999 0.9999 0.9999

The extremely high correlation coefficients (R12 = 0.9999) shows the near-perfect fitting of the isotherm data by the Langmuir model because the adsorption of Li+ on PVC−MnO2 was the monolayer adsorption of the Li+−H+ exchange reaction and the Li4Mn5O12 precursor was formed by the homogeneously structured tetravalent manganese. The standard free energy (ΔG°) was calculated to study the process of lithium uptake from aqueous solution according to eq 4.28

3. RESULTS AND DISCUSSION 3.1. Li+ Adsorption Isotherms on Spherical PVC− MnO2 Ion Sieve. The batch equilibrium isotherm data for adsorption of Li+ on PVC−MnO2 were transformed for estimating the parameters of the Langmuir adsorption model (eq 3).26 Ce 1 1 = Ce + (Langmuir model) Qe Qm KLQ m (3)

(4)

ΔG° = −RT ln b

where R is the universal gas constant, T is the absolute temperature, and b is the Langmuir constant (b = KLQm). The calculated ΔG° is between −8.23 and −7.38 kJ·mol−1 (below zero), indicating the spontaneous nature of Li+ adsorption on PVC−MnO2.29 3.2. Li+ Adsorption Kinetics on Spherical PVC−MnO2 Ion Sieve. To optimize the adsorption technology system for recovering lithium from brine, it is important to model the adsorption uptake data. In this aspect, Li+ adsorption kinetics on the spherical PVC−MnO2 ion sieve is modeled with the pseudo-first-order kinetics Lagergren equation (eq 5).26,30

where Qe (mmol·g−1) is the amount of Li+ adsorbed per gram of MnO 2 in spherical PVC−MnO 2 at equilibrium, Q m (mmol·g−1) is the theoretical maximum monolayer adsorption capacity, Ce (mmol·L−1) is the equilibrium concentration of Li+ in solution, and KL (L·mmol−1) is the Langmuir empirical constant. Figure 3 shows the Li+ adsorption isotherm data and the Langmuir representations for spherical PVC−MnO2 ion sieve at four temperatures (pH 10.10). The equilibrium adsorption capacity (Qe) increases sharply with the Li+ equilibrium concentration (Ce) up to about 2.35 mmol·L−1 and then increases gradually to the maximum Li+ equilibrium adsorption capacities of 3.38, 3.24, 3.07, and 2.86 mmol·g−1 at 324.4, 313.8, 309.0, and 304.5 K, respectively. The equilibrium adsorption capacity increases with the rising adsorption temperature, and this phenomenon is also observed in similar literature,25,27 which indicates that the adsorption on the ion sieve is an endothermic reaction. Also, the Langmuir isotherm parameters obtained from the data fitting are listed in Table 3.

ln(Q e − Q t ) = ln Q e − kadst

(Lagergren equation) (5)

−1

+

in which Qe (mmol·g ) is the amount of Li adsorbed per gram of MnO2 contained in the spherical PVC−MnO2 ion sieve at equilibrium, Qt (mmol·g−1) is the amount of Li+ adsorbed per gram of MnO2 contained in spherical PVC− MnO2 at time t, kads (h−1) is the adsorption rate constant, and t (h) is the contact time. Figure 4 presents the experimental data and model simulation by the pseudo-first-order kinetics Lagergren equation for Li+ uptake on PVC−MnO2 at four temperatures (pH 10.10). The adsorption uptake increases rapidly in the initial 12 h and then increases gradually to approach the equilibrium state at the end of the experiments (72 h). Also, the equilibrium adsorption capacity (72 h) increases slightly with 10923

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Figure 5. Arrhenius plot of ln kads vs 1/T.

Figure 4. Li+ adsorption kinetics data and Lagergren representations for spherical PVC−MnO2 ion sieve at four temperatures (pH 10.10).

up the mathematical model to calculate the film mass transfer coefficient (kf) and pore diffusivity (Dp).32,33

the adsorption temperature, further supporting that the adsorption of Li+ on PVC−MnO2 is an endothermic process. Further, Table 4 summarizes the model parameters and the

mass balance of the liquid phase in batch experiment: 3Wa dC + V k f [C − (Cp)R 0= R p ] = 0 ρp R p dt

Table 4. Kinetics Parameters of PVC−MnO2 at Four Temperatures (pH 10.10) R22 kads (h−1)

324.4 K

313.8 K

309.0 K

304.5 K

0.9760 0.0678

0.9355 0.0663

0.9756 0.0653

0.9667 0.0644

in which V is the solution volume, C is the concentration of Li+ in bulk solution, Cp is the concentration of Li+ in the adsorbent pore, Rp is the radius of the spherical PVC−MnO2, Wa and ρp are the mass and the pellet density of spherical PVC−MnO2, and kf is the film mass transfer coefficient. mass balance in spherical PVC−MnO2 ion sieve:

correlation coefficients obtained by fitting the Li+ uptake data with the Lagergren equation. The correlation coefficients (R22) for the Lagergren equation at four temperatures are higher than 0.90, which shows that the model can be used to describe Li+ adsorption on the spherical PVC−MnO2 ion sieve. Also, the values of the adsorption rate constant (kads) were calculated to be 0.0678, 0.0663, 0.0653, and 0.0644 h−1 at 324.4, 313.8, 309.0, and 304.5 K, respectively, which decreased 1 order of magnitude compared with powder (1.19 h−1).4 The results show that the adsorption rate is seriously affected by the granulation process. This is because the larger particle size would aggravate the diffusion resistance, and PVC could block the structure pore and decrease the hydrophilicity of the lithium ion sieve. This negative effect is easily seen during the granulation process,29 and further research to reduce the negative effect has been in progress. The adsorption rate constant (kads) which increased with temperature was used in the Arrhenius plot of ln kads vs 1/T as shown in Figure 5; the slope of the straight line fitting of the transformed data (R2 = 0.9918) was −Ea/R, according to the well-known Arrhenius equation (eq 6):26 ln kads = ln A −

Ea 1 RT

(7)

εp

∂Cp ∂t

⎛ ∂ 2C ⎞ ∂Q̂ 2 ∂Cp ⎟ p = Dp⎜⎜ + 2 R 0 ∂R 0 ⎟⎠ ∂t ⎝ ∂R 0

+ ρp

(R c ≤ R 0 ≤ R p)

(8)

in which εp is the pellet porosity, Q̂ is the amount of Li+ adsorbed per gram of PVC−MnO2, and Dp is the pore diffusivity. The boundary conditions are Dp

∂Cp ∂R 0

= k f (C − Cp|(t , R p)) (t , R p)

∂Cp ∂R 0

(9)

=0 (10)

(t , R c)

The initial condition is t = 0;

C = 0;

Cp = 0

(11)

Q̂ can be assumed to be Q̂ = WMnO2

(6)

Since the calculated Ea value of 2.17 kJ·mol−1 was less than 20 kJ·mol−1, the adsorption of Li+ on PVC−MnO2 was presumably a diffusion-controlled process.31 In the solution system, the mass transfer process between solid adsorbent and liquid adsorbate includes the following four continuous steps: convection diffusion, membrane diffusion, particle diffusion, and ion exchange or chemical adsorption. For a solid−liquid sorption process, the solute transfer is usually characterized by film diffusion (external mass transfer), intraparticle diffusion (pore diffusion), or both. In order to investigate the mechanism of the Li+ adsorption process, we set

Q mKLCp 1 + KLCp

(12)

in which WMnO2 is the MnO2 content in sphere, and Qm and KL are described in eq 3. We introduce the dimensionless model parameters (Biot number): KfR p (Biot number) Bi = Dp (13) The experimental results and theoretical model predictions are shown in Figure 6. It can be observed that the mathematical model can well describe the batch experimental results. Also, 10924

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Figure 6. Adsorption results and simulated curves of spherical PVC−MnO2 at four temperatures (pH 10.10): solid lines, theoretical model predictions; points, experimental values.

illustrate the adsorption breakthrough and elution curves of Li+ in the PVC−MnO2 column for the six cases of Li+ feed solutions with or without Na+, K+, and Mg2+, respectively. The breakthrough curves of Li+, Na+, and K+ for the columns fed with numbers 1 and 2 feed solutions (columns 1 and 2, respectively) are shown in Figure 7a. The presence of high concentrations of Na+ and K+ in the feed solution resulted in the slightly earlier breakthrough of Li+ from column 2 relative to column 1, and that Na+ and K+ broke through column 2 much earlier than Li+ is evidence of high adsorption selectivity for Li+. The dynamic adsorption capacities (loadings) for Li+ in columns 1 and 2 were 4.14 and 3.65 mmol·g−1, respectively; the reduced loading was due to the competitive adsorptions of Na+ and K+ in the feed solution. The adsorption capacity of PVC− MnO2 in column 1 is less than that of the powder (6.62 mmol·g−1),4 but the spherical PVC−MnO2 ion sieve well maintains the adsorption property of the powder. The elution curves of Li+, Na+, and K+ for columns 1 and 2 shown in Figure 7b demonstrate clearly that the adsorbed Li+, Na+, and K+ can be easily extracted from the spherical PVC− MnO2 adsorbent. Na+ and K+ were completely extracted by 70 mL of 1.0 mol·L−1 HCl, while Li+ was extracted completely by 150 mL. From the elution curves, 4.13 and 3.57 mmol·g−1 Li+ were extracted from columns 1 and 2, respectively; such amounts were evidence of that the Li+ loadings (4.14 and 3.65 mmol·g−1) were almost all eluted. The elution recovered much less of Na+ and K+ (0.12 mmol·g−1 each) at the same time. The maximum Li+ concentrations of the eluent samples were 554 and 494 mmol·L−1 for columns 1 and 2, respectively; for column 2, the maximum Li+ enrichment factor was 15.4 and the molar ratios of Li/Na and Li/K were 25.3 and 20.6, respectively. Using more concentrated HCl for extraction would result in a higher maximum Li+ enrichment factor. Also, the concentrations of manganese in the eluent (1000 mL) measured by atomic absorption spectrometry were 25.5 and

the kinetics parameters are summarized in Table 5. Biot numbers are in the range 170−220, and they decrease with the Table 5. Lagergren Kinetics Model Representations of Li+ Uptake Data for PVC−MnO2 at Four Temperatures (pH 10.10) temp (K)

Bi

kf (×10−5 m·s−1)

Dp (×10−10 m2·s−1)

324.4 313.8 309.0 304.5

220 190 180 170

2.5 2.2 2.0 1.8

1.8 1.8 1.7 1.6

adsorption temperature. Furthermore, the film mass transfer coefficients (kf) are in the range (1.8−2.5) × 10−5 m·s−1 and increase with the adsorption temperature. Therefore, the trends of the Biot number and kf at different temperatures indicate that high temperature favors the film mass diffusion. The pore diffusivity (Dp) is relatively stable in the investigated temperature range, and it is calculated to be in the range (1.6−1.8) × 10−10 m2·s−1, very close to the results reported by Miyai.32 By comparison with the values of kf and Dp, we can easily conclude that the Li+ uptake process is controlled by intraparticle diffusion.32,34 Therefore, optimization of the pore structure of the lithium ion sieve is important for improving the adsorption rate. 3.3. Adsorption and Desorption Behaviors of Li+ in PVC−MnO2 Column. The adsorption and desorption (elution) behaviors of Li+ in the PVC−MnO2 packed column were investigated experimentally. After PVC−MnO2 became saturated by continuous recovery of Li+ from the feed solution, which entered the column from the bottom at 3.0 mL·min−1, the column was washed by deionized water and then eluted by 1.0 mol·L−1 hydrochloric acid solution at a constant flow rate of 1.0 mL·min−1 to recover the adsorbed Li+. Figures 7, 8, and 9 10925

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Figure 7. Adsorption breakthrough and elution curves of Li+, Na+, and K+ for columns 1 and 2.

Figure 8. Adsorption breakthrough and elution curves of Li+ and Mg2+ for columns 3 and 4.

25.9 mg·L−1 for columns 1 and 2, respectively, which corresponded to be about 1.00% loss of the lithium adsorbent. After three cycles, Mn ions cannot be detected in eluent, leading to the conclusion that the adsorbents have good chemical stabilities. The effects of Mg2+ on the selective adsorption were determined employing Mg2+ contained in feed solutions. The breakthrough curves for column 3 (number 3 feed solution) and column 4 (number 4 feed solution) are shown in Figure 8a; those for column 5 (number 5 feed solution) and column 6 (number 6 feed solution) are shown in Figure 9a. The presence of Mg2+ in the feed resulted in the faster breakthrough of Li+ (column 4 vs column 3 and column 6 vs column 5), and the competitive adsorption effect was more notable in column 6 treating a feed containing much more Mg2+. This is because Mg2+ concentrations of column 4 and 6 effluents rose more rapidly relative to Li+ because it was less well adsorbed on PVC−MnO2. The elution curves for columns 3 and 4 are shown in Figure 8b, and those for columns 5 and 6 are shown in Figure 9b. Li+ and Mg2+ were readily extracted from the adsorbent using 1.0 mol·L−1 HCl; Li+ was totally extracted by 150 mL of HCl while Mg2+ was nearly all extracted by 75 mL of HCl. Table 6 summarizes the Li+ loadings, as calculated from the adsorption breakthrough curves, and the amounts of cations extracted from the adsorbent, as calculated from the elution curves, for the six PVC−MnO2 packed columns. The adsorbed

Li+ and presumably Na+, K+, and Mg2+ were almost completely desorbed and eluted due to ion exchange of H+. The presence of other metal ions reduced the Li+ loadings; Na+ and K+ had less competitive adsorption effects than Mg2+ had. The average concentrations of the metal ions in eluent containing different percentages of the Li+ loading were calculated from the elution curves and are presented with the feed in Table 7. The average Li+ concentration in eluent with 95% Li+ loading was much higher than that with 98% loading, due to the smearing phenomena occurring in the lithium elution process. Also, the Li+ concentration of the eluent was much higher than that of the feed solution using the small lab column (15 mm diameter × 150 mm). The concentration factor can be increased using a larger adsorption column. The maximum concentration of Li+ in eluent from column 1 was 554 mmol·L−1, indicating that the HCl eluent can be reused to reduce the cost of Li+ recovery. Through the ion concentrations of the feed solution and eluent for column 2, we can see that the molar ratios of Li/Na and Li/K in the adsorption solution are 0.049 and 0.091, respectively, and they change to 29.2 and 28.9 in eluent. Also, the molar ratios of Li/Mg for columns 4 and 6 are 0.640 and 0.033, respectively, and they change to 43.3 and 16.3 in corresponding eluent. Relative to Na+ and K+, the presence of Mg2+ in the feed resulted in more reductions in Li+ loading and selectivity of the PVC−MnO2 column. The relatively high selectivity for Li+ can be well explained by the ion-sieve effect of the spinel lattice with a three-dimensional 10926

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Table 7. Summary of PVC−MnO2 Column Runs: Compositions of Feed and Corresponding Eluent Solutions composition (mmol·L−1) a

+

column

sample

Li

1

feed 1 elu.(95%) elu.(98%) feed 2 elu.(95%) elu.(98%) feed 3 elu.(95%) elu.(98%) feed 4 elu.(95%) elu.(98%) feed 5 elu.(95%) elu.(98%) feed 6 elu.(95%) elu.(98%)

32.0 280 217 32.0 177 146 32.0 227 192 32.0 198 164 32.0 229 157 32.0 166 141

2

3

4

5

6

Na+

K+

Mg2+

− − − 650 6.03 4.84 − − − −

− − − 350 6.12 4.92 − − − −

−

−

− − − − − − − − − 50.0 4.57 3.68 −

−

−

960 10.2 8.41

a

In this column: elu.(95%), eluent sample containing 95% Li+ loading; elu.(98%), eluent sample containing 98% Li+ loading.

4. CONCLUSION The results of the batch and continuous flow adsorption− desorption experiments have demonstrated that the spherical PVC−MnO2 ion sieve prepared by the antisolvent method has a high capacity for Li+ with a maximum Li+ equilibrium adsorption capacity of about 3.38 mmol·g−1. The adsorption isotherm data are well fitted by the Langmuir model, and the adsorption kinetics are well described by the Lagergren equation. Furthermore, a mathematical model was set up to calculate the film mass transfer coefficient (kf) and pore diffusivity (Dp). The values of kf are in the range (1.8−2.5) × 10−5 m·s−1; values of Dp are in the range (1.6−1.8) × 10−10 m2·s−1. Therefore, the Li+ uptake process is controlled by intraparticle diffusion. The ion-sieve adsorbent has high selectivity for Li+. The presence of Na+, K+, and Mg2+ in the feed solution modestly reduces the adsorbent packed column’s capacity for Li+ due to competitive adsorption; however, the effects on the recovery of Li+ loading and the concentration of Li+ in eluent solution are small. Li+ adsorbed on the spherical PVC−MnO2 ion sieve can be desorbed by exchange with H+ ions in 1.0 mol·L−1 HCl solution. The concentration of Li+ of the feed solution is enriched up to 17 times in the eluent; the maximum concentration of Li+ in the eluent is as high as 554 mmol·L−1 compared to 32.0 mmol·L−1 in the feed solution. Therefore, the spherical PVC−MnO2 ion sieve has a good potential in industrial application for lithium extraction from brine or seawater.

Figure 9. Adsorption breakthrough and elution curves of Li+ and Mg2+ for columns 5 and 6.

Table 6. Summary of PVC−MnO2 Column Runs: Li+ Removed by Adsorption and Metals Recovered by Elution amount recovered (mmol/g(MnO2)) column

Li+ loading (mmol/g(MnO2))

Li+

Na+

K+

Mg2+

1 2 3 4 5 6

4.14 3.65 3.99 3.51 3.09 2.32

4.13 3.57 4.01 3.33 2.99 2.24

− 0.12 − − − −

− 0.12 − − − −

− − − 0.07 − 0.13

(1 × 3) tunnel suitable in size for fixing lithium ions in cubic phase MnO2 nanocrystal obtained from Li4Mn5O12 precursor. The spherical ion sieve had a higher selectivity for Li+ than for Mg2+ in spite of their similar ionic sizes. Since the hydration energy of Mg2+ is about 4 times35 higher than that of Li+, a higher energy may be required for dehydration to enter the micropores of a lithium ion sieve. Since adsorption of Li+ was due to the ion exchange for H+, the higher feed pH was responsible for the higher Li+ loading of column 1 relative to columns 3 and 5.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.Y.); [email protected] (S.S.). Fax: 86-21-64252826. Tel.: 86-21-64252826. Notes

The authors declare no competing financial interest. 10927

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ACKNOWLEDGMENTS The research was supported by NSFC (20906022), the National 863 Program (2012AA061601), STCSM (11dz1205202), and the Fundamental Research Funds for the Central Universities.

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NOTATION Qe = amount of metal ions adsorbed per gram of MnO2 in spherical PVC−MnO2 at equilibrium, mmol/g(MnO2) Qt = amount of metal ions adsorbed per gram of MnO2 contained in spherical PVC−MnO2 at time t, mmol/ g(MnO2) Qm = theoretical maximum monolayer adsorption capacity calculated according to Langmuir equation, mmol/g(MnO2) Q̂ = amount of Li+ adsorbed per gram of PVC−MnO2, mmol/g Ce = concentration of metal ions in bulk solution at equilibrium, mol·L−1 C0 = initial concentration of lithium ions in bulk solution, mol·L−1 Cp = concentration of Li+ in adsorbent pore V = solution volume, mL W = weight of MnO2 in spherical PVC−MnO2 adsorbent, g KL = Langmuir empirical constant, L·mmol−1 R = universal gas constant T = absolute temperature, K ΔG° = standard free energy, kJ·mol−1 kads = adsorption rate constant, h−1 t = contact time, h Ea = apparent activation energy, kJ·mol−1 R0 = radius Rp = radius of spherical PVC−MnO2, m Rc = radius of hollow existing in spherical PVC−MnO2, m Wa = mass of spherical PVC−MnO2 ρp = pellet density of spherical PVC−MnO2, kg/m3 kf = film mass transfer coefficient, m·s−1 Dp = pore diffusivity, m2·s−1 εp = pellet porosity WMnO2 = MnO2 content in spherical PVC−MnO2 Bi = Biot number (Bi = (KfRp)/Dp)

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