A Facile Synthesis of Hexagonal Spinel λ-MnO₂ Ion-Sieves for Highly Selective Li⁺ Adsorption

Abstract

Ion-sieves are a class of green adsorbent for extraction Li⁺ from salt lakes. Here, we propose a facile synthesis of hexagonal spinel LiMn₂O₄ (LMO) precursor under mild condition which was first prepared via a modified one-pot reduction hydrothermal method using KMnO₄ and ethanol. Subsequently, the stable spinel structured λ-MnO₂ (HMO) were prepared by acidification of LMO. The as-prepared HMO shows a unique hexagonal shape and can be used for rapid adsorption-desorption process for Li⁺ adsorption. It was found that Li⁺ adsorption capacity of HMO was 24.7 mg·g⁻¹ in Li⁺ solution and the HMO also has a stable structure with manganese dissolution loss ratio of 3.9% during desorption process. Moreover, the lithium selectivity $({\alpha }_{\mathrm{Mg}}^{\mathrm{Li}})$ reaches to 1.35 × 10³ in brine and the distribution coefficients ($K_{\mathrm{d}}$) of Li⁺ is much greater than that of Mg²⁺. The results implied that HMO can be used in extract lithium from brine or seawater containing high ratio of magnesium and lithium.

1. Introduction

Lithium and its compounds—known as “industrial monosodium glutamate” [1]—are widely used in significant fields such as batteries, ceramics, glass, alloy, lubricants, refrigerants and the nuclear industry [2,3]. The lithium reserves in China are the world’s second-largest, which are primarily distributed in the salt lakes of Qinghai and Tibet [4]. However, the ratio of magnesium to lithium in the salty brine is extremely high, making it difficult to extract and recover lithium using conventional separation technologies [5,6]. Compared with precipitation and solvent extraction methods, ion-sieve adsorption has many technical merits, such as excellent selectivity and relatively low cost [7,8], which is considered to be the most promising environmentally benign technology for extracting lithium from salt lakes [9,10].

Manganese series spinel ion-sieves are widely used in lithium ion adsorption, which primarily includes λ-MnO₂, MnO₂·0.3H₂O and MnO₂·0.5H₂O, after removal of lithium by acidification from precursors LiMn₂O₄ [11], Li₄Mn₅O₁₂ [12,13] and Li_1.6Mn_1.6O₄ [14,15,16], respectively. LiMn₂O₄ (LMO) is commonly used adsorbent precursor⁸, which is fabricated through embedding the target Li⁺ in the Mn-O chemical skeleton to construct composite Li_xMn_yO_z. After extracting Li⁺ by acidification without damages in the structure, of λ-MnO₂ (HMO) with regular vacancy [17]. The cubic spinel structures and adsorption-desorption relationship of HMO and LMO is shown in Figure 1. Oxygen atoms(O), Mn³⁺/Mn⁴⁺ and lithium atoms (Li) occupy 32 e, 16 d and 8 a of the Wyckoff site, respectively [18]. Then, lithium at the 8a position is extracted by hydrogen because of ion exchange process which can adsorption Li⁺ subsequently.

In general, existing methods of preparing LMO can be boiled down to two categories, the solid-phase and the liquid-phase. Yuan et al. [19] utilized Li₂CO₃ and MnCO₃ (Li/Mn molar ratio was 0.5) as raw materials and calcined the mixture at 800 °C in air for 5 h to obtain the LMO. Park et al. [20] prepared spinel LMO by a simple spray mixed Li(NO)₃ and Mn(NO)₃ pyrolysis at 700 °C, with the deficiencies of inhomogeneity and large particle sizes (1 μm). The above-mentioned solid-phase LMO preparation methods often require high energy consumption and involve multiple steps. Besides, they always result in yielding large size particles because of agglomeration which decrease its contact area with solution for Li⁺ extraction. The liquid-phase method (also known as soft-chemical process) for fabrication of LMO usually exhibits high purity, excellent crystal integrity and good dispersion. Tang et al. [21] prepared a nano-chain LMO using a sol-gel method. LiNO₃ and Mn(NO₃)₂ were stirred with the assistance of the starch at 110 °C for 1.5 h, followed by heating at 250 °C for 3 h and a thermal treatment at 700 °C for 3 h. Xiao et al. [22] prepared the ultrafine LMO powder by mixing Mn(NO₃)₂ with ammonia to produce precipitate, then they impregnated the precipitate with LiOH·H₂O and calcined the mixture at 830 °C for 8 h. Zhang et al. [23] prepared cubic phase LMO via a hydrothermal method by reacting Mn(NO₃)₂ with LiOH and H₂O₂ at 110 °C for 8 h. Despite the liquid-phase method being well investigated and developed, simplifying LMO synthetic process and improving the adsorbing ability and selectivity are still challenging. The existing methods mainly use LiOH·H₂O solution or acidic salts as raw materials. To our best knowledge, neutral synthetic routes through one-pot hydrothermal reaction to produce LMO and the corresponded HMO are rarely reported. Besides, HMO synthesized from high-valence manganese always shows higher adsorption capacity and selectivity than that of HMO synthesized from low-valence manganese, which is beneficial for lithium extraction from brine with high Li⁺/Mg²⁺ ratio.

In this study, a series of LMO was prepared by a facile one-pot hydrothermal method using ethanol as reductant, KMnO₄ and LiCl·H₂O as precursors. We first optimized several synthetic parameters (i.e., LiCl·H₂O concentration, mass of KMnO₄, volume of ethanol, reaction time and reaction temperature) in preparing of LMO. Then we prepared the stable HMO by acidification treatment of LMO. The crystallization phase, morphology characteristic and chemical phase of as-prepared ion-sieves were systematically investigated. The Li⁺ adsorption performance of HMO was studied and relevant adsorption kinetic model and adsorption isotherm were fitted. Finally, Li⁺ extraction capacity and selectivity in brine containing high ratio of Mg²⁺ and Li⁺ were studied.

2. Experimental

2.1. Preparation of LMO and HMO Ion Sieve

All chemicals used in this work are AR reagents unless otherwise noted. The detailed synthetic parameters are given in

Table 1. Experimental parameters of synthesis LiMn₂O₄ (LMO) at different schemes.

Experiment Group	LiCl·H2O (mol·L−1)	KMnO4 (g)	Ethanol (V, %)	React. Time (h)	React. Temp. (°C)
1	a	3	7.5	12	160
2	11	b	7.5	12	160
3	11	3	c	12	160
4	11	3	7.5	d	160
5	11	3	7.5	12	e

Note: a = 4, 7, 11; b = 5, 7, 9; c = 2.5, 7.5, 8.75; d = 8, 10, 12; e = 130, 160, 180.

. Briefly, a certain amount of LiCl·H₂O and KMnO₄ were added to 75 mL deionized water. Then, ethanol was dropwise added into the mixed homogeneous solution. The final solution was obtained with the addition of deionized water to 150 mL. Next, the solution was transferred into a polytetrafluoroethylene (PTFE)-lined stainless-steel autoclave, heated at the specified temperatures (130–180 °C) for the specified time and cooled naturally to room temperature. The black precipitate was collected, filtered, washed completely and then dried at 80 °C for 12 h to obtain the as-prepared LiMn₂O₄ (LMO). Subsequently, the obtained LMO was added in hydrochloric acid solution (0.1 mol·L⁻¹) at 20 °C for 24 h until the lithium were completely extracted. The resulting precipitate was filtered, washed completely and dried at 80 °C for 12 h to obtain the λ-MnO₂ (HMO).

2.2. Characterization

The phase composition of the samples was characterized by X-ray powder diffraction (XRD, Mini Flex600, Rigaku Coporation, Tokyo, Japan with monochromatized Cu Kα radiation (λ = 1.54056 Å), operating at 40 kV and 15 mA, with a scanning rate of 20°/min from 10° to 80°. The concentration of each ion was measured by Inductively Coupled Plasma (ICP, Optima 7000DV, Perkin Elmer, Waltham, MA, USA), which was used to examine adsorption/desorption activity of the samples. The morphology of the samples was examined by scanning electron microscopy (SEM, S-4800, Hitachi, Tokyo, Japan) while morphology and crystal lattice were obtained by high resolution transmission electron microscopy (HRTEM, Libra120, Carl Zeiss AG, Jena, Germany). The chemical phase of manganese in the sample was analyzed by X-ray photoelectron spectroscopy (XPS, EscaLab 250Xi, Thermo Fisher, Shang Hai, China), with AlK_α radiation (h_v = 1103 eV), C1s of 20.05 eV to calibration.

2.3. Adsorption Behavior

2.3.1. Adsorption Capacity Test at Different pH Value

The lithium ion adsorption behavior test was measured by stirring (200 rpm) 0.1 g HMO in 500 mL LiCl·H₂O solution (pH value: 4, 5, 6, 7, 8, 9, 10 and 11, respectively), adjusted by a buffer solution composed of 0.1 mol·L⁻¹ NH₄Cl and 0.1 mol·L⁻¹ HCl and 0.1 mol·L⁻¹ NH₄OH) with a uniform initial concentration of lithium ions (50 mg·L⁻¹) at 18 °C for 12 h.

The adsorption capacity is calculated by Equation (1).

$$Q_{\mathrm{t}}=C_0-C_{\mathrm{t}}\times V/W$$

(1)

where C₀ is the initial concentration of metal ions (mg·L⁻¹); C_t is the concentration of metal ions at time t (mg·L⁻¹); V is the volume of solution (L); and W is the weight of HMO ion sieve (g).

2.3.2. Static Kinetic Test

The lithium ion adsorption behavior test was measured by stirring (200 rpm) 0.1 g HMO in 500 mL LiCl·H₂O solution (Ph = 10, adjusted by a buffer solution composed of 0.1 mol·L⁻¹ NH₄Cl and 0.1 mol·L⁻¹ NH₄OH) with a uniform initial concentration of lithium ions (50 mg·L⁻¹) at 18 °C for 12 h.

The data of the HMO adsorption capacity was fitted by a simplified Crank’s single-hole diffusion model to obtain an efficient film coefficient (D_e) by Equation (2) [24,25].

$$\frac{Q_{\mathrm{t}}}{Q_{\infty }}=1-\frac{6}{{\pi }^2}\times exp\left(-\frac{{\pi }^2\times D_{\mathrm{e}}\times t}{r^2}\right)$$

(2)

where Q_∞ is the adsorption capacity at the final time(mg·L⁻¹); D_e is the diffusion coefficient (cm²·s⁻¹); and r is the particle size of the adsorbent (cm).

The pseudo-first-order kinetic model (Equation (3)) and the pseudo-second-order kinetic model (Equation (4)) were used to simulate the saturated adsorption curve, aimed to confirm the kinetic constant of the adsorption process.

$$lg\left(Q_{\mathrm{e}}-Q_{\mathrm{t}}\right)=lg{Q}_{\mathrm{e}}-\left(\frac{K_1}{2.303}\right)\times t$$

(3)

$$\frac{t}{Q_{\mathrm{t}}}=\frac{1}{K_2}\times \frac{1}{Q_{\mathrm{e}}^2}+\frac{1}{Q_{\mathrm{e}}}\times t$$

(4)

where Q_e is the adsorption capacity when it reaches the adsorption equilibrium (mg·L⁻¹); Q_t is the adsorption capacity calculated with Equation (1); K₁ is the adsorption rate constant of the pseudo-first-order kinetic model; and K₂ is the adsorption rate constant of pseudo-second-order kinetic model.

2.3.3. Adsorption Isotherm Test

The lithium ion adsorption behavior test was measured on HMO (0.04, 0.075, 0.11, 0.15 and 0.19 g) in 500 mL initial concentrations (10, 20, 30, 40 and 50 mg·L⁻¹ LiCl·H₂O solution) were added to five flasks respectively, (Ph = 10, adjusted by a buffer solution composed of 0.1 mol·L⁻¹ NH₄Cl and 0.1 mol·L⁻¹ NH₄OH). The flasks were shaken on a shaker at 200 rpm at 18 °C for 12 h.

The adsorption isotherm curve is fitted according to the following isotherm models:

Langmuir isotherm model:

$$Q_{\mathrm{e}1}=\frac{Q_{\mathrm{m}}\times K_{\mathrm{L}}\times C_{\mathrm{e}}}{1+K_{\mathrm{L}}\times C_{\mathrm{e}}}$$

(5)

Freundlich isotherm model:

$$Q_{\mathrm{e}2}=K_{\mathrm{F}}\times C_{\mathrm{e}}^{1/n}$$

(6)

where $Q_{\mathrm{m}}$ is the theoretically calculated maximum adsorption capacity; $K_{\mathrm{L}}$ is the Langmuir constant; $K_{\mathrm{F}}$ is the Freundlish constant; and $n$ is an empirical constant.

2.4. Selective Adsorption Behavior

The selectivity of lithium ions compared with other coexisting ions in brine was adjusted pH value to 10 by 0.1 mol·L⁻¹ NH₄OH, carried out by stirring (200 rpm) 0.1 g ion sieve in 20 mL saline brine at 20 °C for 72 h. The adsorption capacity of metal ion at equilibrium (Q_e), distribution coefficient (K_d), separation factor (${\alpha }_{\mathrm{Me}}^{\mathrm{Li}}$) and concentration factor ($C_{\mathrm{F}}$) are calculated according to the following equations:

$$K_{\mathrm{d}}=C_{0,\mathrm{Me}}-C_{\mathrm{e},\mathrm{Me}}\times V/\left(C_{\mathrm{e},\mathrm{Me}}\times W\right)$$

(7)

$${\alpha }_{\mathrm{Me}}^{\mathrm{Li}}=K_{\mathrm{d},\mathrm{Li}}/K_{\mathrm{d},\mathrm{Me}}(\mathrm{Me}={\mathrm{K}}^{+},{\mathrm{Ca}}^{2+},{\mathrm{Na}}^{+},{\mathrm{Mg}}^{2+},{\mathrm{Li}}^{+})$$

(8)

$$C_{\mathrm{F}}=Q_{\mathrm{e},\mathrm{Me}}/C_{0,\mathrm{Me}}(\mathrm{Me}={\mathrm{K}}^{+},{\mathrm{Ca}}^{2+},{\mathrm{Na}}^{+},{\mathrm{Mg}}^{2+},{\mathrm{Li}}^{+})$$

(9)

where C_{0, Me} is the initial concentrate of ions in brine (mg·L⁻¹); C_{e, Me} is the final concentrate of ions in brine after adsorption (mg·L⁻¹); V is the volume of solution (L); W is the weight of the HMO ion sieve (g); Q_{e, Me} is the saturated adsorption capacity of ions in brine (mg·g⁻¹).

2.5. Desorption Behavior

LMO was renamed LMO-1 after the Li⁺ adsorption of the HMO. The curve of the Li⁺ extraction and manganese dissolution was carried out by stirring (200 rpm) 0.1 g LMO-1 in 500 mL hydrochloric acid solution (0.04 mol·L⁻¹) for 24 h at 20 °C. The extraction ratio of lithium and the dissolution loss ratio of manganese were calculated using Equation (10).

$$R_{\mathrm{Me}}=\frac{C_{t,\mathrm{Me}}\times V}{W_{\mathrm{Me}}}\times 100\%(\mathrm{Me}={\mathrm{Mn}}^{2+},{\mathrm{Li}}^{+})$$

(10)

where $R_{\mathrm{Me}}$ is the extraction ratio of lithium or dissolution loss ratio of manganese; $C_{t,\mathrm{Me}}$ is the element concentration of different times; V is the solution volume and $W_{\mathrm{Me}}$ is the weight of $Me$ in the LMO-1. The influence of hydrochloric acid concentration was studied by stirring (200 rpm) 0.05 g LMO-1 in 100 mL hydrochloric acid solution (0.02–0.1 mol·L⁻¹) for 12 h at 20 °C.

3. Results and Discussion

3.1. Optimization of Synthesis Parameters

The XRD patterns of the products obtained under different conditions are shown in Figure 2. Figure 2a shows intermediate γ-MnOOH (JCPDS cards no. 50-0009) was produced at low Li⁺ concentration. With the increase of Li⁺ (>11 mol·L⁻¹), the target LMO was produced and intermediate γ-MnOOH was disappeared. Figure 2b indicates that with the increase of the amount of KMnO₄, the LMO lattice structure becomes stable gradually but when the amount of KMnO₄ was above 9 g, the impurity (∇) was generated. Figure 2c shows that using lower ethanol volume in the synthesis process resulted in the formation of intermediate Li₄Mn₁₄O₂₇·xH₂O (JCPDS cards no. 41-1379). When the volume fraction increases above 8.75%, the impurity peak (•) was observed. Figure 2d showed that the intermediate Li₄Mn₁₄O₂₇·xH₂O and γ-MnOOH were first formed within a short reaction time and LMO could be obtained after 12-h reaction. Figure 2e reflects the effect of reaction temperature on the LMO. Li₄Mn₁₄O₂₇·xH₂O and γ-MnOOH were produced at the lower temperature and LMO could be synthesized when the reaction temperature over 160 °C. Thus, we found the optimal LMO could be obtained at Li⁺ concentration of 11 mol·L⁻¹, hydrothermal reaction at 160 °C for 12 h, ethanol volume fraction of 7.5%, using 3 g of KMnO₄. We speculate the synthesis is followed by the mechanism illustrated in Figure 3. In LiCl·H₂O solution, KMnO₄ is firstly reduced by ethanol and the intermediates Li₄Mn₁₄O₂₇·xH₂O and γ-MnOOH are formed. Then γ-MnOOH is oxidized by KMnO₄ and Li₄Mn₁₄O₂₇·xH₂O is furthered reduced by ethanol simultaneously. Finally, the lithium ion enters the Mn-O framework to form cubic LMO with the increase of lithium concentration.

3.2. Ion-Sieves Characterization

Figure 4 shows the XRD patterns of the LMO, HMO and the sample after adsorption process (noted as LMO-1). The diffraction peak of LMO corresponds to a cubic spinel HMO structure [space group: Fd3m (JCPDS 35-0782)], with the lattice constants is 8.23 Å. It should be noted that the XRD patterns of HMO and LMO-1 are similar with the diffraction patterns of LMO, with lattice constants of 8.01 Å and 8.23 Å, respectively, indicating that the Li⁺ is free to access the structure and the Mn-O lattice remains stable during the adsorption and desorption process. It is found that the diffraction peak of HMO shifts to a higher diffraction angle than that of LMO, which can be explained by the mechanism showed in Figure 3c. During the Li⁺ desorption process, H⁺ in the solution replaces the original position of Li⁺ in the LMO the ionic radius of H⁺ is smaller than Li⁺, leading to cell shrinkage, which is also reported in literature [26]. The characteristic diffraction peaks of LMO-1 are still sharp and only the intensities decreased compared with the LMO, indicating that the HMO can be used for efficient adsorption of Li⁺.

Figure 5 describes the XPS spectra of LMO and HMO. As showed in Figure 5a, the spectra of the Mn3s orbit shows the binding energy difference of the two peaks was 5.15 eV (ΔE = 5.15 eV), indicating that the valences of Mn in LMO are +3 and +4. The binding energy of Mn³⁺ peak was 641.33 eV and Mn⁴⁺ peaks were 643.76 eV and 642.66 eV, which were obtained by means of peak-differentiation-imitating analysis at the Mn2p3/2 orbit (Figure 5b). The results are in line with a previous report [27]. The average valence of Mn in LMO (+3.65) could be calculated (

Table 2. Average valances of Mn element in LMO and HMO.

Sample	Binding Energy (eV)	Chemical State	Peak Area	Average Valences
LMO	643.76	Mn2p3/2 Mn4+	38,795.51	+3.65
	642.66	Mn2p3/2 Mn4+	40,557.36
	641.33	Mn2p3/2 Mn3+	42,725.95
HMO	—	Mn2p3/2 Mn4+	—	+4

), which is higher than the theoretical valence (+3.5), indicating that proportion of Mn³⁺ in LMO is lower than theoretical. Thus, it can be deduced that LMO has a more stable crystal structure. Figure 5c is the XPS spectra of HMO in the Mn3s orbit and the binding energy difference of the two peaks is 4.78 eV, indicating that the manganese valence in HMO is +4. Furthermore, this is also proven by the peak of HMO in the Mn2p3/2 orbital (Figure 5d).

Figure 6 shows the morphology of LMO, HMO and the ion-sieve after Li⁺ adsorption (LMO-1). The LMO presents regular hexagonal shape with the thickness of 110 nm and the lateral size of ~300–400 nm (Figure 6a). It is apparent that the LMO (Figure 6b) have a smooth surface without agglomeration, while HMO and LMO-1 appear to have a small crack on the surface (Figure 6c). We speculate that it is attributed to the manganese loss after acid treatment that results in partial collapse of the crystal. However, HMO and LMO-1 can still remain their intact hexagonal structure and it is consistent with the XRD results in Figure 4.

Figure 7 shows the HRTEM images of LMO and HMO. Both LMO and HMO were observed as a non-agglomerated particle with a regular hexagonal morphology (Figure 7a,d). The lattice spacing are 0.478 nm and 0.477 nm, respectively, as shown in Figure 7b,f, which agrees with the (111) crystal plane of the XRD pattern in Figure 4. The selected area electron diffraction (SAED) patterns of LMO and HMO can be seen in Figure 7c,g, the dot matrix confirms their cubic single-crystal structures.

3.3. Adsorption Behavior of the HMO

3.3.1. Effect of pH Value on Adsorption Capacity

Figure 8 describes the pH value effect on the Li⁺ adsorption process. The adsorption capacity of HMO was very low in acidic condition. The adsorption capacity of HMO increased sharply and then reached the maximum with the pH value increase in solution, which indicated that alkaline adsorption environment favored the adsorption of HMO. The adsorption-desorption mechanism of LMO can be explained by Figure 1 and Figure 3c. Adsorption Li⁺ at alkaline condition is beneficial to the formation of LMO and desorption of Li⁺ at the acid condition is beneficial to the formation of HMO. The mathematic relationship [12] between adsorption capacity (Q_e) and pH could be described by the equation Q_e = f (C_e, pH). The Q_e (the amount of Li⁺ insertion) increases by the increase of pH. When the pH was greater than 10, the adsorption capacity of HMO hardly increase with the increase of pH value. We speculated that the reduction of Mn⁴⁺ was accelerated under the strong alkaline condition, so the adsorption of the ion sieve was inhibited. Therefore, when pH > 10, the adsorption capacity Q_e tends to be stable. The similar phenomenon was also found by other reseachers [28].

3.3.2. Static Adsorption Test

Figure 9 shows that the adsorption process occurs primarily in the rapid adsorption stage and the exchange of Li⁺ into the spinel lattice dominates the adsorption flat stage.

Table 3. Similar method of adsorption capacity comparison.

Ion Sieve	Raw Materials	Method	Temp. (°C)	t (h)	Crystal Morphology	Q (mg·g−1)	$\frac{\mathit{Q}}{{\mathit{Q}}_{\mathit{t}\mathit{h}}}$ (%)	Ref.
λ-MnO2	Mn(NO3)2, LiOH, H2O2	hydrothermal	110	10	Nanowire	23.7	61.9	[29]
λ-MnO2	MnSO4, (NH4)2S2O8	hydrothermal	150 650	12 6	Nanowire	16.9	49.2	[23]
λ-MnO2	LiNO3, Mn(NO3)2	solid-phase	700	1	Sphere	-		[20]
λ-MnO2	Li2CO3, MnCO3	solid-phase	800	5	-	-		[19]
λ-MnO2	LiCl KMnO4 ethanol	hydrothermal	160	12	Hexagonal	24.7	64.4	This work

compares synthesis method and adsorption capacity of λ-MnO₂ in this paper with those of other paper. Solid-phase [19,20] method is often reacted with high energy consumption. It is apparent that hydrothermal method usually uses strong alkaline LiOH [29] or acidic manganese salt [23] as raw material with the disadvantage of corroding equipment. In this study λ-MnO₂ was obtained by the one-pot hydrothermal method under neutral and mild condition. The adsorption capacity is 24.7 mg·g⁻¹, 64.4% of the theoretical adsorption capacity $Q_{th}=\frac{M_{Li}}{M_{\mathsf{\lambda }-{\mathrm{MnO}}_2}}=\frac{6.94\times 1000}{180.94}=38.3\mathrm{mg}·{\mathrm{g}}^{-1}$; $\frac{Q}{Q_{th}}=\frac{24.7}{38.3}=64.4\%$, which is higher than the 49.2% of the theoretical adsorption capacity reported in the paper [23]; and 61.9% of the theoretical adsorption capacity in the paper. The Crank’s model was used to predict the adsorption rate of Li⁺. The model fitted well with the experimental data. The efficient film coefficient (D_e) were calculated by Equation (2) as 1.35 × 10⁻⁵ cm²∙s⁻¹. The correlation coefficient (R²) was 0.9971. The coefficient of mass transfer (k) can be obtained by efficient film coefficient and physical property of adsorption system. In all, D_e derived from fitting calculation provides a vital parameter of feed height in adsorption tower design [30].

3.3.3. Adsorption Kinetic Test

Figure 10 shows the linear fitting of the pseudo-first-order kinetic model and the pseudo-second-order kinetic model.

Table 4. Dynamic parameters of lithium adsorption.

Temperature	Pseudo-First-Order Kinetic Model			Pseudo-Second-Order Kinetic Model
	${\mathit{K}}_1$	${\mathit{Q}}_{e1}$	${\mathit{R}}^2$	${\mathit{K}}_2$	${\mathit{Q}}_{e2}$	${\mathit{R}}^2$
18 °C	0.115	8.41	0.7678	0.0687	25.3	0.9998

compares the fitted kinetic data of the two models at same temperatures. Under the same test conditions, the two models both predicted the adsorption capacity and the correlation coefficient ($R^2$) of the pseudo-second-order kinetics equation is much larger than the pseudo-first-order kinetic equation ($R^2$ = 0.7678). These data reveal that the adsorption behavior of the HMO ion sieve conforms to the pseudo-second-order kinetics model and the adsorption process is primarily chemical adsorption [31].

3.3.4. Adsorption Isotherm of Li⁺ on HMO

The adsorption constants and the correction factors were obtained by Langmuir and Freundlich equations fittings.

Table 5. Adsorption isotherm constants of Li⁺ on HMO.

Temperature	Langmuir Model			Freundlich Model
	${\mathit{K}}_L$	${\mathit{Q}}_m$	${\mathit{R}}^2$	${\mathit{K}}_F$	$\mathit{n}$	${\mathit{R}}^2$
18 °C	0.415	24.6	0.9999	13.2	6.38	0.9918

lists the various parameter values for both models. Figure 11 show the fitting effect of the two models. The Langmuir isotherm model ($R^2$ = 0.9999) fitting was much better than that of the Freundlich isotherm model ($R^2$ = 0.9918) compared with the experimental data. This result indicates that the HMO has homogeneous adsorption sites.

3.4. Absorption Selectivity of HMO

Table 6. Adsorption selectivity data of metal ions on HMO in brine.

Metal Ion	${\mathit{C}}_0$ (mg·L−1)	${\mathit{C}}_e$ (mg·L−1))	${\mathit{C}}_F$ (L·g−1 × 10−3)	${\mathit{Q}}_e$ (mg·g−1)	${\mathit{K}}_d$ (mL·g−1)	${\mathit{\alpha }}_{\mathrm{Me}}^{\mathrm{Li}}$
Li+	319.3	288.0	19.6	6.26	19.6	1.00
Na+	1810.0	1804.6	0.591	1.07	0.592	36.7
K+	815.8	812.6	0.793	0.647	0.796	27.3
Ca2+	121.8	120.2	2.63	0.320	2.63	8.16
Mg2+	119,600.0	119,590.4	0.0161	1.93	0.0161	1.35 × 103

Experiment conditions: T = 18 °C, pH = 5.64, V = 20.0 mL, W = 0.100 g.

shows the HMO ion sieve adsorption selectivity for Li⁺ compared with other coexisting metal ions in brine, including Na⁺, K⁺, Ca²⁺ and Mg²⁺. According to Table 6, the adsorption capacity of HMO in brine is 6.26 mg·g⁻¹, which is lower than the value of that in the pure Li⁺ solution. We speculated that the acidic environment (pH = 5.64) is not conducive to the free insertion of lithium ions in λ-MnO₂. The distribution coefficients ($K_{\mathrm{d}}$) are in the order of Li⁺ > Ca²⁺ > K⁺ > Na⁺ > Mg²⁺, indicating high selectivity for Li⁺, compared with other metal ions. The ion sieve showed excellent ion selectivity, especially for Mg²⁺, whose separation factor (${\alpha }_{\mathrm{Mg}}^{\mathrm{Li}}$) is 1.35 × 10³. This solves the problem of separating Li⁺ and Mg²⁺ in brine with a high ratio of magnesium to lithium. Na⁺, K⁺, Ca²⁺, Mg²⁺ in solution do not have competitive effect with Li⁺ during ion sieve adsorption process since the concentration factor $\left(C_{\mathrm{F}}\right)$ of Li⁺ is higher than other ions.

3.5. Desorption Behavior of LMO-1

Figure 12 shows the desorption curve of Li⁺ or Mn²⁺ after adsorption. It was observed that the extraction of Li⁺ and Mn²⁺ occurred rapidly at the beginning of the desorption process, almost reaching the maximum extraction rate at 20 min, then slightly rose up to 22.0 mg·g⁻¹ and 38.9 mg·g⁻¹, respectively. The maximum extraction rate of R_Li⁺ is 98.7%. The dissolution of Mn²⁺ (R_Mn²⁺) is only 3.9%, which was calculated by Equation (10). This phenomenon may benefit from the unique layered structure. Integrity shape without defect with a larger surface can sufficiently contact with Li⁺ of solution and maintain adsorption stability, accelerating the absorption and desorption process.

4. Conclusions

A series of LMO was successfully prepared via a facile one-pot hydrothermal method and we optimized synthetic conditions as well. The HMO ion-sieve has unique hexagonal spinel structure with the thickness of 110 nm and lateral size of 300–400 nm. XRD patterns of LMO and HMO confirm their high crystallization degree. The average valence of Mn in LMO is +3.65, which higher than that in theory (+3.5). The adsorption capacity of HMO is 24.7 mg·g⁻¹ in Li⁺ solution and the dissolution of Mn²⁺ is only 3.9%. The adsorption equilibrium isotherms data are well fitted with Langmuir model. Moreover, the distribution coefficients ($K_{\mathrm{d}}$) of HMO is much larger between Li⁺ and Mg²⁺ and the separation factor (${\alpha }_{\mathrm{Mg}}^{\mathrm{Li}}$) was 1.35 × 10³. Therefore, our HMO ion-sieve shows great potential to extract lithium in brine or seawater under high magnesium ratio conditions.