Fabrication of Spinel-Type H₄Ti₅O₁₂ Ion Sieve for Lithium Recovery from Aqueous Resources: Adsorption Performance and Mechanism

Abstract

Lithium (Li) ion sieve is considered to have great potential in the selective extraction of Li⁺ from complex Li⁺-containing brine owing to its cost-effectiveness, excellent adsorption performance, and environmental friendliness. Nevertheless, the defects of complex regulation and control of technological parameters in the preparation process of Li ion sieve and poor recycling efficiency limit its application. In this study, spinel-type H₄Ti₅O₁₂ ion sieves (HTO) were successfully prepared through a high-temperature solid-state method for recovering Li⁺ from aqueous resources. Through the experiment of optimizing the key preparation process parameters of HTO, it was found that the optimum preparation conditions were as follows: lithium ion source of CH₃COOLi‧H₂O, calcination temperature of 800 °C, and acid (HCl) washing concentration of 0.3 mol/L. The uptake of Li⁺ by HTO aligned with the pseudo-second-order kinetic model, which was a chemical adsorption process controlled by reversible Li–H ion exchange reaction. HTO exhibited extremely high regeneration cycle characteristics, and after five cycles, it retained 96.06% of its initial adsorption capacity. The present work highlighted that spinel-type HTO has high industrial application potential in the field of Li⁺ recovery from oilfield brine.

1. Introduction

Lithium (Li), a crucial metal for global advancement, is valued for low density (0.534 g/cm³), strong fatigue resistance, and low redox potential (−3.04 V). These properties make it indispensable in key industries, including energy storage, ceramics, glass, grease, refrigerant and nuclear applications [1]. Lithium reserves are chiefly distributed between geological ores and liquid sources, notably salt lakes and certain types of industrial wastewater. Contemporary lithium production derives roughly 70% of its output from salt lake resources [2]. As we all know, aqueous resources exceed ore deposits in availability and the latter are more expensive for Li⁺ exploitation [3]. Therefore, it is essential to extract Li⁺ from aqueous resources, but this method still faces challenges such as difficult adjustment of preparation process parameters, presence of multiple competing ions (notably Na⁺, K⁺, Ca²⁺, Mg²⁺ and other coexisting cations), and poor recycling efficiency that limit its application [4]. Up to now, the extraction methods of Li⁺ resources mainly include adsorption [5,6], precipitation [7], extraction [8], and membrane separation [9]. Among them, adsorption has emerged as the most viable technique for Li⁺ recovery because of its environmental friendliness, recyclability, cost-effectiveness, and high adsorption selectivity [3,10,11].

Li ion sieves, which exhibit molecular memory properties enabling selective Li⁺ adsorption, have emerged as promising adsorbents for Li⁺ recovery [12]. These substances are primarily classified as either manganese-type or titanium-type Li⁺ sieves [13]. Tian et al. [14] successfully synthesized H_1.33Mn_1.67O₄ for recovering Li⁺ from shale gas flowback produced water and the results showed that the adsorption capacity of H_1.33Mn_1.67O₄ towards Li⁺ was 13.27 mg/g. Although manganese-based Li⁺ sieves are easy to prepare and have good adsorption behavior towards Li⁺, there are risks such as manganese dissolution which may pollute water resources in industrial production. Titanium-derived Li⁺ sieves exhibit several beneficial characteristics due to their strong Ti–O linkages: structural permanence, highly selective adsorption, outstanding acid tolerance, and excellent cyclic stability [15], which can overcome the disadvantages of manganese-based Li⁺ sieves. The spinel structure of Li₄Ti₅O₁₂ is more stable than that of layered Li₂TiO₃ [16]. Li et al. [17] effectively fabricated Yolk-shell C@Li₄Ti₅O₁₂ microspheres, with experimental results demonstrating a peak Li⁺ adsorption capacity of 28.46 mg/g. The adsorption behavior was accurately modeled using both Freundlich isotherm and pseudo-second-order kinetic equations. However, the defect in complex regulation and control of technological parameters in the preparation process of Li ion sieve and poor recycling efficiency hinder the development of H₄Ti₅O₁₂ (HTO). Therefore, there is an urgent need to enhance the titanium-based Li⁺ sieves adsorption performance towards Li⁺ through optimization of the purity, morphology, and surface properties of HTO during its preparation process. Lithium source, calcination temperature, and acid washing concentration are three important parameters in the preparation of HTO. At present, LiOH and Li₂CO₃ are mostly used as Li sources for the preparation of HTO [18,19,20,21,22]. But the regulation law of the selection of Li sources on the microstructure of HTO has not been systematically explored, which makes the selection of Li sources lack reference. In addition, the mechanism of the joint regulation of Li source, calcination temperature, and acid washing concentration on the microstructure and adsorption performance of HTO is still unclear.

In the present work, spinel-structured Li⁺ sieve (HTO) was synthesized via a high-temperature solid-state approach, and the effects of key technological conditions such as lithium source, calcination temperature, and acid washing concentration on the crystal purity, morphological characteristics, and adsorption performance of the HTO were investigated. The adsorption capacity, adsorption selectivity, and cycle performance of the spinel-type HTO in simulated Li⁺-containing oilfield brine were also determined. Additionally, the adsorption mechanism of the spinel-type HTO was further discussed. This study will provide theoretical reference for the optimal preparation of spinel-type Li⁺ sieve and technical support for the separation and recovery of high-value Li resources in aqueous resources.

2. Materials and Methods

2.1. Reagents

Titanium dioxide (TiO₂, anatase type), lithium carbonate (Li₂CO₃), lithium hydroxide monohydrate (LiOH‧H₂O), and lithium acetate dihydrate (CH₃COOLi‧2H₂O) were all analytically pure and obtained from Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China. Hydrochloric acid (HCl), lithium chloride (LiCl), sodium chloride (NaCl), potassium chloride (KCl), magnesium chloride (MgCl₂), and calcium chloride (CaCl₂) were all analytically pure and provided by National Medicine Group Chemical Reagent Co., Ltd., Shanghai, China.

2.2. Preparation of HTO

The HTO was fabricated using a high-temperature solid-state method [19]. Firstly, the lithium source (Li₂CO₃ or LiOH‧H₂O or CH₃COOLi‧H₂O) and the titanium source (nano-TiO₂) were fully mixed according to the ratio of Ti/Li of 5:4.2, and an appropriate amount of anhydrous ethanol was added. Next, the obtained mixture was placed into an agate grinding bowl and ground for 40 min to achieve an even mixture, and placed in a 50 °C oven and dried for 12 h to remove residual ethanol. Then, the dried sample was ground into powder again and passed through a 200-mesh sieve, then placed into a corundum crucible for high-temperature calcination in a tube furnace (SK2-2-12TPA2 tubular furnace, Shanghai Zhuode Technology Co., Ltd., Shanghai, China) for 10 h. Once cooled to room temperature, the resulting powder was ground and sieved through a 200-mesh sieve to obtain Li₄Ti₅O₁₂ (LTO), the precursor of the spinel-type H₄Ti₅O₁₂ lithium ion sieve. Subsequently, the precursor of LTO was placed into a certain concentration of hydrochloric acid according to the solid–liquid ratio of 100 mL/g for 24 h of pickling, followed by repeated washing with deionized water until a neutral pH was achieved. Finally, the obtained product was oven-dried at 50 °C for 12 h to obtain the spinel-type H₄Ti₅O₁₂ (HTO). According to different lithium ion sources, the HTOs were named as HTO-Li₂CO₃, HTO-LiOH, and HTO-LiAc, respectively.

2.3. Characterization

The crystal structures of the HTO before and after adsorption of Li⁺ were analyzed by an X-ray diffractometer (XRD, Rigaku Smart lab SE, Akishima, Japan). The morphologies of the samples were characterized by a scanning electron microscope (SEM, Zeiss Sigma500, Oberkochen, Germany). The functional group of the materials was recorded on a Fourier transform infrared spectrometer (FT-IR, ThermoScientific Nicolet, iS50, Waltham, MA, USA). The surface area and pore distribution of the samples were examined by a 3Fle multifunctional nitrogen adsorption/desorption analyzer (Mike, Atlanta, GA, USA). The element distribution and valence state of the materials were measured with an X-ray photoelectron spectroscope (XPS, Thermo Fisher ESCALAB 250Xi, Waltham, MA, USA).

2.4. Li⁺ Adsorption Experiments

Batch adsorption tests were conducted by using 0.1 g of HTO adsorbent in 100 mL of Li⁺-containing solution (pH = 12) at 25 °C, and the experiments were performed in an XMTD-4000 constant temperature heating magnetic stirrer (Yongguangming Medical Instrument Co., Ltd. Beijing, China) with shaking speed of 500 r/min. After adsorption for a desired time, a 1 mL aliquot of the supernatant was collected and passed through a 0.45 μm pore-size membrane, and then diluted to a suitable multiple for Li⁺ concentration determination. All the experiments were conducted in triplicate. The Li⁺ concentration was quantified using a LICAP PRO ICP-OES instrument (Thermo Fisher Scientific, Waltham, MA, USA). Calculation of the HTO adsorption capacity was performed with Equation (1):

$$q_t={\displaystyle \frac{(C_0-C_t)\times V}{m}}$$

(1)

where q_t is the HTO Li⁺ adsorption capacity at time t, mg/g; C₀ denotes the initial Li⁺ concentration, mg/L; C_t represents Li⁺ concentration at time t, mg/L; V indicates the volume of Li⁺ solution, L; and m is the mass of HTO adsorbent, g.

2.5. Adsorption Kinetics

Adsorption kinetic tests were conducted by immersing 0.1 g of HTO into 100 mL Li⁺-containing solution (initial concentration: 1000 mg/L) and sampling at desired time intervals (0–48 h) to examine the Li⁺ concentration in the solution. Three kinetic models (pseudo-first-order, pseudo-second-order, and Evolich) were applied to analyze the HTO Li⁺ adsorption data and their equations are written as Equations (2)–(4) [23]:

$$\ln (q_e-q_t)=\ln q_e-{\displaystyle \frac{k_1}{2.303}}t$$

(2)

$${\displaystyle \frac{t}{q_t}}={\displaystyle \frac{1}{q_e^2{k}_2}}+{\displaystyle \frac{t}{q_e}}$$

(3)

$$q_t={\displaystyle \frac{1}{\beta }}\ln \alpha \beta +{\displaystyle \frac{1}{\beta }}\ln t$$

(4)

where q_e (mg/g) is the equilibrium Li⁺ uptake by HTO; q_t (mg/g) is the instantaneous adsorption capacity of HTO towards Li⁺; k₁ (min⁻¹) and k₂ (mg/kg/min⁻¹) denote rate constants for pseudo-first-order and pseudo-second-order kinetics, respectively; and t represents the adsorption time (min).

$$q_t=k_{d\mathrm{i}}{t}^{1/2}+C_{\mathrm{i}}$$

(5)

where k_di (mg/(g‧min^1/2)) denotes the intra-particle diffusion rate constant and C_i (mg/g) represents the boundary layer thickness [24,25].

2.6. Adsorption Isotherms

Adsorption isotherm experiments were carried out by immersing 100 mg of HTO into 100 mL Li⁺ solution (100–500 mg/L) for 24 h. The data were fitted using Langmuir (Equation (6)) and Freundlich (Equation (7)) models:

$${\displaystyle \frac{C_e}{q_e}}={\displaystyle \frac{1}{K_L{q}_{\max }}}+{\displaystyle \frac{C_e}{q_{\max }}}$$

(6)

$$\ln q_e=\ln K_F+{\displaystyle \frac{1}{n}}\ln C_e$$

(7)

where K_L (L/mg) denotes the Langmuir equilibrium constant; C_e (mg/g) is the Li⁺ equilibrium concentration; q_max (mg/g) is the HTO maximum adsorption capacity at equilibrium; and K_F (mg/g) and n correspond to the Freundlich coefficient and heterogeneity factor, respectively.

3. Results and Discussion

3.1. Effect of Key Preparation Process Parameters on the Microstructure and Adsorption Behavior of HTO

3.1.1. Effect of Lithium Source

The morphology of HTO-Li₂CO₃, HTO-LiOH, and HTO-LiAc was observed by SEM and the results are presented in Figure 1. HTO-Li₂CO₃ (Figure 1A) had a regular cubic shape, compact structure, and serious particle agglomeration. The particle size of HTO-Li₂CO₃ was about 600–800 nm (Figure 1A). The morphology of each grain of HTO-LiOH was similar (Figure 1B), and the particle agglomeration was also serious (particle size: 400–600 nm). Compared with HTO-Li₂CO₃ and HTO-LiOH, the particle size of HTO-LiAc (300–400 nm) was smaller, and its hierarchy was clearer (Figure 1C).

The specific surface area properties and pore-size distributions of HTO-Li₂CO₃, HTO-LiOH and HTO-LiAc were analyzed via N₂ adsorption/desorption isotherms, as illustrated in Figure 1D–I. Regarding the specific surface area, the three samples displayed these sequences: HTO-LiAc (20.1775 m²/g) > HTO-LiOH (18.2940 m²/g) > HTO-Li₂CO₃ (16.9275 m²/g). The pore size of HTO-based adsorbent with different lithium sources shows that their pore size ranges belonged to the mesoporous range, and their average pore size was about 7 nm. Adsorption capacity is augmented by higher surface area through increased exposure of active sites [26]. Regarding adsorption efficiency, the materials in Figure 1J showed this progression: HTO-LiAc > HTO-LiOH > HTO-Li₂CO₃. In addition, the decomposition temperature of HTO-LiAc may be lower, and the gas production during its calcination process was higher, which made it easier to create a loose and porous structure and endowed it with more active sites. To sum up, among the three common Li sources, the best Li source for preparing spinel-type HTO is CH₃COOLi‧H₂O.

3.1.2. The Effect of Calcination Temperature

During the synthesis of spinel-type HTO by a high-temperature solid-state method, calcination temperature is a crucial control factor. LTO was prepared by calcination at different temperatures of 700 °C, 800 °C and 900 °C with CH₃COOLi‧H₂O as lithium source and nano-TiO₂ as titanium source. The XRD patterns of LTO-700, LTO-800, and LTO-900 are shown in Figure 2A. Compared with the standard PDF card of spinel Li₄Ti₅O₁₂ crystal (PDF#49-0207), it can be seen that spinel LTO was formed at the above three calcination temperatures. Impurity peaks were detected in LTO-700 and LTO-900 at 25.27° and 27.43°, respectively. LTO-800 had the highest purity and the best crystallinity, which indicated that other impurity crystal forms were easily produced when the temperature was too high or too low. On the whole, the peak values of LTO-800 were higher than those of LTO-700 and LTO-900, which suggested that the LTO crystal generated at the calcination temperature of 800 °C was more orderly and had a more stable structure.

The spinel-type HTOs were further obtained by eluting the LTO precursor prepared at 700 °C, 800 °C, and 900 °C with 0.3 mol/L HCl. The SEM images of HTOs were shown in Figure 2B–D. The morphology of HTO-700 appeared irregular and disordered, suggesting that the reaction was incomplete. The shape and size of HTO-800 were uniform, and the agglomeration phenomenon was light. The agglomeration of HTO-900 was serious. Based on the above analysis, the optimum calcination temperature is 800 °C.

3.1.3. Effect of Acid Washing Concentration

The acid washing step is another key step in the process of preparing spinel-type HTO by the high-temperature solid-state method. Following successful synthesis of the LTO-800 precursor, the Li⁺ in the precursor was removed via 24 h of HCl treatment at room temperature to obtain HTO, leaving holes with sizes matching the Li⁺. The effects of acid washing concentration (0.1–0.5 mol/L) on Li⁺ elution rate and Ti⁴⁺ dissolution loss are shown in Figure 3. According to Figure 3, at 0.3 mol/L HCl, the Li⁺ elution rate of LTO-800 was 94.11% which is much higher than that of HTO obtained with 0.1 mol/L HCl (75.84%). Additionally, the Ti⁴⁺ loss rate of LTO-800 precursor was only 0.39%, which was much lower than that of HTO obtained with 0.5 mol/L HCl (0.70%). It was attributed that the high HCl concentration would destroy the structure of LTO precursor. Usually, the ideal Li ion sieves should have small Ti⁴⁺ dissolution and large Li⁺ extraction to increase the adsorption behavior [20,22]. The following experiments employed the optimal 0.3 mol/L HCl to prepare the spinel-type HTO from LTO-800.

3.2. Adsorption Studies

3.2.1. Adsorption Kinetics

As a critical indicator of adsorption efficiency, the adsorption rate directs adsorbent development and enhancement [26,27]. The relationship between adsorption time and Li⁺ removal by HTO is illustrated in Figure 4A. The Li⁺ adsorption capacity of HTO exhibited a time-dependent increase, reaching maximum uptake after 24 h. Three kinetic models (pseudo-first-order, pseudo-second-order, and Evolich) were applied to analyze the HTO Li⁺ adsorption data, with fitting results presented in Figure 4B–D and

Table 1. Adsorption kinetic model rate constants (25 °C).

Adsorption Kinetic Model	Pseudo-First-Order Kinetic Model		Pseudo-Second-Order Kinetic Model		Evolich Kinetic Model
	k1/min−1	R2	k2/g‧(mg‧min)−1	R2	α/mg·(g·min)−1	β/g·mg−1	R2
Li+	0.185	0.985	0.046	0.997	0.12	18.88	0.980

.

As shown in Figure 4B–D and Table 1, the correlation coefficient (R²) values of pseudo-second-order (R² = 0.997) were higher than that of pseudo-first-order (R² = 0.985) and Evolich kinetic model (R² = 0.980), indicating that the pseudo-second-order kinetics provided the most accurate fit to the Li⁺ adsorption data. These results suggest that Li⁺ adsorption by HTO was predominantly chemisorption-driven, mediated by Li⁺–H⁺ ion exchange.

To further study the diffusion mechanism of Li⁺ adsorbed by HTO, experimental data were fitted using the intra-particle diffusion model (Equation (5)) [28], with fitting results presented in Figure 4E and

Table 2. Rate constants for the intra-particle diffusion model.

Parameters	Values
k1 (mg/(g·min0.5))	12.46
C1 (mg/g)	−5.62
R2	0.9953
k2 (mg/(g·min0.5))	6.99
C2 (mg/g)	3.61
R2	0.9497
k3 (mg/(g·min0.5))	2.62
C3 (mg/g)	19.61
R2	0.8158

.

According to Figure 4E and Table 2, q_t against t^1/2 presented three-phase curves and the obtained C_i values were not zero, indicating that besides intra-particle diffusion, there were other diffusion processes involved in the process of Li⁺ adsorbed by HTO. The uptake of Li⁺ by HTO involved three stages including film diffusion, intra-particle diffusion, and chemisorption of pore surface stages. The order of diffusion rate constants (k_di) calculated by the intra-particle diffusion model in the above three stages were in the following order: k_d1 > k_d2 > k_d3. As the adsorption reaction progressed, the reduction in Li⁺ solution concentration led to greater mass transfer resistance and a corresponding decline in adsorption rate. Meanwhile, with the progress of the reaction, the boundary layer effect in the system was enhanced, and the C_i values were increased (C₁ < C₂ < C₃) [29].

3.2.2. Adsorption Isotherms

The Langmuir (Equation (6)) and Freundlich (Equation (7)) isotherm models [30,31] were employed to elaborate the adsorption process of Li⁺ onto HTO and the fitting results are shown in Figure 5A,B and

Table 3. Adsorption isotherm model fitting parameters (25 °C).

Adsorption Isotherm Model	Parameters	Li+
Langmuir	qm/mg·g−1	38.46
	KL/L·mg−1	0.0021
	R2	0.9741
Freundlich	KF/(mg·g−1)(mg·L−1)−1/n	0.2639
	n	1.42
	R2	0.9858

.

The Freundlich isotherm model (R² = 0.9858) demonstrated superior fit to the HTO adsorption data for Li⁺ compared to the Langmuir model (R² = 0.9741), as evidenced by the higher R² value shown in Figure 5 and

Table 4. Comparison of the adsorption performance with other reported adsorbents.

Adsorbent	C0 (mg/L)	pH	qe (mg/g)	Adsorption Time	Ref.
HTO-HEC-60	100	12	27.9	1	[18]
HTO-170 °C-12 h	100	12	30.9	5	[18]
HTO nanosheets	166.6	13	18.75	4	[32]
Yolk-shell structured HTO	347.1	12.73	20.10	6	[17]
HTO-OS	374.8	13	18.81	4	[20]
HTO-NS	374.8	13	28.89	4	[20]
HTO	1000	12	38.46	24	Present study

. Freundlich isotherm analysis revealed an n value exceeding 1, indicating favorable multilayer adsorption of Li⁺ by HTO through chemical interactions. The comparison of HTO and other adsorbents in recent publications is shown in Table 4. It can be seen that the adsorption capacity of HTO is at a moderate level. Considering the long time required for HTO to reach adsorption equilibrium, further efforts should be made to improve the performance of adsorbents in further exploration.

3.2.3. Adsorption Selectivity

Given that in actual oilfield produced water, Li⁺ usually coexists with Na⁺, K⁺, Ca²⁺ and Mg²⁺, which will reduce its adsorption capacity because of the competition of other coexisting ions for adsorption sites [20]. The HTO ion selectivity was tested by mixing 0.1 g of HTO with 100 mL simulation solution containing Li⁺, Na⁺, K⁺, Ca²⁺ and Mg²⁺. Detailed information regarding the simulated oilfield produced water’s cation content (Li⁺, Na⁺, K⁺, Ca²⁺ and Mg²⁺) is exhibited in

Table 5. Adsorption selectivity of HTO in simulated oilfield produced water containing Li⁺, Na⁺, K⁺, Ca²⁺ and Mg²⁺ (Conditions: temperature = 25 °C, pH = 12).

Metal Ions	Li+	Na+	K+	Ca2+	Mg2+
Ionic radius (pm)	76	102	138	100	72
C0 (mg/L)	186.37	25,772.69	7389.41	4397.64	2981.92
Ce (mg/L)	158.83	25,706.89	7371.65	4389.97	2958.17
Kd (mL/g)	173.39	2.56	2.41	1.75	8.03
${\alpha }_M^{L_i}$	1	67.74	71.97	99.24	21.60

. After the adsorption reaction was completed, metal ion concentrations were quantified using LICAP PRO ICP-OES (Thermo Fisher Scientific, USA). The selectivity of HTO towards Li⁺ was evaluated by the coefficient (K_d) and separation factor ($\alpha \frac{L\mathrm{i}}{M}$) using Equations (8) and (9) [21]:

$$K_d={\displaystyle \frac{(C_0-C_e)V}{C_{e}m}}$$

(8)

$$\alpha \frac{L\mathrm{i}}{M}={\displaystyle \frac{K_d^{Li}}{K_d^M}}$$

(9)

where C_e is the equilibrium concentration of Li⁺, mg/L; and $\alpha \frac{L\mathrm{i}}{M}$ represents Li to M ion (Na, K, Ca and Mg) separation factors.

As evidenced in Table 5, Li⁺ exhibited superior K_d value (173.39 mL/g) over competing ions (Na⁺: 2.56, K⁺: 2.41, Ca²⁺: 1.75, Mg²⁺: 8.03 mL/g). High competing ion concentrations decreased the HTO Li⁺ adsorption efficiency compared with that in the single Li⁺ solution, but the decrease was not significant. The differences in K_d values of different metal ions could result from size differences, as Na⁺ (102 pm), K⁺ (138 pm), and Ca²⁺ (100 pm) all possessed larger ionic radii than Li⁺ (76 pm) [19]. As a result, the large cations cannot overcome the steric hindrance and diffuse into the pore channel of HTO, but only anchor on the outer surface of HTO. In particular, the K_d and $\alpha \frac{L\mathrm{i}}{Mg}$ values of Mg²⁺ were 8.03 mL/g and 21.60, respectively. The similar sizes but differing hydration energies (Li⁺: −515 kJ/mol; Mg²⁺: −1922 kJ/mol) explained why Mg²⁺ could not readily exchange with H⁺ in HTO [18]. The $\alpha \frac{L\mathrm{i}}{M}$ values between Li⁺ and Na⁺, K⁺, Ca²⁺ and Mg²⁺ were all greater than 1, which further indicated that the spinel-type HTO had relatively high adsorption selectivity. Therefore, the spinel-type HTO is a potential candidate for recovering Li⁺ in actual Li⁺-containing oilfield produced water.

3.2.4. Recyclability

The regeneration and reuse performance of adsorbent is of great significance to evaluate its practical application in industry, because it will directly affect the cost of adsorbent [21]. In the present work, the recyclability of HTO was investigated and the experimental results are illustrated in Figure 6. The HTO regeneration rate decreased marginally with successive cycles, and the regeneration rate was 96.06% after five times of reuse (Figure 6A). The Li⁺ removal rate in the process of HTO acid washing has been maintained at a high level, with an average Li⁺ removal rate of 93.65% (Figure 6B). At the same time, the acid washing process resulted in low Ti⁴⁺ dissolution (average: 0.42% per cycle) (Figure 6B). Thus, HTO with excellent reusability (high regeneration rate) and chemical stability (low Ti⁴⁺ dissolution rate) can be recycled for Li⁺ recovery.

3.3. Adsorption Mechanism

FT-IR and XRD analyses were conducted on HTO samples pre- and post-Li⁺ adsorption to elucidate the adsorption mechanism, with results displayed in Figure 7. The adsorption peak around 3410 cm⁻¹ corresponding to –OH did not change obviously before and after the adsorption reaction, which demonstrated that –OH was not involved in the HTO adsorption of Li⁺ ions (Figure 7A). The peak at 638.1 cm⁻¹ in HTO was assigned to the vibration of Ti–O–Ti skeleton, and it was blue-shifted by 14.4 cm⁻¹ after Li⁺ adsorption, suggesting that the crystal structure of HTO was ordered after Li⁺ was embedded to replace H⁺, that is, the crystal lattice shrank. The unchanging position of the Ti–O bending vibration peak (477.6 cm⁻¹) before and after adsorption indicates that the HTO spinel framework remained structurally intact during Li⁺ incorporation. In the XRD pattern of HTO-Li⁺, the diffraction peaks at 18.37°, 35.61°, 43.28°, 47.39°, 62.87°, and 66.11° corresponded to the reflections of (111), (311), (400), (331), (440), and (531) crystal planes, respectively, which was consistent with the structure of spinel Li₄Ti₅O₁₂ (PDF No. 49-0207). Compared with HTO, the intensity of all diffraction peaks in XRD pattern of HTO-Li⁺ obtained after adsorption of Li⁺ was significantly increased (Figure 7B), which indicated that the embedding of Li⁺ in the adsorption process repaired the lattice defects caused by protonation and improved the order of crystal structure. After adsorption of Li⁺, the distance between crystal planes of HTO was reduced, and the lattice constant a (a = b = c) was reduced from 8.4006 nm before adsorption to 8.3549 nm, which indicated that the crystal had undergone uniform changes during adsorption, and the ion exchange mechanism was verified. Elemental analysis of HTO and HTO-Li⁺ (at%) determined by XPS showed that the content of Li was enhanced after the adsorption reaction (

Table 6. Elemental analysis of HTO and HTO-Li⁺ (at%) determined by XPS.

Sample/Element	C1s	O1s	Li1s	Ti2p	Cl2p
HTO	17.97	52.12	10.16	19.35	0.39
HTO-Li+	22.65	43.84	15.63	13.3	0.57

), also confirming the successful adsorption of Li⁺.

The above change confirmed the reversible structural transformation ability of HTO, which was the basis of high recyclability of ion sieve. On the other hand, the XRD peak position of HTO-Li⁺ after adsorption was slightly shifted to the right compared with HTO, indicating that Li⁺ intercalation replaced H⁺ in the adsorption process. This change reflected that there was a highly reversible ion exchange in the adsorption process of Li⁺ by HTO.

4. Conclusions

The synthesis of spinel-structured HTO in this work employed a high-temperature solid-state approach, and the optimum preparation conditions were as follows: lithium ion source of CH₃COOLi‧H₂O, calcination temperature of 800 °C, and acid (HCl) washing concentration of 0.3 mol/L. The obtained HTO with good recyclability had high adsorption selectivity for Li⁺ with the coexisting metal ions including Na⁺, K⁺, Ca²⁺ and Mg²⁺ in the simulated oilfield produced water. Combined adsorption tests and characterization analysis showed that Li⁺ uptake by HTO primarily occurred through ion exchange, and the adsorption process was a multi-molecular layer chemical adsorption reaction. The present work establishes novel guidelines for synthesizing, utilizing, and understanding the adsorption mechanisms of spinel-structured HTO, and provides a new solution for the recovery of Li⁺ in oilfield brine.