Preparation and Adsorption Properties of Titanium Series Lithium Ion Sieve

Authors: Chen Wang 1,2, Jiang Lei 1, Pan Qiaozhen 1, Yang Weiwei 1, Zhu Xianrong 1, Chen Linlin 1,2, Zhu Wenshuai 1,2
Affiliations: 1. Jiangsu Tefeng New Material Technology Co., Ltd., Zhenjiang, Jiangsu 212013; 2. School of Chemistry and Chemical Engineering, Jiangsu University

Abstract: Using amorphous hydrated titanium dioxide and lithium hydroxide as sources of titanium and lithium respectively, a titanium lithium ion sieve (HTO) was synthesized through a high-temperature solid-phase method. It was characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), and contact angle measurements. This study explored the effect of HTO on lithium adsorption capacity in salt lake brine with a high magnesium-to-lithium ratio [n(Mg)/n(Li)]. The results indicated that the adsorption capacity of HTO reached 24.8 mg/g after 26 hours, demonstrating excellent selectivity with a separation coefficient (α_g) of 4813.0. After 20 adsorption cycles, the lithium adsorption capacity of HTO decreased by only 4.8%, and the titanium dissolution rate was less than 0.08%. These findings suggest that HTO has superior cyclic adsorption properties and stability. Moreover, HTO has significant potential for lithium extraction from high magnesium-lithium salt lake brine, indicating broad market prospects.

Keywords: Titanium series lithium ion sieve, adsorption, salt lake brine

In recent years, the consumption of fossil fuels and pollution of the natural environment have brought new energy sources into the global spotlight. Lithium, a novel energy metal, is extensively utilized in lithium batteries, ceramics, nuclear energy, aerospace, and other sectors. With the rapid expansion of the lithium-ion battery industry, the demand for lithium across various industries has surged. There are two primary natural sources of lithium: ore and salt lake brine.

Currently, approximately 60% of the world’s lithium resources are found in salt lake brines. Due to the high costs, significant energy requirements, and scarcity of lithium ore, the exploitation of salt lake brines has become a focal point of research. China possesses substantial liquid lithium resources, primarily located in Qinghai and Tibet. However, the lithium content in Chinese salt lake brines is generally low, and the magnesium-to-lithium ratio (n(Mg)/n(Li)) is high. Given that lithium and magnesium share many physical and chemical properties, as they are diagonally adjacent on the periodic table, extracting lithium from these brines poses a significant challenge.

Current extraction methods include adsorption, extraction, calcination, and precipitation. Among these, the adsorption method is considered one of the most effective for recovering lithium from high-magnesium salt lake brines due to its simplicity, low energy consumption, high efficiency, and excellent selectivity. The effectiveness of lithium recovery by adsorption hinges on the adsorbent used. Common organic adsorbents, such as ion exchange resins, are affordable and stable, yet they exhibit poor selectivity for lithium, limiting their industrial application. Recently, inorganic adsorbents have gained popularity due to their cost-effectiveness, stability, and unique ion exchange properties. Aluminum-based adsorbents, known for their low cost, excellent selectivity, and recyclability, have lower adsorption capacities. Conversely, manganese-series lithium ion sieves offer high adsorption capacities but suffer from significant dissolution losses and instability. Titanium lithium ion sieves are preferred for their robust adsorption performance, affordability, and selectivity.

This paper employs amorphous hydrated titanium dioxide and lithium hydroxide as sources of titanium and lithium, respectively, synthesizing a titanium-series lithium ion sieve through a high-temperature solid-phase method to adsorb lithium from salt lake brine. The materials were analyzed using SEM, XRD, and contact angle testing to explore their adsorption, selectivity, and cycling properties.

1 Experimental Part

1.1 Experimental Reagents and Instruments

Reagents: Particulate titanium dioxide (TiO2 with a mass fraction of 43.5%), concentrated sulfuric acid, sodium dodecyl sulfate, ammonia, lithium hydroxide, and hydrochloric acid, all of analytical grade.
Instruments: VISTA-MPX inductively coupled plasma emission spectrometer (ICP), TD-3500 X-ray diffractometer, Thermo Fisher Apreo scanning electron microscope, KSW CM200 optical contact angle goniometer, JW-BK200C specific surface area and porosity analyzer, etc.

1.2 Preparation of Amorphous Hydrated Titanium Dioxide

Add 200 g of metatitanic acid to 450 mL of concentrated sulfuric acid (95%–98% H2SO4) and stir at 120°C until the metatitanic acid is completely dissolved. Cool the mixture to room temperature and add 300 mL of distilled water to form an oxy sulfate solution. Next, add 0.44 g of sodium dodecyl sulfate and stir at 50°C. Gradually add ammonia solution to adjust the pH to 3–4. After maturing for 1 hour, filter and wash the precipitate to obtain amorphous hydrated titanium dioxide.

1.3 Preparation of HTO

Mix amorphous hydrated titanium dioxide with a 10% mass fraction lithium hydroxide solution at a molar ratio of n(Ti4+): n(Li+) = 1:2. Spray dry the mixture in a muffle furnace at 750°C and cool to room temperature over 2 hours to obtain the precursor Li2TiO3 (LTO). Then, slowly add 6 mol/L hydrochloric acid to a conical flask containing LTO and water, continuously shaking. Stabilize the solution at pH=1.5, then filter and wash to obtain the ion sieve H2TiO3 (HTO). Measure the concentration of titanium in the solution using ICP to determine the dissolution rate of titanium.

Ti LL R (%) is calculated as follows:

R= (ρ×V ）/mTi×100%

Where: ρ is the mass concentration of the titanium ions in the acid solution after washing, mg/L; V is the volume of the solution, L; mTi is the mass of the titanium element in the adsorbent, g.

1.4 The Adsorption Experiment

For this experiment, 2 g of HTO were added to a tapered flask containing 500 mL of brine from a Qinghai salt lake with a pH of 5.8. The mass concentrations of each ion in the salt lake brine were as follows: ρ (Li+) = 0.3 g/L, ρ (Na+) = 3.3 g/L, ρ (K+) = 2.0 g/L, ρ (Ca2+) = 0.03 g/L, ρ (Mg2+) = 112.2 g/L. The flask was then placed in a constant temperature water bath shaker. Samples were taken at different time intervals, and the concentrations of various metal ions in the solution were measured using Inductively Coupled Plasma (ICP) spectroscopy.

adsorption capacity qt(mg / g) The calculated formula is as follows:

qt=(ρ0-ρt）V/m （2）

Where: ρ0 is the initial mass concentration of metal ions in solution, mg/L; ρt is the mass concentration of metal ions in solution at different times, mg/L; m is the mass of adsorbent, g.

1.5 Distribution Coefficient and Separation Coefficient

The selectivity of HTO for various metal ions in salt lake brine is represented by the distribution coefficient $K_{d}$ and the separation coefficient $α$ , as expressed by the following formulas:

Kd=(ρ0-ρe）V/ρem （ 3）

M(Li)α =KLi/Kd（ 4）

Where ρe is the mass concentration of Li + after equilibration of the solution, mg/L.

1.6 Cycle Experiment

2 grams of HTO were added to a conical flask containing 500 mL of brine and then placed in a constant temperature water bath shaker for 26 hours. The concentration of lithium ions was measured using ICP. The solution was filtered to collect the adsorbed lithium ion sieve (LTO). Subsequently, 6 M hydrochloric acid was slowly added to a conical flask containing LTO and water. The mixture was continuously shaken until the solution stabilized at a pH of 1.5. After stabilization, the mixture was filtered and washed to recover the HTO. This procedure was repeated for the cycle experiments.

2 Results and Discussion

2.1 Crystal Structure Analysis

Figure 1 displays the XRD diffraction profiles of LTO, HTO, and (L)TO. The diffraction patterns of LTO, as shown in Figure 1, are found at 18.7, 20.4, 36.2, 43.8, 47.9, 57.8, 63.7, and 67.0. These characteristic peaks correspond to the (002), (110), (-131), (-133), (-204), (006), (-206), and (062) planes and are generally consistent with the standard reference (JCPDS 33-0831), indicating a monoclinic phase [14]. Additionally, the high peak heights and narrow half-peak widths suggest a high crystallinity of the sample.

The crystal structure of HTO remains largely unchanged, although the diffraction peaks at (-131) and (-204) became broader, and the peak at (-131) shifted to higher angles. This shift suggests a different bonding interaction between Li and H within the Ti-O layers [3]. The crystal lattice appears to undergo slight contraction, presumably due to the exchange of Li+ ions with larger radii for H+ ions with smaller radii [3,15].

Furthermore, the disappearance of the diffraction peaks at (-133), (006), and (062) after acid treatment indicates the leaching of Li+ from LTO [4]. For both LTO and HTO, the peak shapes are very similar, showing no significant changes, which implies that the structure of the lithium ion sieve remains stable before and after adsorption.

Fig.1 XRD patterns of LTO 、HTO and （L ）TO

2.2 BET Specific Surface Area Test

The BET specific surface area test was conducted for both LTO and HTO. The specific surface areas measured for LTO and HTO were 3.40 m²/g and 14.25 m²/g, respectively. The results show that HTO possesses a larger surface area, likely due to selective preference for acidic adsorption sites during N₂ adsorption.

2.3 Contact Angle Test

The wettability of HTO was investigated, and the results are presented in Figure 2. According to this figure, when deionized water is dropped onto HTO, the contact angle is observed to be 61 degrees, suggesting that HTO exhibits good hydrophilicity. Lithium typically exists in aqueous solutions as hydrated ions. The hydrophilic nature of HTO allows for efficient contact with Li⁺ in brine, facilitating an effective exchange between Li⁺ and H⁺ ions.

Fig.3 SEM images of LTO （a ）、HT O （b ）and （L ）TO （c ）

As demonstrated, the morphology of LTO, HTO, and (L)TO is polyhedral. Following pickling and adsorption processes, the microstructure remained essentially unchanged. The only minor modifications observed were a coarser particle surface and a more loosely structured material, likely due to the dissolution loss of titanium. Furthermore, the particle size of HTO was reduced compared to LTO, which is probably attributed to the exchange of Li+ and H+ ions. The findings indicate that hydrochloric acid, used as an eluent, minimally affects the crystal backbone structure and positively influences the HTO cycle.

2.4 Morphological characterization

The SEM plots of the LTO, HTO, and (L) TO are shown in Figure 3.

Fig.3 SEM images of LTO （a ）、HT O （b ）and （L ）TO （c ）

As demonstrated, the morphologies of LTO, HTO, and (L)TO are polyhedral. After pickling and adsorption, the microstructure remained essentially unchanged. The dissolution loss of titanium caused only minor alterations, such as coarser particle surfaces and a looser structure. Additionally, the particle size of HTO is smaller compared to LTO, likely due to the exchange between Li+ and H+. The results indicate that hydrochloric acid, used as an eluent, has minimal impact on the crystal’s fundamental structure and enhances the cycling performance of HTO.

2.5 Adsorption Properties of HTO

The adsorption capabilities of HTO for lithium in brine were investigated, and the results are illustrated in Figure 4. As depicted in Figure 4, HTO’s adsorption capacity surged in the first 8 hours, reaching 21.5 mg/g. As the adsorption time was extended, the rate of adsorption decreased, and the capacity increased gradually. After 26 hours, the adsorption capacity stabilized at 24.8 mg/g, approaching equilibrium.

Furthermore, the “quasi-first-order kinetic model” and “quasi-second-order kinetic model” are two prominent models used to examine the adsorption mechanisms and rates, as referenced in [4]. Specifically, the proposed first-order kinetic model:

log （qe-qt）=-k1t/2.303+log qe（5）

The proposed second-level kinetic model:

t/qt=t/qe+1/qe2k2（6）

In these two kinetic model equations, (mg/g) represents the adsorption capacity of HTO at equilibrium, while (h $^{-1}$ ) and [g/(mg·h)] are the adsorption rate constants for the two models respectively. The Linefit software was used to fit the data presented in Figure 4 using these model equations. The results are depicted in Figure 5 and Table 1, indicating a high value of 0.9914.

Fig.4 Effect of time on HTO adsorption capacity

Fig.5 Quasi-first-order kinetic curve （a ）and Quasi-second-order kinetic curve （b ）

Table 1 Dynamic parameters

type	k/（h-1）	qe/（mg ·g-1）	R2
Level 1	0.17	18.5	0.953 8
Planned secondary	0.01	28.6	0.991 4

In the proposed first-order kinetic model, R² (0.9538) indicates that the quasi-second-order kinetic model better represents the lithium adsorption process of HTO. The equilibrium adsorption capacity, calculated from the quasi-second-order kinetic equation, is 28.6 mg/g. This value is closer to the experimental results shown in Figure 4, further supporting the dominance of chemical adsorption in the process.

2.6 HTO Selectivity and Cycling Performance

The acid resistance of LTO was assessed by measuring the dissolution rate of titanium. As depicted in Figure 6, the dissolution rate increased with the concentration of hydrochloric acid used for elution. At a hydrochloric acid concentration of 0.5 mol/L, the dissolution rate of titanium was as low as 1.64%. These results demonstrate that LTO possesses excellent acid resistance.

Fig.6 Acid resistance of LTO

In the process of HTO extracting lithium from brine, coexisting metal ions may affect the adsorption performance of lithium (refer to Figure 7a). Therefore, it is essential to investigate the selectivity of HTO. As shown in Figure 7a, the adsorption capacity of Li+ by HTO reaches 24.8 mg/g, significantly higher than that for Na+, K+, Mg2+, and Ca2+. The separation coefficients for Na+, K+, Mg2+, and Ca2+ in the brine are presented in Table 2, further demonstrating HTO’s excellent selectivity. Due to the memory effect of the titanium lithium ion sieve [references 3, 18], the structure’s cavity accommodates only ions that match its radius. Thus, Mg2+ (72 pm), being close to Li+ (76 pm, ionic radius, same below), occupies a few of the Li+ sites, while Ca2+, Na+ (100 pm), and K+ (138 pm) are larger, leading to a higher adsorption capacity for Mg2+ than for Ca2+, Na+, and K+. These studies indicate that although Na+, K+, and Ca2+ are also adsorbed slightly by HTO, the material still exhibits strong selective adsorption for Li+.

Fig.7 Adsorption selectivity（a） and cycle performance of HTO（b）

Table 2 Separation coefficients of various metal ions by HTO

metal ion	Kd/ （mL ·g-1）	αM(Li)	metal ion	Kd/ （mL ·g-1）	αM(Li)
Li+	110.7	1	Mg2+	0.023	4 813.0
Na+	0.95	116.5	Ca2+	30	3.69
K+	0.51	217.1

(T) See Figure 7b. It is evident from Figure 7b that after 20 adsorption-detachment cycle experiments, the lithium adsorption capacity of HTO decreased slightly from 24.8 mg/g to 23.6 mg/g, a modest decline of only 4.8%. Moreover, the titanium dissolution rate remained under 0.08%, demonstrating HTO’s excellent cyclic performance and stability. HTO is also effective across various pH levels in brine, as illustrated in Figure 8. The adsorption capacity increases sharply with rising pH, then stabilizes. When the pH of the brine ranges from 5.2 to 7.4, the lithium adsorption capacity exceeds 20 mg/g. Furthermore, a comparison of adsorption performances among different adsorbents is presented in Table 3. HTO’s adsorption capacity surpasses that of the aluminum-based adsorbent (5 mg/g) and the D751 resin (22.1 mg/g), but is less than that of the manganese-based adsorbent SMO (43.1 mg/g). Although SMO exhibits a higher adsorption capacity, its manganese dissolution loss rate of nearly 5% significantly exceeds that of HTO, adversely affecting the recyclability of the adsorbent. These results underscore HTO’s high adsorption capacity, stability, and substantial industrial and commercial potential.

Fig.8 Effect of pH on adsorption capacity of HTO

Table 3 Comparison of adsorption properties of different adsorbents

adsorbent	initial Li+Mass concentration / (mg · L-1）	adsorption capacity /（mg ·g-1）	eluant	dissolution rate of /%
D751 resin［10］	0.01	22.1	1 mol/L HCl	—
Aluminum-based adsorbent［19］	5 000	5	H2O	—
SMO［20］	500	43.1	0.5 mol/L HCl	<5
HTO	300	24.8	About 0.032 mol/L of HCl	0.08

3 Summary

The titanium series lithium ion sieve was synthesized using the high-temperature solid-phase method. It was characterized using SEM, XRD, and contact angle tests. This sieve employs an adsorption process to separate lithium from various impurity metal ions in high-magnesium, lithium-rich salt lake brines. Our study indicates that HTO exhibits a high adsorption capacity (24.8 mg/g) and exceptional selective performance for high-magnesium, lithium-rich salt lake brines (selectivity coefficient, α_g = 4813.0). This effectiveness is likely due to the unique memory effect of the titanium lithium sieve, where the structural cavities accommodate only ions of matching radius. After 20 cycles, the adsorption capacity decreased by only 4.8%, maintaining a high capacity of 23.6 mg/g. Moreover, the titanium dissolution rate remained below 0.08% per cycle, demonstrating the excellent cyclic performance and stability of HTO.

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