Development of Hydrogel/Aluminum-based Adsorbent Beads and Membranes for Lithium Extraction from Brines

Abstract

Driven by the rapidly expanding market for lithium-ion batteries, the global demand for lithium is projected to surpass 2.4 million metric tons of lithium carbonate equivalent (LCE) in 2030. Lithium extraction from salt-lake brine, which contains 70% of global reserves, is transitioning from inefficient solar evaporation to Direct Lithium Extraction (DLE) technologies. Among DLE methods, adsorption stands out for its superior selectivity and cost-effectiveness in low-grade brines. Nevertheless, the industrial deployment of powder adsorbents is hindered by operational challenges, such as high pressure drop, high material loss, and poor flowability. While current solutions involve immobilizing powders within polymeric binders, this approach introduces new problems: the matrix often causes pore occlusion at active sites and increases mass transfer resistance, thereby compromising lithium (Li⁺) adsorption capacity and kinetics. In this work, a series of hydrogel/aluminum-based adsorbent composite beads and membranes was developed to minimize additional mass transfer resistance. These composites exhibited high Li⁺ extraction capacity, stability, and rapid diffusion kinetics in both batch adsorption and membrane extraction systems. Initially, a lithium aluminum layered double hydroxide adsorbent (LiAl) was synthesized via the coprecipitation of aluminum chloride and lithium chloride in an alkaline solution, achieving a high and stable adsorption capacity of approximately 7.3 mg/g. Notably, aging conditions during fabrication and wet storage were identified as critical factors for maintaining high Li⁺ adsorption capacity, as they preserved the crystallinity of LiAl and reduced particle aggregation. Furthermore, LiAl demonstrated rapid desorption/adsorption kinetics, which were fitted with pseudo-first-order (PFO) and pseudo-second-order (PSO) models. Next, LiAl was granulated using porous and hydrophilic chitosan (CS) as a binder, yielding composite beads with rapid Li⁺ adsorption kinetics. By adjusting bead composition (LiAl, CS, and porogen) and operational parameters (e.g., water-to-solid ratio), a Li⁺ adsorption capacity of 4.4 mg/g was achieved. The adsorption equilibrium and kinetics were well-described by the Langmuir isotherm and the PFO model, respectively, suggesting a monolayer adsorption on relatively homogeneous active sites. Notably, a film-pore diffusion model was applied to elucidate mass transfer mechanisms. A calculated Biot number of approximately 16 at an agitation speed of 120 rpm indicated that intra-particle diffusion was the dominant step. The beads achieved near-equilibrium within approximately 2 h, yielding a high pore diffusion coefficient (Dₚ) of 2.15 × 10⁻¹⁰ m²/s. Furthermore, the composite beads exhibited robust cyclic stability (maintaining 4.4 mg/g after 12 cycles) and selectivity towards Li⁺. This approach demonstrated that the application of a hydrogel binder was effective in maintaining rapid Li⁺ diffusion kinetics in the granulated LiAl beads. Sodium alginate was investigated as an alternative binder to chitosan to enhance Li⁺ adsorption capacity, leveraging its milder gelation process to preserve the LiAl structure. The alginate/LiAl beads exhibited a high adsorption capacity of approximately 7.5 mg/g within a short adsorption time of 2 h over 10 adsorption-desorption cycles. A film-pore diffusion model based on Langmuir adsorption isotherm analysis was applied to elucidate the adsorption kinetics, confirming that Li⁺ uptake was primarily governed by pore-volume diffusion. The beads exhibited a high Dₚ of approximately 1.96 × 10⁻¹⁰ m²/s, indicating the alginate binder imposed minimal resistance to ion transport. Furthermore, the structural instability of the alginate beads caused by the leaching of cross-linking Ca²⁺ was mitigated by the presence of Ca²⁺ ions in the feed solution or by chemical cross-linking with hydrophilic and flexible poly (ethylene glycol) diamine. The composite beads also demonstrated high selectivity for Li⁺ over Mg²⁺, Na⁺, and Ca²⁺ in the simulated brine. To further exploit the rapid kinetics of the hydrogel/LiAl composite and enable simultaneous adsorption and desorption, this study extended its application from batch adsorption to membrane extraction (ME) systems. ME, which combines an organic extractant with a partitioning membrane, is an efficient and sustainable strategy for recovering Li⁺ from salt-lake brines. However, the development of membranes possessing both a high Li⁺ flux and solvent resistance remains a challenge. In this work, a porous chitosan membrane embedded with LiAl was used as a partitioning barrier in the membrane contactor for Li⁺ extraction, with tributyl phosphate (TBP)/kerosene being the extractant. A high Li⁺ flux of 3.63 × 10⁻⁸ mol/(cm² s) was achieved at a feed Li⁺ concentration of 0.2 mol/L. The impacts of individual resistance components in the membrane extraction process on overall mass transfer were analyzed using the resistance-in-series model, and the external mass transfer resistances (liquid boundary layer near the membrane surface and interfacial Li⁺-extractant complexation) were found to be significant for a 19 μm CS/LiAl membrane. The CS/LiAl membrane facilitates preferential Li⁺ transport, while the organic extractant primarily governs the overall Li⁺ selectivity of the extraction process. The versatility of this membrane was further demonstrated using a β-diketone-based organic extractant, which maintained a high Li⁺ flux of more than 3.50 × 10⁻⁸ mol cm⁻² s⁻¹ for 19 days. This study presents an efficient strategy to significantly enhance lithium flux in membrane extraction systems by leveraging the rapid mass transfer in hydrogel/LiAl membranes.