Enhancement and Mechanism of Tannic Acid Modification on Separation of Lithium and Magnesium in a Two-Dimensional Montmorillonite Membrane

Lithium extraction from salt lake is essential for ensuring lithium resource security in China, but the design of high-performance lithium extraction membranes from salt lake faces significant challenges. Two-dimensional (2D) membranes with oriented nanochannels consist of 2D nanosheets stacked layer by layer. These membranes have nanoscale channel heights that can be precisely controlled at the sub-nanometer level, making them suitable for the selective separation of lithium (Li) and magnesium (Mg) ions. Typically, the Li⁺/Mg²⁺ selectivity of membrane requires surface modification with positive charge due to the inherent charge differences between Li⁺ and Mg²⁺. Interfacial polymerization (IP) is commonly used for the membrane surface modification. However, the excessive IP reactions can result in excess positive charge, reducing the Li⁺ transport efficiency. Therefore, the design and development of 2D selective membranes with optimized IP reactions while maintaining high lithium-ion transport efficiency is of great significance. Layered montmorillonite (MMT) mineral has natural layered structure and it can be easily exfoliated into 2D nanosheets, which are ideal building blocks for the assembly of 2D membranes. In this work, hydrophilic tannic acid (TA) with negative charge is applied to modify the surface of 2D MMT nanosheets, enabling an enhanced Mg²⁺ rejection and suppressed transport resistance of Li⁺ transport. Our results provide a valuable reference for the design and fabrication of novel 2D membranes with high Li⁺/Mg²⁺ selectivity.

The MMT nanosheet suspension was prepared by ultrasonic exfoliation. The suspension mixed with auxiliary membrane-forming agents were scraped coated and dried on a substrate to obtain 2D MMT membranes. For the preparation of TA-modified membranes, TA was added to the MMT nanosheet suspension and coated on substrate using the same protocol. As-prepared membranes were then used for IP reactions. The membranes were first immersed in a PEI solution 45 s. After washing and drying, the membranes were then immersed in a TMC solution for 15 s. As-obtained membranes were dried in an oven for subsequent nanofiltration and characterizations. The morphology and surface roughness of the 2D MMT membranes were characterized by a JSM 7100F scanning electron microscope (SEM) and a Multi Mode 8 atomic force microscope (AFM). The surface chemical analyses was conducted by a K-Alpha X-ray photoelectron spectroscopy (XPS). The alignment and orientation of internal 2D nanochannels of the MMT membrane were analyzed by a D8 ADVANCE X-ray diffractometer (XRD) and Xeuss 2.0 X-ray wide-angle diffractometer (WAXS).

According to the morphological and structural characterizations, the 2D membranes fabricated with bare 2D MMT nanosheets exhibit a well-oriented 2D nanochannel structure. The alignment of TA-modified 2D MMT nanosheets were unaffected during IP reaction, generating a well-preserved membrane structure according to the cross-sectional SEM and WAXS.

With the increasing content of TA, the rejection of Mg²⁺ increased simultaneously and reached up to almost 100% with 1.5% addition of TA. A negative Li⁺ rejection was also achieved correspondingly. However, excess amount of TA resulted in a deterioration of both the rejection of Mg²⁺ and selective transport of Li⁺. In addition to the selectivity, the TA modification significantly promoted the membrane flux up to 22.5 L·m^–2·h^–1, which was 49% higher than that of bare MMT membrane. The TA modified MMT membrane exhibited consistently high Mg²⁺ rejection under various feed conditions and high performance stability during a 120 h long-term operation.

The enhancement in membrane selectivity and flux through TA modification was due to the promoted IP reaction and a narrowed pore size distribution, validated by XPS measurements and molecular weight cut off (MWCO) tests. Furthermore, the role of TA in membrane formation was revealed by AFM and XPS measurements. The TA molecules can connect the adjacent MMT nanosheets via hydrogen bonding between its phenolic hydroxyl groups and the Al–OH on edge surface of MMT. The negative charge on TA provides more anchoring site for the PEI monomers, thereby promoting the IP reaction. Energy barrier calculated by Arrhenius equation further validated that the TA modification can reduce the Li⁺ transport resistance from 20.91 kJ·mol^–1 to 18.38 kJ·mol^–1 in the 2D nanochannels.

In this work, a TA-modification method was applied to enhance the separation performance of a 2D MMT membrane system. The Mg²⁺ rejection of the modified MMT membrane with optimized TA content reached almost 100%, with a corresponding negative Li⁺ rejection of –9%. The membrane flux was 22.5 L·m^–2·h^–1, after TA modification, which was 49% higher than that of bare MMT membrane. Moreover, the TA-modified membranes exhibited consistently high Mg²⁺/Li⁺ selectivity and flux under various feed conditions and excellent stability during long-term operation.

Topographical and surface chemical characterizations revealed the role of TA molecules in forming hydrogen bonding with the hydroxyl groups on edge surface of MMT nanosheets. The enhancement mechanism of membrane selectivity via TA modification was attributed to the additional negative charge from TA. The addition of TA enabled efficient PEI monomers adsorption, which can promote the IP reaction and reduce the average pore size. Conductivity test and energy barrier calculations further validated that the TA modification was able to reduce the transport resistance of Li⁺, thus realizing the enrichment of Li⁺.