Fabrication of positively charged nanofiltration membrane with uniform charge distribution by reversed interfacial polymerization for Mg2+/Li+ separation

Scalable Preparation of Ultraselective and Highly Permeable Fully Aromatic Polyamide Nanofiltration Membranes for Antibiotic Desalination

Membranes are important in the pharmaceutical industry for the separation of antibiotics and salts. However, its widespread adoption has been hindered by limited control of the membrane microstructure (pore architecture and free-volume elements), separation threshold, scalability, and operational stability. In this study, 4,4',4'',4'''-methanetetrayltetrakis(benzene-1,2-diamine) (MTLB) as prepared as a molecular building block for fabricating thin-film composite membranes (TFCMs) via interfacial polymerization. The relatively large molecular size and rigid molecular structure of MTLB, along with its non-coplanar and distorted conformation, produced thin and defect-free selective layers (~27 nm) with ideal microporosities for antibiotic desalination. These structural advantages yielded an unprecedented high performance with a water permeance of 45.2 L m-2 h-1 bar-1 and efficient antibiotic desalination (NaCl/adriamycin selectivity of 422). We demonstrated the feasibility of the industrial scaling of the membrane into a spiral-wound module (with an effective area of 2.0 m2). This module exhibited long-term stability and performance that surpassed those of state-of-the-art membranes used for antibiotic desalination. This study provides a scientific reference for the development of high-performance TFCMs for water purification and desalination in the pharmaceutical industry.

A Selective Separation Mechanism for Mono/divalent Cations and Properties of a Hollow-Fiber Composite Nanofiltration Membrane Having a Positively Charged Surface

Positively charged nanofiltration (NF) technology is considered a green and low-cost method for mono/divalent cation separation. Nevertheless, the separation rejection mechanisms of these NF membranes have yet to be extensively investigated. In this work, we fabricated a thin-film composite (TFC) hollow-fiber (HF) NF membrane with a positively charged surface via modification of the nascent interfacial polymerization layer using a branched polyethyleneimine (BPEI)/ethanol solution. Then, we extensively investigated its selective separation mechanism for mono/divalent cations. We proposed and proved that there exists a double-charged layer near the membrane surface, which helps to repel the divalent cations selectively via Donnan exclusion while promoting the fast penetration of monovalent cations. Meanwhile, the membrane skin layer is loose and hydrophilic due to the loose BPEI structure and the abundance of amine groups, as well as the changed fabrication conditions. In this way, we achieved very good mono/divalent cation selectivity and relatively high water permeance for the as-prepared HF NF membrane. We also obtained good anti-fouling, anti-scaling, and acid resistance, and long-term stability as well, which are urgently needed during practical application. Furthermore, we successfully amplified this HF NF membrane and proved that it has broad application prospects in mono/divalent cation separation.

Advanced Lithium Extraction Membranes Derived from Tagged-Modification of Polyamide Networks

Efficient Mg2+ /Li+ separation is crucial to combating the lithium shortage worldwide, yet current nanofiltration membranes suffer from low efficacy and/or poor scalability, because desirable properties of membranes are entangled and there is a trade-off. This work reports a "tagged-modification" approach to tackle the challenge. A mixture of 3-bromo-trimethylpropan-1-aminium bromide (E1 ) and 3-aminopropyltrimethylazanium (E2 ) was designed to modify polyethylenimine - trimesoyl chloride (PEI-TMC) membranes. E1 and E2 reacted with the PEI and TMC, respectively, and thus, the membrane properties (hydrophilicity, pore sizes, charge) were untangled and intensified simultaneously. The permeance (34.3 L m-2  h-1  bar-1 ) and Mg2+ /Li+ selectivity (23.2) of the modified membranes are about 4 times and 2 times higher than the pristine membrane, and they remain stable in a 30-days test. The permeance is the highest among all analogous nanofiltration membranes. The tagged-modification method enables the preparation of large-area membranes and modules that produce high-purity lithium carbonate (Li2 CO3 ) from simulated brine.

Selective electrodialysis process for the recovery of potassium from multicomponent solution systems

Selective electrodialysis is a promising approach to recovering K+ from complex coexisting ionic systems. In this study, the effects of current density, the concentration of K+ and Mg2+, as well as the operating temperature on the separation process of K+ and Mg2+ were explored to investigate the competitive migration of mono- and multivalent ions, offering a guide for the design of selective electrodialysis process, and therefore obtain the desired aqueous solutions containing K+ and Mg2+. The results show that ion concentration played a critical role in determining the selectivity of separation between K+ and Mg2+. High concentrations of K+ and Mg2+ led to a decrease in selectivity but the effect of concentration of K+ on selectivity was more pronounced. Although higher current density increased the flux of ions, their impact on separation selectivity was minimal. Furthermore, higher temperature increased the flux of ions but resulted in a decrease of K+ proportion in the solution. Overall, this study provides good guidance for studying the competitive migration of mono- and multivalent ions and the high-value recycling of potassium resources.

Lithium Concentration from Salt-Lake Brine by Donnan-Enhanced Nanofiltration

Membranes offer a scalable and cost-effective approach to ion separations for lithium recovery. In the case of salt-lake brines, however, the high feed salinity and low pH of the post-treated feed have an uncertain impact on nanofiltration's selectivity. Here, we adopt experimental and computational approaches to analyze the effect of pH and feed salinity and elucidate key selectivity mechanisms. Our data set comprises over 750 original ion rejection measurements, spanning five salinities and two pH levels, collected using brine solutions that model three salt-lake compositions. Our results demonstrate that the Li⁺/Mg²⁺ selectivity of polyamide membranes can be enhanced by 13 times with acid-pretreated feed solutions. This selectivity enhancement is attributed to the amplified Donnan potential from the ionization of carboxyl and amino moieties under low solution pH. As feed salinities increase from 10 to 250 g L^-1, the Li⁺/Mg²⁺ selectivity decreases by ∼43%, a consequence of weakening exclusion mechanisms. Further, our analysis accentuates the importance of measuring separation factors using representative solution compositions to replicate the ion-transport behaviors with salt-lake brine. Consequently, our results reveal that predictions of ion rejection and Li⁺/Mg²⁺ separation factors can be improved by up to 80% when feed solutions with the appropriate Cl^-/SO₄^2- molar ratios are used.