(666d) Development of a Positively Charged Nanofiltration Membrane for Selective Cation Separation from Mine Tailing Wastewater

Selective ion separation is a critical technology for sustainable water treatment and resource recovery. It addresses key challenges associated with water scarcity, environmental contamination, and the circular utilization of valuable elements. In particular, cation-specific separation plays a central role in processes such as the selective extraction of lithium ions (Li⁺) from complex matrices like salt lake brines and seawater, an area of growing importance due to the increasing demand for lithium in energy storage applications. Furthermore, the removal of divalent and multivalent cations, including calcium (Ca²⁺), magnesium (Mg²⁺), and heavy metals, improving process efficiency, and complying with environmental discharge regulations. Advancements in ion-selective membranes, functionalized adsorbents, and electrochemical separation methods are continually enhancing the selectivity, energy efficiency, and scalability of these processes [1,2].

Nanofiltration (NF) membranes based on thin-film composite (TFC) structures have emerged as a highly effective platform for selective ion separation, primarily governed by Donnan exclusion and size sieving mechanisms. Recent advancements in NF membrane fabrication have introduced a range of strategies to enhance ion selectivity, mainly through structural and chemical modification of the polyamide (PA) active layer, including tuning its morphology and surface charge characteristics. Improving ion selectivity fundamentally depends on the intrinsic properties of the membrane material and the precision with which pore size distribution and surface chemistry can be engineered. These factors directly influence both size-exclusion efficiency and ion-membrane interactions. Specifically, the rejection of multivalent cations can be significantly improved by tailoring the structure and surface charge density of the PA layer. A promising and direct approach to enhance cation selectivity is the introduction of positive surface charges on the membrane, thereby strengthening the electrostatic repulsion of multivalent cations via Donnan exclusion. This can be achieved through post-synthetic modification of the PA layer using grafted cationic functional groups or by incorporating positively charged monomers during the interfacial polymerization process [3–5].

In this study, commercial polyether sulfone (PES) membranes were systematically modified to enable the selective separation of multivalent cations from dilute mining tailings. The surface modification involved a multi-step interfacial functionalization process. Initially, the PES membrane was treated with a trimesoyl chloride (TMC) solution in hexane to introduce reactive acyl chloride groups. Subsequently, a branched polyethylenimine (BPEI) aqueous solution was applied to impart a positive surface charge through the formation of a crosslinked polyamide-like interlayer. Finally, ethylenediaminetetraacetic acid (EDTA) was introduced to further tailor the membrane's chemical functionality, resulting in a PES/TMC/BPEI/EDTA composite membrane. The resulting nanofiltration (NF) membranes will be tested at 25 ^oC, pressure (vacuum, permeate), and solution containing cations (Ca, Mg, Mn and Fe). In addiution, they will be characterized by using scanning electron microscopy (SEM) and atomic force microscopy (AFM) to evaluate changes in surface morphology and topographical roughness before and after separation test with solutions.

References

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[5] Z. Zhao, S. Feng, C. Xiao, J. Luo, W. Song, Y. Wan, S. Li, Exploring ions selectivity of nanofiltration membranes for rare earth wastewater treatment, Sep Purif Technol 289 (2022) 120748. https://doi.org/10.1016/j.seppur.2022.120748.