Choline chloride modification of nanofiltration membranes for improving heavy metal ions separation in wastewater

The growing concern over heavy metal ion contamination in industrial wastewater, posing a significant threat to global public health, has led to a heightened interest in devising efficient removal methods. Nanofiltration, while promising, faces a substantial challenge in simultaneously achieving a high rejection of heavy metal cations and maintaining an elevated water permeance. In response, this study utilizes a simple approach by employing choline chloride with a quaternary ammonium group to modify the surface of PEI-TMC membrane, leading to the design of a high-performance PEI-TMC-CC membrane. The choline chloride modification optimized the surface morphology, hydrophilicity, roughness, and combined the Donnan exclusion effect as well as sieving effect, enabling the PEI-TMC-CC membrane to achieve an exceptional rejection of heavy metal ions while retaining a great water permeance. An impressive water permeance 10.0 L m⁻² h⁻¹ bar⁻¹ was performed by the PEI-TMC-CC membrane, nearly three times that of traditional PEI-TMC membranes. Concurrently, the PEI-TMC-CC membrane demonstrated an impressive rejection of heavy metals Cd²⁺, Pb²⁺, Ni²⁺, Zn²⁺, and Cu²⁺, reaching rejections of 98.3 %, 95.9 %, 98.1 %, 98.5 % and 99.1 %, respectively. Moreover, density functional theory (DFT) calculations confirmed the improved hydrophilicity of choline chloride molecules, aligning with experimental results. The modification of traditional nanofiltration membranes using cost-effective, hydrophilic, positively charged chemicals not only leads to significant performance improvements but also holds the potential to advance the development of positively charged nanofiltration membranes tailored for the removing of heavy metals cations from wastewater. This approach demonstrates promising prospects for industrial applications and further development.