OGCN-modified thin-film nanocomposite membranes for efficient capture and enrichment of rare earth ions

Abstract

Conventional methods for recovering ionic rare earth elements are complex, and involve multiple phase transitions that reduce production efficiency. Therefore, simplified and more efficient recovery processes are urgently needed. In this study, we developed thin-film nanocomposite (TFN) membranes modified with oxidized graphite-phase carbon nitride (OGCN) to achieve high-efficiency capture and enrichment of rare earth ions. Graphitic carbon nitride was synthesized via thermal polymerization and then transformed into OGCN nanoparticles through a hydrogen peroxide-assisted hydrothermal process. These nanoparticles were embedded into a polyetherimide (PEI) mixed matrix membrane (MMM), and a TFN membrane (PA-MMM(PEI-OGCN)) was fabricated using piperazine as the aqueous phase and trimesoyl chloride as the organic phase via interfacial polymerization. PA-MMM(PEI-OGCN) exhibited high water flux (38.6 L·m⁻²·h⁻¹) and effective rare earth ion rejection, with La³⁺, Y³⁺, and Nd³⁺ rejection rates all exceeding 95 %. The OGCN modification significantly enhanced the hydrophilicity and surface charge density of the membranes, improving their capacity to capture and enrich rare earth ions. Under different volume reduction factors (VRF), the PA-M2 membrane consistently showed higher rare earth ion rejection compared to the PA-M0 membrane. At a VRF of 10, the PA-M2 membrane increased rare earth ion concentration from 0.242 to 1.285 g/L, and kept rare earth loss in the permeate below 65 mg and significantly lower than the 80 mg observed with the PA-M0 membrane. The PA-M2 membrane demonstrated excellent stability and fouling resistance during long-term operation. In conclusion, OGCN-modified TFN membranes offer great potential for efficient separation and recovery of rare earth ions, and present broad applications in resource sustainability and environmental protection.