Characterization and performance evaluation of polymeric nanofiltration membranes for the separation of strategic elements from aqueous solutions

Nanofiltration (NF) can be used for the separation of mono-and divalent ions as well as for uncharged solutes, such as strategic elements from aqueous solutions. In dependence of pH and feed solution concentration, the element species and membrane charge may vary, which can strongly affect the separation performance. The aqueous solution of interest is acidic, sulfate-rich, and originated from a microbial leaching process of an accrued flue dust from German copper ore smelting with various strategic elements included. The separation of Ge, Mo, Re, Co, Cu, and Zn was evaluated in a cross-flow set-up (flow velocity: 0.5 m s(-1), transmembrane pressure: 10-20 bar). For fundamental understanding, the flux, permeability, and separation performance of the utilized polyamide NF membranes (NF99HF, UTC-60) were investigated with NaCl and MgSO4 and compared to those of the strategic elements. Moreover, the surface characteristics of the membranes were analyzed by AFM, SEM, FTIR, contact angle analysis, and streaming potential measurements. The utilized membranes show differences regarding e.g. morphology, smoothness, and location of the isoelectric point. Nevertheless, the experimental results indicated that Re and Ge can be separated from the multicomponent solution with both tested membranes.