Application of nanofiltration for acidic waters containing rare earth elements: Influence of transition elements, acidity and membrane stability

Acid mine drainage (AMD) is considered the main environmental problem in mining operation due to its acidity, metal content (Fe, Al, Zn, Cu) and onerous associated treatment cost. However, the presence of relatively high levels of added value elements (rare earth elements (REE)) could make its valorisation economically affordable. Membrane nanofiltration (NF) has been postulated as a potential recovery technology because it allows the recovery of a sulphuric acid rich stream and a metal rich stream in one step. In this study, the performance of a semi-aromatic polyamide NF membrane (NF270) in filtering model solutions containing common metals (Ca, Al, Zn) in sulphuric solutions (pH 1.5-3.0) was evaluated. In a second stage, the performance was assessed with the same model solutions but also containing REE. NF270 showed high rejections for metallic ions in solution, allowing acid permeation. Ion rejection data were used to determine membrane permeances to ions using the solution-diffusion model considering reactive transport. The stability of the membrane was also studied by comparing performances of a virgin membrane with an aged membrane exposed to 1 M sulphuric acid for 4 weeks. Physicochemical changes of membrane properties after ageing were also analysed by ATR-FTIR, AFM and XPS.