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14Acid mine drainage (AMD) is considered the main environmental problem in mining operation due to its acidity, 15 metal content (Fe, Al, Zn, Cu) and onerous associated treatment cost. However, the presence of relatively high 16 levels of added value elements (rare earth elements (REE)) could make its valorisation economically affordable. 17Membrane nanofiltration (NF) has been postulated as a potential recovery technology because it allows the 18 recovery of a sulphuric acid rich stream and a metal rich stream in one step. In this study, the performance of a 19 semi-aromatic polyamide NF membrane (NF270) in filtering model solutions containing common metals (Ca, Al, 20 Zn) in sulphuric solutions (pH 1.5-3.0) was evaluated. In a second stage, the performance was assessed with the 21 same model solutions but also containing REE. NF270 showed high rejections for metallic ions in solution, 22allowing acid permeation. Ion rejection data were used to determine membrane permeances to ions using the 23 solution-diffusion model considering reactive transport. The stability of the membrane was also studied by 24 comparing performances of a virgin membrane with an aged membrane exposed to 1 M sulphuric acid for 4 25 weeks. Physicochemical changes of membrane properties after ageing were also analysed by ATR-FTIR, AFM and 26 XPS. 27 28

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Acid mine drainage (AMD) are acidic streams rich in dissolved ferrous and non-ferrous metal sulfates and minor amounts of non-metals. Nanofiltration (NF) has been postulated as a potential technology in the metallurgical and mining industry to recover strong acids as H 2 SO 4 and concentrate metallic ions from acidic mine waters. The performance of semi-aromatic polyamide (NF270) and sulfonated polyethersulfone (HydraCoRe 70pHT) NF membranes were evaluated at different trans-membrane pressures. Different synthetic solutions were filtered under spiral wound configuration at two pHs (2.0 and 2.8): i) a solution of Na 2 SO 4 and ii) a solution mimicking AMD from dams, containing Na 2 SO 4 and Fe 2+ , Zn 2+ and Cu 2+ . NF270 showed metal rejections higher than 90 %, while for HydraCoRe 70pHT they were in between 60 and 70%. Metal rejection values decreased when solution acidity was increased. Chemical composition of the membrane active 2 layer and the aqueous metal-sulfate speciation were found to have a large impact on membrane separation process. Solution-Electromigration-Diffusion-Film model was used to estimate the membrane permeances to ions from the measured ion rejections. Furthermore, a full scale unit vessel containing six spiral wound membrane modules was simulated. NF270 showed a higher capacity for concentrating metal and sulfate ions (100%) than Hydracore 70pHT (50%).