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Pressure-driven membrane processes, such as reverse osmosis (RO) and nanofiltration (NF), are preferred solutions for salinity reduction in ground, surface and sea waters as well as in mining and urban wastewaters. Interest in NF is increasing because it enables a more cost-efficient operation than RO. However, predictive projection tools for NF applications are still lacking. Whereas in RO the solute rejections are usually above 95% and approximately independent of feed composition, the performance of NF membranes is highly influenced by the feed-solution composition. Moreover, in both processes, concentration polarisation (CP) affects the transport of species and membrane scaling. This study presents an extended Solution-Electro-Diffusion model for the description of the transport of solutes through NF membranes in complex multi electrolytes (strong and weak) taking into account CP, namely "Solution-Electro-Diffusion-Film coupled with Reactive Transport" (SEDFRT) model. The model is first validated by using a semi-aromatic polymeric membrane (NF270) and electrolyte mixtures containing two dominant (MgSO 4 and MgCl 2) and two minor salts (NH 4 Cl and NaBr). The membrane permeances to all the species are determined as key parameters to be used in design projections. Finally, the model is applied for the description of the performance of NF270 membrane with model brackish surface water, influenced by urban, industrial and agriculture discharges. [Display omitted] • Nanofiltration (NF) performance depends of feed solution composition. • Difficult to predict properly the performance of NF membranes. • Concentration polarisation affects both species transport and scaling. • Incorporation of reactive transport in the Solution-Electro-Diffusion-Film model. • Model validation with electrolyte mixtures (MgSO 4 /MgCl 2 /NH 4 Cl/NaBr). [ABSTRACT FROM AUTHOR]

• Valorisation of acidic waters using nanofiltration (NF) in the Cu industry. • Characterisation of Duracid membrane as polysulphonamide. • High metal rejections (>85%) and acid recovery (rejection below 40%) with Duracid. • Precipitation of calcium and iron sulphates during the process. • Use of SEDF model as a designing tool for scaling-up. The exhaustion of Cu reserves minable and processable with the available technology is forcing the hydrometallurgical copper industry to seek alternative sources. Following circular economy principles, researchers' attention has focused on the recovery of valuable metals from the acidic waste streams generated. Nowadays, membrane technologies are being selected as the first alternative for the treatment and valorisation of such acidic waste streams. Among them, a new generation of high acidity resistance nanofiltration (NF) membranes offer the alternative for acid recovery while providing a metal-enriched stream. In this work, the extreme-acid resistant Duracid membrane was evaluated for the valorisation of different synthetic acidic waste streams from the hydrometallurgical Cu industry. These waters were characterised by a high acidity (pH 0.5–1.5) and the presence of Fe (11–14 g/L), Zn (0.7–1.4 g/L) and As (0.5–0.7 g/L), among others. Initially, the membrane was characterised by different techniques (SEM, FTIR-ATR, XPS). Experiments were performed under constant and varying permeate flux and feed water composition. Metals were effectively rejected (>90%), whereas H+ easily permeated through the membrane. The experimental results were adjusted to the Solution-Electro-Diffusion-Film (SEDF) model to determine the membrane permeances to species. Empirical mathematical equations were developed and validated to express the dependence of permeances on solution composition. Finally, the prediction capability of the SEDF model, together with the developed empirical equations for the permeances, was proposed as a tool for designing a NF unit to valorise acidic streams from the hydrometallurgical Cu industry. The model predicted gypsum scaling onto the membrane and therefore anticipated the need of applying antiscalants. [ABSTRACT FROM AUTHOR]

• Hydrometallurgical wastewaters as a potential source for chemicals recovery. • Nanofiltration membranes for acid recovery. • Use of Solution-Electro-Diffusion model to determine membrane permeances. The metallurgical industry generates large volumes of toxic effluents characterised, generally, by high acidity and a noticeable content of metals (Fe, Cu and Zn) and non-metals (As, Sb, Bi). The toxicity of these streams makes necessary treatment before its discharge to the environment or reuse. Sustainable management of these effluents must be focused on the recovery of low added valuable by-products (e.g. strong acids) to reduce the wastes generated along with the treatment (e.g. sludge). Nanofiltration offers clear advantages for acid recovery instead of the conventional treatments such as neutralisation and precipitation, due to the high membrane transport ratios of single charged ions and high rejection of multi-charged ions. The performance of a semi-aromatic poly(piperazineamide) membrane (NF270) was evaluated for the treatment of effluents from copper metallurgical process streams of off-gases treatment trains. These streams are characterised by a high acidity (pH < 1) due to a mixture of strong (H 2 SO 4 , HCl) and weak (H 3 AsO 4) acids and the presence of metallic species (Fe, Cu, Zn). The membrane performance was evaluated in terms of acid recovery and metal ions rejection taking into account their aqueous speciation in strong acid media. The transport of the species across the membrane was characterised according to the Solution-Electro-Diffusion model. The membrane permeances to aqueous species (both charged and non-charged) in strongly acidic solutions were calculated. NF270 showed good results for strong acid recovery, exhibiting high rejections of the metallic impurities. The implications of the presence of large amounts of As present as H 3 AsO 4 should involve a selective removal stage using H 2 S or Na 2 S 2 O 3. [ABSTRACT FROM AUTHOR]

Abstract Acid mine waters (AMWs) are characterized by a low pH, high concentrations of transition metals (iron, copper, zinc), and the presence of highly valuable metals such as rare earth elements (REE) at lower concentrations. Sustainable management of AMW implies the reduction of acidity and Fe/Al concentrations and the recovery of valuable metals by ion-exchange processes. Nanofiltration (NF) offers a good opportunity compared to conventional methods to treat AMW, such as good passage of mono-charged ions (e.g. hydrogen sulphate, proton) for further recovery of sulphuric acid and high rejection of multi-charged ions, such as transition metals and REE. Two membranes with a different active layer, a double membrane (poly-piperazinamide/proprietary polyamide) (Desal DL) and a sulphonated polyethersulphone (HydraCoRe 70pHT), were evaluated with a solution mimicking acidic streams generated after the removal of iron by precipitation and its subsequent treatment with ion-exchange resins. The recovery of sulphuric acid and metal rejections were evaluated by changing the sulphuric concentration/pH (1.0–1.5) and by varying the concentration of aluminium in the solution (0.6–2.2 g/L). The transport of ions through NF membranes was described according to the solution-electro-diffusion model coupled with reactive transport of ions, and the membrane permeances to ions were determined as a function of aluminium concentration. Desal DL showed better results than HydraCoRe 70pHT in terms of acid recovery and metal rejections for both tested conditions. Membrane chemistry of the active layer (nature and acid-base membrane properties) and chemical speciation were found to be strong parameters in the separation performance of the membranes. Finally, a process scheme for recovering the REE is proposed. Highlights • Acidic mine waters as a potential source for rare earths elements recovery. • Evaluation of nanofiltration metals for the treatment of acid mine waters. • Study of the effect of aluminium concentration on metal rejection and acid passage. • Determination of membrane permeances by means of solution-electro-diffusion model. • Proposal of a nanofiltration stage in a process for rare earths recovery. [ABSTRACT FROM AUTHOR]

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 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%). [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Nanofiltration (NF), as a selective Mg(II) and Ca(II) separation and concentration treatment, and electrodialysis with bipolar membranes (EDBM) were evaluated for the valorization of seawater desalination reverse osmosis brines (60 NaCl/L) to produce both rich Mg(II) and Ca(II) brines for phosphate recovery and HCl and NaOH as chemicals for desalination treatments. A NF pilot plant, using NF270 membranes at 20 bar, provided a rich Mg(II) (8.3 g Mg(II)/L) and Ca(II) (2.1 g Ca(II)/L) brine on the concentrated stream with enrichment factors of 3.2 for Mg(II) and 2.5 for Ca(II). The NF permeate stream containing 50 ± 2 g NaCl/L was treated to remove residual Mg(II) (760 mg/L) and Ca(II) (415 mg/L) by chemical precipitation with Na 2 CO 3 and NaOH before the EDBM unit. Divalent cations free brine containing NaCl (50 g NaCl/L) were fed into the EDBM stack in order to produce NaOH and HCl under recirculation configuration. Constant voltage and acid and base concentrations at different initial conditions were evaluated to obtain the maximum acid and base concentration (approximately 1 M NaOH/HCl) at 9 V. No substantial effect of initial acid and base concentrations on the overall performance was observed. An energy consumption of 2.6 kWh/kg NaOH and current efficiency of 77 ± 3% were calculated. [ABSTRACT FROM AUTHOR]

• Stringent regulations related to low phosphorous concentrations in wastewater. • Need for seeking alternative sources for phosphrous. • Use of nanofiltration membranes for phosphorous concentration. • Recovery of phosphorous as struvite from secondary treatment effluent. The more stringent regulations related to the reduction of residual phosphorous concentration in treated wastewater and the consideration of phosphorous as a critical raw material are boosting new technologies towards its recovery from waste effluents. The concentration of phosphate in these effluents can be easily increased using nanofiltration (NF) due to its high phosphate rejections (>99%) in order to recover it as a raw material for the fertilizer industry. In this work, different model solutions were used to characterise the transport of phosphate species across a semi-aromatic polyamide membrane (NF270) using the Solution-Electro-Diffusion-Film coupled with reactive transport (SEDFRT) model. Experiments were carried out with waters containing one dominant salt (NaCl or Na 2 SO 4) and one trace salt (dihydrogen or hydrogen phosphate salts) to easily determine membrane permeances to species without simulating the complexity of real solutions. Besides, for the first time the transport of H+ and OH– was also modelled. Results showed a high performance of NF270 when separating phosphate (rejections higher than 90%). Finally, the obtained permeances were used to evaluate the recovery capacity of phosphate as struvite from a model secondary treatment effluent through the SEDFRT model. Results demonstrate the potential of NF as a step forward towards closing the urban phosphorus cycle. [ABSTRACT FROM AUTHOR]

Acidic Mine Waters (AMWs) are characterised by high acidity (pH < 3) as H 2 SO 4 and elevated contents of metals (Al, Fe, Cu, Zn), including rare earth elements (REEs). Due to the exhaustion of minable REE containing-minerals, AMWs are increasingly regarded as an alternative source of REEs. Among the different alternatives for the pre-concentration of AMWs required to make the REE extraction possible, nanofiltration (NF) membranes emerge as a promising technology because they not only successfully reject multivalent ions (metals), allowing its concentration in the retentate stream, but also permit the transport of monovalent ones, such as H+ and HSO 4 −, allowing the recovery of sulphuric acid in the permeate. Despite this potential of NF, there is still a lack of modelling tools for predicting the performance of NF membranes because of its dependence on solution composition, membrane properties and interaction between both. In this study, a prediction tool based on the Solution-Electro-Diffusion model (including the effect of solution composition) was developed and experimentally validated for the application of two polyamide-based NF membranes (NF270 and Desal DL) for the recovery of REEs and H 2 SO 4 from three different synthetic solutions mimicking AMWs (pH 1.0, 60 mg/L REEs and, 25–600 mg/L Al, Cu, Ca and Zn) differing in their Fe concentration (0–2125 mg/L). Metals were effectively rejected (>98%), whereas H 2 SO 4 was transported across the membrane (H+ rejections <30%). The mathematical model was able to predict the performance of both membranes as well as the potential scaling events associated with Fe and Al hydroxides and hydroxy-sulphates. Unlabelled Image • Acidic mine waters as a potential source for chemicals recovery • Nanofiltration membranes for acid recovery and metal concentration • A numerical algorithm as a design tool for NF based separation processes • Use of Solution–Electro–Diffusion model to predict nanofiltration performance • Dependence of membrane permeances to species on solution composition [ABSTRACT FROM AUTHOR]

• Nanofiltration membranes for acid recovery and metal concentration from acidic streams. • Need of resistant nanofiltration membranes in acidic media. • Evaluation of ceramic (TiO 2) and polymeric (MPF–34) acid-resistant membranes. Acid-resistant ceramic and polymeric nanofiltration (NF) membranes have been identified as relevant materials for sustainable management of acidic streams. NF properties such as a high passage of single-charged ions and high rejection of multi-charged ions make NF membranes suitable for acid recovery and metal concentration. In this work, the performance of two acid-resistant membranes: TiO 2 ceramic and MPF–34 (proprietary layer) was tested with solutions mimicking acidic mine waters. Model solutions were composed by Al(III), Fe(III), Ca(II), Cu(II), Zn(II) and rare earth elements (REEs(III)) such as La(III), Dy(III), Sm(III), Nd(III), Pr(III) and Yb(III). The effect of acidity (from pH 1.5 to 1.0), Al(III) (from 0.6 to 1.8 g/L) and Fe(III) (from 0.5 to 2.1 g/L) concentrations was studied. Both membranes allowed the transport of H+ (negative rejections were obtained), but exhibited differences related to the metallic ions transport. While MPF–34 presented metal rejections around 80% and independent on the concentration of the major components (Al(III) and Fe(III)), the TiO 2 membrane provided a sequence of rejection values from 5 to 30%, with highest values for trivalent transition metals. These differences in the sequence of rejections suggested that the chemical properties of the TiO 2 layer played a relevant role, and that they could only be explained by dielectric effects. From the observed rejections, it was estimated that MPF–34 provided concentration factors for metals up to 4.2 and <1 for the H 2 SO 4. [ABSTRACT FROM AUTHOR]