Integration of membrane technologies to enhance the sustainability in the treatment of metal-containing acidic liquid wastes. An overview

The mining and hydrometallurgical industries generate effluents characterised by a high acidity (pH < 3) and a high content of metals (e.g. Fe, Al, Cu and Zn, among others) and non-metals (e.g. As, Sb, Bi), which confers them toxicity that makes necessary their treatment. The conventional treatments of such streams mainly rely on neutralisation/precipitation, solvent extraction, ion-exchange and adsorption. However, these options are often not feasible because of their high consumption of chemicals and the generation of large volumes of sludge. On the other hand, due to the exhaustion of natural resources, circular economy schemes are increasingly promoted for the recovery of valuable elements (e.g. Cu, Zn, rare earth elements) from waste effluents as an alternative to mining the already over-exploited mine sites. Within this new paradigm, the integration of membrane technologies are gaining importance for the valorisation of such effluents since they provide the possibility of: i) reducing the volume of the streams to be treated, ii) favour the selective separation of metal ions from acids, and, iii) the concentration of metals in acidic streams. Membrane technologies are promoting the recovery of valuable dissolved components and the reuse of the acid, reducing the generation and disposal of sludge. Among the different membrane technologies, nanofiltration (NF), diffusion dialysis (DD), reverse osmosis (RO), electrodialysis (ED), forward osmosis (FO) and membrane distillation (MD) are the most promising ones to tackle these challenges and promote circularity.