Synthesis of advanced polyurea membranes for critical resource recovery from acidic mining brines

Abstract

The transition towards sustainable technology demands efficient extraction of critical metals and recovery from waste streams, including Acid Mine Drainage (AMD). Pressure-driven nanofiltration (NF) is emerging as an effective alternative for an energy-efficient closed-loop system that can effectively separate dissolved metals and recycle the acid. This work focused on the synthesis of robust, acid-stable thin-film composite polyurea NF membranes through interfacial polymerization of diethylenetriamine (DETA) and toluene-2,4-diisocyanate (TDI) on four distinct polyether sulfone (PES) substrates. ATR-FTIR confirmed successful deposition of the PU active layer. The membranes maintained a good electropositive surface charge (up to +24 mV at pH ≈3.0), achieving a maximum Fe²⁺ rejection of ~98%. In addition, active layer synthesis using optimized monomer concentrations and ethanol as co-solvent successfully produced looser PU membranes, achieving pure water flux of ~190 LMH and 70% Fe²⁺ rejection. The overall best-performing membrane delivered an optimal balance of ~155 LMH and 89% Fe²⁺ rejection while maintaining good durability. Furthermore, after 14 days of exposure to pH 1.5 H2SO4, the PU-coated membranes retained their separation performance, whereas both commercial membranes (NF-270 & Desal DK) lost ~33% of the average separation efficiency.