Assessment of nanofiltration efficiency for simultaneous concentration of copper and recovery of acid from highly acidic wastewater.

Conventional treatment of acidic printed circuit boards (PCB) wastewater often relies on alkaline neutralization and precipitation, leading to the generation of sludge with significant loss of copper and large consumption of alkaline. Nanofiltration (NF) has emerged as a promising technology for resource recovery from industrial wastewater. However, the evolution of the structure-performance relationship for different types of NF membranes under long-term acidic exposure remains unclear. Moreover, how does the membrane intrinsic separation capability affect its efficiency for copper concentration and acid recovery from acidic PCB wastewater is also a critical gap in existing literature. Herein, three NF membranes with distinct active-layer chemistries were holistically evaluated in terms of physicochemical properties and separation performance under a 100-d exposure in an extremely acidic environment (pH 0.2). Characterization results revealed that the semi-aromatic VNF1 membrane underwent severe hydrolysis thereby resulting in complete loss of separation performance, e.g., water permeance rose to > 200 L·m-2·h-1·bar-1 and complete loss of rejection for all solutes. In contrast, fully-aromatic NF90 membrane and polysulfonamide contained Acidstab-NF membrane maintained their structural integrity and separation performance. Leveraging on their excellent acid stability, these two membranes also demonstrate effective separation of Cu2+ and H+ in the filtration of synthetic PCB wastewater with a water recovery up to 80%. The NF90 membrane achieved >99% rejection of Cu2+ and partial rejection of HCl, leading to continuous buildup of feed osmotic pressure with increasing water recovery, thereby resulting in severe flux decline. In comparison, Acidstab-NF membrane featured a considerable Cu2+ rejection of ∼90% and high permeation of HCl even at high water recovery of up to 80%, which would be more favorable for the simultaneous concentration of copper and recovery of acid in PCB wastewater treatment. Such trade-off between membrane separation efficiency and flux stability suggests that balanced Cu2+ rejection and acid permeation is more important to enable long-term stable operation with more effective resource recovery from highly acidic PCB wastewater.