Comparative evaluation of acid-resistant nanofiltration membranes for heavy metal removal in acidic wastewater

Acid-resistant nanofiltration (NF) membranes offer robustness in efficiently separating valuable resource and recycling waste acid in harsh acidic conditions. Yet, the existing ambiguity in understanding the structure-performance relationship of commercially available acid-resistant membranes leads to an unclear direction for designing innovative acid-resistant membranes and poses challenges in selecting suitable membranes for engineering applications. This work thoroughly examined the structural characteristics of commercially available membranes, assessed their effectiveness in removing heavy metals, and established a clear structure-performance relationship. Key findings include that superior permeability is attributed to the thinnest selective layer, higher separation precision is attributed to small pore size and narrow pore size distribution, exceptional acid stability should be owing to the sulfonamide structure, and medium-strength acid resistance of membranes is due to mixed sulfonamide-amide structures or extended carbon chain structures. Additionally, the work highlighted the contribution of dense and thick selective layers to membrane stability and the influence of surface characteristics on fouling removal. These insights provide valuable guidance for the structural design and selection of acid-resistant membranes in engineering applications, enhancing their effectiveness in diverse acidic environments.