• Specific-chain-length QACs (C12-C16) below CMC formed flocs with ClO 4 − . • Micron-size flocs facilitate selective ClO 4 − removal in following ultrafiltration. • ∼99% ClO 4 − removal from real industrial wastewater with <15% flux decline. • QACs−ClO 4 − co-assembly behavior was unraveled via interaction analysis. Strategies for selective perchlorate (ClO 4 − ) removal from industrial wastewater remain critical but insufficiently explored, as existing technologies prioritize idealized performance metrics over scalability and cost-effectiveness. Herein, a counterintuitive phenomenon is reported that specific-chain-length QACs (C12-C16) can initiate selective ClO 4 − co-assembly into 0.2-0.3 μm flocs starting at sub-CMC concentrations. Thus, we propose a QACs-assisted ultrafiltration (UF) process to treat ClO 4 − wastewater. Our experimental results show that QACs-assisted UF achieves up to ∼99% ClO 4 − removal for specific industrial wastewater, with only ∼10% decline in flux and resist interference from 10-fold excess competing anions. Through state-of-the-art characterizations and meticulous theoretical calculations, we reveal that ClO 4 − triggers interaction transition from repulsion to attraction between QACs with C12-C16 alkyl chains via synergistic van der Waals, hydrophobic, and electrostatic effects, enabling QACs−ClO 4 − co-assembly and spontaneous floc formation. Furthermore, we demonstrate that QACs with C12, C14, and C16 alkyl chains bind ClO 4 − with high K d of 1.4 × 10 4 , 3.4 × 10 4 , and 7.6 × 10 4 M -1 , and ΔG of -23.60, -25.86, and -27.86 kJ/mol, respectively. Overall, QAC-assisted UF provides a low-pressure, interference-tolerant route to achieve near-complete ClO 4 − removal in real industrial wastewater.
Ao et al. (Sun,) studied this question.