School of Civil & Environmental Engineering and ‡Singapore Membrane Technology Centre, Nanyang Technological University, Singapore 639798.
Environ Sci Technol. 2012 Dec 18;46(24):13184-92. doi: 10.1021/es302867f. Epub 2012 Dec 5.
Understanding the effects of chlorine exposure on polyamide (PA) based membranes is essential in membrane lifespan improvement. In this study, NF90 nanofiltration membrane was treated with sodium hypochlorite at different concentrations, pHs and durations. The changes in membrane elemental composition and bonding chemistry obtained from XPS and ATR-FTIR revealed the impacts of two competing mechanisms: N-chlorination and chlorination-promoted hydrolysis. More chlorine was incorporated into the PA matrix at pH <7, at which HOCl is dominant, while chlorine-promoted hydrolysis was more favorable at pH >7 with abundant hydroxyl groups. The membrane surface became more hydrophobic when chlorination was dominant, which in turn caused the water permeability of chlorinated membrane to decrease. Meanwhile, membrane became more hydrophilic and less cross-linked when hydrolysis effects were governing, which made the membrane more permeable for water. Rejection of charged solutes [NaCl, As(V)] improved in most chlorinating conditions due to increased charge density. However, when hydrolysis was severe (≥ 1000 ppm, pH 7 and 9), the enhanced charge repulsion effect could not compensate for the extensive amide bond cleavage, resulting in declined rejection. The lower rejection of neutral boric acid provided strong evidence of a less cross-linked separation layer.
了解氯暴露对聚酰胺 (PA) 基膜的影响对于提高膜寿命至关重要。在这项研究中，使用次氯酸钠在不同浓度、pH 值和时间下处理 NF90 纳滤膜。XPS 和 ATR-FTIR 获得的膜元素组成和键合化学变化揭示了两种竞争机制的影响：N-氯化和氯化促进的水解。在 pH <7 时，HOCl 占主导地位，更多的氯被掺入 PA 基质中，而在 pH >7 时，由于有大量的羟基，氯化促进的水解更为有利。当氯化占主导地位时，膜表面变得更加疏水，这反过来又导致氯化膜的水渗透率降低。同时，当水解作用占主导地位时，膜变得更加亲水和交联度降低，这使得膜对水的渗透性更高。在大多数氯化条件下，由于电荷密度增加，带电溶质（[NaCl、As(V)]）的截留率提高。然而，当水解严重（≥1000ppm，pH7 和 9）时，增强的电荷排斥效应不能弥补广泛的酰胺键断裂，导致截留率下降。中性硼酸的较低截留率为交联度较低的分离层提供了有力证据。
