Effect of halides on polyamide-based membrane flux and monomer degradation during chloramination

Polyamide (PA)-based membranes are widely used in reverse osmosis (RO) and nanofiltration (NF) but can react with chlorine-based disinfectants (free chlorine and chloramines) applied for biofouling control. This study examined how chloramines affected PA layer reactivity and performance when individual and mixed halides (Cl−, Br−, and I−) were present, since these reactants can form a host of different halogenating agents and reactive species. Experiments measured membrane (SWC4-LD) performance via flux, membrane surface chemistry using X-ray photoelectron spectroscopy (XPS), and PA monomer (i.e. benzanilide (BA)) reactivity, under varied halide (0 to seawater range) and pH (4–9) conditions. These experiments were also compared to other experiments that were chlorinated instead. Results indicated that flux differentials were the greatest after chlorination as compared to chloramination with or without Cl−, Br−, and/or I−, implying that chlorination was more detrimental to polyamide structural integrity. The limited role of Br− was supported by its low surface halogenation but contrasted its ability to rapidly transform the BA monomer to by-products. Mass transfer limitations within the rigid 2-D polymer structure likely drove this disparity. Subsequent addition of Cl− further lowered the membrane flux and advanced monomer transformation for all pH conditions, whereas I− did not significantly impact the membrane or monomer. As a consequence, membrane and monomer changes during chloramination were highly dependent on halide type. Such observations are intended to inform how chloramines may be used to reduce biofouling when halides are present in RO- and NF- feed waters.