Decoupling the CT Paradigm in Membrane Chlorination: Equivalent Versus Inequivalent Impacts of Hypochlorite Concentration and Exposure Time on Contaminants Rejection.

CT (i.e., hypochlorite concentration C × exposure time T) value, borrowed from water disinfection that assumes the equivalent contribution of C and T, is widely used to indicate membrane chlorination intensity. Although intentional chlorine treatment is commonly applied to polyamide nanofiltration membranes for surface modification or fouling control, whether C and T play equivalent roles in membrane rejection of diverse contaminants, such as per- and polyfluoroalkyl substances (PFASs), antibiotics, and endocrine disrupting compounds (EDCs), remains unclear. Herein, we investigated the impacts of C and T on contaminant rejection across CT values of 1000-20,000 ppm × h. Results reveal near-equivalent effects on the rejection of PFASs and antibiotics at low chlorination intensities (e.g., ≤ 2000 ppm × h), but significantly inequivalent impacts on the rejection of all contaminants at higher intensities (e.g., CT ≥ 10,000 ppm × h). High-C chlorination induced more severe polyamide degradation than long T chlorination, drastically compromising contaminant rejection (e.g., sulfadiazine rejection of 98.2% vs 59.3% for membranes chlorinated at 2000 ppm × 10 h and 20,000 ppm × 1 h, respectively). Based on these results, a conceptual framework was established to decouple the traditional CT paradigm, revealing the fundamental mechanisms of chlorination-induced changes in membrane rejection.