Persistent free radicals in activated carbon mediate enhanced formation of nitrogenous disinfection byproducts during chloramination.

Abstract

Activated carbons (AC) are widely used in household drinking water purification systems due to their high adsorption capacity. However, the potential impact of persistent free radicals (PFRs) present in AC particularly their interaction with disinfectants, has been largely overlooked. This study is the first to investigate the influence of AC on the formation of nitrogenous disinfection byproducts (N-DBPs). Results demonstrate that exposure to AC significantly increased the formation of haloacetonitriles (HANs) and organic chloramines. Electron paramagnetic resonance (EPR) analysis revealed that PFRs on the AC surface activate monochloramine (NH₂Cl), generating reactive nitrogen species (RNS) and reactive chlorine species (RCS). Importantly, reducing the PFR intensity within the AC markedly diminished its promotion of HAN formation, concurrently suppressing RNS generation. Fourier transform ion cyclotron resonance mass spectrometry analysis further showed that exposure to AC increased the number of detected N-DBPs by 55% (from 58 to 90) and significantly increased nitrogen-containing organic compounds (with an increase of 139 nitrogen-containing CHON formulas), which are inferred as potential precursors for N-DBPs. This study highlights a previously neglected risk associated with PFRs in AC: their role in promoting the formation of toxic disinfection byproducts, with implications for ensuring safe drinking water production.