Persistent free radicals of activated carbon induce highly toxic by-products formation during chlorination

Both activated carbon (AC) and chlorine are commonly used in water treatment processes and thus the exposure of chlorine to AC surface is unavoidable under some circumstances. Therefore, evaluating the effects of AC, particularly the persistent free radicals (PFRs) of AC on the chlorination by-products is critical for drinking water safety. Here, through comparison of the chlorination process of different ACs with similar pore structure but different PFR intensities, it was found that AC with more PFRs generated more unknown by-products with lower relative cell viability (85%). The Fourier transform ion cyclotron resonance- mass spectrometry results showed that the proportions of one-chlorine and nitrogen containing by-products generated by AC with higher PFR intensity were 19.6% and 16.9%, respectively, which were higher than those generated by AC with lower PFR intensity. Furthermore, the polycyclic aromatic compounds and polyphenols with high toxicity were only found in the chlorination products of AC with high PFR intensity. The PFRs of AC could promote the chlorine decay and induce the generation of superoxide radical and chlorine radical, and thus led to the formation of more toxic by-products. This study provides new insights into the risks of water chlorination process and by-product formation in the presence of AC.