Degradation of 1,4-dioxane by reactive species generated during breakpoint chlorination: Proposed mechanisms and implications for water treatment and reuse

Breakpoint chlorination, an important chemical process relevant to chlorine-based advanced oxidation processes for potable reuse and to traditional water treatment, was investigated for its oxidative capacity, generation of reactive species, and potential impacts on organic contaminant degradation. This work describes a newly recognized HO^• radical generation pathway during breakpoint chlorination that may play an important role in water treatment and examines the behavior of the HO^• radical and other related reactive species and their potential formation pathways. Experimental data showed that the removal of 1,4-dioxane (1,4-D) positively correlated with chlorine-to-ammonia molar ratio until a molar ratio of ~1.5–2.0 was reached, above which removal efficiency rapidly decreased. Peroxynitrite (ONOO^–) and peroxynitrous acid (ONOOH) are proposed as important radical sources that lead to the formation of HO^• in the breakpoint process. This is supported by application of tert-butanol as a selective HO^• scavenger and the observation that the amendment of reaction solutions with carbonate species suppressed oxidative capacity to a much greater extent than expected based solely on scavenging of HO^• by H₂CO₃ * /CO₂ and HCO₃^– (apparently due to selective scavenging of ONOOH/ONOO^– by dissolved CO₂). These experiments also provided evidence that reactive species other than HO^• contributed to 1,4-D oxidation. The results of this study suggest that breakpoint chlorination can lead to significant degradation of organic contaminants via ONOOH/ONOO^–-mediated formation of HO^• and other reactive species and may potentially be optimized for enhanced removal of recalcitrant organic contaminants in the context of water reuse, though with due caution to the potential for enhancement of nitrogenous and other disinfection byproduct formation under such conditions.