UV/chlorinated cyanurates as an emerging advanced oxidation process for drinking water and potable reuse treatments

Abstract

Chlorinated cyanurates, prepared by application of hypochlorite to cyanuric acid at different ratios, have been commonly employed for disinfection. Combining UV with chlorinated cyanurates (UV/Cl-cyanurates) can be a novel and effective advanced oxidation process (AOP) because (1) Cl-cyanurates structurally resemble chlorinated amides that feature low reactivity with radicals, and (2) Cl-cyanurates, which bear multiple -Cl, may exhibit high molar absorptivity at 254 nm due to red-shifting absorption. Those chemiphysical properties of Cl-cyanurates may facilitate oxidant photolysis rate and lower radical scavenging rates in an AOP, thereby increasing steady-state concentrations of radicals. In this study, UV spectra measured for Cl-cyanurates highlighted molar absorptivities at 254 nm (∼200 M^-1cm^-1) much higher than free chlorine or H₂O₂, while k_•OH determined using competition kinetics suggests low •OH reactivity (<1.95 × 10⁷ M^-1s^-1) for Cl-cyanurates. Photolysis of Cl-cyanurates forms •Cl (i.e., Cl-N cleavage), and •Cl converts to •OH; formation of •OH during a UV/Cl-cyanurates AOP was evaluated using terephthalate as a probe compound. Experiments systematically investigated the effects of pH, Cl₂ dosage, and cyanuric concentration (three key factors affecting the equilibrium concentrations of chlorinated-cynaurate species) on the efficacy of removing three indicator contaminants by UV/Cl-cyanurates AOP. UV/Cl-cyanurates AOP conducted in phosphate buffers or authentic surface waters highlighted efficiencies up to 170% higher than UV/Cl₂ AOP at neutral pH when the same dosage of oxidants was employed, and the presence of certain levels of background ammonia or chloramines further enhanced its performance. Transformation of cyanuric acid or Cl-cyanurates by reacting with radicals during a UV/Cl-cyanurates AOP treatment was minimum. Toxicity assay indicated that UV/Cl-cyanurates AOP treated water was comparable or less toxicity than UV/H₂O₂ or UV/Cl₂ AOP treated water, and the initial cost estimate indicates UV/Cl-cyanurates AOP is potentially a cost-effective alternative AOP.

Keywords: Advanced oxidation process; Chlorinated cyanurates; N-acetylcysteine thiol reactivity assay; Sodium dichloroisocyanurate.