Revisiting the protocol for determining submicromolar concentrations of ozone in the water treated by advanced...

Ozone is formed at submicromolar concentrations from photolysis of many oxyanions and oxidants in water and contributes importantly to the degradation of emerging contaminants and inactivation of pathogenic microorganisms in the natural and engineered aquatic systems. In this study, we identified and discussed the critical limitations of the commonly-used protocols using cinnamic acid (CNA) as a probe compound to determine the submicromolar-level ozone and proposed a modified protocol that overcomes those limitations. Our experimental investigation demonstrated that the radicals (e.g., HO^•) formed from photolysis of oxyanions and oxidants, other than ozone, could also oxidize CNA and form benzaldehyde, resulting in the overestimation of ozone concentrations by using the commonly-used protocols. Moreover, the benzaldehyde formed from ozone-CNA reactions could be degraded by the radicals, leading to the underestimation of ozone concentrations by using the commonly-used protocols. A new protocol with high accuracy and precision was proposed and the rationales for each operational step of the new protocol were explained in detail and supported with justifications. The new protocol was compared with two commonly-used protocols in determining the concentration of ozone in the same water sample treated by the UV/chlorine process at three different UV wavelengths. The wavelength-dependent overestimation/underestimation of the ozone concentrations by using the two commonly-used protocols was well demonstrated and explained by the overlooked interferences of radicals in the protocols.