Organic Compound Degradation Induced by Br Oxidation with O3 Fine Bubble Injection into Seawater

Abstract

ABSTRACT An advanced technique for the utilization of the dissolved Br in concentrated seawaters discharged from the salt manufacturers in Japan can enable the development of a comprehensive utilization system for seawater resources. Hypobromous acid (HOBr) and hypobromite ion (BrO–) obtained by the oxidative reaction of Br– degrade the many organic compounds and can be utilized as water purification reagents with excellent decolorizing and disinfection capabilities. In this study, we focused on the production of Br oxyacid as an upgrading method for Br using O3 fine bubbles, which has a high oxidation potential, and developed a technique for water purification. When O3 fine bubbles are introduced into the liquid phase, the gas absorption of O3 is accelerated by the increases in gas–liquid interfacial area and residence time of the bubbles and the hydroxyl radical (OH•) generation is enhanced by the increase in contact probability between the dissolved O3 and accumulated OH– at the minute gas–liquid interfaces. Moreover, when O3 fine bubbles are supplied into a liquid phase with coexisting Cl– or Br– as halogen ions, further improvement in oxidation potential in the liquid phase can be expected by the acceleration of the Br oxyacid generation with OH•. When O3 fine bubbles with an average diameter of 50 µm were continuously supplied into the simulated seawater with pH value of 6.1 or ion-exchanged water with pH value of 5.6 at a constant solution temperature (T S) of 298 K, the concentration of active oxygen species (C OS) in the simulated seawater increased to 1.4 times higher than that in the ion-exchanged water. Additionally, in the case where O3 fine bubbles were injected into an aqueous solution of NaCl, MgCl2, CaCl2, KCl, or NaBr to ascertain the factor of C OS increment, the tendency of C OS to increase with the reaction time in the NaBr aqueous solution was more apparent than those in the ion-exchanged water and other salt aqueous solutions, because oxyacids of Br such as HOBr/BrO– were generated by the selective oxidation of Br– during the O3 fine bubble injection into the NaBr aqueous solution. Furthermore, to evaluate the effects of the Br– concentration on the degradation of methylene blue (MB) as an organic compound, MB was degraded by O3 fine bubble injection into NaBr/MB aqueous solutions with different initial NaBr concentrations. The formation of Br oxyacids owing to the acceleration of the reaction between O3 or OH• and Br− during the O3 fine bubble injection led to an enhancement in MB degradation.