## 1 Introduction  
In the aquaculture industry, some pathogenic organisms cause serious diseases in fish that can result in mass deaths. To prevent the outbreak of diseases, and to promote fish production, many types of antibiotics have been used widely in the aquaculture industry. Because a large proportion of antibiotics or their metabolites are excreted through fish urine and feces, these undesirable compounds can easily enter the aquatic environment [1]. Because regulations regarding antibiotic discharge are non-existent, water containing antibiotics is directly discharged into the sea after use. The antibiotics that remain in the aquatic environment can potentially affect the environment and public health, including causing aquatic toxicity, changes to physical health of domestic animals, food safety issues, and increased antibiotic resistance in pathogenic bacteria [1]. Two antibiotics, namely florfenicol (FF) and oxolinite acid (OA), were selected as target compounds for this study. FF is a broad-spectrum antibiotic permitted as a veterinary drug in animals used for food [2; 3]. As a fluorinated derivative of thiamphenicol, FF can inhibit transpeptidation in bacterial protein synthesis, and is effective against gram-negative and gram-positive bacteria [4]. FF demonstrates high efficacy in the control of several bacterial infections in the aquatic environment and is safe for human beings. Therefore, it has been highly recommended in many countries, for use in aquaculture [5]. Oxolinic acid (OA) is a quinolone antibiotic that is widely used in aquaculture, as well as in the poultry and cattle industries. However, OA has been reported as potentially genotoxic, and can cause harm to the environment [6]. Recently, high concentrations of OA have been detected in the aquatic environment, and persistence for long periods in natural water has also been reported [7; 8]. With increasing amounts of antibiotics being detected in the aquatic environment, ozonation, ultraviolet irradiation, electrolysis, and chlorination have been used to remove them [9; 10; 11; 12; 13; 14]. Ozonation has been used for the removal of micropollutants in seawater. Seawater ozonation can protect the aquatic ecosystem from harmful organic substances. The ozone chemistry of seawater is considerably different from that of fresh water because seawater contains a much higher concentration of dissolved anions and cations (e.g., chloride, bromide, magnesium, sulfate), than fresh water [15; 16]. Particularly, the bromide ion is a key ion for ozonation because of its high reactivity towards ozone. The reaction between ozone and the bromide ion leads to the formation of the oxidant, bromine (HOBr/OBr\({}^{-}\)). Seawater contains, on average, 65 mg/L of bromide ions, while fresh water contains less than 1 mg/L. Owing to the high amount of bromide in seawater (65 mg/L), seawater ozonation exhibits a significantly different oxidation mechanism compared to that in freshwater, because ozone reacts rapidly with the bromide ion to produce bromine at a rate of 160 M\({}^{-1}\)s\({}^{-1}\)[17]. Although a high amount of chloride is present in seawater (18,000 mg/L), the rate constant for the reaction of ozone with chloride ions (0.003 M\({}^{-1}\)s\({}^{-1}\)) to produce chlorine, (HOCl/OCl\({}^{-}\)) is lower than that with bromide [17]. Although chlorine is formed during seawater ozonation, it is converted rapidly to bromine as shown by Equation (1). \[\mathrm{HOCl}+\mathrm{Br}^{-}\ \rightarrow\mathrm{HOBr}+\ \mathrm{Cl}^{-} \Big{(}\,\mathrm{k}=1.55\times 10^{3}\ \mathrm{M}^{-1}\mathrm{s}^{-1}\Big{)} \tag{1}\]  
Thus, the primary oxidant in seawater ozonation is bromine (HOBr/OBr\({}^{-}\)), with trace ozone. For the efficient control of antibiotics in seawater by ozonation, the concentration of oxidants in seawater, primarily bromine and trace ozone, must be predicted, and the reaction rate between the antibiotic and each oxidant must be analyzed.