## 3 Results and Discussion
### Total Residual Oxidant in Direct OCI\({}^{-}\)-Injection and Electrolysis
#### 3.2.2 Electrochlorination  
Electrochlorination was performed for both WS and ES with different current densities i.e., 2.2, 9.4, 16.6, and 27.7 mA/cm\({}^{2}\)) and the formation of ClO\({}_{3}\)\({}^{-}\) and ClO\({}_{4}\)\({}^{-}\) was studied. The ClO\({}_{3}\)\({}^{-}\) concentration increased from 0.60 to 4.75 \(\mu\)g/L for WS and from 0.17 to 4.59 \(\mu\)g/L for ES, as the current density was raised from 2.2 to 27.7 mA/cm\({}^{2}\) (Figure 7). The amount of ClO\({}_{3}\)\({}^{-}\) formed in ES increased with the increase of the current density. However, in the case of WS, ClO\({}_{3}\)\({}^{-}\) increase was noted only at a current density of \(>\)16.6 mA/cm\({}^{2}\) (Figure 7). Nonetheless, it seems that the amount of ClO\({}_{3}\)\({}^{-}\) formed during the electrochlorination of seawater depends on both the applied current density and the surface characteristics of electrodes used in the electrolysis. [PERSON] et al. [37] observed 637 \(\mu\)g/L \(\mathrm{ClO_{3}}\)\({}^{-}\) formed during their electrochlorination of seawater from the east coast using Pt/Ti electrodes with 55 mA/cm\({}^{2}\) of current density. Our Ti/RuO\({}_{2}\)-based electrolysis system might produce less ClO\({}_{3}\)\({}^{-}\) than the Pt/Ti-based one.  
Interestingly, in our electrochlorination study with WS or ES, ClO\({}_{4}\)\({}^{-}\) was not detected even at a current density of 27.7 mA/cm\({}^{2}\). In fact, other researchers could observe ClO\({}_{4}\)\({}^{-}\) only when higher current densities were applied for their electrochlorination. In their electrochlorination study with seawater samples from the east coast of Korea, [PERSON] et al. [38] could generate 27 \(\mu\)g/L of ClO\({}_{4}\)\({}^{-}\) with a current density of 110 mA/cm\({}^{2}\); in their experiment, they used Pt/Ti electrodes. From the results of our study and others, it can be concluded that the formation of inorganic CBPs such as ClO\({}_{3}\)\({}^{-}\) and ClO\({}_{4}\)\({}^{-}\) is very dependent on the surface characteristics of electrodes or electrode-surface-specific reactions. When electrode material has a lower potential for oxygen evolution, the electrode easily produces oxygen, which reacts with bromine and chloride to form BrO\({}_{3}\)\({}^{-}\) and ClO\({}_{3}\)\({}^{-}\) formation [37].  
Nonetheless, the amount of ClO\({}_{3}\)\({}^{-}\) produced from the electrochlorination of seawater was about 170 times lower than that from the direct OCI\({}^{-}\)-injection. This phenomenon could be explained by the difference in the amount of OCI\({}^{-}\) used for the ClO\({}_{3}\)\({}^{-}\) formation between the direct OCI\({}^{-}\)-injection system and the electrochlorination of seawater. If a large amount of OCI\({}^{-}\) is added to seawater at once, the bromine ions in seawater will rapidly react with OCI\({}^{-}\)/HOCl to form OBr\({}^{-}\)/HOBr (reactions 1-5 in the Supplementary Materials). However, some of the OCI\({}^{-}\) still might be able to participate in reactions that form ClO\({}_{3}\)\({}^{-}\). In the case of electrochlorination, OCI\({}^{-}\) is gradually produced on the electrode surface. In this case, OCI\({}^{-}\) generated on the electrode surface might preferentially react with Br\({}^{-}\) and form OBr\({}^{-}\)/HOBr. Comparing to the direct OCI\({}^{-}\)-injection system, thus, a relatively smaller amount of OCI\({}^{-}\) will be available for the ClO\({}_{3}\)\({}^{-}\) formation during seawater electrolysis.  
In short, unlike conventional chlorination, electrochlorination produces much less or no inorganic chlorination byproducts, which makes it an environmentally friendly and sustainable technology.  
Figure 6: Effect of OCI\({}^{-}\) concentration on formation of ClO\({}_{4}\)\({}^{-}\) (**a**) WS, (**b**) ES (volume: 0.4 L; temperature: 20 \(\pm\) 1 \({}^{\circ}\)C; salinity: 28.9 PSU for WS and 28.7 PSU for ES; pH: 8.3 for WS and 8.0 for ES).