Influence of Pore Size and Ozone Concentration on Reactive Oxygen Species (ROS) Generation Efficiency in Ozone-Based Advanced Oxidation Processes

Abstract

In this study, the effects of pore size and ozone concentration on the efficiency of reactive oxygen species (ROS) generation via advanced oxidation processes (AOPs) based on ozone oxidation were investigated. The ozone gas used in the process was generated by a dielectric barrier discharge (DBD) reactor, with oxygen and air as feed gases at a flow rate of 11 L/min. Ozone generation was carried out for 15 minutes under a high-voltage electric field to induce the ionization of oxygen molecules. The resulting ozone concentration was determined using the iodometric titration method. The results showed that when oxygen was used as the feed gas, the maximum ozone concentration reached 15.77 mg/L, whereas the use of air resulted in a significantly lower concentration of 4.70 mg/L, corresponding to a difference of 235.5%. When ozone was introduced into water through porous diffusers to generate fine bubbles, reactive oxygen species were produced through ozonation and ozonolysis reactions, particularly in the presence of organic compounds in the water. The oxidation–reduction potential (ORP) was measured to assess ROS generation efficiency. When oxygen was used as the feed gas, ORP values for both nano- and micro-porous diffusers increased significantly and reached similar maximum values of approximately 820 mV, indicating effective ROS production. In contrast, when air was used as the feed gas, distinct differences were observed. The micro-porous diffuser exhibited a gradual increase in ORP, reaching a maximum of 569 mV. Meanwhile, the nano-porous diffuser showed a rapid increase, attaining the same ORP level (569 mV) within 5 minutes and continuing to rise to a maximum of 682 mV. In addition, electrical conductivity (EC) and total dissolved solids (TDS) were evaluated after ozone was dispersed into water using porous diffusers. Under the condition of high ozone concentration (15.77 mg/L), obtained when oxygen was used as the feed gas, similar trends were observed for both diffuser types. EC and TDS decreased consistently in the same direction. For the nano-porous diffuser, EC decreased from 271 to 248 µS/cm, while TDS decreased from 173 to 135 mg/L. The micro-porous diffuser showed a similar pattern, with EC decreasing from 273 to 254 µS/cm and TDS decreasing from 172 to 138 mg/L. In contrast, under the condition of low ozone concentration (4.70 mg/L), obtained when air was used as the feed gas, the results differed significantly. The nano-porous diffuser still resulted in decreases in both EC and TDS, with values similar to those observed when oxygen was used as the feed gas. However, the micro-porous diffuser exhibited an increase in EC from 273 to 306 µS/cm, while TDS decreased only slightly from 172 to 160 mg/L. The experimental results indicate that both ozone concentration and the pore size of the diffuser significantly influence the efficiency of reactive oxygen species (ROS) generation. This effect is particularly evident when low-concentration ozone (4.70 mg/L), produced using air as the feed gas, is applied. Under these conditions, nano-porous diffusers were found to enhance ROS generation more rapidly and effectively than micro-porous diffusers. Overall, the results demonstrate that both ozone concentration and diffuser pore size have a significant impact on the efficiency of ROS generation. This effect is mostly prominent at low ozone concentrations (4.70 mg/L), produced using air as the feed gas. Under these conditions, nano-porous diffusers were found to promote ROS generation more rapidly and effectively than micro-porous diffusers.