Influence of static pressure on toluene oxidation efficiency in groundwater by micro-nano bubble ozonation

Micro-nano bubble ozonation has been widely applied in the purification of drinking water due to its superior characteristics such as high mass transfer rate and long resistance time. However, its application in groundwater remediation is limited, partially due to the unclear effect of static water pressure on the oxidation efficiency. This study constructed a batch reactor to investigate the influence of static pressure on toluene oxidation by ozone micro-nano bubble water. To achieve constant pressure, weight was added above the mobile reactor roof, and the initial concentrations of toluene and dissolved ozone were 1.00 mg L^-1 and 0.68 mg L^-1 respectively. Experimental results demonstrated that as the static water pressure increased from 0.0 to 2.5 m, the average microbubble diameter decreased significantly from 62.3 to 36.0 μm. Simultaneously, the oxidation percentage of toluene increased from 40.3% to 58.7%, and the reaction rate between toluene and hydroxyl radical (OH·) increased from 9.3 × 10⁹ to 1.39 × 10¹⁰ M^-1 s^-1, indicating that the shrinkage of micro-nano bubbles generated an abundance of OH· that quickly oxidized toluene adsorbed at the bubble interface. A greater enhancement of oxidation efficiency for nitrobenzene, as compared to p-xylene, was observed after the addition of 2.5 m water pressure, which verified the larger contribution of OH· under static pressure. Although the improvement of oxidation efficiency was reduced under acid and alkaline environments, as well as in practical groundwater matrices, the overall results still demonstrated the promising application of micro-nano bubble ozonation in groundwater remediation.