Remediation of NAPL-contaminated porous media using micro-nano ozone bubbles: Bench-scale experiments

Abstract

Aqueous solutions of micro-nano bubbles (MNBs) containing ozone gas were injected through a NAPL-contaminated glass bead column. The glass column (15 cm × 2.5 cm) was packed with glass beads: the first 12 cm was packed with coarse glass beads while much finer glass beads were used to pack the remaining 3.0 cm of the column. Decane was used as the representative NAPL, to which an oil-soluble fluorescence tracer was added. The fluorescence tracer was considered as a constituent of the NAPL that readily reacts with ozone. Air and ozone-containing oxygen were used to generate MNB solutions, and injected through the column. In addition, H2O2 was introduced to the O3-containing MNB (O3-MNB) solution to investigate the effect of hydroxyl free radicals on the NAPL removal. An ozone gas sparging experiment was also conducted for comparison. After 72 h of O3-MNB application, a significant mass of n-decane (27.6% of the initial mass applied) was removed from the column. H2O2 injection into the column during O3-MNB application was effective in increasing the n-decane mass removal by 22%, compared to the O3–MNB experiment. The rate of NAPL removal during O3-MNB flushing was significant, although slower than ozone sparging. During O3-MNB application, fast decay of fluorescence was observed; whereas, during co-injection of H2O2 and O3-MNB solutions, only a slight change in the fluorescence was observed. This indicates that oxidative degradation of NAPL during H2O2 and O3-MNB injection takes place only at the NAPL-water interface due to the reactivity of hydroxyl free radical, whereas ozone diffusion into NAPL induced the decay of the fluorescence tracer in the bulk NAPL. The removal characteristics during MNB application and ozone gas sparging were investigated based on the analysis of NAPL using mass spectrophotometer. When O3-MNB and H2O2 were co-injected, only n-decane was detected in the NAPL; while when O3-MNB was used for flushing, oxidative products were found in the NAPL. More hydrophilic compounds were found in the NAPL after ozone sparging. This implies different removal mechanisms depending on the kind of oxidation agent, and the state of oxidizing fluid. Based on the findings in this study, the application of O3-MNB could be a feasible option for cleaning up NAPL-contaminated aquifers.