Enhanced toluene remediation in low-permeability zone by injecting rhamnolipid-coated ozone micro-nano bubble water combined with groundwater pumping

Organic contaminants in groundwater always tend to accumulate in low-permeability zone (LPZ), which are difficult to be completely remediated. Rhamnolipid-coated ozone micro-nano bubbles have very tiny diameters ranging from 1 nm to 100 μm, which have potential to permeate into LPZ to oxidize organic contaminants. However, they are easily attached on porous media, limiting the transport distance of ozone and reducing the remediation efficiency. This paper carried out two-dimensional sand tank experiments to investigate the toluene removal rate in LPZ by the injection of rhamnolipid-coated ozone micro-nano bubble water (ROMBW) combined with groundwater pumping. Results showed that the final removal rate of toluene in LPZ reached 96.2 % with the combined technology, which was higher than ROMBW injection without groundwater pumping (92.0 %). Besides, the addition of rhamnolipid also slightly increased the toluene removal rate in LPZ by 0.8 %. A numerical model was successfully developed to simulate the final toluene concentration in the sand tank with the average error of 2.17 mg/L, which proved that the improved removal rate of toluene in LPZ was mainly attributed to the permeation of more nanobubbles into LPZ. Furthermore, the model also indicated that the total injected volume of ROMBW was the most important factor that controlled the remediation efficiency, and a short screen length around LPZ as well as the close distance between well screen and LPZ improved the remediation efficiency. The findings of this study could guide the application of ozone micro-nano bubble oxidation in heterogeneous aquifer remediation.