Investigation of transport and deposition behavior and mechanisms of micro-nano-bubbles in porous media

Abstract

Micro-nano-bubbles (MNBs) offers a sustainable solution for remediating organics-contaminated groundwater. The transportation of MNBs in idealized porous media systems has been investigated through a series of experiments to deepen the understanding of MNBs migration. Based on an evaluation of the stability of MNBs, it was determined that dissolved gas concentration can serve as an indirect means for measuring MNB concentration. A 2D microfluidic chip was developed to simulate porous media formed by uniform-sized particles, allowing for direct observation of the retention mechanisms of MNBs. The results indicated that MNB retention primarily occurs through surface deposition (above 99 %), with minimal evidence of hydrodynamic bridging. Column experiments using glass beads and quartz sand as porous media were conducted at 20 ± 0.5 °C to assess the effects of particle size and gradation of porous media and groundwater chemistry on MNB transport and deposition. For glass beads with a particle size of 0.5 mm, the retention rate of injected MNBs (with an average size of 235 nm) in the porous medium increased from 47.04 % to 59.43 % with decreasing flow rate (from 7.81 × 10^-2 cm/s to 1.95 × 10^-2 cm/s). Tests performed with different types of porous media demonstrated that at a flow rate of 3.91 × 10^-2 cm/s, irregular quartz sand had a greater ability to capture MNBs (55.07 %) than uniformly shaped glass beads (49.10 % for 0.5 mm glass beads and 42.04 % for 1.5 mm glass beads). Decreases in groundwater pH and increases in NaCl concentratrion influenced MNB migration by reducing the absolute value of the zeta potential of both the MNBs and the porous media. A convection–dispersion model, incorporating attachment and detachment processes, was successfully fitted to the breakthrough curves of MNBs under various experimental conditions. The R² obtained from the fit exceeding 0.96 in all cases, further indicating that surface deposition is the dominant mechanism of MNB retention. This study provides valuable insights into transportation behavior of MNBs.