Effect of pore size distribution and colloidal fines of porous media on the transport behavior of micro-nano-bubbles

Abstract

The key of successful application of micro-nano-bubbles (MNBs) for groundwater remediation is understanding the transport characteristics of MNBs in porous media. The effect of pore size distribution and colloidal fines on transport behavior of MNBs in saturated porous media was studied in this research. At first, the diameter of MNBs and pore distribution of porous media were measured. Then, the hydraulic conductivities of ten different kinds of porous media were tested by distilled water and MNBs solution. Finally, one-dimensional column transport experiments of MNBs in saturated porous media were conducted. The test results showed that the diameter of MNBs was mainly distributed in 61–172 nm and 1.635–3.270 µm. The size of pore in glass beads was bigger than the diameter of MNBs, while the size of most pores in mixture of silt and sand or kaolinite was smaller than the diameter of MNBs. When transporting in porous media, MNBs had no effect on the hydraulic conductivity of coarse, medium or fine glass beads. However, MNBs had detrimental effect on hydraulic conductivity of the mixture of silt and quartz sand or kaolinite. Advection-dispersion theory was suitable for fitting the breakthrough curve of MNBs. The values of retardation factor (R) and pseudo first order rate coefficients for attachment (μ) were largest in the mixtures of silt and kaolinite, followed by silt as well as mixtures of silt and quartz sand, and smallest in glass beads. The particle size of porous media, the relative size of MNBs and pores, and colloidal fines significantly affected the transport behavior of MNBs in porous media. Furthermore, the deformation and dissolution of MNBs as well as unfavorable surface interactions between sand particles and bubbles could promote the transport of MNBs in saturated porous media. Thus, MNBs injection and transport are expected to be effective in remediation of contaminated site composed of sand or fine-grained soil with few colloidal fines.