Factors influencing the dissolution rate of residual DNAPL in unconnected pores based on PIV technology

Abstract

Objective The leakage of dense nonaqueous phase liquids (DNAPL) with a density greater than that of water into the underground environment becomes a long-term pollution source. Previous researchers have studied the effects of flushing fluid flow rate, cosolvent concentration, and media properties on DNAPL removal efficiency through methods such as column experiments, sandboxes, and numerical simulations. Methods However, the effect of pore-scale flow rate on the dissolution rate of residual DNAPL in the pores remains unclear. In this study, ethanol flushing solution was injected into a micropore model to simulate the removal process of residual PCE from pores. Microscale particle image velocimetry (PIV) was used to obtain the distribution of the water phase velocity field in the pore channel, and the factors affecting the dissolution and removal rate of residual PCE in different pore structures were analysed. Results The experimental results indicate that the dissolution rate (R) of residual PCE in unconnected pores is influenced by several factors, including the water phase flow rate, the cross-sectional flux (q) within the pore, the angle (α) between the direction of pore opening and the direction of water phase flow, and the flux gradient (I). Based on fitting the experimental data, the quantitative relationship between dissolution rate and influencing factors is obtained as follows: R=3 876.79q(-0.016a+2.28/I)2. A larger value of q corresponds to a higher renewal rate of the water phase in pore channels near unconnected pores, leading to an increased dissolution rate of residual PCE. When α is larger (α>90°), more flushing fluid enters the unconnected pores, further enhancing the dissolution rate of residual PCE. On the other hand, a larger value of I leads to a faster attenuation of the vertical component of the water phase flux entering the unconnected pores. This results in lower flow velocities near the interface, causing a decrease in the dissolution rate of residual PCE. Conclusion Based on micropore PIV technology, multiple factors, such as pore flow rate and medium pore structure, have been quantitatively revealed to jointly affect the dissolution rate of DNAPL in pores. This finding offers a novel approach to enhance our the understanding of the dissolution mechanism of residual DNAPL in pores and to quantitatively assess the removal efficiency of residual DNAPL under real-site conditions.

Keywords

• pore velocity
• micropore-model
• dissolution rate
• unconnected pore