Mechanism of direct-push injection impact on porous media structure and contaminant removal performance

This study focused on the mass transfer limitations in the remediation of organic contaminants in porous media with low hydraulic conductivity. It investigated how direct-push injection (DPI) technology enhances the degradation of phenol (C₆H₅OH) by potassium permanganate (KMnO₄). Three representative aquifer media with low-to-medium hydraulic conductivity were selected, and experiments were conducted under both ambient pressure control (APC) and DPI conditions. Nuclear magnetic resonance (NMR) was employed to monitor the degradation process in real time, while the pore structure evolution was comparatively analyzed. The results showed that under DPI conditions of 0.3 MPa and 0.6 MPa, the overall C₆H₅OH degradation efficiency was enhanced by up to 13.60 % and 32.63 %, respectively. This improvement originated from two synergistic mechanisms. On one hand, DPI reinforced the dominance of convective transport as the primary transport mechanism of KMnO₄ and enhanced its mechanical dispersion, thereby improving the overall mass transfer efficiency. On the other hand, the external pressure applied by DPI effectively modified the pore structure, improved inter-pore connectivity, and enlarged the contact area between KMnO₄ and C₆H₅OH. Together, these effects accelerated the oxidation process, reduced intermediate product accumulation, and ultimately enhanced the overall degradation efficiency. Moreover, DPI promoted the penetration of KMnO₄ into small pores, which is particularly beneficial for low hydraulic conductivity media dominated by fine pores. This study elucidates the mechanism by which DPI enhances oxidant transport and reaction efficiency, and clarified the dual influence of pore structure and injection pressure on the effectiveness of DPI.