Bimolecular reactive transport in a filled single fracture-matrix system considering the nonequilibrium sorption

Abstract

Comprehending the transport mechanism of bimolecular reactive solutes in fracture-matrix systems is crucial for improving the efficiency of collaborative remediation using hydraulic fracturing and in-situ chemical oxidation in low-permeability contaminated sites. Previous studies have neglected the effects of physical nonequilibrium (PNE) caused by sedimentation and chemical nonequilibrium (SNE) caused by rete-limited sorption. In this study, a two-dimensional numerical model that considers mobile-immobile mass transfer in the fracture and two-site nonequilibrium sorption in both fracture and matrix is established to investigate the behavior of bimolecular reactive solutes in a filled fracture-matrix system. The results indicate that the PNE and SNE lead to two-stage characteristics and severe tailing effects in contaminant remediation. High tailing occurs when the equilibrium sorption fraction and fracture-mobile/fracture-immobile porosity ratio are relatively small and the longitudinal hydrodynamic dispersion coefficient is relatively large, requiring consideration of multi-nonequilibrium processes. Additionally, the solute concentration in the matrix is most sensitive to the matrix porosity (θk), less sensitive to fracture aperture (2b), and reagent bulk diffusivity (DOA). The remediation area (Sr) in the matrix has an exponential relationship with θk and a linear relationship with 2b and DOA. The quantitative relationship between Sr and key parameters facilitates the prediction of remediation effectiveness.