A numerical model to simulate the NAPL source zone remediation by injecting zero-valent iron nanoparticles

Abstract

A continuum-scale numerical model is developed to simulate the flow, mass-transfer, and reactive processes taking place during the injection of zero-valent iron nanoparticles (nZVI) in a water-saturated porous medium, containing a chlorinated non-aqueous phase liquid (NAPL) at its residual saturation (source zone). Firstly, the transport of the nanoparticles (NP) is modelled by coupling the nanoparticles transport with their deposition in a porous medium. A systematic sensitivity analysis is done to examine the individual effect of each parameter on the spatial and temporal evolution of nanoparticle concentration along the column length. The parameters quantifying the kinetics of attachment/detachment between the nanoparticles and the porous medium (sand grains) are estimated via inverse modeling. Secondly, the kinetics of the reaction of nZVI with dissolved tetrachloroethylene (PCE) is based on the numerical predictions of the statistical shrinking-core model that couples the mass-transfer with the reactive processes taking place at a nanoparticle scale. The continuum NP transport model is then extended to include the reactive flows by combining the dynamics of PCE ganglia dissolution and nZVI reactions with mass balances for the residual PCE saturation, the dissolved PCE concentration, and the nZVI concentration in the aqueous phase. Sensitivity analyses are performed on the NP transport-NAPL remediation model with a view to clarify the effects of dimensionless parameters (dimensionless flow velocity, Damköhler numbers) on the fate of nZVI and trapped PCE ganglia. Finally, the residual PCE remediation efficiency is numerically predicted as a function of the injected NP mass. This efficiency is compared with the respective experimental results of a PCE source zone remediation test performed in a sand column, by using as adjustable parameters the kinetic coefficients of the nanoparticles deposition on sand grains. The remediation efficiency of the PCE source zone is maximized when the injected nZVI maintains its reactivity throughout its flow in the porous medium. To design an efficient NAPL source zone remediation strategy, the parameters must be set to values which will ensure that the nZVI is kept to a higher than zero concentration throughout the length of the porous medium.