Behavior characteristics of bimolecular reactive transport in heterogeneous porous media

In order to study the mechanism of bimolecular reactive solute transport in heterogeneous porous media, the chemical reaction (CuSO₄ + Na₂EDTA^2-→CuEDTA²) was carried out by laboratory experiments and numerical simulation in heterogeneous porous media. Three different kinds of heterogeneous porous media (S_d² = 1.72, 1.67 and 0.80 mm²) and flow rates (1.5, 2.5 and 5.0 mL/s) were considered. The increase of flow rate would promote the mixing between reactants, resulting in a greater peak value and a slighter "tailing" of product concentration, while the increase of medium heterogeneity would result in a more significant "tailing". It was found that the concentration breakthrough curves of reactant CuSO₄ had a peak in the early stage of the transport, and the peak value increased with the increase of flow rate and medium heterogeneity. The concentration peak of CuSO₄ was caused by the delayed mixing and reaction of reactants. The IM-ADRE (The advection-dispersion-reaction equation considering incomplete mixing) model could well simulate the experimental results. The simulation error of IM-ADRE model for the concentration peak of product was less than 6.15%, and the fitting accuracy for "tailing" increased with the increase of flow. The dispersion coefficient increased logarithmically with the increase of flow, and was negatively correlated to the heterogeneity of the medium. In addition, the dispersion coefficient of CuSO₄ simulated by IM-ADRE model was one order of magnitude larger than that simulated by ADE model, indicating that the reaction promoted dispersion.