Development and evaluation of a mathematical model for the transport and oxidation-reduction of CoEDTA

Oxidation-reduction reactions influence the subsurface mobility of a wide variety of toxic contaminants. In the work reported here, we develop a new model for coupled transport and oxidation-reduction, and we test the model against published data on the movement of CoEDTA (cobalt ethylenediaminetetraacetic acid) through columns packed with β-MnO₂-coated sand. The model solves equations for the advective-dispersive transport of three aqueous species: Co(II)EDTA, Co(III)EDTA, and dissolved oxygen. These transport equations are linked with nonlinear kinetics expressions that describe (1) oxidation of Co(II)EDTA to Co(III)EDTA by β-MnO₂, (2) inhibition of Co(II)EDTA oxidation due to precipitation of Mn₂O₃, an insoluble reaction product that occludes the β-MnO₂ surface, and (3) regeneration of the redox-reactive β-MnO₂ surface through oxidation of the Mn₂O₃ precipitate by dissolved oxygen. Comparison of experimental and calculated results demonstrates that the model describes the response of the coupled hydrological and geochemical processes to changes in flow rate, influent concentrations of Co(II)EDTA, and β-MnO₂ surface coverages.