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 v ariety of toxic contaminants. In the work reported here, we develop a new m odel for coupled transport and oxidation-reduction, and we test the model a gainst published data on the movement of CoEDTA (cobalt ethylenediaminetetr aacetic acid) through columns packed with beta -MnO2-coated sand. The model solves equations for the advective-dispersive transport of three aqueous s pecies: Co(II)EDTA, Co(III)EDTA, and dissolved oxygen. These transport equa tions are linked with nonlinear kinetics expressions that describe (1) oxid ation of Co(II)EDTA to Co(III)EDTA by beta -MnO2, (2) inhibition of Co(II)E DTA oxidation due to precipitation of Mn2O3, an insoluble reaction product that occludes the beta -MnO2 surface, and (3) regeneration of the redox-rea ctive beta -MnO2 surface through oxidation of the Mn2O3 precipitate by diss olved oxygen. Comparison of experimental and calculated results demonstrate s that the model describes the response of the coupled hydrological and geo chemical processes to changes in flow rate, influent concentrations of Co(I I)EDTA, and beta -MnO2 surface coverages.