Importance of flow and particle-scale heterogeneity on CoII/III EDTA reactive transport

The effect of flow and particle-scale heterogeneity on the applicability of batch-derived reaction networks for Co(II/III)EDTA interaction with synthetic and natural Fe oxide-containing subsurface sediments was evaluated using batch and column systems. At pH 4.5 and 6.5, Co(II)EDTA reactions with sediments involved adsorption (within 0.2 to 2 h), followed by iron dissolution, forming Co2+ and Fe(III)EDTA (within 10 to 300 h) and oxidation, forming Co(III)EDTA (within 5 to 50 h). Reaction parameters that were based upon numerous batch experiments, in some cases, poorly described reactive transport in columns. For five sediment/pH combinations, column breakthrough could be well described with systematic change in some reaction parameters which included: slower metal-EDTA adsorption rates; a greater number of adsorption sites; slower iron dissolution; and faster oxidation. Additional simulations were used to determine the significance and uncertainty in determining these reaction parameters in the 15-reaction system with 22 species. Simulations that incorporated particle-scale heterogeneity with spatial distributions of adsorption site density, affinity, and rate could reproduce the column/batch trends that were observed, indicating that spatial averaging across reactive and nonreactive surface sites results in different single-valued reaction parameters needed to describe the column system as an equivalent porous media. The impact of solute advection and additional mass transfer in columns on accurately predicting column behavior with batch-derived parameters was not responsible for the batch to column disparity. This study demonstrates that while the nature of the reaction network (i.e. reaction identity and stoichiometry) is identical in batch and column systems, development of transport-relevant reaction parameters is needed for nonlinear reactions as particle-scale heterogeneities can significantly influence the apparent manifestation of the reaction network during advective flow.