Evaluating Cation Exchange as a Driver for Manganese Release During Carbonate Dissolution

Abstract

The Central Oklahoma Aquifer (COA) is a heterogeneous siliciclastic aquifer with clays and small amounts of Mn-containing dolomite scattered throughout. Areas of COA groundwater are well-known for the presence of naturally occurring heavy metals, including toxic levels of As, Cr, and U. When leached from aquifer rocks, these metals typically undergo oxidation-reduction reactions to more soluble and, in the case of Cr, more toxic forms. We hypothesized that Mn2+ released from dolomite during dissolution is available to serve as a redox couple in these reactions. This study aims to experimentally quantify and geochemically model the role of carbonate dissolution and cation exchange in the release of this Mn2+. Synthetic Mn calcite ± Na-saturated clay was reacted under lower (pH≈6) and higher (pH≈8) pH conditions at 25°C and 60°C for 14 days with periodic measurements of pH, alkalinity, and cation concentrations followed by SEM characterization of the final Day 14 reaction products. The experimental samples' aqueous chemistry data provided the basis for PHREEQCI thermodynamic modeling, which was used to calculate and predict solution chemical responses to pH, starting chemistry, type of solid solution, and the presence or absence of exchange. Experimental results were complicated and less straightforward to interpret. Overall, the results of both experimental reactions and modeling predictions indicated carbonate dissolution in the presence of clay/exchanger does result in cation exchange, which then enhances the dissolution of additional carbonates. Temperature and pH tended to have the most significant effect on the extent of dissolution and, therefore, cation concentrations and exchange, though effects of starting chemistry and type of solid solution present were also noted in modeling solutions. Results of experimental reactions and modeling were confirmed by SEM images of final experimental reaction products. These results suggest that aquifer trace element concentrations may be linked to the co-occurrence of Mn carbonates and Na-exchanged clays along recharge flowpaths.