REACTION MECHANISMS AND KINETICS FOR OXIDATION OF FERROUS IRON BY H2O2 IN NET ALKALINE MINE DRAINAGE

This investigation examines ferrous iron oxidation by hydrogen peroxide (H₂O₂) in iron-rich, net alkaline mine drainage (NAMD). The purpose was to determine the reaction mechanism and aqueous chemistry of direct mixing between anoxic NAMD and H₂O₂, a potential oxidant for in-situ removal of metals. An abandoned underground mine from which NAMD is pumped in northern West Virginia was used as a representative, "fresh" source of water for these experiments. Its chemistry exhibits an approximate average dissolved Fe concentration (mainly ferrous) of 103 mg/L, pH 6.2, and 460 mg/L CaCO₃ alkalinity. Lab simulation of Fe oxidation was performed by injection of concentrated (8 M) H₂O₂ at six different dosages into a uniform flow of NAMD, simulating "plug flow" of mine groundwater. Aliquots were withdrawn immediately after vigorous reagent mixing, then held without air entry and sampled at from 0.1 to 120 hours for chemical analysis of solutes, pH, ORP, DO, and alkalinity. Fe was rapidly precipitated as Fe(OH)₃ soon after injection in all samples, reducing dissolved Fe to below detection in all but the 2 lowest dosages. At these low dosages, Fe precipitation was partial and initially rapid, then much slower after 2 hours. Results analyzed using PHREEQC indicate that Fe precipitation-induced acidity causes conversion of HCO₃ to CO₂, resulting in elevation of partial pressure and in some samples outgassing of CO₂. Results suggest that reaction occurs via a mechanism similar to that of Fenton's reagent, with very rapid oxidation by OH radical and subsequent slower reaction of residual iron with dissolved O₂ generated by early decomposition of H₂O₂. Companion field simulations at the mine site mirror lab simulations on a larger scale. The results suggest that H₂O₂ has potential for very rapid ferrous iron oxidation/precipitation but will require very effective mixing and careful control of dosage to preclude undesirable oxidation effects.