Proton generation by dissolution of intrinsic or augmented aluminosilicate minerals for in situ contaminant remediation by zero valence state iron

Metallic, or zero-valence-state, iron is being incorporated into permeable reactive subsurface barriers for remediating a variety of contaminant plume types. The remediation occurs via reductive processes that are associated with surface corrosion of the iron metal. Reaction rates for these processes vary widely with both the form of iron and the contaminant but have previously been shown to increase when certain aquifer materials are present in the mix. Knowledge of such geochemical effects is important for planning an in situ remediation, as well as understanding the transport and fate of the contaminant within the barrier. The increase in reaction rate has been hypothesized to be due to the presence of aluminosilicate minerals in some aquifer materials that can dissolve and participate in the reaction sequences. Current results show that a variety of aluminosilicate minerals, including kaolinite, montmorillonite, and a range of feldspars, can undergo dissolution in these disequilibrium systems and provide protons as electron accepters at a rate sufficient to maintain and/or enhance the reactions. The proposed reactions are illustrated for aluminosilicate dissolution and chromate reduction. A potential benefit for TCE dechlorination is also depicted. These aluminosilicates may occur naturally in the aquifer geology and become mixed with the iron during emplacement, or they can be added to the iron as amendments prior to emplacement. Both scenarios provide greater confidence that contaminant reactions will be complete before the reactants exit the barrier and may allow the engineering of thinner barriers in situations constrained by cost or the presence of physical structures.