Quantum chemical modeling of redox reactivity, degradation pathways and persistence for aqueous phase contaminants

Abstract

Models used to predict the fate of aqueous phase contaminants are often limited by their inability to address the widely varying redox conditions in natural and engineered systems, as well as by their dependence on existing experimental data for structurally similar compounds. Here, a novel approach based on quantum chemical calculations is developed, which can be applied to assess the environmental fate of any contaminant of interest without previous knowledge of the compound. It identifies the thermodynamic conditions necessary for redox-promoted degradation, and predicts degradation pathways as well as contaminant persistence. Hexamethylphosphoramide (HMPA), a widely used solvent and poorly characterized groundwater contaminant, is used as a test case. The development of an analytical technique based on liquid chromatography / time-of-flight mass spectrometry enables the detection of various degradation products that are reported here for the first time. The oxidation of HMPA is estimated to require at least iron-reducing conditions at low to neutral pH, and nitrate-reducing conditions at high pH. Furthermore, the transformation of HMPA by permanganate, a common groundwater remediation agent, is predicted to proceed through sequential N-demethylation. Experimental validation confirms the predicted pathways of HMPA oxidation by permanganate to phosphoramide via the formation of less methylated as well as singly and multiply oxygenated reaction intermediates. Pathways predicted to be thermodynamically or kinetically unfavorable are similarly absent in the experimental studies. Theoretical and experimental investigations, using 18O-labeled water to determine the source of oxygen in the products of HMPA oxidation, reveal a novel mechanism in addition to the one reported in the literature for methyl oxidation. The strategy of calculating Gibbs free energies of activation can be generally used to determine the primary degradation pathway when two or more pathways are thermodynamically favorable. In this study, however, the determined kinetic parameters show that both HMPA oxidation pathways proceed at similar reaction rates. Hydrolysis of the P-N bond in HMPA is the only thermodynamically favorable reaction that may lead to its degradation under strongly reducing conditions. Through calculation of aqueous Gibbs free energies of activation for all potential reaction mechanisms, it is predicted that HMPA hydrolyzes via an acid-catalyzed A2@P mechanism at pH < 8.2, and an uncatalyzed concerted backside SN2@P-b mechanism at pH 8.2 - 8.5. The estimated half-lives of thousands to hundreds of thousands of years over the groundwater-typical pH range of 6.0 to 8.5 indicate that HMPA will be persistent in the absence of suitable oxidants. At pH 0, where the hydrolysis reaction is rapid enough to enable measurement, the experimentally determined rate constant and half-life are in excellent agreement with the predicted values. The newly developed methodology will enable scientists, regulators, and engineers to estimate the favorability of contaminant degradation at a specific field site, suitable approaches to enhance degradation, and the persistence of a contaminant and its reaction intermediates.