Abiotic attenuation, which involves chemical reactions between contaminants and a soil/sediment constituent, is an important sink for many contaminants in the environment. If engineered correctly, the process can also be an inexpensive, semi-passive approach to control plume migration in soil and groundwater. Under anoxic subsurface conditions, redox-labile chemicals are especially susceptible to reductive attenuation processes. Hence, sites where these contaminants are present have the greatest opportunities for successful application of enhanced abiotic attenuation. Many soil constituents have been identified that can either reduce or catalyze the reduction of these contaminants, including ferrous iron-containing minerals, ferrous iron complex, natural organic matter, and black carbon. However, the reactivity of both the reductants and contaminants can vary by many orders of magnitude, depending on the type and nature of the reactants and the geochemical conditions. 

The objective of this project is to develop a methodology for predicting (1) the abiotic reduction rates of munitions compounds in the solid matrix of any given geochemical state of a solid, and (2) the longevity and enhancement frequency necessary to control plume migration.

The linear free energy relationship (LFER) model can quantitatively predict abiotic reduction rate constants of nitro compounds. It measures the reduction potential distribution and electron exchange capacity of a soil using a chemical redox titration. However, all LFERs established to date for abiotic redox transformation are based on a single reductant. That is, they can predict contaminant degradation rates if and only if the identity and concentration of the reductant involved are known.

The project team has tested the model using a data set of measured rate constants for six nitro benzenes that vary over five orders of magnitude in a system containing H₂S and dissolved organic matter (H₂S/DOM) with pH varying from 5.5 to 8.6 with remarkable success, whereas presently available LFERs fail. The new feature is that pH + pe is used rather than pe alone to quantify the redox potential. The quantity of reductant in the soil is determined by relating the change in pH + pe to the quantity of chemical reductant titrant added in the redox titration. This provides both the LFER parameter (pH + pe) and the soil reductant quantity over the entire range of soil reduction enhancement. This improved model can predict the increase in reduction rate constant of energetic compounds at various levels of soil enhancement.

In this project, the team plans to measure the reaction rate constants of eight model compounds of diverse reactivity in 10+ soils titrated to different redox potentials in order to calibrate and validate the LFER model. They will evaluate the feasibility of enhancing soil reactivity using flow-through soil columns. The model will be used to predict the observed contaminant distributions throughout the column over time.

A calibrated and validated predictive model will permit practitioners to develop designs for more cost-effective long-term remediation. The methodology can be directly incorporated in the groundwater models that are used to design and evaluate remediation strategies. The data requirements are not excessive, as it only requires redox titrations of a number of soil samples. The model can be used to predict the quantity of reductant needed and also to evaluate the reduction rates of the reduction intermediates and new munitions compounds. Results of this research will help confirm that enhanced abiotic attenuation can be a viable long-term remedial option. (Anticipated Project Completion - 2026)

Cárdenas-Hernández, P.A., K.A. Anderson, J. Murillo-Gelvez, D.M. Di Toro, H.E. Allen, R.F. Carbonaro, and P.C. Chiu. 2020. Abiotic Reduction of 3-Nitro-1,2,4-triazol-5-one (NTO) and the Hematite–Fe(II) Redox Couple, Environmental Science and Technology, 54(19):12191-12201. doi.org/10.1021/acs.est.0c03872.

Cárdenas-Hernández, P. A., K.P. Hickey, D.M. Di Toro, H.E. Allen, R.F. Carbonaro, and P.C. Chiu. 2023 Linear Free Energy Relationship for Predicting the Rate Constants of Munition Compound Reduction by the Fe(II)-Hematite and Fe(II)-Goethite Redox Couples. Environmental Science & Technology, 57(36):13646–13657. doi.org/10.1021/acs.est.3c04714.

Di Toro, D.M., K.P. Hickey, H.E. Allen, R.F. Carbonaro, and P.C. Chiu. 2020. Hydrogen Atom Transfer Reaction Free Energy as a Predictor of Abiotic Nitroaromatic Reduction Rate Constants: A Comprehensive Analysis. Environmental Toxicology and Chemistry, 39(9):1678-1684. doi.org/10.1002/etc.4807.

Hickey, K. P., P.A. Cárdenas-Hernández, D.M. Di Toro, H.E. Allen, R.F. Carbonaro, and P.C. Chiu. 2023 Thermodynamic Two-Site Surface Reaction Model for Predicting Munition Constituent Reduction Kinetics with Iron (Oxyhydr)oxides. Environmental Science & Technology, 57(33):12411–12420. doi.org/10.1021/acs.est.3c02651.

Hickey, K.P., D.M. Di Toro, H.E. Allen, R.F. Carbonaro, and P.C. Chiu. 2020. A Unified Linear Free Energy Relationship for Abiotic Reduction Rate of Nitroaromatics and Hydroquinones Using Quantum Chemically Estimated Energies. Environmental Toxicology and Chemistry, 39(12):2389-2395. doi.org/10.1002/etc.4867.

Hickey, K.P., J. Murillo-Gelvez, D.M. Di Toro, H.E. Allen, R.F. Carbonaro, and P.C. Chiu. 2022. Modeling the Reduction Kinetics of Munition Compounds by Humic Acids. Environmental Science and Technology, 56(8):4926-4935. doi.org/10.1021/acs.est.1c06130.

Murillo-Gelvez, J., D.M. Di Toro, H.E. Allen, R.F. Carbonaro, and P.C. Chiu. 2021. Reductive Transformation of 3-Nitro-1,2,4-Triazol-5-One (NTO) by Leonardite Humic Acid and AQDS. Environmental Science and Technology, 55(19):12973-83. doi.org/10.1021/acs.est.1c03333.

Murillo-Gelvez, J., K.P. Hickey, D.M. Di Toro, H.E. Allen, R.F. Carbonaro, and P.C. Chiu. 2019. Experimental Validation of Hydrogen Atom Transfer Gibbs Free Energy as a Predictor of Nitroaromatic Reduction Rate Constants. Environmental Science and Technology, 53(10):5816-5827. doi.org/10.1021/acs.est.9b00910.

Murillo-Gelvez, J., K.P. Hickey, D.M. Di Toro, H.E. Allen, R.F. Carbonaro, and P.C. Chiu. 2023. Electron Transfer Energy and Hydrogen Atom Transfer Energy-Based Linear Free Energy Relationships for Predicting the Rate Constants of Munition Constituent Reduction by Hydroquinones. Environmental Science and Technology, 57(13):5284–5295. doi.org/10.1021/acs.est.2c08931.