It is now widely recognized that abiotic processes play an important role in the natural attenuation (NA) of groundwater contaminants, and this development has created demand for new and improved methods of measurement and/or enhancement of abiotic NA processes. The overall aim of this project was to develop several of these methods into “core concepts” that will help to form a foundation for field applications of abiotic NA, and related varieties of “in situ chemical reduction” (ISCR).
The three methods developed under this project included: (i) standardized and validated performance metrics, (ii) enhancement of performance by mediation or activation, and (iii) rigorous yet practical quantification of reductant demand. Each of these objectives was pursued through a combination of conceptual model development, data mining, laboratory experiments, and modeling.
With respect to performance metrics (i), this project demonstrated that simultaneous consideration of rate constants normalized to mass and surface area (kM and kSA, respectively) of the reducing mineral phase is sufficient to quantify some major effects, such as sulfidation of iron and iron oxides. However, kM and kSA are not easily applicable to field conditions. In principle, a field measurable parameter like magnetic susceptibility could serve as a surrogate for magnetite mass concentration, but magnetite does not appear to be directly responsible for contaminant reduction under most conditions.
With respect to mediation/activation (ii), it was determined that addition of most shuttle compounds (e.g., quinones) had limited benefits, but stimulating the formation of “reactive mineral intermediate” (RMI) phases by adding Fe(II) to iron oxides can greatly enhance rates of contaminant reduction. It was concluded that these RMIs may serve as mediators in abiotic natural attenuation (ANA) processes under in situ conditions.
With respect to quantification of reductant demand (iii), a comprehensive conceptual model was developed with quantitative definitions of the efficiency and selectivity of contaminant reduction using sulfidation of (nano)zerovalent iron. These concepts should be applicable to other in situ chemical reduction technologies and related ANA processes.
The three core concepts developed in this project include protocols and metrics that should enable Department of Defense site managers to perform more precise and quantitative assessment of the potential contributions of (enhanced) ANA in the remediation of sites contaminated with chlorinated solvents and munitions residues.
Cai, S., B. Chen, X. Qiu, J. Li, P. G. Tratnyek, and F. He. 2020. Sulfidation of Zerovalent Iron by Direct Reaction with Elemental Sulfur in Water: Efficiencies, Mechanism, and Dechlorination of Trichloroethylene. Environmental Science & Technology, 51(1):645-654. [DOI: 10.1021/acs.est.0c05397].
Fan, D., Y. Lan, P. G. Tratnyek, R. L. Johnson, J. Filip, D. M. O'Carroll, A. N. Garcia, and A. Agrawal. 2017. Sulfidation of Iron-based Materials: A Review of Processes and Implications for Water Treatment and Remediation. Environmental Science & Technology, 51(22):13070–13085. https://pubs.acs.org/doi/pdf/10.1021/acs.est.7b04177
Gong, L., X. Qiu, P. G. Tratnyek, L. Chengshuai, and F. He (2021) FeNX(C)-Coated Microscale Zero-Valent Iron for Fast and Stable Trichloroethylene Dechlorination in both Acidic and Basic pH Conditions. Environ. Sci. Technol. 55(8): 5393–5402. [10.1021/acs.est.0c08176]
He, F., L. Gong, D. Fan, P. G. Tratnyek, and G. V. Lowry. 2020. Quantifying the Efficiency and Selectivity of Organohalide Dechlorination by Zerovalent Iron. Environmental Science: Process & Impacts, 22(3):528-542.
Kocur, C. M. D., D. Fan, P. G. Tratnyek, and R. L. Johnson. 2019. Predicting Abiotic Reduction Rates using Cryogenically Collected Soil Cores and Mediated Reduction Potential Measurements. Environmental Science & Technology Letters, 7(1):20-26.
Qin, H., X. Guan, J. Z. Bandstra, R. L. Johnson, and P. G. Tratnyek. 2018. Modeling the Kinetics of Hydrogen Formation by Zerovalent Iron: Effects of Sulfidation on Micro- and Nano-scale Particles. Environmental Science & Technology, 52(23):13887-13896.
Qin, H., X. Guan, and P. G. Tratnyek. 2019. Effects of Sulfidation and Nitrate on the Reduction of N-nitrosodimethylamine (NDMA) by Zerovalent Iron. Environmental Science & Technology, 53(16):9744-9754.
Qin, C., J. Zhang, C. Zhang, Y. He, and P. G. Tratnyek (2021) Abiotic Transformation of Nitrobenzene by Zero Valent Iron under Aerobic Conditions: Relative Contributions of Reduction and Oxidation in the Presence of Ethylene Diamine Tetraacetic Acid. Environ. Sci. Technol. 55(10): 6828-6837. [10.1021/acs.est.1c00653]
Torralba-Sanchez, T. L., E. J. Bylaska, A.J. Salter-Blanc, D. E. Meisenheimer, M. A. Lyon, and P. G. Tratnyek. 2020. Reduction of 1,2,3-Trichloropropane (TCP): Pathways and Mechanisms from Computational Chemistry Calculations. Environmental Science: Processes & Impacts, 22(3):606-616.