Abiotic and Biotic Mechanisms Controlling In Situ Remediation of NDMA

摘要

This project was initiated to investigate whether in situ coupled abiotic/biotic degradation of N-nitrosodimethylamine (NDMA, an emerging contaminant) could be used as a permeable reactive barrier for remediation at the Aerojet, California site, where groundwater contains up to 36-ppb NDMA. The sediment mainly used in experiments is from the Aerojet groundwater aquifer (260-ft depth), with additional sediments from other groundwater aquifers (i.e., not shallow soils, Ft. Lewis, Washington, 60-ft depth, Puchack, New Jersey, 273- ft depth). Different in situ remediation processes were compared in this study: a) biostimulation (oxic, anaerobic, iron-reducing, sulfate-reducing), b) abiotic iron-reducing environment (by dithionite reduction of sediment or zero valent iron addition), c) coupled abiotic/biotic remediation (iron-reducing environment), and d) sequential iron-reducing, then oxic, biostimulation. The overall goal was to understand and optimize the combined effects of abiotic and biotic processes to degrade NDMA to nontoxic products. Iron-reducing conditions were created by chemical reduction of a sediment using sodium dithionite, and in a few cases, a natural reduced aquifer sediment or sediment with zero valent iron addition. When subsurface sediments are chemically or naturally reduced, abiotic surface phase(s) rapidly degraded NDMA (8-hour half-life for high reduction, slower for low reduction) to nontoxic dimethylamine (DMA). Experiments showed up to 80% conversion of NDMA to DMA, with further degradation to nitrate (up to 40% molar conversion), formate (trace concentration) and carbon dioxide (up to 82% molar conversion). Methylamine and formaldehyde (likely intermediates) were not detected. Experiments with 100 and 10 ng/L NDMA starting concentration had a rapid degradation half-life (4.7 hours), and NDMA was degraded to <3 ng/L (detection limit). Although degradation to DMA is sufficient for remediation (DMA is not toxic at <5 mg/L), because NDMA mass was degraded further to more toxic intermediates, NDMA mineralization (i.e., to CO2) was considered the lowest risk product, and was the main focus of this study.