Sulfidated nano zerovalent iron (S-nZVI) for in situ treatment of chlorinated solvents: A field study.

Sulfidated nano zerovalent iron (S-nZVI), stabilized with carboxymethyl cellulose (CMC), was successfully synthesized on site and injected into the subsurface at a site contaminated with a broad range of chlorinated volatile organic compounds (cVOCs). Transport of CMC-S-nZVI to the monitoring wells, both downgradient and upgradient, resulted in a significant decrease in concentrations of aqueous-phase cVOCs. Short-term (0-17 days) total boron and chloride measurements indicated dilution and displacement in these wells. Importantly however, compound specific isotope analysis (CSIA), changes in concentrations of intermediates, and increase in ethene concentrations confirmed dechlorination of cVOCs. Dissolution from the DNAPL pool into the aqueous phase at the deepest levels (4.0-4.5 m bgs) was identifiable from the increased cVOCs concentrations during long-term monitoring. However, at the uppermost levels (∼1.5 m above the source zone) a contrasting trend was observed indicating successful dechlorination. Changes in cVOCs concentrations and CSIA data suggest both sequential hydrogenolysis as well as reductive ß-elimination as the possible transformation mechanisms during the short-term abiotic and long-term biotic dechlorination. One of the most positive outcomes of this CMC-S-nZVI field treatment is the non-accumulation of lower chlorinated VOCs, particularly vinyl chloride. Post-treatment soil cores also revealed significant decreases in cVOCs concentrations throughout the targeted treatment zones. Results from this field study show that sulfidation is a suitable amendment for developing more efficient nZVI-based in situ remediation technologies.

Fate and transport of sulfidated nano zerovalent iron (S-nZVI): A field study.

Treatment of nano zerovalent iron (nZVI) with lower valent forms of sulfur compounds (sulfidation) has the potential to increase the selectivity and reactivity of nZVI with target contaminants and to decrease inter-particle aggregation for improving its mobility. These developments help in addressing some of the long-standing challenges associated with nZVI-based remediation treatments and are of great interest for in situ applications. Herein we report results from a field-scale project conducted at a contaminated site. Sulfidated nZVI (S-nZVI) was prepared on site by first synthesizing carboxymethyl cellulose (CMC) stabilized nZVI with sodium borohydride as a reductant and then sulfidating the nZVI suspension by adding sodium dithionite. Transmission electron microscopy (TEM) coupled with energy dispersive X-ray spectroscopy (EDS) of CMC-S-nZVI, from synthesis barrels, confirms the presence of both discrete spherical nZVI-like particles (∼90 nm) as well as larger irregular structures (∼500 nm) comprising of iron sulfides. This CMC-S-nZVI suspension was gravity fed into a sandy material and monitored through multiple multi-level monitoring wells. Samples collected from upstream and downstream wells suggest very good radial and vertical iron distribution. TEM-EDS analysis from the recovered well samples also indicates the presence of both nZVI-like particles as well as the larger flake-like structures, similar to those found in the injected CMC-S-nZVI suspension. This study shows that S-nZVI stabilized with CMC can be safely synthesized on site and is highly mobile and stable in the subsurface, demonstrating for the first time the field applicability of S-nZVI.

Long-Term Field Study of Microbial Community and Dechlorinating Activity Following Carboxymethyl Cellulose-Stabilized Nanoscale Zero-Valent Iron Injection.

Nanoscale zerovalent iron (nZVI) is an emerging technology for the remediation of contaminated sites. However, there are concerns related to the impact of nZVI on in situ microbial communities. In this study, the microbial community composition at a contaminated site was monitored over two years following the injection of nZVI stabilized with carboxymethyl cellulose (nZVI-CMC). Enhanced dechlorination of chlorinated ethenes to nontoxic ethene was observed long after the expected nZVI oxidation. The abundance of Dehalococcoides (Dhc) and vinyl chloride reductase (vcrA) genes, monitored using qPCR, increased by over an order of magnitude in nZVI-CMC-impacted wells. The entire microbial community was tracked using 16S rRNA gene amplicon pyrosequencing. Following nZVI-CMC injection, a clear shift in microbial community was observed, with most notable increases in the dechlorinating genera Dehalococcoides and Dehalogenimonas. This study suggests that coupled abiotic degradation (i.e., from reaction with nZVI) and biotic degradation fueled by CMC led to the long-term degradation of chlorinated ethenes at this field site. Furthermore, nZVI-CMC addition stimulated dehalogenator growth (e.g., Dehalococcoides) and biotic degradation of chlorinated ethenes.

Contributions of Abiotic and Biotic Dechlorination Following Carboxymethyl Cellulose Stabilized Nanoscale Zero Valent Iron Injection.

A pilot scale injection of nanoscale zerovalent iron (nZVI) stabilized with carboxymethyl cellulose (CMC) was performed at an active field site contaminated with a range of chlorinated volatile organic compounds (cVOC). The cVOC concentrations and microbial populations were monitored at the site before and after nZVI injection. The remedial injection successfully reduced parent compound concentrations on site. A period of abiotic degradation was followed by a period of enhanced biotic degradation. Results suggest that the nZVI/CMC injection created conditions that stimulated the native populations of organohalide-respiring microorganisms. The abundance of Dehalococcoides spp. immediately following the nZVI/CMC injection increased by 1 order of magnitude throughout the nZVI/CMC affected area relative to preinjection abundance. Distinctly higher cVOC degradation occurred as a result of the nZVI/CMC injection over a 3 week evaluation period when compared to control wells. This suggests that both abiotic and biotic degradation occurred following injection.