Field test of electrokinetically-delivered thermally activated persulfate for remediation of chlorinated solvents in clay.

In situ chemical oxidation (ISCO) has demonstrated success in remediating soil and groundwater contaminated with chlorinated volatile organic compounds (CVOCs). However, its performance is often hindered in low-permeability or heterogeneous media due to an inability to effectively deliver the oxidants. This field-scale study investigated the novel approach of applying electrokinetics (EK) to enhance the delivery of persulfate in a low-permeability media and the ability of electrical resistance heating (ERH) to thermally activate the delivered persulfate. Results showed that 40% of the mass of total sulfur delivered was due to EK mechanisms, demonstrating that EK has the potential to enhance oxidant delivery. ERH may have activated some of the persulfate, but catalytic reactions with reduced forms of iron likely resulted in appreciable persulfate decomposition prior to ERH. Significant decreases (>80%) in the aqueous concentration of CVOCs was observed before and after ERH initiation, attributed to in situ transformation and physical processes (e.g., dilution). In situ transformation of CVOCs was assessed by compound-specific isotope analysis (CSIA) of 1,2-dichloroethane (1,2-DCA) samples collected after ERH application. Enrichment of 13C was only measured in the well with appreciable persulfate breakthrough, confirming dechlorination of 1,2-DCA. Results from this field study demonstrate that EK and ERH can be used for persulfate delivery and activation for remediation of CVOCs in low-permeability media.

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.