Controls and predictions of geogenic redox-sensitive contaminants in Danish groundwater

Pacific Northwest National Laboratory conducted a proof-of-principle test at the Fort Lewis Logistics Center to determine the feasibility of using the innovative remedial technology In Situ Redox Manipulation (ISRM) to treat groundwater contaminated with dissolved TCE. ISRM creates a permeable treatment zone in the subsurface to remediate redox-sensitive contaminants in groundwater. The permeable treatment zone is created by injecting a chemical reducing agent (sodium dithionite with pH buffers) into the aquifer through a well to chemically reduce the naturally occurring ferric iron in the sediments to ferrous iron. Once the reducing agent has been given sufficient time to react with aquifer sediments, residual chemicals and reaction products are withdrawn through the same well. Redox-sensitive contaminants such as TCE, moving in a dissolved-phase plume through the treatment zone, are destroyed. TCE is degraded via reductive dechlorination within the treatment zone to benign degradation products (acetylene, ethylene). Analyses of sediment samples collected from post-test boreholes showed a high degree of iron reduction, which confirmed the effectiveness of the treatment zone.

Environmental Context. Groundwater remediation is generally a costly, long-term process. In situ remediation using permeable reactive barriers, through which the groundwaters pass, is a potential solution. For redox-sensitive contaminants in groundwater, a metallic iron barrier (zerovalent iron, ZVI) can immobilize or degrade these dissolved pollutants. Scrap iron materials are a low-cost ZVI material but, because of the wide variation of scrap metal compositions, testing methods for characterizing the corrosion behaviour need to be developed. Abstract. Zerovalent iron (ZVI) has been proposed as reactive material in permeable in situ walls for contaminated groundwater. An economically feasible ZVI-based reactive wall requires cheap but efficient iron materials. From an uranium treatability study and results of iron dissolution in 0.002 M EDTA by five selected ZVI materials, it is shown that current research and field implementation is not based on a rational selection of application-specific iron metal sources. An experimental procedure is proposed which could enable a better material characterization. This procedure consists of mixing ZVI materials and reactive additives, including contaminant releasing materials (CRMs), in long-term batch experiments and characterizing the contaminant concentration over the time.

Although permeable reactive barriers (PRB) technology appears to be a very suitable and cost effective option, the extent to which remediation results will be realized, greatly depends on the long-term integrity of the system. The formation of mineral precipitates is possibly a major factor in the long-term performance of PRB. Precipitates may passivate reactive surfaces by blocking electron-transfer sites, and thereby reduce the long-term reactivity of the granular iron to degrade groundwater contaminants. To evaluate the potential passivation impacts of inorganic groundwater chemistry, column experiments containing zero-valent iron (Fe0) were performed under anoxic conditions to treat two contrasting Danish groundwater types spiked with trichloroethylene (TCE). For most of the experiments using Danish groundwater types, a soft low alkalinity groundwater produced slightly higher TCE dechlorination rate than did a hard high alkalinity groundwater. Compared to a soft low alkalinity baseline groundwater, it was also found the dechlorination of TCE in the column was enhanced in the presence of 1 mM CaCO3 and 1 mM NaHCO3. The dechlorination of TCE in the presence of 1 mM KNO3 and 1 mM Na2SiO3 was found to decrease considerably compared with the baseline solution. The results suggest that the composition of field groundwater is likely to strongly affect the ability of Fe0 barriers to degrade TCE.

Cr(VI) reduction by Fe(II) sorbed to silica surfaces