In situ abiotic/biotic permeable reactive barriers (PRBs) offer potential for the cost-effective removal of heavy metal ions from contaminated groundwater. In these systems, reactive material is introduced into a "permeable wall" placed in the groundwater flow path to remove targeted contaminants. Of interest in this project was the use of particulate iron sulfide (FeS) in PRB applications. FeS has a high capacity for non-redox active metals such as Cd(II) in which highly insoluble cadmium sulfides (CdS(s)) form by favorably exchanging for Fe in FeS. For redox-active metals such as arsenic (As), FeS serves as an effective reductant, converting oxidized forms of As(V) to the more reduced forms of As(III) and subsequently removing it by adsorption or formation of mixed-metal sulfide phases. Concerns remain, however, related to the longevity of materials in PRBs and the impact of changing geochemical conditions (e.g., pH and redox) on long-term sequestration properties. A natural consequence of redox reactions and introduction of oxygenated water is the formation of more oxidized forms of FeS. In situ microbiological processes could provide a cost-effective way to rejuvenate FeS for long-term use and reuse and to maintain reducing conditions. Also, the successful design and performance evaluation of FeS PRBs will require the application of reactive transport models, based on a clear understanding of the metal ion sequestration mechanisms (e.g., reduction, sorption and precipitation) and their impact on transport properties (e.g., porosity and hydraulic conductivity) under realistic geochemical conditions.

The overall objective of this project was to evaluate the effectiveness of FeS materials for sequestration of a targeted oxyacid, As, and heavy metal cation, Cd, under anoxic conditions in PRB applications.

In recognition of the potential of FeS for treating contaminated groundwater, two different forms of FeS have been developed for treating heavy metal-contaminated groundwater plumes in PRB systems, a nanoscale form of FeS for direct injection and FeS-coated sand for emplacement in PRB walls. To test the effectiveness of reduced FeS for long-term sequestration of heavy metal ions and oxyacids, Cd and As were selected. FeS performance was investigated using batch reactor and column reactor systems under various geochemical conditions. Information on the properties of FeS and mechanistic information on the removal mechanisms by FeS of both As and Cd were obtained using molecular-scale surface techniques including synchrotron-based x-ray absorption spectroscopy (XAS), x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), and microscopic tools such as transmission electron microscopy (TEM) and scanning electron microscopy (SEM) with energy dispersive x-ray spectroscopy (EDXS). Two emplacement methods, colloidal injection and physical packing of FeS-coated sand in porous media, were also evaluated. The ability to rejuvenate FeS from oxidized forms of iron that were expected to be representative of reacted FeS using a variety of sulfate-reducing microorganisms were also assessed. Finally, a reactive transport model was developed using batch uptake and column breakthrough data for calibration along with the mechanistic information obtained.

This work developed two forms of reactive FeS media for field-scale application: a nanoscale FeS for direct colloidal injection into the subsurface at a contaminated site and FeS-coated sand for a trench-and-fill PRB application. The colloidal form was tested and the conditions established for optimal effective dispersal and coating of a packed column of clean sand. An extension for establishing the appropriate conditions for natural sand will require applying the protocols developed for site-specific materials. The results indicate, in general, that by changing solution conditions (pH and ionic strength) it is possible to optimize the process for effective distribution of the material in sandy porous media without clogging the formation. Likewise, a procedure to effectively coat sand with FeS was developed. Reactivity studies with both nano-scale FeS and FeS-coated sand indicate that these materials will be effective for removing As from contaminated groundwater in PRB applications.

This research has led to (1) a detailed understanding of the mechanisms of the reaction of As and Cd under changing geochemical conditions; (2) an assessment of the potential to rejuvenate FeS by biological means; (3) a determination of the feasibility of using various forms of FeS for PRB applications; (4) an understanding of the impact of metal removal mechanisms on transport properties in porous media; and (5) the development of a reactive transport model for design of PRB-FeS systems. The overall results provide the tools needed to design and apply the FeS PRB media for effective long-term treatment of mixed-metal ion plumes at appropriate Department of Defense sites.