PNNL-13501 - Pacific Northwest National Laboratory

Study Control Number: PN00090/1497
Use of Reduced Metal Ions in Reductive Reagents
John S. Fruchter, Jim E. Szecsody
In the past 15 years, groundwater cleanup of chlorinated solvents has involved installation of reactive subsurface iron
barriers in over 25 sites in the U.S. One method used, which relies upon the reduction of iron in natural sediments, is
applicable to deep aquifers (more than 100 feet). In this laboratory study, we modified geochemically the sediment
reduction method to make the process more efficient and, ideally, less expensive.
Project Description
The purpose of this study was to determine a more
efficient method for introducing chemical reducing
capacity into the subsurface. After an initial reduction of
aquifer sediments with sodium dithionite, reduced cations,
such as ferrous iron (Fe(II)) or titanous ion (Ti(III)) would
be injected into the aquifer. The expected results are an
understanding of the proportions of Fe(II) and Ti(III)
metal ions to use and the chemical conditions needed for
achieving a successful reduced zone. In this way, a
reactive barrier can be developed that is more effective
for destroying dissolved contaminants such as chlorinated
solvents and explosives.
Over the past several years, a major effort at Pacific
Northwest National Laboratory was the development of
reductive barrier remediation techniques for use on
shallow and deep (>50 feet below ground surface)
contaminant plumes. The most promising method is in
situ redox manipulation. The basis of the method is the
use of a strong reducing agent (dithionite) to reduce
normally trivalent structural iron in the soil matrix to the
more reactive divalent form. The presence of the
immobile divalent iron in the treated zone creates a
chemically active barrier to the transport of reducible
species, which can last for decades before renewal.
Despite these successes, dithionite addition alone is an
inefficient and expensive way to add reducing capacity to
the aquifer. Because the reduction of some contaminants
(trichloroethylene for example), the surface has a catalytic
or surface coordination role in reduction, the presence of
specific metals on the iron oxide surface can increase the
electron transfer (reduction) rate. This concept has been
applied to zero-valent iron walls, with the incorporation
of trace amounts of Cr and Ni. A more efficient method
would be to inject dithionite to produce reducing
conditions in the aquifer, then add one or more reduced
metal ions directly. We propose to amend the in situ
redox manipulation technique through addition of
additional reduced metal ions (e.g., Fe(ii), Ti(III)) to the
redox injection. Cationic adsorption of reduced metal
ions on the soil matrix should the produce a more reactive
barrier.
Introduction
In this study, we investigated electron donors or reaction
catalysts potential chemical additions.
Fe(II) and Mn(II) additions (electron donors) resulted in a
significant increase in trichloroethylene dechlorination.
Trichloroethylene dechlorination with the Fe(II) addition
was predictable and more efficient than the Mn(II). The
additions of Ti(III), Ni(II), Pd catalyst, and EDTA did not
enhance the trichloroethylene dechlorination rate in
sediments, even though these compounds are used in
engineered systems. Because the addition of an electron
shuttle had only a slight positive effect, the rate of
trichloroethylene degradation is not limited by electron
transfer, so an additional electron donor rather than
catalyst is likely more efficient. While an Fe(II) addition
is expected to be more cost efficient than the use of
reduced sediment alone, amending the redox treatment at
the field scale involves additional hydraulic and
geochemical considerations. Upscaling these results of
using small to large experimental systems is needed to
design viable field-scale injection schemes.
Trichloroethylene dechlorination with reduced sediment
relies on the oxidation of ferrous iron [adsorbed Fe(II),
Fe(II)CO3, reduced smectite clays; Szecsody et al. 2000a,
b]. In addition, adsorbed or structural Fe II on an Fe III -
oxide or clay surface is necessary for dechlorination,
either as a catalyst, a semiconductor, or to provide surface
Eh conditions (Scherer et al. 1999; Wehrli 1992). The
trichloroethylene degradation pathway using zero-valent
iron (Roberts et al. 1996) and reduced sediments
(Szecsody et al. 2000b) is reductive elimination producing
chloroacetylene, then acetylene:
Earth System Science 251