Environmental Engineering Reference
In-Depth Information
barriers constructed using granular elemental iron (ZVI (Fe
o
) or Fe
2+
) have
been used successfully in North America and Europe for the elimination of
a variety of contaminants including chlorinated solvents, heavy metals, and
radionuclides (Roehl et al., 2005a). The most commonly used mechanisms
are redox and sorption reactions. PRBs allow the passage of groundwater
through the reactive zone of the barrier, but either immobilize (i.e., precipita-
tion, sorption, ion-exchange, surface complexation, solid-solution formation)
or chemically transform (i.e., oxidation, reduction, and degradation) contam-
inants to a more desirable (i.e., less toxic, more readily biodegradable) state
(USEPA, 1999a,b).
Apart from iron-based materials, other types of reactive materials suitable
for use in PRBs for the removal of inorganic and organic compounds from
groundwater are available in a number of publications. These are briefly
summarized in Table 1.3.
Readers interested in case examples of the use of PRBs are directed to
Chapters 2, 4, 6, and 8 in this topic. As will become evident, variants of this
technology are being trialed globally and while this approach to remedia-
tion has been found to be attractive, the technology has failed in a number
of instances. Failures have been attributed to the clogging of walls either due
to chemical reactions resulting in precipitation of insoluble compounds or
enhanced microbial activity leading to the growth of algae, or other prod-
ucts that clog the permeable pores.
1.4.1 Potential Problems Associated with the Long-Term
Performance of PRBs
Designing a PRB system for a given contaminant, should include a feasibility
study (e.g., column experiment) to determine the flow velocity in the bar-
rier and effect geochemistry of groundwater on reactive material, and the
retention time required to treat the groundwater. The design and selection
of the reactive material has to be thick enough to allow a decrease in the con-
taminant concentrations to an acceptable level (the remediation target), and
a longer lifetime before breakthrough. While PRBs have proven to be quite
effective with the remediation of a range of contaminants, long-term sustain-
ability and efficacy of the barriers have been major uncertainties. Despite
these being addressed by a number of researchers (Gu et al., 2002; Henderson
and Demond, 2007; Liang et al., 2003), microgeochemical processes within
the barrier could play an influential role in the long-term performance, as
they have been found to affect contaminant removal both within and down-
gradient of the barrier matrix. For example, PRB demonstration studies in
Germany and Australia show that depending on aquifer chemistry, micro-
organisms can reduce the porosity of the barrier by forming a biofilm (Taylor
and Jaffe, 1990) which reduces pore space by clogging (Vandevivere and
Baveye, 1992) or by contributing to mineral precipitation, or by producing
gas bubbles that restrict water flow.
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