Sequenced Permeable Reactive Barrier for the Pretreatment of Nitrate and Remediation of Trichloroethene - Permeable Reactive Barrier: Sustainable Groundwater Remediation - page 110

the resulting contaminant plume in order to protect downgradient sensitive
receptors may be more achievable and, if required, may allow for more time
to develop an effective source remediation solution.
Zero-valent iron (ZVI) permeable reactive barriers (PRBs) have been proven
to be successful in removing a broad range of contaminants, including many
chlorinated solvents (Roberts et al. 1996, Farrell et al. 2000). The first applica-
tion of a ZVI PRB was constructed in northern California in 1994, and it con-
tinues to operate successfully to this day. The use of ZVI PRBs to remediate
plumes of chlorinated solvent has become more common with hundreds of
PRBs having been installed around the world (Adventus 2011).
There are two primary pathways for the dechlorination of chlorinated
ethenes in ZVI PRBs: β-elimination and hydrogenolysis (IRTC 2005).
β-Elimination is a reductive elimination in which halide ions are released
from the molecule. Hydrogenolysis is the replacement of a halogen by a
hydrogen ion. Batch experiments have indicated that the dominant degra-
dation pathway is β-elimination (Arnold and Roberts 2000). This pathway
is also the preferred pathway as it results in the chlorinated ethene degrad-
ing directly to ethane, ethene, and acetylene, thereby circumventing the
production of intermediate daughter products (Eykholt 1998, Arnold and
Roberts 1999).
Geochemical parameters that may affect the effectiveness of ZVI when
treating chlorinated solvents have also been researched with the Interstate
Technology Regulatory Council (ITRC) (2005) that summarized concentra-
tions of nitrate, dissolved organic carbon (DOC), metals, and silica as some
the most important. Of particular interest with respect to the work presented
here are nitrate and DOC.
Research has found that nitrate can passivate iron (inhibit iron perfor-
mance) by causing a thin layer of iron oxide to coat the iron. Some iron
oxides such as goethite and maghemite have been proven to inhibit iron cor-
rosion, resulting in iron passivation. For this reason, it is believed that as iron
oxides form, the number of reactive sites available to nitrate and chlorinated
solvents reduce, resulting in the advancement of both nitrate and chlorinated
solvent profiles in the iron (Farrell et al. 2000).
Research has shown that certain types of DOC can also passivate the iron
by coating reactive sites (ITRC 2005).
A potential solution to elevated concentrations of nitrates in the ground-
water could be the use of an upgradient parallel denitrification PRB. A study
by Vogan et al. (1993) concluded that sequenced PRB treatments were a
potential technique to treat mixed nitrate and trichloroethene (TCE) plumes.
A literature review did not identify any field tests of denitrification and
ZVI PRBs in series. However, a column experiment conducted by Vogan et al.
(1993) attempted to replicate a sequenced PRB for the removal of nitrates by
Nitrex TM (proprietary media incorporating waste cellulose solids) followed
by the removal of TCE by ZVI. The experiment used groundwater contain-
ing 11 mg/L nitrate as N and 30 mg/L TCE. Complete removal of nitrates