Environmental Engineering Reference
In-Depth Information
32.2.5 Permeable Reactive Barrier
A PRB is an
in situ
technology used to treat perchlorate-contaminated groundwater in full-
scale systems. Some of the commonly used reactive materials for barriers include soybean and
other edible oils, woodchips, pecan shells, cottonseed, chitin, limestone, and other compost-
ing materials. Many of these materials can provide both electron donors and the necessary
nutrients for microbial growth. Soluble electron donors such as lactate, acetate, and citrate
may be added to the barrier materials to further stimulate biodegradation of perchlorate.
A PRB is an
in situ
treatment zone of reactive material that degrades or immobilizes
contaminants as groundwater lows through it. PRBs are installed as permanent, semiper-
manent, or temporary units across the low pattern of a contaminant plume. Contaminants
in groundwater that low through a PRB are degraded chemically or biologically (FRTR,
2005). Barriers are made of reactive material that targets speciic contaminants (USAFCEE,
2002; USEPA, 2005). To treat groundwater contaminated with perchlorate, the reactive
barrier may be inoculated with anaerobic bacteria that can convert perchlorate into chlo-
ride and oxygen (USAFCEE, 2002). For treatment of perchlorate-contaminated groundwa-
ter, the PRB system is backilled with reactive material that includes an electron donor
to stimulate reduction of perchlorate and organic substrates to nourish the microorgan-
isms (USAFCEE, 2002; Craig and Jacobs, 2004; Beisel et al., 2004). Proper installation of
PRBs requires access to depths of the contaminated groundwater and barriers formed by
trenches that surface excavation or trenching equipment may not be able to reach (FRTR,
2005). Additional maintenance may be required to unclog the barrier fouled biologically
or clogged with chemical precipitates (USAFCEE, 2002; USEPA, 2005).
32.2.6 Phytotechnology
This is an emerging technology for perchlorate remediation. It involves use of plants to
remove contaminants by natural processes occurring within the plant body. Selection
of the best plant species is critical to achieving the treatment goals. Research is cur-
rently under way to identify the mechanism involved in perchlorate removal using this
approach. A few bench-scale studies have indicated the suitability of certain plant species
for remediation of perchlorate-contaminated media. Phytotechnology includes various
mechanisms such as rhizosphere biodegradation, phytovolatilization, phytostabilization,
and phytoextraction (FRTR, 2005). Rhizodegradation or rhizosphere degradation proceeds
via activities of microorganisms present in the soil surrounding the roots. The natural
substances released by plant roots provide nutrient material to the microbial population,
which in turn degrade the contaminants present in soil. The mechanism of remediation
of perchlorate-contaminated media by phytotechnology is not yet established. However,
studies conducted at bench scale have indicated possible suitability of certain plant species
for perchlorate removal (Motzer, 2001; Schnoor et al., 2004; Susarla et al., 1999).
32.2.7 Membrane Technologies
Semipermeable or permeable membrane technologies have been used for perchlorate
removal. Electrodialysis and reverse osmosis are examples of membrane technologies that
have been examined for perchlorate removal from groundwater, surface water, and waste-
water. These are briely discussed below.
