Environmental Engineering Reference
In-Depth Information
placed treatment media. The media degrade, sorb, precipitate, or remove
chlorinated solvents, metals, radionuclides, and other pollutants. The sub-
stantial deviation from common remediation techniques is that the contami-
nant plume, and not its source, is treated (Schad and Grathwohl 1998).
The concept of PRBs was first developed in North America, with pioneer-
ing work conducted at the University of Waterloo in Canada. Initially, the
activities including first pilot field tests focused on “funnel-and-gate” sys-
tems and abiotic reductive dehalogenation of chlorinates and recalcitrant
compounds by elemental iron (Gillham 1993, Starr and Cherry 1994, Vidic
and Pohland 1996, Sivavec et al. 1997, Tratnyek et al. 2003). During the 1990s
research activities on PRBs increased significantly leading to a number of
new approaches in terms of PRB design, suitable reactive materials, and tar-
get contaminants.
Amongst the first and most widely studied metal compounds treated by
PRBs are chromate (Blowes and Ptacek 1992, Powell et al. 1995, Blowes et al.
2000) and uranyl (Cantrell et al. 1995, Dwyer et al. 1996, Bostick et al. 2000),
which are usually treated by reductive processes using, for example, elemen-
tal iron. The use of PRBs for groundwater protection or remediation has also
been studied in other fields, for example, the treatment of metals-containing
mine waters (Morrison and Spangler 1992, 1993, Waybrant et al. 1998, Benner
et al. 1999, Naftz et al. 1999). The treatment of inorganic anions and cations
can be grouped into abiotic reduction and immobilization (mostly by ele-
mental iron), biologically mediated reduction and immobilization (bacterial
sulfate reduction and precipitation of metals as sulfides), and adsorption and
precipitation reactions (Blowes et al. 2000).
The selection of the reactive material to be used in a PRB depends on the
type of contaminant and the remediation approach (contaminant removal
mechanism). In general, contaminants can be removed from polluted water
using the following processes:
• Precipitation: Immobilization of contaminants by formation of insol-
uble compounds (minerals), often after first reducing the contami-
nant to a less soluble species. The immobilized contaminants remain
in the barrier material.
• Sorption: Immobilization of contaminants by adsorption or complex
formation. The immobilized contaminants remain in the barrier
material.
• Degradation (of organic pollutants): Application of chemical or bio-
logical reactions that lead to the decomposition of contaminants and
the formation of harmless compounds which are either retained in
the barrier or released downstream.
Frequently, groundwater treatment can involve a combination of these
processes which cannot be individually distinguished. Nowadays, the most
widely used approaches for PRBs can be grouped into two categories: reductive
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