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7.3.5.4 Organic Ligand-Catalyzed Titanium (III) Reduction
Similarly to iron, titanium ions (Ti3+) reduce perchlorate, but the reaction is slow.
Other metal ions, including ruthenium(II), vanadium(II), vanadium(III), molybde-
num(III), dimolybdenum(III), and chromium(II) also reduce perchlorate, but the tita-
nium ion has been studied more intensively. The process requires acidic conditions,
with pH of approximately 4 [80]. Laboratory research has identified catalysts in the
form of organic ligands that can speed up the reduction reaction time significantly,
with a resulting half-life of minutes rather than hours or days. The ligands are
usually in the form of ethylene-diamine-tetraacetic acid (EDTA) or hydroxyethyl
EDTA (or HEDTA). The ultimate products of the Ti(III)-perchlorate reaction are
titanium dioxide and chloride salts, non-toxic and environmentally benign products.
The reactant Ti(III) is inexpensive and readily available. The product Ti(IV) can be
reduced to Ti (III) by electrochemical or chemical means and then reused in the
process [81].
7.4 Summary and Conclusions
As with all contaminants, the physical nature of perchlorate provides the best
clues to successful remediation technologies. As discussed in previous chapters,
perchlorate is:
"* Very soluble in water and thus migrates readily in groundwater
"*  Highly oxidized, and therefore resistant to chemical oxidation reactions
"* Chemically stable due to the high activation energy of reduction reactions,
and thus resistant to most chemical reduction reactions
"* Capable of being thermally degraded
"* Biodegradable by a limited number of microorganisms and only under
anoxic conditions
Because of its high solubility, problems with perchlorate at most sites occur in
groundwater rather than in soil, and the groundwater plumes tend to be large and
disperse due to perchlorate's mobility.
Technology selection for perchlorate, as for any other contaminant, depends on a
combination of factors. The following considerations summarize the site character-
istics, logistics, and regulatory drivers to be considered when selecting the appropri-
ate treatment technology.
* Where is the perchlorate located? Is it only in the surface soil, the vadose
zone, the aquifer, or some combination of these subsurface regions? Is it in
a remote location? Is it in the middle of a busy area with subsurface utili-
ties? How will power and water be supplied to the site? How will the treat-
ment system be operated or new equipment or materials such as treatment
media be delivered to the treatment system?
"* What are the regulatory drivers? The cleanup levels in different states wil
dictate the size of the groundwater plume or soil volume to be remediated
"* If the perchlorate is in the groundwater, how deep is the groundwatei
table? What is the aquifer thickness? How fast is the groundwater flow.
ing, and in what direction? What type of soil is in the aquifer (sandy, or
clay, or other)? What are the pH, temperature, oxidation/reduction poten.
tial, organic content, concentrations of competing anions, and concentra.
tions of other materials such as calcium, iron, or manganese that may foul
equipment?
"* Whether to use in situ or ex situ treatment depends on several factors. Can
the aquifer sustain the type of biological growth needed for perchlorate
destruction using in situ bioremediation? What is the size of the contami-
nant plume and what is the concentration of perchlorate? For example, in
situ bioremediation using injection of nutrients makes sense in a shallow
aquifer with natural reducing conditions, moderate levels of organic
content, and a high concentration of perchlorate. However, the same biore-
mediation process would be ineffective and costly if the aquifer were deep,
had almost no organic content, was highly oxidized, and the plume was
very large and disperse. In such cases, new methods of installation of deep
biobarriers are being developed, which may be effective. However, extrac-
tion/treatment options continue to be used until these barrier treatments
become more feasible and cost effective.
*   For ex situ treatment, what is the concentration of perchlorate itself? For
groundwater with concentrations of perchlorate below 10 pg/L, the less
selective (and more cost effective) ion exchange resins, or even granular
activated carbon, may be used. For concentrations between 10 and
10,000 pg/L, nitrate- and perchlorate-selective ion exchange resins are
cost-effective. For concentrations over 1,000 gg/L, bioreactors such as
the fluidized bed reactor are cost effective. For each type of treatment
medium, the design must also include consideration of the groundwater
pH, temperature, concentrations of competing anions (chloride, sulfate,
carbonate, nitrate) and concentrations of other materials. For example,
even with low concentrations of perchlorate, if the concentrations of
competing anions are high, then the less selective ion exchange resins
would be ineffective. High concentrations of competing anions do not
make bioreactors ineffective, but they must be designed to meet the
increased requirement for more nutrients and increased hydraulic resi-
dence time for the microorganisms to reduce the anions in addition to the
perchlorate.
* Existing infrastructure should also be considered. For example, is the
perchlorate in a drinking water aquifer, and if so is it in the capture zone of
a public water supply? If the perchlorate is in the drinking water supply, it
may be more cost effective to simply add a treatment component onto the
existing treatment train, since the water is already being pumped out of the
ground.
"* For soil treatment, what are the comparative cost advantages of different
forms of bioremediation, thermal destruction, or disposal? How much can
the soil be disturbed? How quickly does remediation need to occur? Can
the soil conditions support bioremediation (temperature, nutrients, pH), and
what kind of amendments will enhance the degradation process? Are there
any issues with invasive species?
"* What problems might occur as a result of using the process, such as mobi-
lization of other contaminants if the pH is lowered or raised, or toxic by-
products that may be created in the process of biological reduction (such as
chlorate) or thermal destruction (such as dioxins and furans)?
Several of the more well known sites with perchlorate in groundwater have deep
aquifers in sandy soils. In the shallower soils, barrier/treatment trenches may be
effective for the destruction of perchlorate. In deeper aquifers, bioremediation via
injection might be more effective, but if the plume is disperse and the groundwater
movement is too fast, groundwater extraction and treatment may be the only effec-
tive option.
The most common perchlorate treatments currently in use employ ex situ tech-
nologies. Two treatment technologies currently dominate the field of perchlorate
treatment: ion exchange and biological treatment. During the early years of
perchlorate treatment in the 1990s, ion exchange resins were considered the best
method for removing perchlorate from groundwater. However, in 2000, research
and development efforts demonstrated that bioreactors, most notably the fluidized
bed reactors operated under anaerobic conditions, could cost-effectively destroy
high concentrations of perchlorate. Lately, selective ion exchange resins of vari-
ous types have been developed that economically treat a lower concentration of
perchlorate. Ion exchange resins also have the added feature that they have
already been NSF-approved for drinking water treatment. Although operating
experience has shown bioreactors to be effective, anecdotal information suggests
that municipalities often opt for ion exchange resins to avoid the use of microor-
ganisms in the treated water. This anecdotal information is supported in part by
the lack of full-scale implementations of bioreactors in municipal water supply
systems to date [42].
Ion exchange resins are used for treatment of drinking water supplies and for
treatment of groundwater containing 10 pg/L to between 1,000 and 10,000 pg/L
perchlorate. Bioreactors are typically used for treatment of groundwater containing
1,000 to as much as 500,000 pg/L perchlorate. Other ex situ treatment technologies
are still being researched, but to date the treatment costs are either not cost-compet-
itive or have not garnered sufficient regulatory approvals for use compared with ion
exchange resins and bioreactors.
Certain forms of in situ bioremediation are becoming more common, including
injection of amendments and recirculation well systems. The applicability of these
remedial actions depends largely on site conditions, including the permeability of
the soil, depth to groundwater, and lateral extent of contamination. Other biological
treatments such as phytoremediation, will likely be used only in specialized cases.
Research into and application of soil treatment technologies is less common.
Composting has proven effective, as has thermal destruction. However, it is not
likely that more innovative soil treatment technologies will be developed due to the
relatively small number of sites with surface soil contamination.
Table 7.6 summarizes the major features of each of the perchlorate treatment tech-
nologies described in this chapter. For each technology, the table shows the range
of perchlorate concentrations known to have been tested that were successful in
removing or destroying perchlorate, along with advantages and limitations of the
technology and the current status (as of this writing).
Table 7.6 Summary Comparison of Perchlorate Treatment Technologies
Known
Effective
Treatment        Favorable
Technology      Concentrations  Characteristics    Limitations    Current Status
Separation
Ion Exchange      10 to 100,000   NSF-approved     Regeneration      Full scale
Resin             Ag/L (depending  Can be used in  creates waste    implementation
on the type of  GAC-type         brine that may be  at many sites
resin)          treatment vessels,  costly to treat or
dispose of.
Disposal of non-
regenerable resins
is costly.
Granular          1 to 10 pg/L    NSF-approved,    Ineffective at    Full scale
Activated Carbon                   removes other   perchlorate      implementation
inorganic and    concentrations   at one or two
organic          higher than 100  sites
compounds.       ug/L.
Traditional
treatment, can be
used in
emergency
situations.
Carbon can be
regenerated.
Cationic          1 to 1,000 pg/L  Can be used in  Not NSF-          Pilot scale at
Substance Coated                  GAC-type         approved. No      one or more
Media                             treatment vessels,  large-scale    sites.
manufacturing
facilities in place.



Library of Congress subject headings for this publication:
Perchlorates -- Environmental aspects,
Water -- Purification -- Perchlorate removal,
Soil pollution,
Soil remediation,