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18.104.22.168 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 . 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 . 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 . 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.