Direct Chemical Oxidation
There is a concern that when using traditional incineration to destroy hazardous wastes ex situ, stack emissions could result in significant releases of harmful contaminants. This is especially true when dealing with mixed wastes (i.e., a combination of organic contaminants mixed with radioactive contaminants). One alternative to incineration is Direct Chemical Oxidation (DCO). Still under development, DCO is a non-thermal, near ambient (atmospheric) pressure, aqueous-based technology that destroys organic contaminants in hazardous or mixed wastes. DCO uses solutions of the peroxydisulfate ion, the strongest known chemical oxidant other than fluorine-based chemicals, to convert organic solids and liquids into benign carbon dioxide, water, and constituent minerals.
Peroxydisulfate has also been used alone or with a catalyst in decontamination and etching solutions to remove dissolved plutonium oxide (PuO2) from nuclear equipment. The expended oxidant may be regenerated to minimize secondary wastes. Off-gas volumes are minimal, allowing retention of volatile or radioactive components in the process fluid.
The DCO process is attractive when a small amount of organic material must be removed from a large amount of an inert solid matrix, such as sludge, soil, sand, or filter material. DCO can treat a wide variety of organic wastes (liquids and solids; water-soluble or not) and waste matrices (soils, sands, clays; ceramic substrates; steel machinery, etc.) contaminated with organic constituents. The rate of the oxidation reaction can be selected (through concentrations and temperature) to provide either complete destruction of organic substrates, or merely decontamination and etching of metal, ceramic or plastic debris. DCO is also well suited for certain types of decontamination, using peroxydisulfate alone or with another chemical oxidant.
Limitations and Concerns
Wastes containing particulate radionuclides, mercury, and certain volatile constituents, may result in significant off-gas treatment challenges.
Destruction of some organic solids, such as paper, cloth, and styrene resins, is possible, and other plastics and inorganic debris will be partially oxidized and decontaminated. However, reactions with polyethylene and PVC are slow, so surface oxidation to decontaminate rather than destroy the matrix is a more practical goal.
Applications to large quantities of bulk organic matter or combustible debris will require large amounts of oxidant, which will contribute to secondary wastes or require significant recycling.
Though peroxydisulfate readily destroys dilute organic contamination in aqueous solutions, the water present also dilutes the oxidant, making recycling more difficult.
Wastes containing finely divided aluminum or iron powders can be oxidized so rapidly that unsafe conditions can occur.
Though the process is amenable to oxidant regeneration and reuse, the cost effectiveness of recovery must be evaluated based upon the waste to be treated.
Because the reactions take place at low temperature and in a liquid state, the times required for the reactions are much longer than for thermal systems, and typically, more secondary waste is generated by the oxidizing agents.
A radioactive material license from the Nuclear Regulatory Commission (NRC) or its applicable agreement state is required to treat mixed wastes.
Because the primary process hazard associated with the DCO process arises from the contact between the oxidant and reducible materials, potential end users must take care to store and handle the oxidant appropriately, using the proper personal protective equipment and procedures.
Highly reactive reducing agents such as aluminum, iron and zinc powders, metallic alkali metals, lithium hydride, and sodium oxide should be removed from DCO feeds.
Ceramic, earthenware, or glass-lined vessels should be used for the oxidation reactors.
The process is designed for re-oxidation of the oxidant by electrolysis for recycling, but this system should be evaluated for cost-effectiveness based on the wastes to be treated and the potential complexity of recovery of the oxidant from the process residuals.
This ex-situ process is being developed for the treatment of liquids and solids contaminated with organic contaminants. It is being developed at the Department of Energy (DOE) for the treatment of mixed radioactive wastes. The DCO process is best suited to destroy organic contaminants dispersed in inert matrixes, such as soil or water, which are not efficiently treated by incineration. The DCO process may also be an excellent choice for decontaminating debris. Systems have also been used to remove SVOCs and trinitrotoluene (TNT), and they have been tested on chemical warfare agents.
Technology Development Status
DCO was tested at pilot scale in 1998. To the best of our knowledge, there have been no other treatability studies. Although DOE states that it can be scaled up directly for deployment, it will be necessary to better define ancillary system requirements. Patents are pending for important applications of the DCO technology.
Other Resources and Demonstrations
See Report of the Secretary of Energy Advisory BoardÕs Panel on Emerging Technological Alternatives to Incineration, December 2000, Secretary of Energy Advisory Board, U.S. Department of Energy.
beginning in 1992, the DCO process was developed for applications in
waste treatment, chemical warfare agent demilitarization and
and environmental remediation by the Lawrence Livermore National
(LLNL). The integrated DCO process was demonstrated at pilot-plant
LLNL waste streams or surrogates containing chlorinated solvents. A
spectrum of materials has been successfully oxidized using
including: acetic acid, ethylene glycol, tributyl phosphate, kerosene,
(TNT) and other explosives, surrogates of biological or
warfare agents, paper and cotton, PCBs,
pentachlorophenol, and ion exchange
for a technical description of explosives in different media and the
use of some analytical techniques.