Direct
Chemical Oxidation
Description
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.
Applicability
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.
Web
Links
http://www.osti.gov/bridge/servlets/purl/16711-dIObiK/native/
http://www.p2pays.org/ref/24/23814.pdf
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.
Also,
beginning in 1992, the DCO process was developed for applications in
mixed
waste treatment, chemical warfare agent demilitarization and
decontamination,
and environmental remediation by the Lawrence Livermore National
Laboratory
(LLNL). The integrated DCO process was demonstrated at pilot-plant
scale using
LLNL waste streams or surrogates containing chlorinated solvents. A
broad
spectrum of materials has been successfully oxidized using
peroxydisulfate,
including: acetic acid, ethylene glycol, tributyl phosphate, kerosene,
methyl
chloroform, trinitrotoluene
(TNT) and other explosives, surrogates of biological or
chemical
warfare agents, paper and cotton, PCBs,
pentachlorophenol, and ion exchange
resins.
See http://clu-in.org/characterization/technologies/exp.cfm#86
for a technical description of explosives in different media and the
use of some analytical techniques.