Visual display of the Renovation of pesticide contaminated rinse waters

				


DISCUSSION


For both reactors tested, the atrazine data appear to fit a first-order model,
but
the flat-plate reactor is a closer approximation of the safer, cheaper kind
of low light
intensity (1-10 x 10-9 einsteins/sec cm2) system that will be practical for
scale-up.
Using the rate equation determined by linear regression on the logarithmically
transformed data, the half-life for atrazine in this system is approximately
4000
minutes. To lower an initial 10 ppm concentration of atrazine to below 10
ppb,
degradation must proceed for 10 half-lives. However, utilizing an annular
design of
1.8 cm diameter x 1 m long with a standard 40W fluorescent black light will
increase
the light intensity by a factor of six and the illuminated surface area by
a factor of 45.
With such a reactor, the t1/2 for atrazine would be approximately 15 minutes.
With
a starting concentration of 10 ppm, about 2.5 hr would be required to bring
the
atrazine concentration down to 1 0 ppb, and could continuously process liquid
at a rate
of 1 mL/min.  An additional 50-60 minutes would be required to reduce the
concentration to less than 1 ppb, for a flow rate of 0.75 mL/min. Electric
power
costs (at 60/kWh) work out to $1.60/gallon or $88 to treat the liquid in
a 55 gallon
drum to the 1 0 ppb level. Formic acid degradation did not appear to follow
first-order
kinetics, but the rate was about 49 times faster than the zero-time rate
for atrazine.
It is expected that the degradation rates of most compounds will fall within
these
bounds. Use of this technology in treating drinking water contaminated with
pesticides might not be accepted by the public unless the levels could be
decreased
to 10% of Wisconsin's Preventative Action Limit (PAL). For atrazine, the
PAL is 0.3
ppb and therefore 0.03 ppb is likely to be an acceptable level.  For a well
contaminated at the enforcement standard (3 ppb), about seven half-lives
would be
needed.
Many studies have shown the diminished phytotoxicity of the dechlorinated,
hydroxy-analogs of atrazine, and in the standard setting process in Wisconsin
that
only chlorinated metabolites are included in the state standards. Thus the
assertion
appears to have been accepted that hydrolysis reduces the human toxicity
of atrazine


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and its chlorine-containing metabolites.
These calculations suggest that photocatalytic degradation is a relatively
slow
process, but faster degradation is expected with the titania pellets. Improvements
in
catalyst efficiency and optimization of reactor design promise to increase
the rate
significantly. Earlier studies in this laboratory indicate that improvements
in quantum
efficiency may be obtained with platinum doping. A pellet-filled reactor
similar to that
described above would be inexpensive to build and operate.
Current work is now focusing on degradation of mixed pesticide rinsates
collected from a commercial applicator in Wisconsin and of other pure pesticides.
Experiments are being designed to determine rate equation parameters for
compounds
of interest and for mixed wastes. The use of hydrogen peroxide and other
electron
acceptors in these systems is also of interest.


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