53    SENSITIZED PHOTOOXIDATION OF BROMACIL: PILOT,
BENCH, AND LABORATORY SCALE STUDIES
Talbert N. Eisenberg, Graduate Instructor
E. Joe Middlebrooks, Provost & Vice President for Academic Affairs
V. Dean Adams, Director
Center for the Management, Utilization,
and Protection of Water Resources
Tennessee Technological University
Cookeville, Tennessee 38505
INTRODUCTION
The fate of herbicides, pesticides, and refractory organics in the environment is of great interest and
importance. Their occurence in natural waters presents a serious problem to public health and safe
drinking water. Their toxicity and bioaccumulation in the environment merit public concern, and their
removal prior to reaching natural waters is of the utmost importance. Bromacil, one of the most
important herbicides for non-cropland and citrus control of grasses and weeds, is a potent and
specific inhibitor of photosynthesis and is slightly toxic and refractory.1'23 The half life in silt loam
soils is approximately 5 to 6 months.4 Losses from soils and water due to photodecomposition and
volatilization are negligible.5.*
Sensitized photooxidation of bromacil has proved to be effective in small scale laboratory stud-
ies;7'8'9 however, the process has not been evaluated in a full scale system. The objective of this study
was to develop information to design a full scale sensitized photooxidation system. The specific
objectives were to: determine the feasibility of sensitized photooxidation for bromacil detoxification
in a full scale field situation; determine the effects of mixing, sunlight intensity, pH value, reactor
depth, sensitizer and sensitizer concentration, initial substrate concentration, dissolved oxygen concentration, water and air temprerature, suspended solids and turbidity, and hydraulic detention time
on detoxification efficiency; screen a variety of dyes and stains for potential sensitizers and determine
the effect of pH value on the reaction rate for the sensitizers; and develop mathematical models to
predict and optimize detoxification efficiencies.
THEORY
With the absorption of light, a molecule rises from its ground state of lowest energy to an excited
state of higher energy in which one of the electrons is at a higher energy level.10 The activated
molecule expends the energy of excitation in one of several ways. The molecule may either emit
radiation in the form of fluorescence or phosporescence, lose its energy as heat by collision with other
molecules, dissociate, or take part in chemical reactions. In many chemical reactions, the photosensitizing action of dyes is responsible for key life processes and naturally occuring chemical reactions.
The photosensitizing action of dyes results from the ability of dyes to act either as strong oxidizing
or as strong reducing agents, in the presence of reducing or oxidizing substances, with subsequent
regeneration.8 In certain reactions, dyes are predestined sensitizers because the most reactive triplet
state 3D is produced in dyes with high efficiency by intersystem crossing from the first excited singlet
state 'D. 3D is reactive not only in redox reactions, but 3D can also transfer the energy of the triplet
state to other molecules and initiate specific reactions. In dye-sensitized photooxidations, either an
oxygen transfer process or a hydrogen-abstraction reaction is involved. In the oxygen transfer process, the excitation of the dye to the singlet state is followed by intersystem crossing to the triplet state.
Kautsky, Egerton, and others11 have shown that photooxidation can occur in systems with separated
dyes and substrates, indicating the production of an oxidizing volatile species. This species may be
either hydrogen peroxide (in the presence of water) or a semireduced oxygen molecule 02■- which can
act as an intermediate in the production of hydrogen peroxide. The reaction between oxygen and dye
leads to a semioxidized dye radical which is rereduced to the ground state by the oxidizable reactant.
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