36    SENSITIZED PHOTOOXIDATION OF BROMACIL: A DYE
SURVEY OF POTENTIAL PHOTOSENSITIZING
AGENTS
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 occurrence in natural waters presents a serious problem to public health and safe
drinking water. The toxicity and bioaccumulation in the environment of these compounds merit
public concern, and their removal prior to reaching natural waters is of the utmost importance.
Bromacil is one of the most important herbicides for non-cropland and citrus control of grasses and
weeds [1]. It is a potent and specific inhibitor of photosynthesis [2,3] and is slightly toxic and
refractory. The 48 and 72-hr LC50 for rainbow trout are 71 and 28 mg/L, respectively. The half life in
silt loam soils is approximately 5 to 6 months [4]. Losses from soil due to volatilization and phot ode-
composition are negligible [5]. Losses from water due to photodecomposition and volatilization are
also negligible [6].
Photodecomposition of bromacil in sunlight is negligible because bromacil does not absorb light in
the visible spectral region [7]. Furthermore, photolytic cleavage by visible light (e.g., at 600 nm, E =
45 kcal) occurs only on weak covalent bonds [8]. The energy necessary to break chemical bonds
between C-C or C-O amounts to 80 and 88 kcal/mole, respectively, and only ultraviolet radiation with
energy higher than these values (lambda < 325 nm) can disrupt these bonds in a photolytic reaction.
Solar ultraviolet radiation below 270 nm is absorbed by ozone and air and does not reach the earth's
surface. For photochemical reactions, it is usually visible light between 400 and 700 nm and ultraviolet
radiation between 390 and 350 nm which are active.
Photochemical activity with visible light is associated with colored substances (e.g., dyes). Dye
photosensitizers absorb energy in the visible spectral region, and transfer the energy either directly or
indirectly to a substrate, which results in the photodecomposition of the substrate. Acher and Saltz-
man [7], Watts [9], Eisenberg et al. [10] and others have shown that addition of dye photosensitizers
to aerated aqueous solutions of bromacil leads to a quantitative and fast sunlight photochemical
reaction. The objective of this study was to evaluate 69 different dyes, stains, and indicators at acidic,
neutral, and alkaline pH values for the degradation of bromacil.
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 [11]. 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 phosphorescence, 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 occurring 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
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