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Case Study in Environmental Chemistry....
Case Study 3: Photolysis of Terbufos
Authors: Sarunya Hengpraprom and Cindy Lee, Environmental Engineering and Science, Clemson University.
Abstract : This case study considers the rate at which exposure to sunlight can transform a pesticide known as terbufos. Transformation of a contaminant by photolysis will change its behavior in a given situation. In some cases, a product of photolysis is just as toxic or more toxic than the parent compound. In other cases, the products of photolysis are rendered nontoxic and the transformation can be considered beneficial. The experiment described in this case study produced some of the first basic kinetics data available to evaluate the importance of photolysis as a process to remove terbufos from the environment. For more detailed information about this research, see Lee, C. M.; Anderson, B.; and Elzerman, A. W. 1999. Photochemical oxidation of terbufos. Environmental Toxicology and Chemistry. 18(7):1349-1353.
For further detail choose the appropriate section
Kinetics of Photochemistry
Both direct and indirect photochemistry processes can be defined as a first-order reaction with respect to the disappearance of the compound. On this basis, the overall photolysis rate for a given compounds is expressed as a summation of direct and indirect photolysis rates (14):
-d[C]/dt = kp[C] = {kd + ks} [C] Equation 3
where [C] is the molar concentration of compound of interest,
kp is the overall photolysis rate constant (time-1),
kd is the direct photolysis rate constant(time-1), and
ks is the indirect photolysis rate constant(time-1).
The rate of direct photolysis can further be expressed in terms of the rate of light absorption by the compound and the efficiency of the chemical transformation process (14):
kd = fka Equation 4
where f is the efficiency of the chemical transformation (commonly called the quantum yield) (unitless), and
ka represents the light absorption rate of the compound (time-1).
The quantum yield (f) estimates can be determined through measurements using a reference compound. The quantum yield can be calculated from the results of the experiment by:
f = (kp/kr) x [(åelrLlr)/(åelLl)] x fr Equation 5
where kp is the overall photolysis rate constant (time-1),
kr is the reference photolysis rate constant (time-1),
er is the reference molar absorption coefficient,
e is the molar absorption coefficient,
fr is the reference quantum yield, and
Ll is the solar irradiance applicable to shallow depths in a water body (millieinsteins/cm2d).
The light absorption rate (ka) is a function of both the absorbance of the compound and the intensity of the incident light. The ka is generally defined as:
ka = å kal = 2.3 å (elEol) Equation 6
where Eol is irradiance (photon/cm2s),
el is scalar molar absorptivity (L/mole cm).
The rate of indirect photolysis is more complicated than that of direct photolysis, since it involves the presence of photosensitizer. However, Zepp (14) expressed the rate of indirect photolysis at low concentration of compound and near surface conditions as below:
ks = 2.3 å (QlelEol)[S] Equation 7
where Ql is the proportionality constant between photosensitizer quantum yield and compound concentration [C],
[S] is the photosensitizer concentration (mole/L).
The corresponding half-life, t 1/2 (time-1) for the first-order photolysis reaction (either direct or indirect) can be presented as:
t 1/2 = 0.693/kp Equation 8