Case Study in Environmental Chemistry....

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.

Principles of Photochemistry

A simplified conceptual model of environmental photochemistry would include three components: 1) light source, 2) compound of interest, and 3) other active system components termed photosensitizers (11). Light possesses both a particle and wave characteristic. As a wave, light is a combination of oscillating electric and magnetic fields perpendicular to each other and to the direction of the propagation of the wave. The distance between two consecutive maxima is the wavelength (l ). The wavelength is inversely proportional to the frequency, n , which is commonly expressed by the number of complete cycles passing a fixed point in 1 second (12):

l = C/n (Equation 1)

where C is the speed of light in a vacuum, 3.0 x 10⁸ ms^-1. The unit of wavelength is nanometer (nm).

The particle form of light is termed a photon. The photon can be envisioned as a reactant in a chemical reaction (11). The energy (E) of each photon is related to frequency n and the wavelength l of the light as expressed by (12):

E = hn = hc / l (Equation 2)

where h is the Planck constant, 6.63 x 10^-34 J.s.

As can be seen in equation 2, the shorter the wavelength of the light, the greater the energy it transfers to a molecule when absorbed. Note that the energy of photon is dependent on its wavelength. The wavelength of sunlight falls in the range of 280-750 nm, which is in the region of ultraviolet light, visible light, and infrared light. Ultraviolet light is high in energy content, visible light is of intermediate energy, and infrared energy is low in energy (12).

Once molecules absorb the light (usually in the ultraviolet, visible, or infrared region), they immediately undergo a change in the organization of their electrons. The electrons can move from a ground state (most stable level) to an excited state (less stable), where bond-breaking or bond-making process occurs. However, molecules do not generally remain in the excited state, and therefore do not retain the excess energy provided by the photon for very long (12). Within a tiny fraction of a second, they must either use the energy to react photochemically or return to their ground state either by emitting a photon or by converting the excess energy into heat that is quickly shared among several neighboring molecule as a result of collisions. Thus, molecules normally cannot accumulate energy from several photons until they receive sufficient energy to react (12). All the excess energy required to drive a reaction usually must come from a single photon. Therefore, for a chemical reaction to occur energy from the light source must have high enough energy to initiate the reaction. The phenomenon in which energy from the light source initiates the reaction is called a photochemical reaction (14). The photochemical reaction may take place either directly or indirectly. In direct photolysis, the organic molecule itself can absorb sufficient light energy to initiate the transformation. Indirect photolysis, on the other hand, involves light energy transfer through an intermediate, the photosensitizer (13).