Applications of Chemical Kinetics in Environmental Systems - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 311

TABLE 6.3
Photodegradation of Organics by TiO 2 /UV Process
Phenol, TOC reduced by 35% in 90 min
Acetic, benzoic, formic acids, ethanol, methanol, TOC reduced by > 96% in 10 min
Degradation of aniline, salicylic acid, and ethanol
1,2-Dimethyl-3-nitrobenzene and nitro- o -xylene from industrial wastewater, > 95%
TOC removed in 30 min
Degradation of pentachlorophenol
Degradation of other organic compounds (trichloroethylene, chlorobenzene,
nitrobenzene, chlorophenols, phenols, benzene, chloroform)
Degradation of a pesticide, atrazine

Source: From Legrini, O., Oliveros, E., and Braun,A.M. 1993. Chemical Reviews ,
93, 671-698.

of organic compounds by TiO 2 in the presence of H 2 O 2 is significantly affected by
pH and solution ionic strength.
The large-scale development of semiconductor-promoted reactions awaits opti-
mal design of the photoreactor, photocatalyst, and radiation wavelengths. The critical
issue in this regard is a suitable design for the treatment of a sample that can simul-
taneously absorb and scatter radiation. Work in this area appears to be promising and
is summarized in Table 6.3.

6.2.2.4 Photochemical Reactions in Natural Waters

Natural waters are a mixture of several compounds, and in most cases transient radi-
calsorspeciesareproducedbyphotochemicalreactions.Zepp(1992)hassummarized
a variety of transient species. Their effects on photochemical reactions are such that
the rates of reactions can vary by orders of magnitude depending on the type and con-
centration of the transient. DOCs(humic and fulvic compounds), inorganic chemicals
(nitrate, nitrite, peroxides, iron, and manganese), and particulates (sediments, biota,
and minerals) can produce transients in natural waters upon irradiation.The transients
participate in and facilitate the redox reactions of other compounds in natural waters.
There are a variety of transients that have been identified in natural waters. Some
of the important ones are (i) solvated electron, (ii) triplet and singlet oxygen, (iii)
superoxide ions and hydrogen peroxide, (iv) hydroxyl radicals, and (v) triplet excited
state of dissolved organic matter. The level of steady-state concentrations of some
of these species and their typical half-lives in natural waters are given in Table 6.4.
Solvated electron (e aq ) is a powerful oxidant, observed in natural waters during irradi-
ation. It reacts rapidly with electronegative compounds (both organic and inorganic).
Its reaction with O 2 is the primary pathway for the production of superoxide anions
in natural waters. The major source of e aq is aquatic humic compounds. The quantum
yield for their production is approximately 10 − 5 . Dissolved organic matter in natural
waters is also known to absorb photons to generate singlet and triplet excited states.
These then decay by transferring energy to dissolved oxygen to produce singlet oxy-
gen. Singlet oxygen is an effective oxidant and the quantum yield for its formation is