21    MODELING UV PHOTOCATALYSIS OF ORGANIC
CONTAMINANTS ON TITANIUM DIOXIDE
Nan-Miii Wu, Ph.D. Candidate
H. Ted Chang, Professor
Pritzker Department of Environmental Engineering
Illinois Institute of Technology
Chicago, Illinois 60616-3793
INTRODUCTION
Photocatalytic degradation using UV activated titanium dioxide (Ti02) is one of the key technologies for the cleanup of hazardous wastes. The photocatalytic process has several advantages
including: (1) capable of degrading a wide variety of contaminants; (2) destruct contaminants to
innocuous end products; (3) immune to contaminants toxicity; (4) fast reaction rate; (5) ease of
process. The principle of photocatalytic degradation is based on the activation of Ti02 surface
using near ultraviolet (UV) light (1 < 365 nm). When Ti02 particles are illuminated by UV, electron and hole pairs are generated. In aqueous solution, the holes can be scavenged by hydroxide
ions (OH") to form hydroxyl free radical ("OH). The free radicals are strong oxidizing agent capable of triggering the degradation of organic and inorganic contaminants.1 Previous studies
have shown that many hazardous organic compounds including halogenated organic compounds
(HOCs) can be mineralized to innocuous end products.2-3
In general, there are two types of Ti02 commercially available—porous and nonporous. Several studies employed nonporous Ti02 to investigate the feasibility and chemistry of photocatalytic reactions.4-5 Langmuir isotherm was found adequate to describe the experimental data in
these studies. This result was due in part to the nonporous structure of the Ti02 used which limited the reaction on the Ti02/liquid surface (i.e., geometric surface of the Ti02). The result also
indicated that adsorption of the organic contaminants may be the rate-limiting step in the reaction.
In another application of Ti02 semiconductor, porous Ti02 was used to produce hydrogen
from water.6-7 The advantage of porous Ti02 was the high pore surface area which provided a
higher photocatalytic reaction rate than nonporous Ti02. The porous structure of the Ti02 was
examined using electron microscope. The higher hydrogen production rate was attributed to the
high pore surface area of the porous Ti02.
In this research, porous TiOz is used in the photocatalytic oxidation of organic contaminants. It
is hypothesized that the high porous surface area of the Ti02 can adsorb a large amount of the
contaminants. The porous structure also allows UV light to penetrate the TiOz particle. As a result, the degradation rate for organic contaminants can be increased. In this paper, a mechanistic
model incorporating key reaction and transfer mechanisms is presented. The concept and the approach to the model are discussed first. The model features and the solution techniques are then
given. To conclude this paper, simulations using hypothetical values for model parameters are
presented to demonstrate the features of the model.
MODEL DEVELOPMENT
The concept for photocatalytic reaction in porous Ti02 particle is presented in Figure 1. The
sequence of events for organic degradation photocatalytically can be described as follows.
1. Organic molecules move from bulk liquid to near the surface of the Ti02 particle.
2. Organic molecules are transported through the stagnant liquid film.
50th Purdue Industrial Waste Conference Proceedings. 1995, Ann Arbor Press, Inc.. Chelsea, Michigan 48118.
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