Introduction

Nature has many built in checks and balances to maintain a healthy environment. But the industrial growth of the past two centuries has been adversely affecting this balance. Thus, there is an urgent need to regulate our environment-polluting activities before irreversible damage is done to the environment.

The generation of wastes (hazardous and nonhazardous) is inevitable in our technological society. Until very recently, no serious thought was given to the proper management of wastes from the manufacturing sector. Large-scale landfill disposal or ocean dumping are two examples of the previous improper management of hazardous wastes. These methods do not remove the pollutants from the environment but only delay their catastrophic effects. Pragmatic wisdom dictates the necessity of using waste disposal methods in which the pollutants are degraded into environmentally benign species prior to their discharge into the environment. Increased public awareness of the consequences of improper disposal of hazardous materials has placed the responsibility for safe disposal of such wastes squarely on the industries that produce them.

An ideal waste treatment process will completely mineralize all the toxic species present in the waste stream without leaving behind any hazardous residues. It should also be cost-effective. None of the treatment technologies at present approach this ideal situation. There are a number of waste disposal methods currently in practice with varying degrees of success. Figure 1 is a schematic representation of different treatment technologies either currently available or in varied stages of development. At present, the disposal of the bulk of the industrial wastes is based on the processes developed on phase-separation principles [1-3], even though none of them is completely satisfactory. The incineration of organic wastes is the other widely practiced method. This in principle should destroy the toxic pollut-

*Current affiliation: Lamar University, Beaumont, Texas

Figure 1 Treatment methods for hazardous wastes.

ant completely, but the incineration of many hazardous organic wastes releases other toxic species into the air [4]. An example of this is the incineration of PCBs, which has resulted in the release of dioxins that are as toxic as the PCBs themselves. Incineration as practiced today is thus also not an ideal waste treatment process. Biotreatment of municipal wastes has been practiced, but similar biotreatment of industrial wastes are not common methods of waste management.

Photochemical degradation of wastes containing toxic species has many attractive features. The process does not require exogenous chemical additions, thus eliminating the consequence of residual chemical species in the effluents. Ultraviolet irradiation as an alternative to chlorination for the disinfection of drinking water is probably the best example of a photochemical detoxification process. Though there have been some attempts to treat toxic wastes with direct UV irradiation [5,6], their success has not been such that large-scale commercial waste treatment facilities using direct UV light irradiation have been built. One reason for this may be the absence of a fully developed photochemical treatment process for the management of large volumes of hazardous industrial wastes. The other reason may be the incommensurate cost of the direct photochemical degradation processes. If the process can be made less expensive and more specific, then toxic wastes could be processed through photochemical degradation.

Photocatalytic degradation of wastes rather than direct photochemical decomposition shows promise. Recently, a number of investigators have reported the degradation of hazardous chemical species in aqueous medium using microheterogeneous media as a photocatalytic agent rather than direct photolysis [7-12], The potential for the development of waste treatment processes based on photocatalytic reactions in microheterogeneous media appears to be very promising. Here, we describe the use of two such microheterogeneous media, namely, colloidal-semiconductor sols and micellar systems, as photocatalytic agents.

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