The promise offered by semiconductor-sensitized photocatalytic detoxification of aquatic environments has been demonstrated with photodegradation of various pollutants including halogenated organics, nitroaromatic compounds and dyestuffs [1], as well as microorganisms [2]. As the photocatalysts, the dispersed titanium dioxide and microheterostructures comprising two photoactive semiconductors as well as sorbent and semiconductor (e.g., TiO₂/SiO₂) or magnetic core and titania shell [3] have proved to be particularly advantageous for water treatment. Further advancement of photocatalytic systems permitting to enhance the photogeneration efficiency and to facilitate the separation of dispersed photocatalyst from the treated solution can be achieved by performing the photocatalytic reaction in the foam formed by bubbling air or another gas (oxygen, chlorine, ozone, etc.) through the aqueous solution containing a small amount of surfactant; in some cases the role of foam-producing agent can be played by amphiphilic pollutants. The proposed approach makes possible (i) to use the detergent properties of pollutants, (ii) to lift the diffusion limitation for the paticipation of gaseous compounds in the photocatalytic process, and (iii) to concentrate both pollutant and colloidal photocatalyst in the active zone of photoreactor .

As the foaming agents, the photochemically-stable alkylsulphates C_nH_2n+1CH(CH₃)₃OSO₃Na (n = 6¸ 16) were used. The concentrated foam consisting of polyhedral gas compartments was generated by forcing the air through a suitable designed nozzle (Fig. 1) and then through the aqueous solution containing surfactant (0.2%) and TiO₂ Hombikat UV-100 (0.5%). The photodegradation of model pollutants (salicylic acid, dichloracetic acid, 2-chlorophenol, SO₂), dyes (Rhodamine 6G, Chrome Deep Blue, Eosine, Methylene Blue, Thionine), microorganisms (Escherichia coli cells) was investigated employing the 120W medium-pressure Hg lamp as a light source.

The typical photodegradation curves shown in Fig. 2 evidence that the rate of photolysis drastically enhances in presence of dispersed TiO₂ entrapped in the foam. Of particular importance is the fact that concentration of both dispersed titania and pollutant (organics, microorganisms) occurs in the foam (Table). The foam medium also facilitates the adsorption of pollutants on the surface of photocatalyst as is clearly exemplified by adsorption isotherms obtained in aqueous solution and in foam.

The performed investigations evidence that the employment of concentrated foams as an environment for photocatalytic processes opens up fresh opportunities for increasing the efficiency and the degree of sophistication of photochemical water treatment systems simultaneously extending the area of possible application of photo-assisted detoxification technologies.

Figure 1 Photoreactor construction: (1) compressor, (2) foam generator, (3) aqueous solution containing TiO2 and surfactant, (4) quarts tube, (5) photoreactor, (6) foam, (7) UV lamp.

Figure 2 Destruction efficiency for (1,1') Thionine, (2,2') Chrome Deep Blue, (3,3') Methylorange, (4,4') Eosine during photocatalytic oxidation in (1'-4') bare; (1-4) in TiO2-loaded concentrated foam.

Figure 3 Absorption isotherms of salicylic acid on Hombikat UV100 (2) in aqueous solution; (1) in foam.

References

[1] M. Hoffmann, S.Martin, Wonyong Choi, D. Bahnemann. Chem. Rev. 1995, 95, p.69-96.
[2] Chang Wei, K. Rajeshwar et al. Environmental Sci&Technol, 1994, 28, No5, p.934-938.
[3] D. Shchukin, D. Sviridov, A. Kulak. Int. J. Photoenergy, 1999, 1, No 1, p. 65-68.

[List of Contributions]

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emails: kulak@igic.bas-net.by , < a href="mailto:sviridov@chem.bsu.unibel.by">sviridov@chem.bsu.unibel.by

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Last updated April 26, 2000