Photocatalytic water treatment: solar energy applications
Introduction
Persistent organic chemicals are present as pollutants in wastewater effluent from industrial manufacturers and normal households, and in landfill leachates. They can be found in ground water wells and surface waters. In all cases they have to be removed to protect our water resources or to achieve drinking water quality. Therefore, many processes have been proposed over the years and are currently being employed to destroy these toxins. The so-called photocatalytic detoxification has been discussed as an alternative method for clean-up polluted water in the scientific literature since 1976 (Carey et al., 1976). Considerable public attention has been focused on this possibility of combining heterogeneous catalysis with solar technologies to achieve the mineralization of toxins present in water (e.g., Hecht, 1990). Compilations of substances which can mineralized using photocatalysis are available (Legrini et al., 1993, Blake, 1994). Several reviews have been published discussing the underlying reaction mechanisms and illustrating examples of successful laboratory and field studies (Kamat, 1993, Fox and Dulay, 1993, Herrmann et al., 1993, Bahnemann et al., 1994, Serpone and Pelizzetti, 1989, Pelizzetti and Schiavello, 1991, Ollis and Al-Ekabi, 1993, Hoffmann et al., 1995). While the overall stoichiometry of most mineralizations appears to be understood details of the complex reaction mechanism are, however, still only known in a few selected cases.
Section snippets
Initial processes
Semiconductors (e.g., TiO2, ZnO, Fe2O3, CdS and ZnS) can act as sensitizers for light-induced redox processes due to their electronic structure which is characterized by a filled valence band and an empty conduction band. Absorption of a photon of energy greater than the bandgap energy leads to the formation of an electron/hole pair. In the absence of suitable scavengers, the stored energy is dissipated within a few nanoseconds by recombination (Rothenberger et al., 1985). If a suitable
Reactor types
The artificial generation of photons required for the detoxification of polluted water is the most important source of costs during the operating of photocatalytic wastewater treatment plants. This suggests to use the sun as an economically and ecologically sensible light source. With a typical UV-flux near the surface of the earth of 20–30 W m−2 the sun puts 0.2–0.3 mol photons m−2 h−1 in the 300–400 nm range at the process disposal (Bahnemann, 1994). Principally these photons are suitable for
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