TY - CHAP

T1 - Photocatalytic detoxification of polluted aquifers: Novel catalysts and solar applications

AU - Bahnemann, D.W.

AU - Bockelmann, D.

AU - Goslich, R.

AU - Hilgendorff, M.

N1 - Publisher Copyright: © 1994 by CRC Press, Inc. Copyright: Copyright 2019 Elsevier B.V., All rights reserved.

PY - 1994

Y1 - 1994

N2 - Persistent organic chemicals are present as pollutants in wastewater effluents from industrial manufacturers, dry cleaning facilities, or even normal households. They can be found in groundwater wells and surface waters where they have to be removed to achieve drinking water quality. 1,2 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 cleanup of polluted water in the scientific literature since 1976. 3 Lately, 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. 4-7 Several reviews have recently been published discussing the underlying reaction mechanisms of photocatalytic detoxification and illustrating examples of successful laboratory and field studies. 8-10 While the overall stoichiometry of most mineralizations appears to be understood, 11 details of the complex reaction mechanism are still not known. Anatase, titanium dioxide, the material with the highest photocatalytic detoxification efficiency, is a wide bandgap semiconductor (E g ≈ 3.2 eV). 12 Thus, only light below 400 nm is absorbed and capable of forming the e −/h + pairs 13 which are a prerequisite for the process. Therefore, only the ultraviolet (UV) part, i.e., 5% of the solar energy reaching the surface of the Earth, could be utilized, in principle, when 350TiO 2 is the photocatalyst. Hence, it is evident that for solar applications other materials have to be found or developed that exhibit similar efficiencies as anatase TiO 2, but possess spectral properties more closely adapted to the terrestrial solar spectrum. For a solar application of photocatalytic detoxification, it is essential that the incident sunlight is effectively utilized. Therefore, parts of the investigations presented in this chapter concentrate on the synthesis and characterization of photocatalysts that absorb in the visible part of the solar spectrum and simultaneously improve the photocatalytic detoxification properties in this spectral region. The absorption of photons by semiconductors leads to the formation of an equal number of positive and negative charge carriers (e −/h + pairs). While the fate of the hole which induces the desired oxidation process has been studied in detail, most authors did not examine the role of the cathodic process, i.e., the reactions of. It is generally assumed that molecular oxygen acts as the oxidant. 8-10 However, hydrogen peroxide (H 2O 2), which should be formed during O 2 reduction, is found only in trace amounts when TiO 2 is used as the photocatalyst. 14 Further reduction of H 2O 2 leads to the formation of OH radicals. In fact, it has been shown that the rate of photodegradation can be considerably enhanced when H 2O 2 is used as the oxidant. 15 Separation of the anodic and cathodic process, in principle, is not possible in microheterogeneous photocatalytic systems containing semiconductor particles. Hence, it cannot be decided whether hydroxyl radicals are formed via the oxidation of water or the reduction of molecular oxygen, i.e., whether electrons or holes are more important for the initial step of pollutant degradation which is generally believed to be the reaction of OH with the substrate molecule S. Since the efficiency of a complex process is always limited by the slowest reaction step, it is necessary to distinguish between the two possibilities discussed above and to study them separately. In the following, we will therefore also present evidence from photoelectrochemical investigations with separated anode and cathode which have been carried out to further elucidate the underlying reaction mechanisms. Various reactor designs have been tested in laboratory studies where chemical engineering problems characteristic for the different reactor types, such as mass-transfer limitations, have been exploited in detail. 16,17 Here, we will show results obtained with two different solar detoxification reactors at test sites in Almeria, Spain and Campinas, Brazil. Many of the commercially used reactors for photocatalytic cleanup processes, e.g., parabolic trough reactors which have been successfully utilized in solar test fields in Spain and the United States, employ light concentrating systems. Since during the normal use periodical variations of the temperature in the reactor are also encountered, a systematic study of the light intensity and the temperature dependence of photocatalytic mineralizations is required. Here, we present our results of a respective laboratory investigation using titanium dioxide (Degussa P25) as the photocatalyst and dichloroacetic acid as the test compound. Finally, it should be noted that photocatalysis has been compared with other methods for the destruction of toxic chemicals in aqueous solutions on economical 18 and technical 19 grounds. The authors of the respective papers agree that the method bears considerable potential and should be pursued.

AB - Persistent organic chemicals are present as pollutants in wastewater effluents from industrial manufacturers, dry cleaning facilities, or even normal households. They can be found in groundwater wells and surface waters where they have to be removed to achieve drinking water quality. 1,2 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 cleanup of polluted water in the scientific literature since 1976. 3 Lately, 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. 4-7 Several reviews have recently been published discussing the underlying reaction mechanisms of photocatalytic detoxification and illustrating examples of successful laboratory and field studies. 8-10 While the overall stoichiometry of most mineralizations appears to be understood, 11 details of the complex reaction mechanism are still not known. Anatase, titanium dioxide, the material with the highest photocatalytic detoxification efficiency, is a wide bandgap semiconductor (E g ≈ 3.2 eV). 12 Thus, only light below 400 nm is absorbed and capable of forming the e −/h + pairs 13 which are a prerequisite for the process. Therefore, only the ultraviolet (UV) part, i.e., 5% of the solar energy reaching the surface of the Earth, could be utilized, in principle, when 350TiO 2 is the photocatalyst. Hence, it is evident that for solar applications other materials have to be found or developed that exhibit similar efficiencies as anatase TiO 2, but possess spectral properties more closely adapted to the terrestrial solar spectrum. For a solar application of photocatalytic detoxification, it is essential that the incident sunlight is effectively utilized. Therefore, parts of the investigations presented in this chapter concentrate on the synthesis and characterization of photocatalysts that absorb in the visible part of the solar spectrum and simultaneously improve the photocatalytic detoxification properties in this spectral region. The absorption of photons by semiconductors leads to the formation of an equal number of positive and negative charge carriers (e −/h + pairs). While the fate of the hole which induces the desired oxidation process has been studied in detail, most authors did not examine the role of the cathodic process, i.e., the reactions of. It is generally assumed that molecular oxygen acts as the oxidant. 8-10 However, hydrogen peroxide (H 2O 2), which should be formed during O 2 reduction, is found only in trace amounts when TiO 2 is used as the photocatalyst. 14 Further reduction of H 2O 2 leads to the formation of OH radicals. In fact, it has been shown that the rate of photodegradation can be considerably enhanced when H 2O 2 is used as the oxidant. 15 Separation of the anodic and cathodic process, in principle, is not possible in microheterogeneous photocatalytic systems containing semiconductor particles. Hence, it cannot be decided whether hydroxyl radicals are formed via the oxidation of water or the reduction of molecular oxygen, i.e., whether electrons or holes are more important for the initial step of pollutant degradation which is generally believed to be the reaction of OH with the substrate molecule S. Since the efficiency of a complex process is always limited by the slowest reaction step, it is necessary to distinguish between the two possibilities discussed above and to study them separately. In the following, we will therefore also present evidence from photoelectrochemical investigations with separated anode and cathode which have been carried out to further elucidate the underlying reaction mechanisms. Various reactor designs have been tested in laboratory studies where chemical engineering problems characteristic for the different reactor types, such as mass-transfer limitations, have been exploited in detail. 16,17 Here, we will show results obtained with two different solar detoxification reactors at test sites in Almeria, Spain and Campinas, Brazil. Many of the commercially used reactors for photocatalytic cleanup processes, e.g., parabolic trough reactors which have been successfully utilized in solar test fields in Spain and the United States, employ light concentrating systems. Since during the normal use periodical variations of the temperature in the reactor are also encountered, a systematic study of the light intensity and the temperature dependence of photocatalytic mineralizations is required. Here, we present our results of a respective laboratory investigation using titanium dioxide (Degussa P25) as the photocatalyst and dichloroacetic acid as the test compound. Finally, it should be noted that photocatalysis has been compared with other methods for the destruction of toxic chemicals in aqueous solutions on economical 18 and technical 19 grounds. The authors of the respective papers agree that the method bears considerable potential and should be pursued.

UR - http://www.scopus.com/inward/record.url?scp=85051971746&partnerID=8YFLogxK

U2 - 10.1201/9781351069847

DO - 10.1201/9781351069847

M3 - Contribution to book/anthology

SN - 9781315890746

SP - 349

EP - 368

BT - Aquatic and Surface Photochemistry

CY - Boca Raton

ER -