Organic pollutants, photocatalytic

Keywords Environmental homogeneous photocatalysis Photoreduction of transition metal complexes NO-carriers Photodegradation of organic pollutants Photocatalytic cycles stimulated by iron, copper and chromimn complexes. 

The primary target of studies on photocatalytic semiconductor suspensions has been water cleavage by visible light. Suspension-based photocatalytic processes are also useful for the removal of inorganic (metal ions) and organic pollutants, the reduction of CO2, the photodestruction of bacteria and viruses, and various organic reactions an example is the use of Pt-loaded CdS for the photocatalytic racemization of L-lysine [210]. 

The oxidative degradation of organic pollutants in water and air streams is considered as one of the so-called advanced oxidation processes. Photocatalytic decomposition of organics found widespread industrial interest for air purification (e.g., decomposition of aldehydes, removal of NO , ), deodorization, sterilization, and disinfection. Domestic applications based on Ti02 photocatalysts such as window self-cleaning, bathroom paints that work under illumination with room light, or filters for air conditioners operating under UV lamp illumination have already been commercialized. Literature-based information on the multidisciplinary field of photocatalytic anti-pollutant systems can be found in a number of publications, such as Bahnemann s [237, 238] (and references therein). 

Patsoura, A., Kondarides, D.I., and Verykios, X.E. (2007) Photocatalytic degradation of organic pollutants with simultaneous production of hydrogen. Catalysis Today, 124 (3-4), 94-102. 

This section covers environmental applications of nanomaterials insofar as they are directly applied to the pollutant of interest. The photocatalytic degradation of organic pollutants and remediation of polluted soils and water are discussed here. The high surface areas and photocatalytic activities of semiconductor nanomaterials have attracted many researchers. Semiconductor nanomaterials are commercially available, stable, and relatively nontoxic and cheap. Prominent examples that are discussed are metal oxides such as Ti02 and ZnO and a variety of Fe-based nanomaterials. 

The photocatalytic activity of ZnO nanomaterials for the degradation of some organic pollutants in water [173] (e.g., dyes [174]) was explored by several groups to achieve environmental benefits. Recent studies have indicated that ZnO can be used under acidic or alkaline conditions with the proper treatment [175,176]. ZnO nanomaterials were used as photocatalysts for the degradation of phenol [177] and chlorinated phenols such as 2,4,6-trichlorophenol [178]. ZnO nanomaterials were also used for the degradation of Methylene Blue [179], direct dyes [180], Acid Red [181], and Ethyl Violet [182],... 

Wang, P., et al., A one-pot method for the preparation of graphene-Bi2Mo06 hybrid photocatalysts that are responsive to visible-light and have excellent photocatalytic activity in the degradation of organic pollutants. Carbon, 2012. 50(14) p. 5256-5264. 

Byrne JA, Eggins BR, Byers W, Brown NMD (1999) Photoelectrochemical cell for the combined photocatalytic oxidation of organic pollutants and the recovery of metals from waste waters. Appl Catal B Environ 20 L85-89... 

Activated carbon is often used as a catalyst support because of its high surface area and porosity. For example, Uchida et al. (1993) utilized activated carbon adsorbent as support for TiOz photocatalysts. It is reported that the adsorptive action of the activated carbon support enables the organic pollutants to concentrate around the loaded Ti02, resulting in a high photocatalytic degradation rate. 

These catalytic membranes have been successfully used in a photocatalytic membrane reactor for the mineralization of the phenol, one of the main organic pollutants in wastewater, demonstrating that these catalytic membranes are stable and recyclable [42]. 

In recent years some studies were performed using submerged membrane modules coupled to photocatalytic systems for the removal of organic pollutants such as fulvic acid [84], bisphenol-A [85], para-chlorobenzoate [86]. 

One of the main objectives in the use of a membrane process coupled to a photocatalytic reaction is the possibility of recovering and reusing the catalyst. Moreover, when the process is used for the degradation of organic pollutants, the membrane must be able to reject the compounds and their intermediate products, while if the photocatalysis is applied to a synthesis, often the membrane have to separate the product(s) from the environment reaction. Therefore, in a PMR the choice of a suitable membrane is essential to obtain an efficient system. 

Effect of 03. Adding ozone in dioxygen or air is a very efficient means of enhancing the photocatalytic rates of the removal and, above all, the mineralization of organic pollutants both in air and in water, even if the wavelengths are intentionally selected so as not to excite ozone (39 41). This substantial effect is attributed to the difference in electron affinity between 03 (2.1 eV) and 02 (0.44 eV). Consequently, in the presence of ozone, the electrons photopromoted to the Ti02 conduction band can be captured more easily, either directly ... 

Competition Between Pollutants. Competition between several organic pollutants may affect the photocatalytic degradation rate of each species, depending on whether the process is limited by the irradiation or by the total organic matter. The factors intervening in the competition are the respective concentrations, the partition coefficients between the fluid phase and the adsorbed phase, and the relative reactivities with respect to the active species. Consequently, interference effects may or may not be observed. 

Effect of Water Vapor on Photocatalytic Air Treatment. Several studies have reported on the effects of water vapor on the photocatalytic treatment of air (101-108). The effect of water vapor very much depends on the type of pollutant and, obviously, on the partial pressure of water against that of the pollutant. On one hand, water can compete with the adsorption of organic pollutants, especially those that are structurally related, such as alcohols. On the other hand, water can behave as a reactant in some of the successive steps of the degradation of organics and, in particular, can limit the formation of products that inhibit the photocatalytic activity. Water can be at the origin of the formation of hydroxyl radicals however, the importance of these radicals in gas-phase photocatalytic reactions is being debated on (109-111). The conclusion is that some humidity seems necessary for optimum photocatalytic activity. 

For the treatment of air and water, photocatalysis over Ti02 is a technique that is simple, robust, easily automated, and flexible. It is versatile because organic pollutants, some inorganic pollutants, and microorganisms can be eliminated or inactivated, respectively. In water, some ions can also be transformed. Its operating cost arises mainly from the UV lamps and the corresponding electrical consumption. Provided that the needs and location of the treatment allow it, this cost can be reduced substantially by the use of solar irradiation instead of UV lamps. For these reasons, photocatalytic water purification appears well adapted to isolated communities, in particular if no competent labor force is available. 

Comninellis C. Electrocatalysis in the electrochemical conversion/combustion of organic pollutants for waste water treatment. Proceedings of the Symposium on Water Purification by Photocatalytic, Photoelectrochemical and Electrochemical Processes. Vol. 94-19. Pennington, NJ The Electrochemical Society, 1994 75-86. 

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