Solar light, photoreactions

The photosynthetic reaction in nature that creates biomass represents an attractive model for photoreactions, i.e., one would like to be able to take C02 from the atmosphere and fix it —convert it to a useful product using the energy of solar light. 

Sayama K, Mukasa K, Abe R, Abe Y, Arakawa H (2002) Photo-catalytic water splitting system into H2 and 02 under visible light irradiation mimicking a Z-scheme mechanism in photosynthesis. Solar Light Energy Conversion Team, Photoreaction Control Research Center, Advanced Industrial Science and Technology (AIST), Ibaraki, Japan... 

The photochemical reactions induced by sunlight are of essential relevance to pollution abatement only when they lead to oxidative degradation of waste organic material. This can be done either through direct or indirect photoredox reactions. The direct photodecomposition of organic pollutants under simlight is not very common and applies mostly to aromatic compoimds. Much more common is indirect photoreactivity in which solar light is... 

Photocatalytic reactors can also be powered by solar light as 4-5% of the wavelengths of the solar spectrum can excite Ti02. For low intensities, there is a first order relation between the rate of photoreaction and the irradiation intensity. At higher levels of irradiation, the relation between the photoconversion rate and the irradiation intensity becomes of fractional order. Increased inefficiencies with increasing irradiation are a significantly limiting factor in solar photoreactor applications. 

Much effort has gone into development of catalysts for photochemical reactions, initially with the objective of converting solar energy into storable fuels (typically H2 from the photolysis of water) but, more recently, mainly for the destruction of noxious pollutants such as chlorocarbons. There are two ways in which a catalyst may be involved in a photochemical reaction it may simply provide a surface on which the reactants can be adsorbed, so that, when a molecule of one reactant is activated by absorption of light, a molecule of the other is held in close proximity to facilitate reaction (a catalyzed photoreaction)-, or it may itself be excited by the absorption of light and then activate the adsorbed molecules (a sensitized photoreaction). The latter mode is the more relevant to the theme of this chapter, and is exemplified by the photocatalytic properties of titanium dioxide, Ti02-14 15... 

All these photoreactions caused by uv light prevent H-bonding between base pairs which is fundamental to the replication and propagation of life (Figure 9.1). Some of these damages can be repaired, others are permanent and lethal to life. Therefore the importance of ozone screen in cutting off these deleterious radiations from the solar spectrum. 

All benchmark chemicals, anthracene (AN), chlorpyrifos (CP), 2,6-di-t-butylphenol (DBP), y-hexachlorocyclohexane (HCH), and trichloroethylene (TCE) absorb UV light between 200-300 nm, but only anthracene and chlorpyrifos absorb solar photons (> 295 nm) rapidly enough to undergo direct photoreactions at significant rates. Table 15.2 summarizes the UV spectral properties of AN, CP, DBP, and two other compounds, p-nitroanisole (I) (PNA) and dinitramide ion (II) (DN ion), both of which directly photolyze in sunlight. 

KzX is usually expressed per meter and depends on wavelength and water composition. Typically, ultraviolet radiation penetrates less deeply than visible radiation. Attenuation coefficients vary over many orders of magnitude in natural waters, with the highest values (least light penetration) in inland water bodies and the lowest values (highest penetration) in open seawater. The photic zone for solar ultraviolet radiation, which is very important for halocarbon photoreactions, ranges from tens of meters in the open ocean and clear lakes to only a few centimeters in some inland wetlands. The spectral properties of water bodies are linked to water composition. Baker and Smith (20) developed algorithms that relate K,k to certain parameters such as chlorophyll a concentrations. 

Compounds that do not absorb light within the surface solar spectrum are also subject to photochemical modification through indirect or sensitized photoreactions. In this case, the ROS or intermediates produced by direct photoreactions of a lightabsorbing constituent such as CDOM can react secondarily with the nonabsorbing compounds. DMS and COS, two trace gases of some importance to the atmosphere, are thought to be destroyed and created, respectively, by sensitized photoreactions in marine surface waters. 

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