Photochemistry of Environmental Aquatic Systems
Rod Zika1987, ACS Symposium Series
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Abstract
Sunlight that arrives at the surface of the earth contains substantial amounts of energy. When surface waters, which contain either natural or anthropogenic chromophores, are exposed to sunlight, light initiated chemical reactions often occur. Sunlight induced photochemical reactions in surface waters may broadly be defined as environmental aquatic photochemistry. Within aquatic photochemistry, it is possible to envision reactions involving either inorganic and/or organic molecules. These chemicals could be either natural or anthropogenic and may participate in either homogeneous or heterogeneous reactions. Very often in these environments, a complex array of primary and secondary photoprocesses are occurring simultaneously. Surface waters are diverse in nature. They might be near shore or inland wetland environments or mid-oceanic oligotrophic water. Until recently, sunlight induced photochemistry was not recognized as an important pathway for the transformation of natural and anthropogenic chemicals in surface waters. It is now well established that photochemically mediated processes are important in most, if not all, areas of aqueous phase environmental chemistry. Both direct, primary, and indirect photoprocesses have been documented in natural waters.
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Chromophoric dissolved organic matter (CDOM) in surface waters is a photochemical source of several transient species such as CDOM triplet states ((3)CDOM*), singlet oxygen ((1)O2) and the hydroxyl radical (OH). By irradiation of lake water samples, it is shown here that the quantum yields for the formation of these transients by CDOM vary depending on the irradiation wavelength range, in the order UVB > UVA > blue. A possible explanation is that radiation at longer wavelengths is preferentially absorbed by the larger CDOM fractions, which show lesser photoactivity compared to smaller CDOM moieties. The quantum yield variations in different spectral ranges were definitely more marked for (3)CDOM* and OH compared to (1)O2. The decrease of the quantum yields with increasing wavelength has important implications for the photochemistry of surface waters, because long-wavelength radiation penetrates deeper in water columns compared to short-wavelength radiation. The average steady-...
This paper is published as part of a themed issue of Photochemical & Photobiological Sciences containing papers in honour of
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Do photochemical transformations of dissolved organic matter produce biorefractory as well as bioreactive substrates
Photochemical processes appear to have an important impact on the cycling of dissolved organic matter (DOM) in a wide variety of aquatic environments. Numerous photoproducts of DOM, including dissolved inorganic carbon and low-molecular-weight organic compounds, have been identified, and the types of photoproducts and extent of phototransformation of DOM are dependent upon its chemical composition. Heterotrophic bacteria are the primary consumers of DOM, and photochemical transformations of DOM can affect its bacterial utilization in a variety of ways. Numerous studies spanning fresh and marine waters have demonstrated enhanced bacterial growth following exposure of DOM to solar radiation. These studies indicate that photochemical processes can enhance the microbial utilization of biorefractory DOM. Other studies in fresh and marine waters, however, have demonstrated that exposure of DOM to solar radiation can result in a reduction in bacterial growth rates, suggesting that photochemical and microbial processes "compete" for the same substrates. It appears that some photoproducts of DOM are resistant to microbial degradation, suggesting that photochemical processes could be involved in the production as well as destruction of biorefractory DOM.
Advances in marine photochemistry 1983–1987
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The area of marine photoc•emistry ubiquitous they have received a great deal of has advanced dramatically during the last four at tention in recent years being the subject of years. The incre_ase in meetings dedicated to the recent reviews on their structure, photophysics, subject and in related publications oes a clear photochemistry and free radical character sign of the growing interest and importance of the topic. The experimental approaches, mechanistic process description and quantitative estimation of photo-processes have become far more sophisticated and detailed than before. A clearer recognition has emerged on the role and importance of p•otochemistry in affecting the biota, chemical speciation and degradation, and atmospheric sources. glucinol based polymer from a benthic marine Copyright 1987 by the American Geophysical Union. macrophyte. Although riboflavin photochemically degraded rapidly to lumichrome and other minor Paper number 7R0317. products it was a far more efficient sensitizer 8755-1209/87/007R-0317515.00 than the other t•o tested. Dunlap and Susic
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