Photochemical reactions aquation

It is well established that important photochemical reactions are mediated by humic material in the aquatic environment (Zepp et al. 1981a,b), and that these are particularly signihcant for hydrophobic contaminants. Partial reductive dechlorination of the persistent insecticide mirex associated with... 

Collectively, these examples illustrate the diversity of transformations of xenobiotics that are photochemically induced in aquatic and terrestrial systems. Photochemical reactions in the troposphere are extremely important in determining the fate and persistence of not only xenobiotics but also of naturally occurring compounds. A few illustrations are given as introduction ... 

The role of aquatic microorganisms in affecting photochemical reactions has not been carefully studied in the past probably because of the lack of knowledge that such reactions do take place. Yet algae constitute the bulk of biomass in many aquatic systems. They are known to collect pesticidal chemicals because of their large surface areas. 

Hoigne, J., Y. Zuo, and L. Nowell, Photochemical Reactions in Atmospheric Waters Role of Dissolved Iron Species, in Aquatic and Surface Photochemistry (G. Helz, R. Zepp, and D. Crosby, Eds.), Chap. 4, pp. 75-84, Lewis, Boca Raton, FL, 1994. 

Faust, B. C., Aquatic photochemical reactions in atmospheric surface, and marine waters Influences on oxidant formation and pollutant degradation . In The Handbook of Environmental Chemistry, Vol. 2, Part L, P. Boule, Ed., Springer, Berlin, 1999, pp. 101-122. 

Mixed-ligand Crm complexes have a particularly rich substitutional photochemistry in that two (or more) reaction modes are normally observed. Data for the well-studied class of acidoamine complexes are presented in Table 2. The dominant photochemical reaction for [CrX(NH3)5]2+ complexes in aqueous solution is NH3 aquation, with X- aquation occurring to a lesser extent (equation 31). In contrast, the latter pathway is the favored thermal reaction of these compounds. Such behavior again illustrates that the reactivity of ligand field excited states can differ sharply from that of the ground state. 

Most of the reported photochemical reactions of lanthanide complexes involve some type of redox behavior.147 Photolysis (254—405 nm) of Eu2+ in acidic aqueous solution, for example, results in photooxidation of the metal and generation of H2 (equation 50).148 While the excited states responsible for this reaction nominally arise from 4/ -+ 5d transitions localized on the metal, strong mixing of the 5rf-orbital with ligand orbitals endows these states with appreciable CTTL character. Photoreduction of aquated Eu3+ can also be driven with UV ( 254 nm) light149 and forms the... 

Leifer (1988) reviewed fundamental theory and practice of the kinetics of aquatic reactions to express relevant direct and indirect photochemical reactions in natural water. 

Once produced PAHs can be transported through the atmosphere or the water column if directly discharged via uncombusted petroleum. In the air, PAHs partition between the gas and particle phases, can undergo photochemical and oxidation reactions, be washed out by precipitation and deposit to aquatic surfaces by both wet and dry deposition. Once in the aquatic system, PAHs partition between the dissolved and particulate phases, can undergo photochemical reactions and bioaccumulate in the lower trophic levels. 

In addition to the direct absorption as a biological hazard, UV can have additional indirect effects on organisms.26,27 A number of UV photochemical reactions occur in solutions, both within cells and in the external aquatic environment. In the presence of UV, water itself is hydrolyzed, producing hydroxyl ions. Related reactions involving dissolved substances and mediated by UV lead to the formation of peroxides, super oxide, and other radicals. These reactive products are toxic by causing oxidative damage to biological molecules.28-31... 

Owen ED. Principles of photochemical reactions in aqueous solution. In Faust SD, Hunter JV, eds. Organic Compounds in Aquatic Environments. New York Marcel Dekker Inc., 1971 387-423. 

Faust BC. Aquatic Photochemical Reactions in Atmospheric, Surface and Marine Waters Influences of Oxidant Formation and Pollutant Degradation. In Boule P, ed. Environmental Photochemistry. Berlin Springer, 1999 101-122. 

Photochemical processes in aquatic systems are not limited to substances in solution. Photochemical reactions may involve particles. One can distinguish two mechanisms ... 

This chapter reviews past studies of other investigators and presents previously unpublished laboratory studies relating to photochemical reactions of halocarbons in aquatic environments. Kinetics considerations relevant to the modeling of halocarbons in natural waters are briefly considered. Then direct and indirect photoreactions that provide sinks for halocarbons are examined with consideration of the effects of sorption onto dissolved NOM and suspended sediments. Photoreductive dehalogenation is emphasized. Finally, the role of photochemically produced reactive oxygen species in the production of halocarbons in the aquatic environment is discussed. The main emphasis is on field and laboratory studies that provide predictive capability and a mechanistic understanding of the processes. 

Apart from the inherent efficiency of the reactions leading to the light-induced formation of a ROS as summarized by the relevant apparent quantum yield and action spectrum, the observed rate of production will depend on other factors that affect the photon exposure including water column composition and depth (Chapter 3), time of day (i.e., solar zenith angle), season, latitude (Chapter 2), and physical transport processes (Chapter 4). For more details regarding the fundamental equations used to define the rates of primary and secondary photochemical reactions and their application to aquatic systems, the reader is referred to recent reviews on this topic [41,42]. 

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