Indirect Photolysis in Surface Waters

In addition to chemical reactions brought about directly by light energy, there are other atmospheric chemical reactions that can occur in the dark, but that typically involve reactive chemical species previously produced photo-chemically (cf. indirect photolysis in surface waters). The rates of such thermal, or dark, reactions depend on the temperature and on the concentrations of the reactive chemicals involved. For example, consider the rate of the dark reaction between the photochemically produced compounds ozone (03) and nitric oxide (NO). The rate is proportional to the concentration of each reactant... 

Photolysis Abiotic oxidation occurring in surface water is often light mediated. Both direct oxidative photolysis and indirect light-induced oxidation via a photolytic mechanism may introduce reactive species able to enhance the redox process in the system. These species include singlet molecular O, hydroxyl-free radicals, super oxide radical anions, and hydrogen peroxide. In addition to the photolytic pathway, induced oxidation may include direct oxidation by ozone (Spencer et al. 1980) autooxidation enhanced by metals (Stone and Morgan 1987) and peroxides (Mill et al. 1980). 

Photolysis. Photolysis of a chemical can proceed either by direct absorption of light (direct photolysis) or by reaction with another chemical species that has been produced or excited by light (indirect photolysis). In either case photochemical transformations such as bond cleavage, isomerization, intramolecular rearrangement, and various inter-molecular reactions can result. Photolysis can take place wherever sufficient light energy exists, including the atmosphere (in the gas phase and in aerosols and fog/cloud droplets), surface waters (in the dissolved phase or at the particle-water interface), and in the terrestrial environment (on plant and soil/mineral surfaces). 

Triazine herbicides absorb sunlight weakly at wavelengths >290 nanometers (nm), thus, dissipation of the triazine herbicides in the atmosphere and in surface waters via photodegradation occurs mainly by indirect photolysis or photosensitized reactions. 

Coal tar constituents present in surface waters may be degraded by direct and indirect photolysis. Estimated aqueous photolysis half-lives of 8.4, 71, and 21 hours have been reported for phenanthrene, naphthalene and fluoranthene, respectively (Zepp and Schlotzhauer 1979). Other coal tar constituents which may undergo aqueous photolysis are acenaphthalene, anthracene, benzene, quinoline, phenol, cresol, and carbazide. In a microcosm study, PAHs leached from creosote-impregnated wood pilings were degraded in aquatic environments by photolysis and microbial degradation, while sorption to sediment was not significant (Bestari et al. 1998). Photolysis in water is not expected to be a major route of the environmental fate of creosote constituents, particularly for the less soluble compounds. 

PROBABLE FATE photolysis no direct photolysis, indirect photolysis too slow to be environmentally important, photooxidation half-life in water 2.4-12.2 yrs, photooxidation half-life in air 7.4 hrs-2.5 days oxidation not important, reaction with photochemically produced hydroxyl radicals gives a half-life of 18 hrs hydrolysis hydrolysis (only in surface waters) believed to be too slow to be important, first-order hydrolytic half-life 10 yrs volatilization not expected to be an important transport process sorption sorption onto particulates and com-plexation with organics are dominant transport processes biological processes bioaccumulated in many organisms, biodegraded rapidly in natural soil, some biotransformation, all biological processes are important fates... 

A variety of photolytic processes are possible and from the discussion above it is apparent that in surface waters, indirect effects depend primarily on the DOM. This component will reduce the rate of direct photolysis, but there are many examples where this reduction is compensated for by indirect reactions. However, under environmental conditions, it is not always possible to differentiate among the different options when... 

Pereira VJ, Weinberg HS, Linden KG, Singer PC (2007) UV degradation kinetics and modeling of pharmaceutical compounds in laboratory grade and surface water via direct and indirect photolysis at 254 nm. Environ Sci Technol 41 1682-1688... 

Describe the general kinetic approach that can be used to quantify indirect photolysis involving well-defined photooxidants in (a) the water column of surface waters and (b) the atmosphere. 

Table 18.5 shows indirect photolysis half-lives of azo dyes in natural waters at 40°N at the surface and 1 meter depth using measured rate constants and singlet oxygen concentrations. They apply only if singlet oxygen and/or radical oxidation are the only mechanisms for degradation. 

Air t,/2 = 8 h, based on a rate constant k = 3.0 x 10-11 cm3 molecules-1 s-1 for the reaction with 8 x 10-5 molecules/cm3 photochemically produced hydroxyl radical in air (GEMS 1986 quoted, Howard 1989). Surface water estimated t,/2 = 3.2 d in Rhine River in case of a first order reduction process (Zoeteman et al. 1980) midday t,/2(calc) = 45 min in Aucilla River water due to indirect photolysis using an experimentally determined reaction rate constant k = 0.92 h-1 (Zepp et al. 1984 quoted, Howard 1989) estimated t,/2 = 3.2 d for a river 4 to 5 m deep, based on monitoring data (Zoeteman et al. 1980 quoted, Howard 1989). 

Surface water midday t,/2 12 min in Aucilla river due to indirect photolysis using experimentally determined rate constant k = 3.6 Ir1 (Zepp et al. 1984) ... 

PROBABLE FATE photolysis direct photolysis is slow, indirect photolysis may be important, vapor phase aldrin residues expected to react with photochemically produced hydroxyl radicals with a half-life of 35.46 min oxidation reacts to form dieldrin, photooxidation by ultraviolet light in aqueous medium 90-95°C forms 25% CO2 in 14.1 hr, 50% CO2 in 28.2 hr, 75% CO2 in 109.7 hr, photooxidation half-life in air 0.9-9.1 hrs hydrolysis too slow to be an important process volatilization an important process, evaporation rate from water 3.72x10 m4ir, will volatilize from soil surfaces sorption an important process, adsorption to sediment is... 

PROBABLE FATE photolysis photooxidation to DDE occurs slowly, indirect photolysis may be important oxidation photoxidation occurs, photooxidation half-life in water 7-350 days, photoxidation half-life in air 7.4 days hydrolysis may be an important process under certain conditions, first-order hydrolytic half-life 22 yrs volatilization is an important process, some will evaporate from soil and surface water into the air sorption is an important process, will adsorb very strongly to soil if released to the soil, will adsorb very strongly to sediments if released to water biological processes biotransformation and bioaccumulation are important processes, may be subject to biodegradation in flooded soils or under anaerobic conditions, may be significant in sediments... 

PROBABLE FATE photolysis no direct photolysis, half-life from surface waters 3500 hr, indirect photolysis is too slow to be important, photodegradation by hydroxyl radicals will occur with a half-life of 23.8 hrs oxidation not an important process, photooxidation half-life in air 4.7 days-46.6 days hydrolysis too slow to be important under natural conditions, first-order hydrolytic half-life 1163 days volatilization possible, but not important sorption sorption onto particles and biota and complexation with humic substances principal transport mechanism, little adsorption to soil or sediment is expected to occur biological processes bioaccumulation, biodegradation, and biotransformation by many organisms (including humans) are very significant fates... 

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