Indirect photolytic process photolysis

Distilled rather than natural water is often used as the solvent for determination of quantum yields for two major reasons. First, the total absorbance of the solution at the wavelength of irradiation should not exceed 0.02. Second, and more important, the presence of natural water constituents (e.g., humic material, nitrate) could enhance the total photolytic transformation rate by indirect photolytic processes as described in Chapter 16. Zepp and Baughman (1978) have argued that for many chemicals d>,r obtained in distilled water is nearly the same as that observed in natural waters (at least in uncontaminated freshwaters), because concentrations of natural water constituents that could undergo reactions with or quench photolysis of excited pollutants are generally very low. Furthermore, the effects of molecular oxygen, which may act as a quencher, can also be studied in distilled water. 

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... 

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). 

PROBABLE FATE photolysis , no direct photolysis, indirect photolysis is too slow to be important, atmospheric and aqueous photolytic half-life 144-200 days oxidation not an important process, photooxidation half-life in water 44-584 days, photooxidation half-life in air 2.9-29 hrs hydrolysis too slow to be important (half-life of several years) volatilization not a likely transport process, should not evaporate from soil or water sorption sorption onto particles and biota and complexation with humic materials are most important transport processes, attaches strongly to soil particles biological processes bioaccumulation and metabolization by many organisms, and biodegradation are all very important fates... 

PROBABLE FATE photolysis, direct photolysis is improbable, indirect photolysis is too slow to be important, aqueous photolytic half-life 100 yrs oxidation could occur, but could probably not compete with degradation, photooxidation half-life in air 2.5 days hydrolysis too slow to be important, half-life is greater than 100 days volatilization not expected to be a likely transport process, will volatilize under windy conditions or from shallow waters sorption sorption onto particulates and complexation with organic substances are dominant transport processes, expected to adsorb if released to soil biological processes bioaccumulated and metabo-... 

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