Indirect photodegradation

No information was found on the transformation of diisopropyl methylphosphonate in the atmosphere. Based on the results of environmental fate studies of diisopropyl methylphosphonate in distilled water and natural water, photolysis (either direct or indirect) is not important in the transformation of diisopropyl methylphosphonate in aquatic systems (Spanggord et al. 1979). The ultraviolet and infrared laser-induced photodegradation of diisopropyl methylphosphonate in both the vapor or liquid phase has been demonstrated (Radziemski 1981). Light hydrocarbon gases were the principal decomposition products. Hydrogen, carbon monoxide (CO), carbon dioxide (C02), and water were also detected. 

Chen Y, Hu C, Hu X, Qu J (2009) Indirect photodegradation of amine drugs in aqueous solution under simulated sunlight. Environ Sci Technol 43 2760-2765... 

The laser flash photolysis of aromatic diisocyanate based polyurethanes in solution provides evidence for a dual mechanism for photodegradation. One of the processes, an N-C bond cleavage, is common to both TDI (toluene diisocyanate) and MDI (methylene 4,4 -diphenyldiisocyanate) based polyurethanes. The second process, exclusive to MDI based polyurethanes, involves formation of a substituted diphenylmethyl radical. The diphenylmethyl radical, which readily reacts with oxygen, is generated either by direct excitation (248 nm) or indirectly by reaction with a tert-butoxy radical produced upon excitation of tert-butyl peroxide at 351 nm. 

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. 

The (indirect) photodegradation of chemicals (i.e., reactions with reactive species formed by photochemical processes) has been recognized as the major transformation pathway for chemicals in the troposphere. The electrophilic addition of tropospheric radicals constitutes the principal... 

Some of the topics addressed here have been reviewed by other authors. Studies of photoinduced processes (direct or indirect) of chlorophenols carried out before 1998 have been covered comprehensively [8] this work will only cursorily be treated here. A detailed overview of the photochemical behavior of phenylurea herbicides was in press at the time of writing this article [9]. The related subject of the photodegradation of pharmaceuticals in the aquatic environment has been reviewed very recently [10]. 

It has to be stressed that the models proposed until now for the photodegradation of the polyhalophenols are based on indirect evidence only. As yet, real-time mechanistic studies may have been hindered by low aqueous solubilities and low quantum yields. In particular, this means that the mechanisms involving initial homolytic cleavage are not supported by direct evidence. The observation of preferential formation of 3,5-dichlorocatechol... 

The photoassisted activity is especially useful for degradation of strongly hazardous substances or recalcitrant pollutants, which are resistant to removal in chemical and/or biochemical processes. Transition metal photocatalysts are able to stimulate the indirect photodegradation of molecules that resist direct photolysis. Thus, they can activate species not absorbing sunlight, or those whose excited states populated by simlight absorption are chemically inefficient. 

Photodegradation u = 0.89 d based on a global, seasonal and diurnal average OH radical concn of 1 x 10 molecule cm in air and indirect photolysis U = 2.4 d (Peterson Staples 2003)... 

From Table 2-10, it is evident that polycyclic aromatic hydrocarbons such as B[a]p are likely to directly photodegrade because double bonds in aromatic rings can absorb light. Indirect degradation of B[a]p can also occur via attack by l02. 

Reactive species formed by the CDOM absorption of UVR can then undergo indirect, or secondary, photochemical reactions. In fact, the many possible reactions caused by photosensitized transient intermediates probably account for most of the photodegradation of CDOM that we observe. The many different photoprocesses involved in CDOM photochemistry make for a very complex pathway of reactions beginning with the initial absorption of light energy and ending with the final products of these multiple reactions. 

The effect of concentration, temperature, and solvent on the photodegradation of benzothiazolinospiropyrans using flash photolysis has been presented. trans-Cyclo-octene is shown to be the major product in the photolysis of m-cyclo-octene with a quantum yield of 0.34. Hamanoue and co-workers " report on intersystem crossing and lowest triplet states of 4-chromanone, chromone, and flavone and of some nitroanthracene derivatives." For the latter long build-up times for triplet-triplet absorption were observed (72—86 psec) and represent the effect of the rates of internal conversion T aa/ T,). Indirect intersystem crossing to higher levels of the triplet manifold is most important in these systems. 

PROBABLE FATE photolysis no direct photolysis, indirect photolysis is too slow to be important, photooxidation half-life in air 4.5 hrs-1.9 days, if emitted to atmosphere, will be subject to photodegradation by hydroxyl radicals with a half-life of 14 hr oxidation not an important process hydrolysis hydrolysis only in surface waters but too slow to be important, first-order hydrolytic half-life 107 yrs volatilization not expected to be an important transport process sorption adsorption onto solids and particles and complexation with organics are important transport processes, will adsorb strongly to sediment and particulate matter biological processes bioaccumulation by many organisms, biodegradation and metabolization are all important fates... 

---