Reaction pathways for the photodegradation of the organophosphorus cyanophos in aqueous solutions.

Photodegradation in aqueous solutions is an important pathway for many agrochemicals such as pesticides. In the present work, the photochemical transformation of cyanophos (CYA) was investigated in aqueous solutions using UV light within the 254-313 nm range as well as solar light. The study was performed in order to have a deep insight into the mechanistic pathways for the photochemical disappearance of CYA. Upon UV irradiation of an aerated solution of CYA, the degradation quantum yield was found equal to 1.8 x 10(-2). It is independent of the excitation wavelength but varies with oxygen concentration. It increased by a factor of 2 from oxygen-saturated to oxygen-free solution. Photosensitized experiments were performed using acrylamide and hydroquinone as energy acceptor and energy donor substrates, respectively. They show that both singlet and triplet excited states were involved in the photochemical behavior of CYA. The laser flash photolysis experiments clearly showed the involvement of the triplet excited state which was efficiently quenched by molecular oxygen and acrylamide with the rate constants 1.97 x 10(9) and 2.71 x 10(9) mol(-1) L s(-1), respectively. The photoproducts structures were proposed according to the mass spectral data using the LC/MS technique. The analytical study shows that various processes such as hydrolysis, homolytic bond dissociations and Photo-Fries process occur.

Fulvic acid-mediated phototransformation of mecoprop. A pH-dependent reaction.

Elliott soil fulvic acid sensitized the phototransformation of mecoprop in aqueous medium. The reaction was selective, leading to the main formation of 4-chloro-2-methylphenol in aerated and in deoxygenated neutral solution. In turn, 4-chloro-2-methylphenol underwent a fulvic acid-mediated phototransformation. A pH decrease from 8 to 2.2 led to an increase of the rate of mecoprop loss by a factor of 10. The roles of hydroxyl radicals, fulvic triplets and singlet oxygen were investigated using the scavenging technique. At pH 6.5 where mecoprop was in the anionic form, triplet excited states and hydroxyl radicals were the main oxidant species contributing to 40 +/- 10% and 20 +/- 5% of the reaction, respectively. At pH 2.2 where mecoprop was protonated, fulvic triplets and hydroxyl radicals contributed to 75 +/- 10% and 16 +/- 5%, respectively. The enhancement of the reaction upon acidification might result from two phenomena tending to the same effect: (i) the capacity of protonated MCP to reach intra-humic microdomains and there interact with reactive species, and (ii) a greater photosensitizing property of FA in acidic medium.

Photochemistry of 6-chloro and 6-bromopicolinic acids in water. Heterolytic vs. homolytic photodehalogenation.

The photochemistry of 6-chloro and 6-bromopicolinate ions ( and , respectively) was investigated by product studies and ns laser flash photolysis (LFP). In deoxygenated pH 5.4 water, yields 6-hydroxypicolinic acid (70%) and a substituted pyrrole. In 2-propanol-water (1 : 1) mixture, the reaction yields, very unselectively, 6-hydroxypicolinic acid, 2-carboxypyridine, pyridine and bipyridines. Photolysis of aqueous leads to 6-hydroxypicolinic acid (78%) and hydroxybipyridines. Oxygen suppresses the photolysis of but does not affect that of . By LFP, we detected a short-lived transient at the pulse end from (lambda(max)= 305 nm, k=(2.8 +/- 0.2)x 10(5) s(-1), epsilonphi= 2200 +/- 200 dm3 mol(-1) cm(-1)). This is quenched either by oxygen or methyl acrylate and thus assigned to the triplet excited state. The triplet excited state of is detected at pH 1 only (lambda(max)= 320 nm, k > 3 x 10(7) s(-1)). The radical ion Cl2- could be successfully detected by photolysing in 2-propanol-water (1 : 1) in the presence of Cl-. Similarly, Br2- could be detected by irradiating aqueous in the presence of Br-. These results show that the photodehalogenation of is heterolytic in water and mainly homolytic in 2-propanol-water mixtures while that of is both heterolytic and homolytic in water. A mechanism in which the triplet excited state undergoes homolysis of the C-X bond and subsequent electron transfer from the carboxypyridyl radical to the halogen atom to form an ion pair may account for these observations.

Mineralisation of Monuron photoinduced by Fe(III) in aqueous solution.

The degradation of Monuron (3-(4-chlorophenyl)-1,1-dimethylurea) photoinduced by Fe(III) in aqueous solution has been investigated. The rate of degradation depends on the concentration of Fe(OH)2+, the most photoreactive species in terms of *OH radical formation. These *OH radicals are able to degrade Monuron until total mineralisation. The primordial role of the speciation of Fe(III)-hydroxy complex in aqueous solution, for the efficiency of the elimination of pollutant, was shown and explained in detail. The formation of Fe(II) in the irradiated solution was monitored and correlated with the total organic carbon evolution. Degradation photoproducts were identified and a mechanism of degradation is proposed.

Photochemistry of the three carboxypyridines in water: a pH dependent reaction.

The photochemistry of ortho, meta and para-carboxypyridines (pK(a)(1)= 1.0-2.1 and pK(a)(2)= 4.7-5.3) in aqueous medium was studied by laser-flash photolysis and product studies. At pH < pK(a)(1), hydroxylated compounds are produced with low quantum yields. Within the pH range 4-7, ortho and meta isomers undergo dimerization together with decarboxylation with a quantum yield showing a very sharp maximum around pK(a)(2)([small phi](max)= 0.09 and 0.01, respectively) while the para isomer is photostable. End-of-pulse transients assigned to triplet states were detected by laser-flash photolysis at pH < pK(a)(1) and pH > 4. Additionally, the carboxypyridinyl radicals were detected as secondary intermediates at pH < pK(a)(1) and 4 < pH < 7 and the OH-adduct radicals at pH < pK(a)(1). This is in favour of an electron transfer reaction between triplet and starting compound producing a charge transfer species. The radical anion would escape as carboxypyridinyl radical while the radical cation may add water at pH < pK(a)(1) yielding the OH-adduct radical or may undergo decarboxylation at pH > 4. The high quantum yield of phototransformation of the ortho isomer at pH > 4 is due to an easy decarboxylation process. A reaction scheme is proposed accounting for the dependences of [small phi] on both the pH and the carboxypyridines concentration. This study points out the distinct pattern of reactivity of carboxypyridines depending on the ionisation state of starting compounds and isomeric substitution.