Relationship between photosensitizing and emission properties of peat humic acid fractions obtained by tangential ultrafiltration.

Peat humic acid was fractionated by tangential ultrafiltration into six nominal molecular weight (NMW) fractions, HA5-10, HA10-20, HA20-50, HA50-100, HA100-300 and HA > 300, which were purified by dialysis using a 0.5 kDa membrane. The absorbing and emission properties of the separated fractions were compared and their ability to generate singlet oxygen under light excitation was evaluated, using furfuryl alcohol (FFA) as a singlet oxygen scavenger. The absorbance, the emission intensity, and the apparent first order rate constants of FFA loss were normalized per mole of organic carbon (a*, IF*, and k*, respectively). The fraction absorbance decreased with NMW, except for HA > 300 which was less absorbing than HA100-300. The low NMW fractions and the HA > 300 fraction generally showed lower k* and IF* values compared to the HA50-100 and HA100-300 fractions. A plot of k* versus IF* indicates that the first order rate constant of FFA photo-oxygenation increased with the intensity of fluorescence at 380, 430, and 500 nm (R2 = 0.77-0.84). This shows that the distribution of fluorescent centers among fractions paralleled that of photosensitizing centers. Plotting k* or IF* versus a* at365 nm reveals the apparent relative quantum efficiency of the different fractions. Higher values for low NMW fractions and HA50-100 are either due higher percentages of absorbing centers able to produce singlet oxygen or exhibit fluorescence or to lower quenching processes.

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.

Inhibition of humic substances mediated photooxygenation of furfuryl alcohol by 2,4,6-trimethylphenol. Evidence for reactivity of the phenol with humic triplet excited states.

To probe the reactivity of 2,4,6-trimethylphenol with humic triplet excited states, we investigated its influence on the humic substances-mediated photooxygenation offurfuryl alcohol. Elliott soil humic and fulvic acids were employed for these experiments. When added in the concentration range of 10(-4) - 10(-3) M, 2,4,6-trimethylphenol inhibited furfuryl alcohol photooxygenation to an extent depending on its concentration. The inhibiting effect decreased as the oxygen concentration was increased. By postulating that 2,4,6-trimethylphenol competes with oxygen for reaction with humic triplet excited states and with furfuryl alcohol for reaction with singlet oxygen, we obtained kinetic laws describing the consumption profiles of furfuryl alcohol and 2,4,6-trimethylphenol. Experimental rates of 2,4,6-trimethylphenol and furfuryl alcohol loss could be satisfactorily fitted with 1.09-1.16 for the ratio k2/k3, where k2 and k3 are the reaction rate constants of humic triplet excited states with oxygen and 2,4,6-trimethylphenol, respectively. These types of experiments could be extended to a variety of substrates to measure their reaction rate constants with humic triplet excited states.

Photolysis of fluometuron in the presence of natural water constituents.

Phototransformation of the herbicide fluometuron (1 microM) in natural sunlight was investigated in neutral Milli-Q water and in synthetic waters containing either fulvic acids, nitrate ions or both in order to mimic reactions taking place in aquatic environments. Fluometuron degradation followed a pseudo-first order kinetics. The reaction was faster in synthetic than in Milli-Q water. Fulvic acids (10 mg l(-1)) increased the rate of fluometuron photolysis by a factor 2.5 and nitrates (25 mg l(-1)) by a factor 15. Identification of major photoproducts was conducted under laboratory conditions using LC-ESI-MS. Numerous photoproducts were detected and tentatively characterized. In the presence of nitrates, hydroxylation of the aromatic ring with or without hydrolysis of CF(3) into CO(2)H and oxidation of the urea chain leading to demethylation were observed. In the presence of fulvic acids, hydroxylation of the aromatic ring was the major reaction route.