Relationship between Photosensitizing and Emission Properties of Peat Humic Acid Fractions Obtained by Tangential Ultrafiltration

International audiencePeat humic acid was fractionated by tangential ultrafiltration into six nominal mol. wt. (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 sepd. fractions were compared and their ability to generate singlet oxygen under light excitation was evaluated, using furfuryl alc. (FFA) as a singlet oxygen scavenger. The absorbance, the emission intensity, and the apparent first order rate consts. of FFA loss were normalized per mol of org. carbon (a*, IF*, and k*, resp.). 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* vs. IF* indicates that the first order rate const. 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* vs. a* at 365 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