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, HA(5-10), HA(10-20), HA(20-50), HA(50-100), HA(100-300) 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*, I-F*, and k*, respectively). The fraction absorbance decreased with NMW, except for HA(>300) which was less absorbing than HA(100-300). The low NMW fractions and the HA(>300) fraction generally showed lower k* and I-F* values compared to the HA(50-100) and HA(100-300) fractions. A plot of k* versus I-F* indicates that the first order rate constant of FFA photo-oxygenation increased with the intensity of fluorescence at 380, 430, and 500 nm (R-2 = 0.77-0.84). This shows that the distribution of fluorescent centers among fractions paralleled that of photosensitizing centers. Plotting k* or I-F* versus a* at 365 nm reveals the apparent relative quantum efficiency of the different fractions. Higher values for low NMW fractions and HA(50-100) are either due higher percentages of absorbing centers able to produce singlet oxygen or exhibit fluorescence or to lower quenching processes.