RESUMO
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.
