Effect of micelle-like aggregates of humic acids on singlet oxygen experiments

LUCIANO CARLOS; BRIAN W. PEDERSEN; MÓNICA C. GONZALEZ; PETER R. OGILBY; DANIEL O. MARTIRE

La Serena

Congreso; X Latin-American Meeting on Photochemistry and Photobiology (IX ELAFOT); 2010

Humic acid (HA) form micelle-like aggregates in solution with a hydrophobic interior and hydrophilic periphery. In soil environments, such aggregates can play important roles in the transport, bioavailability and biodegradability of hydrophobic organic pollutants. On the other hand, photolysis of HA leads to the formation of a complex mixture of reactive species. Among these species, singlet oxygen [O2 (1aDg)] is argued to be responsible for the degradation of many organic pollutants. In this work, the effect of the formation of HA aggregates on the O2 (1aDg) lifetime was ivestigated. Additional assays were carried out in the presence of hydrophylic (TMPyP) and hydrophobic (TPP) porphyrins as the sensitizers, and rubrene and NaN3 as the O2 (1aDg) quenchers. Also, the shape and the size of the HA aggregates and sensitizers location in the aggregates were analyzed by microscope techniques. All the HA dispersions were made in D2O and the pH range 8.5-10.5. The singlet oxygen phosphorescence at 1270 nm was detected both in cuvette and in microscope experiments. In the latter case, the laser beam was focused either on the micelle-like aggregates or on the bulk phase. The results showed the formation of micelle-like aggregates above the critical micelle concentration (cmc = 0.4 g L-1, as determined by using the fluorescence probe pyrene). These aggregates have no uniform size (0.5 - 30 m average diameter). In the presence of both TMPyP and TPP, microscope images showed that TMPyP is located in the exterior surface of the aggregates, whereas TPP is mainly inside the aggregates. Quenching of O2 (1aDg) experiments performed in the cuvette showed an increase of 1/tau with increasing [HA] in the range 0.05-1.6 g L-1. The plot shows two linear regions. The intersecetion of the two slopes of the straight lines corresponds to a HA concentration close to the cmc. The same behavior was obtained when TMPyP or TPP were added to the HA dispersion. Addition of NaN3 (1.0 mM) lead to complete quenching of the O2 (1aDg) at all HA concentrations tested. Microscope experiments showed that only when TMPyP was used as the sensitizer and the laser beam was focused on the aggregates, singlet oxygen was detected (tau = 13.6 microseconds). The environmental consequences of these results are discussed.