Compositional differences between soil humic acids extracted by various methods as evidenced by photosensitizing and...

Humic acids (HAs) were isolated from Elliott soil provided by the International Humic Substances Society (1BS102M) by three commonly used methods: (i) 0.1M NaOH (EI-HA), (ii) neutral 0.1M Na(4)P(2)O(7) (L-HA) and subsequent 0.1M NaOH (S-HA), and (iii) 0.1M NaOH+0.1M Na(4)P(2)O(7) (NP-HA). The objective was to evaluate the impact of these extractants on the photosensitizing properties of the isolated HAs. HAs were analyzed for their elemental composition, functional acid groups content, absorption and emission properties, electrophoretic characteristics and ability to produce singlet oxygen using furfuryl alcohol (FFA) as a scavenger. L-HA was slightly more aromatic and oxygenated than the other HAs and contained a higher portion of long-wavelength fluorophores and macromolecules showing low molecular size (MS) and high electrophoretic mobility. L-HA also gave a rate of FFA photooxygenation between 1.25- and 1.6-fold higher than the other HAs. This suggests that the free humic macromolecules ionized at pH 7 and/or weakly bounded on mineral surfaces via cation bridges are of relatively low MS and contribute significantly to the photosensitizing and long-wavelength emitting properties. Differences among the other HAs were more subtle, but the parallel evolution of the reactivity and electrophoretic characteristics was observed. Photochemical and electrophoretic measurements seem to be sensitive indicators to evaluate differences among the extraction procedures of HAs.