Photodegrading properties of soil humic acids fractionated by SEC-PAGE set-up. Are they connected with absorbance?

Humic acids (HAs) extracted from soils of different origins were fractionated using size exclusion chromatography-polyacrylamide gel electrophoresis set-up. For each HAs, three fractions showing distinct electrophoretic mobilities and molecular sizes (MS) were obtained. Bulk HAs and their fractions (25 mg L−1) were then compared for their ability to photosensitize the degradation of 2,4,6-trimethylphenol upon irradiation at 365 nm. Rate coefficients were deduced from pseudo-first order kinetics. For five of the six series, the rate coefficient varied in the order: high MS < medium MS < low MS. Except for one HA, the general tendency was a linear increase of the rate coefficient with 1–10−A, where A is the absorbance of HAs or fractions solution at 365 nm. It is an indication that the ratio of photosensitizing to non-photosensitizing chromophores is roughly constant among fractions.