Comparison of the photoinductive properties of commercial, synthetic and soil-extracted humic substances

The photochemical behavior of a fulvic acid (FA) and several humic acids (HAs) were studied. These synthetic, commercial or soils-extracted humic substances were compared according to their abilities to sensitize the formation of singlet oxygen and to photoinduce the transformation of aromatic compounds such as 2,4,6-trimethylphenol (TMP) and 1,1-dimethyl-3-phenylurea (fenuron). Singlet oxygen was detected by ESR experiments, and quantum yields of singlet oxygen formation were measured by using 1,2-difuranyl-2-hydroxyethanone (furoin) as a probe molecule. The photoinductive capacities were evaluated from the kinetics of TMP and fenuron disappearance. The humic substances that showed the greatest ability to generate singlet oxygen were also most efficient in photoinducing the transformation of TMP and fenuron. The efficiencies were found to increase in the following order: synthetic HAs << Podzol HA < Rendzina HA < Aldrich HA < Ranker HA < Ranker FA. Though humic substances formed under the same ecological conditions exhibited similar photoinductive properties, the structural characteristics (IR, elemental analyses, solid-state (CNMR)-C-13) showed no apparent correlations with the photochemical behavior.