Reactive species produced on irradiation at 365 nm of aqueous solutions of humic acids

In order to characterize the reactive species produced on irradiation at 365 nm of humic acid (HA) solutions, the photoinduced transformation of aromatic compounds was studied as a function of several parameters (concentrations of substrates, HA and oxygen, pH of the solution) and in the presence of additives such as isopropanol, ethylenediaminetetraacetic acid (EDTA), H2O2 and Fe2+. No consumption of the substrates was observed in deoxygenated medium. In oxygenated solutions containing commercial HA, the rates of phenol disappearance were significant; the rate laws were dependent on the substrate and HA concentrations. In weakly oxygenated medium, the consumption rates were lower and other reactive species were involved. EDTA and citric acid led to partial inhibition of the reactions, indicating that the reactive species were related to metallic cations contained in HA. In acidic solution, hydroxyl radicals were produced by a photo-Fenton reaction. HA synthesized from phenol was incapable of photoinducing the transformation of the substrates, whereas HA synthesized from phenol-Fe3+ produced transformation of the substrates. The formation of singlet oxygen on irradiation of commercial HA was characterized previously. Unexpectedly, no consumption of phenolic substrates through singlet oxygen reaction was observed.