Impact of UV254-radiation on aquatic humic substances

Aqueous solutions of humic substances (humic and fulvic acids) were exposed to UV-radiation at 254 nm (UV254). The induced changes in the dissolved organic carbon content (DOC), in the absorbance of radiation at 272 nm (A272), and in the solid state 13C NMR spectral data were recorded. During the first 40-h exposure of the humic acid solution, the DOC-normalised UV272-absorbance (A272/DOC ratio) was approximately constant, indicating a fairly high mineralisation rate as compared to the rate of the degradation of UV272-absorbing structural subunits. During prolonged exposure, however, the mineralisation rate was clearly retarded. The corresponding normalised values for the fulvic acid solution demonstrated that the relative “rate of mineralisation” was constantly lower. Fulvic acid was more susceptibile for UV-initiated degradation than humic acid. The NMR spectra revealed that the high-molecular-weight matter in both of the exposed samples changed to a more aliphatic character and that more carboxyl and carbonyl carbon atoms appeared.