Characterisation of lake-aquatic humic matter isolated with two different sorbing solid techniques: tetramethylammonium hydroxide treatment and pyrolysis-gas chromatography/mass spectrometry

Abstract

Tetramethylammonium hydroxide (TMAH) treatment followed up with pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) was applied to characterise the structural composition of lake aquatic humic matter (HM). The HM samples were isolated with two different chromatographic methods: (1) non-ionic sorbing solid (standard XAD technique) with preacidification of a water sample to pH 2; and (2) weakly basic anion exchanger (DEAE, diethylaminoethyl cellulose) without any preacidification. A detailed structural analysis of degradation products was performed and compositional differences between different HM samples were evaluated. The main degradation products, in addition to varying proportions of different nitrogen and sulphur compounds, were methyl derivatives of different phenols, alkylphenols, phenolic acids and aliphatic mono- and dicarboxylic acids, which speak for the importance of ester linkages between the different structural subunits of aquatic HM. Despite all HM samples yielded qualitatively quite similar organic compounds, some fundamental quantitative differences existed. As a general rule, the DEAE isolate and two main XAD fractions: conventional fulvic (FA) and humic (HA) acids, seem to consist of almost similar organic matter dominating by different phenols and aliphatic acids. The other two special XAD fractions appear to belong to different structural categories yielding more alkylbenzenes/styrenes, alkanes and aliphatic monocarboxylic acids. The structural analyses and statistical calculations on the different HM samples support the postulation that the structural composition of the DEAE isolate corresponds to a certain average of the four different acidic XAD fractions.