Multi-method characterization of lake aquatic humic matter isolated with two different sorbing solids

Abstract

Aquatic humic solutes were isolated from lake water by two different methods: non-ionic sorbing solid (XAD technique) and weakly basic anion exchanger (DEAE cellulose). The humic solutes removed by the DEAE procedure from the fresh water sample at natural acidity consist of practically equivalent quantities of organic acids (ca. 80% of the dissolved organic carbon (DOC)) to the total amount of the various organic acids (so-called `hydrophobic' acids and `neutrals' and `hydrophilic' acids) obtained by the XAD technique at preadjusted acidity (pH 2) with different adsorbents. According to the 13 C and 1 H NMR spectra, the structural composition of acidic humic solutes obtained by the DEAE isolation procedure appears to be a combination of certain `hydrophobic' and `hydrophilic' acidic solutes obtained by the multi-stage XAD procedure. For setting up the structural composition of an average humic solute molecule, it is important to realize the significant (mostly also predominating) role of alkenyl, cycloalkyl, ester or amide groups as structural subunits in the complicated web of aggregated solutes. Although, the strongly acidic treatments connected to the isolation and fractionation of organic solutes by the XAD technique are somewhat coincidental, especially from the structural chemistry point of view, structural compositions of the different aquatic humic solute fractions derived by the XAD technique seem to be, to a certain degree, real products, not just accidental ones, of the isolation method. With a computational method developed, specially for the complex mixture of humic solutes, it was possible to estimate the most probable carbon distributions to an average humus molecule.