Synthesis of amino-phenolic humic-like substances and comparison with natural aquatic humic acids: A multi-analytical techniques approach

Monitoring of the synthesis of low and high molecular weight humic-like molecules by oxidative reaction between catechol and one amino-acid (glycine) or one dipeptide (triglycine) was conducted by Fourier transform infrared spectroscopy (FTIR) and fluorescence measurements. The results suggest that a well-defined energy threshold should be reached to allow the formation of humic substances. This is substantiated by the occurrence of the humic-like fluorescence peak (around 350/450 nm for excitation–emission wavelengths), referred to as “alpha” component. In addition, nitrogen incorporation was found to be one of the fundamental processes governing the behaviour of fluorescence substances. The study of the synthesis of these model molecules appears to be a promising route to obtain a variety of materials approaching the properties and the variability of natural substances but in a somewhat more reproducible way. In this report, an approach relying on several complementary techniques is proposed. It involves elemental analysis, potentiometric titration, Fourier transform infrared spectroscopy, high-performance liquid chromatography, pyrolysis gas chromatography–mass spectrometry and UV/visible absorption spectrophotometry. Model synthetic molecules were characterised and compared to natural humic substances extracted from the materials suspended in waters of a river. All combined analytical results led to the conclusion that protein-like structures could be used to describe humic-like structures and could be related to recent supramolecular association concepts proposed for the structural description of humic substances. Synthetic molecules display the main chemical features of naturally occurring humic acids and could therefore be suitable for investigating the reactivity of humic substances.