Analytical pyrolysis and computational chemistry of aquatic humic substances and dissolved organic matter

Humic acids (HA), fulvic acids (FA), non-humic substances (NHS) and dissolved organic matter (DOM) in a bog lake water are investigated by analytical pyrolysis. The applied thermal methods are direct, in-source pyrolysis-field ionization mass spectrometry in the high electric field (Py-FIMS), and Curie-point pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) in combination with library searches. Based on the identified building blocks and together with complimentary analytical data, proposals for a general concept of the basic molecular structures of humic macromolecules in water are put forward. Computational chemistry is utilized for structural modeling and geometry optimization of DOM. Molecular mechanics calculations are performed to evaluate the conformation of structural, three-dimensional models and to determine the total energy and the partial contributions from bond-, angle-, dihedral-, van der Waals-, stretch-bend-, and electrostatic energies. Quantitative structure-activity relationship (QSAR) properties are calculated and allow the correlation of molecular structures with properties such as mass, surface area, volume, partial charges (electronegativity), polarizability, refractivity, hydrophobicity, and hydration energy. The principal aim and long-term strategy are to develop step by step improvements of the presented model structures of organic matter in water which explain the molecular composition as well as their ecological meaning, dynamic character, and structure-property relationships in natural and contaminated aquatic and terrestrial systems. In a first integrated approach, the dissociation and association processes of humic substances are simulated at nanochemistry level and are proposed as concepts for future collaboration incorporating results of additional chemical, biological, spectroscopic and microscopic methods. (C) 1999 Elsevier Science B.V. All rights reserved.