Structure of molecular weight fractions of bayer humic substances. 1. Low-temperature products

The humic substances in Bayer liquor from a bauxite alumina refinery operat ing at 145-150 degrees C have been separated by dialysis into seven fractio ns of different molecular weight and characterized by acidity, nuclear magn etic resonance (NMR) spectroscopy, infrared spectroscopy, pyrolysis gas chr omatography mass spectrometry (py-GC/MS), and thermogravimetric techniques including modulated differential scanning calorimetry. The higher molecular weight fractions contained more aromatic carbon than the lower molecular w eight fractions (e.g., >300 000 Da, 56.4%; < 1200 Da, 33.6%). Acidity, NMR, and infrared spectroscopy indicate that carboxylic groups are more prevale nt in lower molecular weight fractions (e.g., <1200 Da, 20.1%) than higher molecular weight fractions (e.g., >300 000 Da, 9.8%). Some fractions have p articularly interesting structures and properties. For example, lignin-deri ved humic substances seem to concentrate in the 6000-12 000 Da fraction, as indicated by lignin guaiacyl residues. Involatile polycondensed material c oncentrates in the molecular weight fraction >300 000 Da. This fraction cou ld be best described as soluble char with unusual properties such as the ca pacity to hold occluded water above 200 degrees C.