Spectroscopic and chemical characterizations of molecular size fractionated humic acid.

A sample of humic acid was divided by ultrafiltration into five fractions of different molecular size (F1; 300 000: F2; 100 000-300 000: F3; 50 000-100 000: F4; 10 000-50 000: F5; 1000-10 000 daltons). Characterization by IR, and CPMAS C-13 NMR spectroscopy indicated that the molecules of the fraction of >/=100 000 daltons were primarily aliphatic, while the smaller molecules of the >/=10 000 dalton fraction were predominantly aromatic. Titration (pH) data were consistent with an increase in the number of carboxylate groups per unit mass as molecular size became smaller. A comparative study with unpurified and purified (by treatment with ion exchange elution, acid precipitation and alkaline dissolution) humic acid samples showed chemical alteration with some loss of carboxyl groups in the humic acid.