Spectroscopic and chemical characterizations of molecular size fractionated humic acid

A sample of humic acid was divided by ultrafiltration into five fractions o f different molecular size (F1: 300 000: F2: 100 000-300 000: F3: 50 000-10 0 000: 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 fracti on of greater than or equal to 100 000 daltons were primarily aliphatic, wh ile the smaller molecules of the greater than or equal to 10 000 dalton fra ction 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 dissol ution) humic acid samples showed chemical alteration with some loss of carb oxyl groups in the humic acid. (C) 1999 Elsevier Science B.V. All rights re served.