Humic Acid: Aggregate, Floc, Macromolecule, Micelle, Polymer, or Supramolecule?

Two humic acids, extracted from an Alfisol forest soil and a lignite mined in Greece, were purified of ash components, size fractionated by membrane dialysis, and polarity fractionated by adsorption chromatography on XAD-8 resin to determine binding mechanisms of humic acid constituents. Humic acids, contained in 12,000 to 15,000 dialysis membranes, were successively dialyzed against 0.1 M HCl, 0.2M HF, 0.1M sodium citrate at pH 10, and 0.1 M NaOH to remove ash components and disaggregate and size fractionate with increasing pH. A part of humic acid did not permeate the dialysis membranes during the acid treatment, while the humic acid that permeated during the sodium citrate and NaOH treatments, was polarity fractionated by a decreasing pH gradient on XAD-8 resin. The resulting fractions were characterized by mass balance, elemental analyses, pH titration, and FT-IR spectrometry. Certain fractions of the lignite humic acid had characteristics very similar to molecular condensates formed by radical coupling of phenolic acid precursors of lignin. Significant differences between the composition and fractionation patterns of the soil and lignite humic acids were interpreted in terms of binding mechanisms within the humic acid precipitates.