Molecular size of humic substances. Supramolecular associations versus macromolecular polymers (Reprinted from Advances in Environmental Research, vol 3, pg 508-521, 2000)

This work illustrates recent experiments conducted by low-and high-pressure size exclusion chromatography (HPSEC) on humic substances of different ori gin and molecular structure. Solutions of humic substances were either inje cted as such into the HPSEC system that operated with eluents which differe d in composition but were constant in ionic strength, or were previously mo dified with mineral and organic acids and then eluted in the HPSEC operatin g with a constant solution. The findings indicate that humic substances beh ave as associations of relatively small molecules rather than as macromolec ular polymers. The weak association of humic molecules into supramolecular structures is stabilized by weak hydrophobic forces. Interactions with mine ral and organic acids of different degrees of aliphaticity may reversibly d isrupt the supramolecular association and separate smaller sub-units. The r ecognition that soil humus possesses supramolecular rather than polymeric p roperties is of importance in further understanding the function and reacti vity of soil organic matter in the environment.