Soils IssuesMOLECULAR AGGREGATION OF HUMIC SUBSTANCESWershaw, Robert L.Author Information U.S. Geological Survey, Mail Stop 408, Federal Center, Denver, CO 80225. E-mail [email protected] Received Aug. 3, 1999; accepted Aug. 13, 1999. Soil Science: November 1999 - Volume 164 - Issue 11 - p 803-813 Buy Abstract Humic substances (HS) form molecular aggregates in solution and on mineral surfaces. Elucidation of the mechanism of formation of these aggregates is important for an understanding of the interactions of HS in soils and natural waters. The HS are formed mainly by enzymatic depolymerization and oxidation of plant biopolymers. These reactions transform the aromatic and lipid plant components into amphiphilic molecules, that is, molecules that consist of separate hydrophobic (non-polar) and hydrophilic (polar) parts. The nonpolar parts of the molecules are composed of relatively unaltered segments of plant polymers and the polar parts of carboxylic acid groups. These amphiphiles form membrane-like aggregates on mineral surfaces and micelle-like aggregates in solution. The exterior surfaces of these aggregates are hydrophilic, and the interiors constitute separate hydrophobic liquid-like phases. © 1999 Lippincott Williams & Wilkins, Inc.