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Department of Plant, Soil and Insect Sciences, University of Massachusetts, Amherst, MA 01003
* Corresponding author (bx{at}pssci.umass.edu or bx{at}psis.umass.edu)
Received for publication June 23, 2004. Clay–humic complexes are commonly distributed in natural environments. They play very important roles in regulating the transport and retention of hydrophobic organic contaminants in soils and sediments. This study examined the structural changes of humic acid (HA) after adsorption by clay minerals and determined phenanthrene sorption by clay–humic complexes. Solid- and liquid-state 13C nuclear magnetic resonance (NMR), for the first time, provided direct evidence for HA fractionation during adsorption on mineral surfaces, that is, aliphatic fractions were preferentially adsorbed by clay minerals while aromatic fractions were left in the solution. The ratio of UV absorbance of HA at 465 and 665 nm (E4 to E6 ratio), which is related to aromaticity, corroborated with the NMR results. For both montmorillonite and kaolinite, adsorbed HA fractions had higher sorption linearity (N) and affinity (Koc) than the source HA. The Koc of adsorbed HA for the clay–humic complexes could be up to several times higher than that of the source HA. This large increase may be contributed by the low polarity of the bound HA. Moreover, for each mineral, the N values of adsorbed HA increased with increasing HA loading. It is believed that HA may develop a more condensed structure on mineral surface at lower HA loading level due to the stronger interactions between HA and mineral surface as a result of close contacts.
Abbreviations: CP, cross-polarization • HA, humic acid • HOCs, hydrophobic organic compounds • HS, humic substances • Kd, distribution coefficient at a given concentration • KF, Freundlich coefficient • Koc, organic carbon–normalized sorption coefficient • MAS, magic angle spinning • N, Freundlich exponent or nonlinearity parameter • NMR, nuclear magnetic resonance • OC, organic carbon • SOM, soil organic matter • TOSS, total sideband suppression
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