Adsorption and transformation of salt marsh related humic acids by quartz and clay minerals

Abstract

Humic acids isolated from living and dead Spartina alterniflora plants and the marsh mud of a Georgia salt marsh estuary were adsorbed up to 12% on quartz particles, 85% on kaolinite and 90% on montmorillonite. The results of, for example, elemental analyses (up to 13.5% higher C content in humic acids remained after interaction with clay minerals) and Fourier transform infrared (FTIR) spectra indicate that aliphatic structures of the heterogenous humic acid molecules were preferentially adsorbed. However, since the ash contents of humic acids remaining in solution rose to as high as 45%, this fraction also apparently reacts with clay minerals. Up to 13.7% and 4.8% of humic acids were re-extracted from kaolinite and montmorillonite, respectively, using 0.1 N H 2SO 4. A mixture of 0.1 M NaOH + 0.1 M Na 4P 2O 7, enhanced the yields of 34.6% for kaolinite and 68.4% for montmorillonite. These numbers indicate that appreciable amounts of humic acids were transformed into humins which cannot be extracted by alkali from the mineral matrix. All re-extracted humic acids were rich in ash, which was composed of the clay mineral used. Although we found previously that the salt marsh related humic acids can be decomposed by microorganisms, their adsorption on mineral surfaces, especially on clay minerals, may contribute to their geochemical stability in the salt marsh environment.