Atomic Status of Al and Fe Reacted with Humic Acid as Determined by XAFS and Solid-State NMR.

Andisols in Japan accumulate large amounts of humic substances. Both Al and Fe are associated with humic substances, and Al has a more prominent ability to accumulate humic substances than Fe. To elucidate the mechanism governing the differences in the ability of Al and Fe to accumulate humic acid, we investigated the atomic status of Al and Fe associated with humic acid.

Al/Fe nitrate solutions were mixed with humic acid solution to accomplish metal to COOH ratio ranging from 0.1 to 2, at pH 4 and 6 and aged for 24 h and 180 d. After aging, the concentrations of humic acid and Al or Fe were determined after filtration and precipitated amounts of humic acid, Al, and Fe were calculated. After 180 d, the mixture was freeze-dried for spectroscopic analyses. Atomic status of Al and Fe associated with humic acid was evaluated by solid-state ²⁷Al nuclear magnetic resonance (NMR) and Al K-edge and Fe K-edge X-ray absorption fine structure (XAFS). The XAFS data acquisition was done at beamlines 11A and 12C at the Photon Factory, High Energy Accelerator Research Organization.

Larger amounts of humic acid were precipitated by the addition of Al than Fe at the same pH and ratio of metal to humic acid. The amounts of precipitated humic acid were not changed from 24 h to 180 d for the Al-humic acid system, but they decreased significantly for the Fe-humic acid system. The NMR and XAFS analyses showed that the Al-humus complex was predominant in the precipitate of Al-humic acid mixture after 180 d of reaction. On the other hand, the presence of the Fe hydroxide phase was suggested by XAFS analyses. The ability to accumulate humic substances was lower for Fe than Al because the precipitation of Fe hydroxide takes precedence over formation of Fe-humus complex.