Aluminum complexation by soil humic acids, with special reference to chelating ability
2015
Katsumi, N. (Ishikawa Prefectural University, Nonoichi, Ishikawa (Japan)) | Yonebayashi, K. | Okazaki, M.
We investigated the complexation of Al by soil humic acids (HAs) extracted from two Andisols, two Entisols, and an Inceptisol at a constant Alsup(3+) concentration at pH 4.0 using batch techniques. The quantity of Al bound to 1 g of soil HAs decreased in the order Andisol HAs > > Entisol HAs almost equal Inceptisol HAs. The quantity of Al bound to HAs showed a highly positive correlation with the degree of humification and the COOH content of the HAs and a highly negative correlation with the H/C atomic ratio of the HAs. Al-HA complexes were found to contain 1 mol of Al and 1 mol of carboxyl groups with acid dissociation constants of <3.2 (dissociated at pH 4.0). The competition for Al between HAs and several chelating agents was investigated; specifically, HAs were added to complexes of Al and iminodiacetic acid (IDA), nitrilotriacetic acid (NTA), oxalic acid, and citric acid. The Al-HA complexes were more stable than the Al-IDA complex and less stable than the Al-NTA, Al-citric acid, and Al-oxalic acid complexes. These results imply that binding strength between Al and HAs is relatively low under acidic condition, suggesting that HAs in the complex are able to perform ligand-exchange with organic acids in acidic soil.
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