Aluminum complexation by soil humic acids, with special reference to chelating ability

摘要

We investigated the complexation of Al by soil humic acids (HAs) extracted from two Andisols, two Entisols, and an Inceptisol at a constant Al~(3+) concentration at pH 4.0 using batch techniques. The quantity of Al bound to 1g of soil HAs decreased in the order Andisol HAs ＞＞ Entisol HAs ≈ 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.