From the viewpoint of the phytotoxicity and mobility of Al
3+ released from soil minerals due to soil acidification, the interaction between Al
3+ and acrylic acid (AA) and polyacrylic acid (PAA) as a model compound of fulvic acid was investigated. The interaction was examined at pH 3 so as to avoid the hydrolysis of Al
3+. The interaction between Al
3+ and AA was weak. However, the interaction between Al
3+ and PAA was strong and depended on the initial (COOH in PAA)/Al molar ratio (
RP) of the solution. For the range of 1/
RP, the interaction between Al
3+ and PAA can be divided into three categories: (1) 1:1 Al-PAA-complex (an Al
3+ combines to a carboxyl group), (2) intermolecular Al-PAA-complex (an Al
3+ combines to more than 2 carboxyl groups of other Al-PAA-complexes) in addition to the 1:1 Al-PAA-complex and (3) precipitation of intermolecular complexes. In conclusion,
RP is an important factor affecting the behavior of Al
3+ in acidic soil solution.
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