Complexation of Aluminum Ions with Formic Acid Under Acidic Condition: 27Al NMR, ESI-MS, and DFT study

To understand the behavior of aluminum ions (Al3+) in strongly acidified soil environments, complexation between Al3+ and formic acid (FA), a natural monocarboxylic acid, at pH 3 was investigated using NMR, ESI-MS, and DFT techniques. In the presence of FA, two new Al-27 NMR peaks appeared at high magnetic fields, in addition to a peak due to Al(H2O)(6)(3+), suggesting the formation of monodentate 1 : 1 (AlFA(2+)) and 1 : 2 (AlFA(2)(+)) complexes. The trends observed in the Al-27 NMR analysis of the Al-FA complex was well represented by DFT calculations (i. e., move of the chemical shifts to the high magnetic field side, additivity low of the Al-27 NMR chemical shift, and change in electron density of each Al3+ nucleus). The average conditional formation constants of the Al-FA complexes were determined to be log K-1=1.17 and log K-2=0.91 for the 1 : 1 and 1 : 2 complexes. The formation constant of the acetate complex was also determined to be log K=1.80 (1 : 1 complex). These results strongly suggest that natural monocarboxylic acids produced from the decomposition of organic compounds and certain bacterial activities show potential for the detoxification of Al3+, that is, complex formation with toxic Al species in acidified soil.