Effect of Coordination Structure of Aluminum on Its Activation from Minerals and Oxisols During Their Acidification

The types and contents of phyllosilicate minerals in soils play an important role in soil acidification, as soil acid buffering capacity varies with the composition of the phyllosilicate minerals. In addition to aluminum-oxygen (Al-O) octahedrons, a certain number of Al-O tetrahedrons exist in phyllosilicate minerals due to the isomorphic substitution of silicon ion (Si4+)by aluminum ion (Al3+) in Si-O tetrahedrons of minerals. However, the effect of the two coordination structures of Al on the release of Al during mineral acidification has not yet been investigated. Therefore, the differences in Al activation in phyllosilicate minerals and soils with different Al coordination structures were investigated through constant-pH experiments and 27 Al magic-angle spinning nuclear magnetic resonance (MAS-NMR) measurements. The results of( 27) Al MAS-NMR spectra showed that kaolinite contained Al-O octahedrons, phlogopite and illite contained Al-O tetrahedrons, and vermiculite composite contained both octahedral and tetrahedral Al. At pH < 5.1, the content of Al released from minerals during simulated acidification followed the order: illite > vermiculite composite > phlogopite > kaolinite, which was consistent with the orders of cation exchange capacity and content of tetrahedral Al of the minerals. According to the rate constants, the Al release rates were in the order of phlogopite > illite > vermiculite composite > kaolinite at pH 4.8. Except for phlogopite, the Al release rates in these minerals increased with decreasing suspension pH. Therefore, the Al release contents and rates were greater in phlogopite, illite, and vermiculite composite containing Al-O tetrahedrons than in kaolinite containing only Al-O octahedrons. Two Oxisols derived from basalt with different ages were selected for similar studies. The (27) Al MAS-NMR spectra of the Oxisols showed that the 0.01-million-year (Ma) Oxisol contained both octahedral and tetrahedral Al, while the 1.33-Ma Oxisol contained only Al-O octahedrons. The contents of both exchangeable and soluble Al released from the 0.01-Ma Oxisol were greater than those from the 1.33-Ma Oxisol when the two soils were acidified to the same pH. The results from minerals and soils confirmed that Al was more readily released into solution and exchangeable sites as soluble and exchangeable Al in Al-O tetrahedrons than in Al-O octahedrons during the acidification of soils and minerals. The findings of this study will provide useful references for investigating the mechanisms of solid phase Al release and for mitigating soil acidification and inhibiting Al activation in different soil types.