Effect of coordination nature of aluminum on its activation from minerals and Oxisols during their acidification

The types and amounts of phyllosilicate minerals in soils play an important role in soil acidification, as the acid buffering capacity of the soil 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 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 containing different Al coordination structures were investigated through constant pH experiments and 27Al magic-angle spinning nuclear magnetic resonance (MAS-NMR) measurements. The results of 27Al MAS-NMR spectra showed that kaolinite contained Al-O octahedron, phlogopite and illite contained Al-O tetrahedron, and vermiculite-composite contained both octahedral and tetrahedral Al. At pH < 5.1, the amount of Al released from the minerals during simulated acidification followed the order: illite > vermiculite-composite > phlogopite > kaolinite, which was consistent with the cation exchange capacity and tetrahedral Al of the minerals. According to the rate constants, Al release rates were in the order of phlogopite > illite > vermiculite-composite > kaolinite at pH 4.8. Except for phlogopite, Al release rates in these minerals increased with decreasing suspension pH. Therefore, the amount and rate of Al release in phlogopite, illite, and vermiculite-composite containing Al-O tetrahedron were greater than those in kaolinite containing only Al-O octahedron. Two Oxisols derived from basalt with different ages were selected for similar study. 27Al MAS-NMR spectra of the Oxisols showed that the 0.01 million years (Ma) Oxisol contained both octahedral and tetrahedral Al, while the 1.33 Ma Oxisol contained only Al-O octahedrons. The results showed that the amounts of both exchangeable and soluble Al released from 0.01 Ma Oxisol were greater than those from 1.33 Ma Oxisol when the soils were acidified to the same pH. The results from minerals and soils confirmed that Al in Al-O tetrahedrons was more readily released into solution and exchangeable sites as soluble Al and exchangeable Al than that in Al-O octahedrons during the acidification of soils and minerals. The results of this study will provide useful references for investigating the mechanisms of solid phase Al release in different soil types and for mitigating soil acidification and inhibiting Al activation.