Effect of high-temperature dehydroxylation on acidification-induced aluminum activation in montmorillonite

PurposePhyllosilicate minerals are the important components in soils and an important source of activated aluminum (Al) during soil acidification. However, the mechanisms for Al activation in phyllosilicate minerals were not understood well. This study was designed to investigate the role of hydroxyl groups on the surface of montmorillonite in acidification-induced activation of solid aluminum.Materials and methodsMontmorillonite was treated at high temperatures to remove the hydroxyl groups from its surface, and then XRD, FTIR, and Al-27 NMR of montmorillonite before and after high-temperature treatment were determined. After that, Al activation during acidification of montmorillonite treated at different temperatures was investigated using the constant pH titration method.Results and discussionThe results showed that hydroxyl groups on the surface of montmorillonite can effectively be removed when the temperature was raised to 670 degrees C. After high-temperature treatment, the crystallinity of the montmorillonite was reduced. The montmorillonite layer space was reduced, the pH and cation exchange capacity (CEC) of the mineral were decreased, and consequently the amount of acid consumed during acidification was significantly reduced. The decrease of CEC and surface hydroxyl groups in the mineral was responsible for the reduction in acid consumption during mineral acidification. After high-temperature treatment, the Al coordination structure of montmorillonite was transformed from six-coordination to four-coordination. The removal of surface hydroxyl groups inhibited the activation of Al during montmorillonite acidification, and the inhibition effect was increased with increasing temperatures below the complete dehydroxylation point.ConclusionsThe activation of Al during montmorillonite acidification was inhibited, suggesting that the hydroxyl groups at the edge of montmorillonite played an important role in Al activation process.

Automated summary

The purpose of this study is to investigate the role of hydroxyl groups on the surface of montmorillonite in acidification-induced activation of solid aluminum. The results showed that the removal of surface hydroxyl groups inhibits the activation of aluminum during montmorillonite acidification.