Li Ke-wei,Lu Hai-long,Nkoh Nkoh Jackson,Xu Ren-kou
Purpose Phyllosilicate 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 methods Montmorillonite was treated at high temperatures to remove the hydroxyl groups from its surface, and then XRD, FTIR, and 27 Al 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 discussion The results showed that hydroxyl groups on the surface of montmorillonite can effectively be removed when the temperature was raised to 670 °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. Conclusions The 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.
Nkoh Jackson Nkoh,He Xian,Lu Hai-long,Li Ke-wei,Shi Ren-yong,Li Jiu-yu,Xu Ren-kou
Purpose This study aims to evaluate the effects of chitosan and D-fructose 1,6-bisphosphate (F16BP) trisodium salt on the acidity of selected Ultisols (Ultisol-QY1, Ultisol-QY2, Ultisol-LX, and Ultisol-YT) and their resistance to soil acidification. Materials and methods The effect of adsorbed chitosan and F16BP on soil acidity was evaluated by interacting different concentrations of the organic substrates with 4.0 g soil under different agitation times (2 to 264 h, 200 rpm, 25 °C). Also, the soils were amended with the organic substrate and incubated for 14 days to determine their effects on soil physicochemical properties. Afterwards, the soil solid phases from the adsorption studies and incubated soil were acidified with different concentrations of nitric acid to simulate soil acidification and activate soil aluminum (Al). Results and discussion The adsorption interactions of chitosan and F16BP with Ultisols demonstrated different potentials in their ability to displace mineral-bound hydroxyl groups and ameliorate soil acidity. The R-PO 4 3− group of F16BP was more effective in displacing the hydroxyl groups through a ligand exchange mechanism and thus showed greater ameliorating effect on soil acidity than chitosan. Compared to lime and F16BP, chitosan-treated soils exhibited the least decrease in pH during soil acidification due to the significant improvement in soil pH buffering capacity (pHBC). Specifically, when amended with chitosan, the pHBC of Ultisol-QY1, Ultisol-QY2, Ultisol-LX, and Ultisol-YT was increased by 262.2%, 88.9%, 337.7%, and 320.6% as opposed to 22.1%, 18.4%, 27.4%, and 83.0% for these Ultisols by F16BP, respectively. Thus, F16BP is a better material to improve soil pH, while chitosan is a better option to improve soil pHBC, inhibit soil acidification, and retard the activation of phytotoxic Al 3+ . By the nature of their functional groups, we deduced that F16BP ameliorated soil acidity by displacing the hydroxyl groups on soils due to adsorption of R-PO 4 3− group through ligand exchange mechanism, while chitosan directly improved pHBC by providing abundant R-NH 2 groups to accommodate excess exogenous H + . Conclusions Chitosan is a promising soil amendment material for use in the management of acidic soils due to its high N content and potential to inhibit soil acidification. Given the readily available raw materials and ease of producing chitosan, long-term incubation studies should be carried out to assess the effect of chitosan on soil pH variations, carbon, and N mineralization.
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