In situ observations of the occlusion of a clay-sugar compound within calcite

Abstract

Organo–clay complexes could be adsorbed and subsequently occluded into soil mineral matrices under local supersaturated solution conditions, leading to inaccessibility of microorganisms and their extracellular enzymes, which plays an important contribution to stabilization of soil organic matter (SOM) and affects their biogeochemical cycle. However, the underlying molecular mechanisms remain poorly understood. Here we apply Raman spectroscopy to analyze the LAPONITE®-sugar (monosaccharide glucose (Glu) and 5/20 kDa dextran (Dex-5/20) polysaccharides) interactions and use in situ atomic force microscopy (AFM) to observe their occlusion processes within calcite. As shown by Raman spectra, the LAPONITE®-sugar complexes form with the mix of sugars and LAPONITE®, and the degree of elution is mediated by the molecular weight of sugars and more Glu would be eluted compared with Dex-5 and Dex-20. Then the LAPONITE®-sugar complexes could be occluded within calcite observed by AFM, and the occlusion of the LAPONITE®-sugar complexes within calcite hillocks are influenced by molecular weight with the trend of Dex-20 > Dex-5 > Glu. The binding force between sugars and calcite (104) surfaces are measured by AFM-based dynamic force spectroscopy (DFS) to record the molecular-scale interactions, and high-molecular weight sugar such as Dex-20 exhibits strongest binds with calcite surfaces as shown by DFS data. These in situ nanoscale observations and single-molecule determinations in a model system may provide insights into the clay-SOM-calcite fixation mechanisms by sugar in alkaline soils, with potential implications for global carbon cycling.