Process And Chemical Mechanisms Of Occlusion Of Representative Organic Substances Within Calcite And Iron Oxides

Soil organic matter(SOM)and organic pollutants can interact physiochemically with mineral matrices to be selectively preserved and occluded in the growth or aggregation process of soil minerals.Inclusion organics within growing minerals and pore spaces of mineral aggregates are potentially inaccessible to plant root cells and soil microorganisms via the limitation of their movement.Moreover,these processes would be mediated by various factors,such as the properties of SOM or organic pollutants,mineral species and soil solution conditions.Previous studies mainly observe these processes at microscopic levels,but the microscopic mechanisms that control the process are poorly understood.In our current study,we applied time-resolved in situ atomic force microscopy(AFM)and Raman spectra to observe the microscopic process of occlusion of SOM such as humic substances or sugars and organic pollutants such as nanoplastics mixed with PMG by model minerals including calcite,clay and iron oxides.We also applied dynamic force spectroscopy(DFS)to quantify the molecular-scale interactions between SOM or organic pollutants and calcite.The main results are listed as following:1.Direct observations of the occlusion of soil organic matter within calcite.Calcite play important roles for the fixation of SOM in alkaline soils.Here,using timeresolved in situ AFM to observe how calcite,a representative mineral in alkaline soils,interacts with humic substances,we show that following adsorption,humic substances are gradually occluded by the advancing steps of spirals on the calcite(101 4)face grown in relatively high supersaturated solutions,through the embedment,compression and closure of humic substance particles into cavities.This occlusion progress is inhibited by phytate at high concentrations(10-100 μM)due to the formation of phytate-Ca precipitates on step edges to prevent the step advancement,whereas phytate at relatively low concentrations(≤ 1 μM)and oxalate at high concentrations(100 μM)have little effect on this process.These in situ observations may provide new insights into the organo-mineral interaction,resulting in incorporation of humic substances into minerals with a longer storage time to delay degradation in soils.This will improve our understanding of carbon cycling and immobilization in soil ecological systems.2.In situ observations of the occlusion of a clay-sugar compound within calciteOrgano-clay complexes could be adsorbed and subsequently occluded into soil mineral matrices by calcite and clays collectively.Here we apply Raman spectroscopy to analyze the laponite-sugar(monosaccharide glucose(Glu)and 5/20 k Da 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(10 4)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 singlemolecule determinations in a model system may provide insights into the clay-SOMcalcite fixation mechanisms by sugar in alkaline soils,with potential implications for global carbon cycling.3.Nanoscale imaging of simultaneous occlusion of nanoplastics and glyphosate within Soil MineralsNanoplastics are widely distributed in crop soils,and they can combine with other pollutants.Minerals could interact with nanoplastic-pollutants complexes to limit their movement in soils.Here we use time-resolved AFM to observe how model soil minerals interact in situ with different functional groups of polystyrene(PSFG)mixed with PMG.Our results show that the PSFG-PMG complexes are occluded within calcite and iron hydroxide particles through growing hillocks and aggregation,respectively.Among them,minerals could occlude more PSFG with carboxyl and sulfonic groups.Moreover,more particles would be occluded with the increase of PMG concentration.The free energies of binding between functional groups of polystyrene and calcite surfaces measured by AFM-based dynamic force spectroscopy in the presence of different concentrations of PMG account for molecular interactions involved in the occlusion process and the effect of PMG concentrations.These in situ nanoscale observations and molecular-scale energetic measurements in a simple model system may provide insights into the immobilization of both nanoplastics and pesticides by soil minerals,with potential implications in multiple pollutant sequestration.