Effect Of Calcium Carbonate On The Stability Of Cinnamon Soil Nanoparticles

As the smallest and most active organic-inorganic complex in soil,soil nanoparticles（NPs）have an important impact on the migration of nutrients,organic pollutants and heavy metals.Due to their diversity and heterogeneity,researchers have mainly investigated the effect of solution chemistry factors on the stability of soil nanoparticles,but little attention is paid to the influences of particle material composition.Calcium carbonate is an important cementing material of soils developed from loess,which plays a crucial role in the formation of loess granular structure.However,its microscopic mechanism is still unclear.This paper mainly studied the colloidal properties of calcium carbonate,and the differences in the properties of nanoparticles in three types of eluviated cinnamon soil,typical cinnamon soil and carbonate cinnamon soil with different calcium carbonate contents.The influence of calcium carbonate on the aggregation of cinnamon soil nanoparticles was further explored by removing calcium carbonate with hydrochloric acid,and the reason for the cementation of calcium carbonate was clarified from the perspective of surface electrochemical.The main results are as follows:（1）Calcium carbonate nodule colloids and calcium carbonate nodule nanoparticles were obtained through the physical extraction method and chemical dispersion method,respectively,in contrast with the artificial nano-calcium carbonate.The mineral composition,zeta potential and colloidal stability of those fine particles were determined by X-ray diffractometer,zeta potential analyzer and dynamic light scattering apparatus.The results showed that the initial particle diameters of calcium carbonate nodule colloids,calcium carbonate nodule nanoparticles and artificial nano-calcium carbonate were 224.24,88.01 and 98.50 nm,respectively.However,the polydispersity of calcium carbonate nodule colloids and calcium carbonate nodule nanoparticles were higher than that of artificial nano-calcium carbonate.The content of calcite in calcium carbonate nodule colloids was 70.3%,followed by quartz,feldspar and illite.Calcium carbonate nodule nanoparticles were mainly contained calcite and illite with a content of 48%and 45%,respectively.The testing calcium carbonate colloids were all negatively-charged,and the absolute values of zeta potential increased with the increase of solution p H.The critical coagulation concentrations of calcium carbonate nodule colloids in Na Cl and Ca Cl₂ solutions were 538.01 and 2.08 mmol L^-1,respectively.The critical coagulation concentrations of calcium carbonate nodule nanoparticles in Na Cl and Ca Cl₂ solutions were 82.18 and 1.11 mmol L^-1,respectively.Nevertheless,the critical coagulation concentrations of artificial nano-calcium carbonate were 80.37 and 1.59 mmol L^-1,respectively.The differences in mineral composition of the three calcium carbonate colloids were the primary reason for the discrepancy in their aggregation behaviors,followed by particle diameter and solution chemical conditions.（2）Eluviated cinnamon soil nanoparticles（ECS-NP）,typical cinnamon soil nanoparticles（TCS-NP）and carbonate cinnamon soil（CCS-NP）were obtained by probe-sonication and high-speed centrifugation.The particle composition,surface properties,and the aggregation kinetics of soil nanoparticles were investigated.Correlation analyses between soil/soil nanoparticles properties and critical coagulation concentrations were further discussed.The critical coagulation concentrations of ECS-NP,TCS-NP and CCS-NP in Na Cl solution were 318.69,130.34 and 46.73 mmol L^-1,respectively;and the corresponding critical coagulation concentrations in Ca Cl₂solution were 1.71,1.10 and 0.96 mmol L^-1,respectively.The particle size,specific surface area,and Hamaker constant of soil nanoparticles followed the order of ECS-NP<TCS-NP<CCS-NP,while the yield,zeta potential（in absolute values）,and the critical coagulation concentrations were in the reverse trend.Soil Ca CO₃ content and particle diameter,specific surface area and zeta potential of soil nanoparticles were significantly correlated with the critical coagulation concentrations.Redundancy analyses manifested that the soil Ca CO₃ content explained 88.4%of the critical coagulation concentrations,while the specific surface area of soil nanoparticles explained 98.7%.The higher the content of calcium carbonate in soil,the worse the stability of the nanoparticles,and the easier the particles to aggregate.The composition of soil nanoparticles played an important role in dominating the stability（aggregation and dispersion）of soil nanoparticles.（3）HCl was used to remove calcium carbonate from typical cinnamon soil and carbonate cinnamon soil,and the carbonate-removed typical cinnamon soil nanoparticles and carbonate-removed carbonate cinnamon soil nanoparticles was extracted.Their particle size distribution,zeta potential,critical coagulation concentrations,and charge density were compared with TCS-NP and CCS-NP.The results showed that the yield and surface negative charge of carbonate-removed typical cinnamon soil nanoparticles and carbonate-removed carbonate cinnamon soil nanoparticles increased,and the diameter and specific surface area decreased.The critical coagulation concentrations of carbonate-removed typical cinnamon soil nanoparticles in Na Cl and Ca Cl₂ solutions increased by 1.17 and 1.21 times,respectively.Carbonate-removed carbonate cinnamon soil nanoparticles in Na Cl and Ca Cl₂ solution increased 2.55 and 1.38 times,respectively,showing stronger stability.After removing the soil calcium carbonate,soil nanoparticles were not easy to aggregate,which confirmed the role of calcium carbonate in promoting the formation of loess aggregates.Based on this study,it can be concluded that calcium carbonate has the same colloidal properties as clay minerals,organic matter,and other cements.It is also found that the higher the content of calcium carbonate in the soil is,the more easily the soil nanoparticles aggregates,which promotes the formation of aggregates.Moreover,the stability of soil nanoparticles is effectively improved by removing soil calcium carbonate.In a word,the experimental results in this paper can provide theoretical reference for the mechanism of calcium carbonate promoting aggregate stability in soil in arid and semi-arid regions.