Molecular Adsorption Mechanism Of Heavy Metals By Different Density Fractions In Soil

Soil heavy metal pollution control is an important ecological environment problem in China.In the past,many studies used different particle sizes to group soil components,which could not effectively reduce the complexity of soil.It is still difficult to reveal the adsorption mechanism of soil components on heavy metals at the micro molecular level.The effective separation of components with different structure and chemical properties?particulate organic matter,clay minerals,oxidized minerals,etc.?in soil is conducive to the scientific and effective disclosure of the mechanism of heavy metal adsorption and fixation on the molecular level.In this thesis,the particle density fraction?PDF?method is used to separate the particulate organic matter?POM?,light-weight organo-mineral complex?OMC-L?and heavy-weight organo-mineral combined organic matters?OMC-H?from two types of red soils and black soils,the carbon and nitrogen content and iron oxide composition of different density components were determined by element analysis and chemical extraction analysis,and batchexperiments was conducted on the adsorption of copper(Cu²⁺),Chromium(Cr³⁺)on the bulk soil?BS?and PDF of the investigated soils.The theoretical maximum adsorption capacity of heavy metals was obtained by isotherm adsorption model fitting.Meanwhile,the typical adsorption samples were selected for Scanning Transmission X-ray Micro-spectroscopy?STXM?analysis to reveal the molecular mechanism of heavy metals adsorption by density components of red soil and black soil.The main conclusions are as follows:?1?OMC-H accounted for more than 80%of the total soil mass,and the decomposition degree of organic matter in OMC-H was the highest,mainly nitrogen-containing organic matter,while the decomposition degree of organic matter in POM and OMC-L was relatively low.All the density components contain amorphous iron and crystal iron.After using 1.8 g/m³ as the density gradient to separate POM and OMC,there were still organic mineral complexes present in POM.After further refining the density gradient range?1.6 g/m³,2.0 g/m³?,the proportion of organic mineral complexes in POM decreased significantly.The widely adopted density separation gradient had not been able to completely separate minerals such as iron oxide in granular organic matter,which deserved further improvement.?2?The theoretical maximum adsorption capacity of soil and its density components on Cu was determined,and the molecular mechanism of Cu adsorption by soil and its density components was revealed.The theoretical maximum adsorption capacity of OMC-L and POM for Cu in four soils was much higher than that of OMC-H,and the adsorption behavior of four density components for Cu was more better fitted using the Langmuir isotherm adsorption model.The isotherm adsorption curve and theoretical maximum adsorption capacity of OMC-H were similar to BS.OMC-H was the main contributor of adsorption and fixation of Cu in soil,and Cu in red soil was likely to be fixed by organic mineral complex formed by the combination of carboxylic carbon and iron aluminum oxide.Cu in black soil was mainly Cu²⁺.The adsorption of Cu depended on the content of organic matter.The higher the aromatic carbon content was,the higher the adsorption capacity of Cu.Aluminosilicate minerals were not the main contributors of Cu adsorption in black soil.In the soil microsites lack of carbon,amorphous Fe Al oxides adsorbed some Cu²⁺on the surface of secondary clay minerals due to their high activity specific surface area,but organic matter still played a key role in Cu²⁺adsorption.?3?The theoretical maximum adsorption capacity of Cr³⁺by soil and its density components was described,and the molecular mechanism of adsorption of Cr²⁺by soil and its density components was clarified.The density component with the highest theoretical adsorption capacity of Cr in the four soils was OMC-L.The organic matter amorphous iron oxide complex may had specific sites for adsorption and fixation of Cr.The adsorption mechanism of Cr in soil was complex.The isotherm adsorption model of different density components in the same soil was different.The three density components played an important role in the adsorption process of Cr in soil.The distribution of Cr in black soil was heterogeneous.Clay minerals and oxidized minerals such as calcium and manganese were not the important substances for adsorption and fixation of Cr.The adsorption of Cr was related to organic carbon,but the total carbon content was not the only driving force for adsorption and fixation of Cr.The contribution of fatty carbon and phenol carbon to adsorption and fixation of Cr was small,and the affinity of carboxyl carbon to Cr was higher.