Study On Soil Organic Components From Tropical Soils In Hainan And Their Adsorption Mechanism For Lead

Soil organic matter,as the most important component and active substance of soil,can adsorb and complex soil heavy metals through its surface charge and functional groups,which plays a vital role in reducing or eliminating soil heavy metal pollution.The composition and properties(elements,group composition,composition,and physical and chemical properties)of soil organic components varies greatly,which determines their ability and mechanism of soil organic components to bind heavy metals in the environment,and affects the distribution and molecular morphology of heavy metals on the surface of soil organic components,and ultimately makes their behavior and mechanism of passivation and immobilisation heavy metals in the soil differently.The spatial distribution,composition and properties,and adsorption mechanism for lead of soil organic components were studied,to reveal the mechanism of heavy metal adsorption by soil organic components from three aspects:driving mechanism,process mechanism and composition mechanism.This study has important theoretical value and application potential for recognizing,understanding and regulating the behavior of adsorption and immobilisation heavy metals by soil organic components in soil.In this project,the Udic Ferralsol of granite soil parent materials development under different land use patterns in Hainan was selected.1)The distribution characteristics of soil organic components in soil profile and aggregates were clarified using the method of continuous separation of soil organic components.2)The composition and properties of the main soil organic components were studied by combining traditional laboratory analysis with the advanced analysis techniques such as SEM,FTIR,XRD,and TG.3)The adsorption characteristics of lead with different soil organic components were studied by adsorption test and adsorption model.4)Continuous desorption test,XAFS and XPS were used to study the chemical and molecular morphology of lead adsorbed by different soil organic components.The main objectives of this study are to clarify the molecular mechanism of binding of lead to different soil organic components,understand the environmental behavior and functional mechanism of soil organic components at the molecular level.The main results of this study are as follows:1)The total soil organic carbon content in the 0-30 cm layer soil ranked in the order of natural secondary forest(Y,21.69 mg g^-1)>paddy field(S,12.63 mg g^-1)>rubber forest land(J,6.04 mg g^-1)>vegetable land(C,4.94 mg g^-1).The contents of total organic matter and soil organic components decreases with the increase of depth.POM is distributed in 0-30 cm layer of the soil with abundant organic residues.GRSP is mainly distributed in the 0-20 cm soil layer.HA is also distributed in the lower layer because of its great mobility.The0-10 cm layer of paddy fields soil contains a large amount of BC.With the increase of grain size,the carbon content of total organic matter and soil organic components decreased gradually.POM and BC are mainly distributed in large aggregates(250-2000?m)and medium aggregates(50-250?m).However,HA,FA and GRSP are mainly distributed in microaggregates(<10?m)and small aggregates(10-50?m).2)Elementary composition:The order of carbon content of soil organic components was:BC(47.62%)>HA(38.64%)>POM(24.05%)>GRSP(7.15%).The nitrogen content in V-BC was the highest(2.36%),followed by R-BC(1.64%)and T-BC(0.73%).The ratios of H/C(2.13%)and(O+N)/C(2.32%)ratios of POM were higher than HA and BC,However,HA and BC has higher carbon content,humification coefficient and aromatization degree.Surface functional groups:The POM has high polysaccharide carbon and low aromatic carbon content.HA and BC have a large number of phenolic hydroxyl groups,carboxyl groups-C=O,aromatic ring carbon skeletons and hydroxides,while POM,GRSP and BC have significant Si-O?Si-O-Si absorption peaks.Material composition:POM consists of plant and animal residues,black carbon and soil minerals,and HA is consists of cellulose and hemicelluloses,and GRSP contains more soluble minerals at high temperature.Moreover,BC is composed of soluble aliphatic and aromatic compounds that are resistant to decomposition,as well as soil minerals.The order of ash content was:GRSP(54.70-67.57%)>POM(32.62-37.06%)>BC(19.25-29.88%)>HA(6.64-9.21%).Physicochemical property:The POM has a rough structure and a different shape,and BC has multiple voids of different sizes.While,HA and GRSP have smooth and dense structures due to acidic precipitation?The order of cation exchange capacity was BC(131.51 cmol kg^-1)>HA(159.86 cmol kg^-1)>POM(62.22 cmol kg^-1)>GRSP(4.31 cmol kg^-1).3)With the increase of initial concentration,adsorption time and p H value,the adsorption capacity of POM,HA,GRSP and BC to lead were increased and then stabilized.The adsorption process of soil organic components to lead were correspond with the second order kinetic model.Except for R-BC,the isothermal adsorption behavior of soil organic components to lead were correspond with the Langmuir model,which is monolayer adsorption.The adsorption rate,adsorption capacity,maximum adsorption capacity and adsorption form of lead by soil organic components were very different.The adsorption rate of POM(0.616 kg g^-1 h^-1)and GRSP(0.605 kg g^-1 h^-1)was higher than that of HA(0.358 kg g^-1 h^-1)and BC(0.314 kg g^-1 h^-1).The order of the maximum adsorption amount is BC(60.67mg g^-1)>HA(62.57 mg g^-1)>POM(40.46 mg g^-1)>GRSP(21.39 mg g^-1).The lead contents of physical absorption of POM(4-8%)and BC(4-9%)was higher than that of HA(1%)and GRSP(1%),and the lead contents of ion exchange of HA(28.07 mg g^-1)and BC(17.18 mg g^-1)is higher than that of POM(9.53 mg g^-1)and GRSP(10.88 mg g^-1).Moreover,the proportion of hydrogen bonding of POM(43-58%)is the highest,while the proportion of complexed of GRSP(4%)is the lowest.4)With the increase of lead ions,the order to reach the maximum is as follows:first the physical adsorption,then the complex,and finally the hydrogen bonding and the ion exchange.Pb species in Pb-loaded HA were in form of Pb(C₂H₃O⁾₂₂(77.6%)and Pb O(22.4%).In contrast,the percentages of Pb species in Pb-loaded POM included Pb-loaded montmorillonite(76.7%),Pb(C₂H₃O⁾₂₂(14.3%),Pb-loaded Al₂O₃(6.8%)and Pb-loaded goethite(2.2%).Pb species in Pb-loaded GRSP were in form of Pb-loaded montmorillonite(47.5%),Pb(C₂H₃O⁾₂₂(33.3%)and Pb-loaded Al₂O₃(19.9%).Pb species in Pb-loaded GRSP were in form of Pb-loaded montmorillonite(45.1%),Pb(C₂H₃O⁾₂₂(40.6%),Pb-loaded Al₂O₃(11.1%)and Pb-loaded goethite(3.2%).Surface complexation and ion exchange were occurred through the phenolic hydroxyl and carboxyl functional groups of HA with lead ions.The adsorption mechanisms include co-precipitation,ion exchange and complexation by organic functional groups and mineral oxides on POM,GRSP or BC with lead ions.5)The contribution rates of physical adsorption,ion exchange,hydrogen bonding and complexation were 3.8-7.8%,28.6-44.4%,40.4-50.7%and 11.1-17.1%,respectively.The content of lead by physical adsorption,ion exchange and hydrogen bond was positively correlated with the content of SSA,CEC and surface groups(hydroxyl and carboxyl groups)of black carbon.Lead ions are complexed to form Pb-O bonds with O=C-O and C=O groups on the surface of black carbon.In this study,the content distribution of soil organic components in soil profile and aggregates was determined by the method of continuous separation of soil organic matter.The differences in the composition and properties of soil organic components and the adsorption characteristics of lead were clarified.The molecular mechanism of lead adsorbed by different soil organic components was revealed.The results of this study have important theoretical significance for evaluating the environmental capacity and environmental effects of different soil organic components on lead,and provide data reference and theoretical support for evaluating,predicting and controlling the risk of heavy metal pollution.