Adsorption Characteristics And Mechanism Of Heavy Metals By Soil Humus

In this paper,we take several main types of soils in China（dark brown soil,brown coniferous forest soil,yellow brown soil,yellow soil and latosol）as research objects,and study the adsorption of heavy metal ions(Cu²⁺,Pb²⁺,Zn²⁺,and Cd²⁺)on soil humus components（HU and HA）under different ionic strength(0.01⁰.1 mol·L^-1),concentration of Cu²⁺(0⁸00mg·L^-1),solution pH（2.0⁸.0±0.1）,reaction temperature（25oC⁵5oC）and contact time（5¹440 min）.The surface morphology,element distribution and functional group structure of the soil humic substances before and after adsorption were analyzed which combined with scanning electron microscopy（SEM）,X-ray energy dispersive spectroscopy（EDS）,solid state ¹³C CPMAS NMR spectroscopy,Fourier transform infrared spectroscopy（FTIR）and X-ray photoelectron spectroscopy（XPS）.Meanwhile,The extended coordination structure（EXAFS）of X-ray absorption was used to investigate the local coordination structure of adsorbed Cu²⁺.The results are as follows:1.It has similar functional group structure in HU and HA of brown coniferous forest soil and yellow soil.The quasi-second-order kinetic equation（R²>0.99）can better match with the adsorption of Cu²⁺by soil HU and HA.It is known from the thermodynamic fitting parameters that the Langmuir equation has a accurate fitting effect on soil HU to adsorb Cu²⁺（R²>0.96）,however,the Freundlich equation fits the adsorption Cu²⁺of HA exactly（R²>0.98）.The maximum adsorption amount of Cu²⁺in brown coniferous forest soils HU and HA was 175.4and 224.2 mg g^-1,which was more than the maximum adsorption in yellow soil(166.7 and217.4 mg g^-1),the adsorption is an easy non-uniform surface reaction.Cu²⁺has similar coordination sites and coordination structures on the surface of soil HU and HA.In the Cu-O/N first coordination layer of soil HU and HA samples,the number of O or N atoms coordinated to the central Cu atom is about 4,the atomic spacing is about 1.93¹.94?;in the Cu-C second coordination layer,about 2 C atoms are coordinated with Cu atoms,and the atomic spacing is about 2.79².84?.Compared with the surface morphology of the two humic components we will find changs before and after the adsorption reactions,The carboxyl group,hydroxyl group and aromatic N participated in the adsorption reaction of Cu²⁺and Cu²⁺was mainly adsorbed on the surface of HU and HA of C,N,O organic functional groups are combined in the form of inner layer composite rather than the mineral components.2.In order to find out the differences between the adsorption of Cu²⁺by humins and humic acid,we demonstrated and analyzed humins and humic acid of three typical soil,and the humins have higher aliphaticity and less polarity.The quasi-second-order kinetic equation can more accurately describe the adsorption kinetics of soil humin and humic acid,and the Langmuir model faultlessly fits the adsorption isotherm.In terms of the amount of adsorption that humin is much less than that of humic acid.It is confirmed that Cu²⁺is bound to O/N and C atoms on the surface of Humin in the form of inner complex.3.Under different conditions such as pH value,ionic strength,contact time,reaction temperature,metal ion type and concentration,and the addition of exogenous organic acid,the adsorption of metal ions by dark brown soil humin was studied,the results show that:（1）Whether single or many metal ions coexisting,the order of adsorption amount is still Pb²⁺>Cu²⁺>Zn²⁺>Cd²⁺.When a variety of metal ions coexist,they are competitors for the adsorption sites on the surface of humin,that is,the adsorption amount for a single metal ion is larger than that for the metal ion coexists with other metal ions;the more kinds of other metal ions coexisting with a certain metal ion,the lower the adsorption amount of the metal ion to the metal ion;The amount of adsorption of a certain metal ion by humin is also related to the type of coexisting metal ions.（2）The adsorption amount of metal ions on humins increases accompanied with pH,but the ionic strength has the opposite effect.（3）As reaction time goes on,the concentration of Cu²⁺and the temperature increases,the adsorption amount enhances;the adsorption kinetics accords with the quasi-second-order kinetic equation,which is the physical adsorption that requires energy and endotherm,and belongs to the association reaction from the reaction mechanism;The adsorption isotherm conforms to the Langmuir and Freundlich equations,but the former has a better fitting effect which is a spontaneous,endothermic process that degrees of freedom is increased.When humin combined with EDTA and organic acids（including citric acid,tartaric acid and oxalic acid）,the amount of adsorption to metal ions is reduced,and the order of reduction is EDTA>tartaric acid>oxalic acid>citric acid.In addition,we compare with unpurified crude humin,the adsorption of metal ions by humin decreased significantly after purification,which indicating that the coexistence of minerals reduced the adsorption capacity of humin on metal ions.