Complexation Of Cadmium With Different Molecular Organic Acids And The Effect Mechnism On Cadmium Immobilization/Release In Soils

Organic acids widely exist in soil.Organic acids in soil form complexes with cadmium（Cd）,which have a critical impact on the adsorption and dissolution of Cd in soils,consequently affecting Cd bioavalability.However,there are many kinds of organic acids in soil,and their structures vary greatly.At present,there is still a lack of systemic investigation on Cd chemical behavior as affected by organic acids in soils.In the study,the typical high molecular organic acids（HMWOAs）（humic acid,fulvic acid）and low molecular organic acids（LMWOAs）（oxalic acid,citric acid,malic acid,amino acid,etc.）were chosen to compare the compleaxation ability of organic acids and Cd²⁺,to indentify the effect of orgainic acids on Cd²⁺onto soil minerals,to illustrate the mechanisam of Cd dissolution as affected by orgainic acids with diffenrent molecular weight and to verify the effect of Cd uptake in maiz as affected by organic acids.The main results are as follows:（1）The complexation patterns of high molecular weight organic acids（HA）and fulvic acid（FA）to cadmium ions were analyzed by DFT theoretical calculations,and the complexation ability of FA to Cd was confirmed to be higher than that of HA.The DFT theoretical calculations revealed that the two oxygen atoms on the carboxyl groups in HA and FA formed two O-Cd bonds with Cd,respectively,and FA containing more carboxylic acids were more likely to form stable complexes with Cd ions than HA containing more phenolic hydroxyl groups.The simulation by WHAM 7.0 software revealed that more than 50%of Cd complexed with fulvic acid through bidentate coordination and more than 60%of Cd complexed with humic acid through monodentate coordination,which further confirmed that fulvic acid complexed with Cd more than humic acid complexed with Cd.（2）The complexation patterns of amino acids and low-molecular fatty acids with Cd were resolved with 11 typical low-molecular weight organic acids,and it was confirmed that the complexation ability of low-molecular fatty acids to Cd was higher than that of amino acids.Among the amino acids,glycine（Gly）,leucine（Leu）,alanine（Ala）and phenylalanine（Phe）were bonded to Cd with[COc,COc]structure,while glutamic acid（Glu）and threonine（Thr）were bonded to Cd with[COc,COs,COc]group structure.The order of stability of amino acid complexes with Cd is:Thr>Glu>>Gly>Ala>Leu>>Phe.Among the low molecular fatty acids,citric acid,oxalic acid and malic acid are bonded to Cd with[COc,COs]structural groups,while lactic acid and fumaric acid are bonded to Cd with[COc,COc]structures.The order of stability of low molecular fatty acids complexes with Cd was citric acid>oxalic acid>malic acid>fumaric acid>lactic acid.Compared with amino acids,low-molecular fatty acids have a higher complexing ability to Cd.（3）The pattern that high molecular weight organic acids（HMWOAs）promote Cd²⁺adsorption by soil acerite and montmorillonite,while low molecular weight organic acids（LMWOAs）inhibit Cd²⁺adsorption was revealed.As the concentration of HMWOAs（HA and FA）increased,the surface charge of goethite and montmorillonite decreased and Cd²⁺adsorption increased.The adsorption of Cd by needle iron ore increased from 3.66 mg/g to 4.82-4.93 mg/g,and the adsorption of Cd by montmorillonite increased from 35.64 mg/g to 64.52-92.64 mg/g,indicating that HMWOAs promoted the adsorption of Cd²⁺by needle iron ore and montmorillonite by the following mechanism:on the one hand,goethite and montmorillonite adsorbed HMWOAs by themselves,and through complexation with Cd²⁺,and on the other hand,by increasing the negative charge on the mineral surface and increasing the electrostatic adsorption of Cd²⁺on the mineral surface.On the contrary,the adsorption of Cd by acanthite decreased to 0.008-0.009 mg/g and that of montmorillonite decreased to 2.67-5.32 mg/g in the presence of LMWOAs,indicating that LMWOAs inhibited the adsorption of Cd²⁺on the surfaces of acanthite and montmorillonite by the following mechanism:LMWOAs mainly existed in the liquid phase and reduced its adsorption on the mineral surfaces by forming stable complexes with Cd²⁺The mechanism is that LMWOAs is mainly present in the liquid phase and reduces its adsorption on the mineral surface by forming stable complexes with Cd²⁺.（4）The mechanism of the conversion of soil Cu,Pb and Cd to stable forms under the action of high molecular weight organic acids（HMWOAs）and the release of Cu,Pb and Cd from soil under the action of low molecular weight organic acids（LMWOAs）was elucidated.Taking Cd in soil at the Pb-Zn smelting contaminated site as an example,FA and HA（1000 mg/L）resulted in a decrease of Cd concentration in soil solution by 4.15-25.55%and 40.20-66.76%;while the oxidizable and residual Cd content in the soil increased.The mechanism was that HMWOAs increased the content of SOM in the soil and promoted the partitioning of heavy metals onto SOM;secondly,HMWOAs increased the content of amorphous iron oxide in the soil,and the combined effect of HMWOAs and amorphous iron oxide led to the increase of heavy metals in the residual state in the soil.In contrast,LMWOAs promoted the release of Cd from the soil,with oxalic acid and threonine（1000mg/L）increasing Cd in the soil solution by 1.52-1.58 fold and 0.84-1.14fold;and the content of weak acid extractable,reducible and oxidizable states in soil decreased significantly;the mechanism of action was that LMWOAs caused the dissolution of organic matter and amorphous iron in soil and increased the solution DOM content,thus converting Cd,Cu and Pb in the soil solid phase to the soil solution.In terms of Cd morphology,LMWOAs promoted the conversion of soil Fe-Mn-bound and residual Cd to the exchange and organically bound Cd,increasing the effective Cd concentration in the soil by 13.26-36.02%,while HMWOAs promoted the conversion of soil exchangeable and Fe-Mn-bound Cd to the organic-bound and residual Cd,decreasing the effective Cd concentration by 3.27-18.73%.（5）The effect pattern of different molecular weight organic acids on Cd uptake in wheat and the transformation of soil Cd morphology was investigated.The results of the pot experiments showed that LMWOAs promoted the accumulation of Cd in wheat,and the Cd concentration in the plant above ground was as high as 5.8-7.72 mg/kg,which was 6.81-42.17%higher than the control.In contrast,HMWOAs inhibited the uptake of Cd in wheat,and Cd in the aboveground part of wheat treated with HMWOAs decreased by 11.05-42.54%compared with the control.Compared to the control,LMWOAs increased the available Cd of the soil by 13.26-36.02%,attributed to the conversion of the Fe-Mn bound and residual Cd to the unstable fraction,increasing the availability of Cd.On the contrary,HMWOAs treatment decreased the available Cd content by3.27-18.73%,converting the soil exchangeable and Fe-Mn-bound Cd to residual Cd and decreasing the Cd availability.The results of the study provide a scientific basis for the application of LMWOAs in heavy metal removal remediation of contaminated soil and HMWOAs in heavy metal stabilization remediation of contaminated soil.47 figures,29 tables,and 171 references.