The Effects And Mechanism Of Immobilization And Phytoextraction On The Remediation Of Alkaline Cadmium Contaminated Soils

Cadmium(Cd)pollution of soils in wheat producing areas in northern China has seriously threatened the safety of wheat production.At present,numerous researches have focused on the remediation of Cd contaminated acidic paddy soils in southern China,while there are few researches on the remediation of Cd contaminated alkaline wheat fields in the northern China.The remediation materials,products and technology models suitable for acid paddy soils are not completely suitable for the remediation of Cd contaminated alkaline wheat fields.Therefore,strengthening the remediation of Cd contaminated wheat fields and the reduction of the Cd accumulation in wheat grain are of significant importance to ensure wheat safe production.This paper studied the immobilization effects and mechanism of mercapto-modified clays on alkaline Cd contaminated soils;the phytoextraction efficiency and environmental effect of S,S-ethylenediamine disuccinic acid(EDDS)assisted Tagetes patula L.(T.patula),Phytolacca americana L.(P.americana),and Solanum nigrum L.(S.nigrum)extraction in alkaline Cd contaminated soils,and the combination effects of phytoextraction and immobilization,as well as the inhibition mechanism of MnSO4 on wheat Cd accumulation.The main results were as follows:(1)The application of mercapto-modified clays in alkaline Cd contaminated soil promoted the conversion of exchangeable Cd to Fe/Mn oxide-bound Cd,decreased the leaching efficiency of Cd in leachate by 75.98-77.70%,but had little effect on Cu and Zn elements.The immobilization effect of mercapto-modified clays on Cd was quick(after 1 d of soil aging)and effective(44.89-62.39%),and it was not affected by the leaching of heavy metal extractant.Soil sterilization treatment changed the structure and function of bacterial community.Compared with the mercapto-modified palygorskite(MPAL)treated natural soil,the stable Cd fractions significantly increased(36.62-50.00%)in MPAL treated sterilized soil.The immobilization effect of MPAL on Cd in sterilized soil was significantly better than that in natural soil.In addition,MPAL application had little effect on the bacterial community structure and diversity of soil.Wheat pot experiments showed that the application of MPAL promoted the migration of Cd from large particles(>0.25 mm)to small particles(<0.048 mm),accompanied by a decrease in the concentration of available Cd in macro-aggregates.In Cd contaminated alkaline soil,the application of 0.1%MPAL reduced the concentration of Cd in grains of two wheat varieties from 0.57 and 0.44 mg·kg-1 to 0.10 and 0.09 mg·kg-1.(2)In alkaline Cd contaminated soil,the concentration of heavy metals in the soil solutions significantly increased in 7 d after the EDDS application,then decreased rapidly with EDDS degradation,and disappeared after 30-35 d of application.The activation effect of EDDS on Cd in the soil solutions was better for a one-time application than a two-time application.The phytoextraction efficiency of hyperaccumulator in alkaline soil was T.patula(3.43%)>S.nigrum(2.30%)>P.americana(0.07%).After applying EDDS in a proper method,the phytoextraction efficiency of T.patula,S.nigrum,and P.americana slightly increased,by 1.38%,1.35%and 0.52%,respectively.In addition,the changes of pH value,heavy metals concentrations and enzyme activities of soil were correlated to the application period of EDDS.(3)The two-year successive pot experiments showed that when T.patula were harvested in the first season,EDDS was not completely degraded,and the EDDS-Cd complex could not be absorbed by wheat roots in the second season.The application of EDDS increased the soil pH,but had no effect on Cd fractions.EDDS application enhanced the Cd extraction amount of T.patula in the first season,but had no effect on the decreasing of Cd in wheat grains of subsequent season.The application of MPAL had no effect on soil pH,but significantly increased the stable Cd fractions,and decreased the available Cd content in soil.The application of 0.1%MPAL decreased the Cd concentration of low Cd accumulation wheat grain from 0.35 mg·kg-1 to 0.05 mg·kg-1,which are below the China national standard limit of 0.1 mg·kg-1(GB 2762-2017);and MPAL application had no effect on the Fe,Mn,Cu and Zn contents of wheat grain.The immobilization of MPAL on Cd in soil was not affected by the pre-application of EDDS.Compared to the single MPAL application treatments,the combination of EDDS enhanced T.patula extraction and MPAL immobilization treatments did not significantly decrease the concentration in wheat grains.(4)Soil application of 0.05-0.2%MnSO4 significantly decreased the concentration of Cd in wheat grains by 24.16-63.44%.The application of MnSO4 significantly increased the contents of Mn in wheat root,and inhibited wheat root Cd uptake by the ion antagonism between Mn and Cd.The upward transportation of Cd from nodel to internodel and from panicle to wheat grain was reduced.Nodes 2-4 can restrict the transport of both Cd and Mn.Nodel and the panicle can inhibit Cd transport,but have no effect on Mn transport.The overall spatial distribution of ionomes of different wheat tissues is consistent with the growth morphology of wheat plants.MnSO4 application significantly changed the ionomes of the roots,nodes,glumes,and wheat grains,and the differences in the ionomic responses among the roots are the most remarkable.