Effect Of Iron-based Materials On Cd Speciation Transformation In Soil And Microbial Community Characteristics

According to the National Soil Survey Bulletin in 2014,cadmium(Cd)-contaminated soil topped the list of exceedance rates and has a high threat to human health.Iron-based materials are one of the main materials applied in the remediation of Cd-contaminated soil.Iron-based materials application not only change Cd speciation and soil properties,but also alter the microbial community.Most research explored the mechanisms of Cd speciation transformation in terms of material characterization and soil properties.The most study in microbial community characteristics was based on the results of the equilibrium stage on Cd speciation transformation.In this study,two types of iron-based materials with goethite(GE)and 2-line ferrihydrite(MGE)as the main components were prepared based on iron salts and lignin.The effect of Cd speciation transformation,soil properties and microbial community by iron-based materials in soil was investigated in the early,middle,and later stages.The mechanism of microorganisms in Cd speciation transformation was also explored.(1)MGE and GE application could promote the transformation of Cd in soil from exchangeable speciation to iron-manganese oxide-bound and residual speciation.DTPA-Cd decreased by 63.81%and 7.95%,respectively.Soil p H,organic matter(OM),cation exchange capacity(CEC),ammonium nitrogen(NH₄⁺-N)and available potassium(AK)all increased significantly in the MGE group,while OM,CEC,NH₄⁺-N and AK did not change obviously in the GE group.(2)After iron-based materials application,the diversity and richness of the bacterial community decreased in the early stage and then increased in the middle and later stages,but no significant change was observed for the fungal community.Compared with the CK group,the relative abundance of Proteobacteria,Bacteroidota and Sphingomonas increased,and Trichoderma and Mortierella decreased after MGE application.In the GE group,the relative abundance of Sphingomonas decreased,and Trichoderma,Mortierella increased.Network analysis showed that the complexity of bacterial and fungal community was all increased in the MGE and GE groups.The fungal community in the MGE group was relatively stable.(3)HCl-Fe²⁺,HCl-Fe³⁺,Fe_d and Fe_o in soil all increased after MGE application,while HCl-Fe³⁺and Fe_o did not change significantly in the GE group.Pearson correlation analysis showed that Cd speciation transformation was closely related to p H and Fe speciation.Spearman correlation analysis indicated that NH₄⁺-N,F1,F3,F5 and Fe species have a significant impact on microbial community structure.(4)Combined with Cd speciation transformation,iron speciation change,p H and the relative abundance of Proteobacteria and Sphingomonas increased in the MGE group,it is speculated that the mechanism of Cd speciation transformation after iron-based materials application in soil mainly included adsorption,reduction,oxidation and coprecipitation.When iron-based materials were added to the soil,Cd was adsorbed on the surface of the materials.On the one hand,the lignin component in MGE stimulated the growth of iron reducing bacteria(Fe RB)which accelerated the iron redox reaction and promoted the reduction of iron minerals to Fe²⁺.Due to aerobic conditions,Fe²⁺was oxidized and hydrolyzed,which led to an increase in soil p H.The increasing of p H was beneficial for Cd convert to stable stages.On the other hand,secondary minerals which was formed in the iron redox reaction were able to encapsulate the Cd adsorbed on the material surface within the mineral structure,further promoting Cd transformed from exchangeable to stable speciation.