The Mechanism Of Ferrihydrite Phase Transition Mediated By Pe+pH On Cd Availability In Paddy Soil

The risk of Cd pollution in paddy soil has attracted more and more attention.Paddy soil is a special soil with alternating dry and wet conditions formed under artificial intervention.The soil p H and Eh are often in a fluctuant state due to the repeated alternating flooding and drainage processes,which will lead to a series of physical and chemical changes in redox-sensitive elements or compounds in paddy soil,such as the phase transition of ferrihydrite,that could affect the Cd availability.However,the mechanism of how the comprehensive system pe+p H（pe refers to-log（e^-）activity,pe=Eh（m V）/59.2）of p H and Eh in paddy soil drives ferrihydrite phase transition and affects Cd availability is still unclear.Therefore,this study focused on the phase transition of ferrihydrite driven by pe+p H changes of paddy soil and the mechanism of Cd migration and transport in paddy soil-rice system.The adsorption and desorption efficiency of Cd on ferrihydrite and its secondary minerals（goethite and hematite）were measured in this study.The phase transition characteristics of ferrihydrite(the changes of free Fe,crystalline and amorphous Fe as well as dissolved Fe²⁺concentration)and its effect on Cd availability under unsteady pe+p H were analyzed by chemical analysis,scanning electron microscopy（SEM）,X-ray diffraction（XRD）,X-ray photoelectron spectroscopy（XPS）and isotope fractionation.The innovative results are as follows:（1）There were significant differences in the adsorption and desorption capacity of ferrihydrite and secondary minerals goethite and hematite on Cd,which affected the accumulation of Cd in rice.The adsorption capacity of three kinds of iron minerals to Cd:ferrihydrite>goethite>hematite.The material characterization results show that the surface of ferrihydrite is composed of fine nanoparticles,and the crystallinity increases significantly after Cd adsorption.The desorption amount of Cd on hematite was the highest,and the adsorption sites on the surface of rice roots under this treatment were preferentially saturated,resulting in a decrease in the net flux of Cd in the root tip.The concentrations of Cd in rice roots under the treatments of ferrihydrite-Cd,goethite-Cd,and hematite-Cd precipitates were 2.38 mg?kg^?1,2.94 mg?kg^?1,and 3.25 mg?kg^?1,respectively.The results showed that the phase transition of ferrihydrite generated crystalline iron oxides,which reduced the adsorption of Cd.（2）The unsteady pe+p H value of paddy soil significantly affected the phase transition of ferrihydrite in soil and controlled the mobilization of Cd.At low pe+p H,the content of Ca Cl₂-extractable Cd decreased,while the concentration of Fe-Mn oxide bound Cd increased.Compared with the oxidation environment,the amorphous iron oxides in the soil of Hengyang（HY）and Guangzhou（GZ）under reduced conditions increased by 96.8%and 80.1%,respectively,and the free Fe²⁺in soil solution increased by98.3%and 101.9%,respectively.In addition,XRD and XPS characterization results also confirmed that the crystallinity of iron minerals in soil was decreased and the proportion of Fe²⁺increased under low pe+p H conditions.（3）The fluctuant redox environment of paddy soil altered the redox of iron and induced phase transition of ferrihydrite that furthermore affected the Cd availability.The results showed that Cd availability mainly depended on water management and Fh concentration:the extractable Cd in soil decreased by 28.2%at reduction stage,and Fh application further promoted Cd stabilization in Fe-Mn（hydro）oxides.In addition,isotopically heavy Cd were preferentially enriched from soil to extractable phase(?^114/110Cd_{extractable Cd-soil}=0.39～0.62‰)and from soil to rice(?^114/110Cd_grain-soil=0.40～0.66‰),while light Cd were enriched in Fe/Mn oxides(?^114/110Cd_{Fe/Mn oxides bound Cd-extractable Cd}=?0.65～?0.14‰),and Fh application further increased the degree of Cd isotope fractionation.The expression of transporters involved in Cd transport,such as Os NRAMP1,Os NRAMP5,Os HMA3,Os HMA2 and Os LCT1,were suppressed with the decrease of pe+p H.These results provide new insights into the behavior of Cd in soil-rice system,which is mediated by the redox dependence of Fe and irrigation-related changes under different pe+p H conditions.（4）The study explored the dynamic process and mechanism of Cd stabilization with Fe or S in paddy soil under unsteady pe+p H by regulating water management.It can be found that the fate of Cd in paddy soil was strongly affected by pe+p H,and low pe+p H was favorable to Cd immobilization.More amorphous iron oxides formed at low pe+p H can adsorb Cd and reduce the Cd availability.However,when pe+p H was below 3.9,part of iron oxides was dissolved due to the reduction of Fe³⁺into Fe²⁺,resulting in the release of attached Cd.When pe+p H was lower than 3.2,S^2–produced by SO₄^2-reduction began to play a leading role in the immobilization of Cd through Cd S precipitation.Therefore,in the process of flooding for reducing Cd mobility,it was attributed to the contribution of iron oxides at first,and then S^2-began to play a dominant role as pe+p H continuously decreased.In addition,it should be noted that there was a certain competitive relationship between Fe²⁺and Cd²⁺in the combination with S^2-.