Study On The Dynamic Process And Mechanism Of The Effect Of Iron And Sulfur On The Availability Of Cadmium In Paddy Fields

Cadmium（Cd）is a toxic and carcinogenic heavy metal widely distributed in the environment.Due to human activities,Cd pollution in rice fields has gradually intensified,and the safety issues of agricultural products caused by excessive Cd levels have attracted much attention.Cd in rice fields has high bioavailability and is easily absorbed by rice roots and transported to aboveground tissues.Rice is currently the staple food for over half of the global population and has become the primary source of Cd intake for people in these regions.Controlling Cd pollution in paddy soil and ensuring agricultural product safety have become critical issues for national food safety.At present,it is urgent to clarify the characteristics and main control factors of Cd transformation in the soil to reduce the harm of Cd to agricultural products and reduce the health risks of Cd to humans.Under the conventional mode of rice cultivation,alternate flooding and drainage management is used.Flooding and drainage lead to changes in soil redox potential（Eh）and p H,which in turn cause drastic changes in Cd bioavailability.Sulfur is an essential element for plant growth and is important in plant growth.The phenomenon of sulfur deficiency caused by fertilizer application methods and planting structures is increasing,and the application of sulfur fertilizer is gradually receiving attention.As the fourth richest element on Earth,Fe is widely present in various mineral phases,such as natural oxides/hydroxyl oxides.Due to its large surface area and large functional groups,Fe oxides are often used to regulate Cd activity in rice fields.Currently,the research on the bioavailability of Fe-S on Cd mainly focuses on the single effect of elements.In contrast,the research on the coupling effect of Fe-S on Cd during the redox conversion process is insufficient.In this study,Cd-contaminated soil was taken as the research object.Through soil flooding and drainage experiments,rice pot experiments,Fe-S co-precipitation addition experiments,and different Cd compound oxidation experiments,diffusive gradient in thin film（DGT）technology,soil solution sampling technology,and chemical extraction methods were combined to reveal the mechanism of the influence of Fe-S on the activity of Cd in soil.The main results are as follows:（1）Through the flooding and drainage experiment,the various rules and control factors of Cd activity in three different types of soils（Tianjin Chao soil:TJ for short,Hunan paddy soil:HN for short,Sichuan yellow-brown soil:SC for short）were investigated.During a 60-day incubation period（40 days of flooding and 20 days of drainage）,it was found that the Cd activity(C_DGT-Cd)detected by LDHs-DGT in the three soils decreased rapidly during the flooding stage and increased rapidly during the drainage stage.The activities of C_DGT-Cd in TJ,HN,and SC soils decreased by25.2%,26.1%,and 87.8%during flooding treatment for 10 days,while the activities of C_DGT-Cd in SC soils decreased to 2.0μg·L^-1.After waterlogging treatment,Eh significantly decreased,and the content of C_DGT-Fe in the soil increased,but there was a significant difference in the increase in the three soils.The content of C_DGT-Fe in SC soil increased rapidly after 4 days of flooding,while that in TJ soil began to increase after 20 days.The higher Mn/Fe ratio and lower organic matter content were the reasons for the slow increase of C_DGT-Fe in TJ soil.The concentration of SO₄^2-first increases and then decreases with the prolongation of flooding time,and long-term flooding results in sulfate reduction promoting Cd fixation.High Fe and high SO42 reduction are the main factors for the rapid decline in Cd activity.After drainage,the content of C_DGT-Fe decreases rapidly,C_DGT-S increases,and the activity of C_DGT-Cd increases.It is inferred that it should be caused by the disintegration of Fe ^II-Fe ^III LDH and sulfur-containing secondary iron ore.（2）The effects of different forms of sulfur on Cd activity in HN paddy soil were further explored through long-term rice pot experiments and short-term soil incubation experiments.During the rice growth period,sulfur（S）and sulfate(SO₄^2-)treatments both promote the formation of metal sulfides（Cd S and Fe S）and reduce the activity of Cd in the soil.Treatment with S and SO42-significantly reduced the accumulation of Cd in rice roots by 31.5%and 16.6%,respectively.From the short-term soil incubation experiment,both S and SO₄^2-promoted the release of Cd during the initial stage of sulfur application（within 48 hours）,resulting in a 12.2%and 31.1%increase in soil C_DGT-Cd content compared to the control（CK）,respectively.Treatment with S and SO₄^2-significantly increased the C_DGT-Fe content in the soil in a short period of time,resulting in a 17.3%and 73.3%increase in C_DGT-Fe content,respectively.Path analysis showed that although the mechanism of the influence of different forms of sulfur on Cd activity varies at different stages,they all indirectly affect Cd activity by regulating Fe forms.（3）Adding Cd compounds under flooding and drainage conditions further verified the mechanism of sulfur action on Cd activity in the soil.Adding different forms of Cd compounds significantly affects Cd activity in HN paddy soil.Adding Cd S increases soil Cd activity,and the more added,the stronger the effect.However,adding Cd CO₃ has almost no effect on soil Cd activity.After 10days of flooding and adding the same dose of Cd(Cd:2 mg kg^-1 and 10 mg kg^-1),the C_DGT-Cd content in Cd S treatment increased by 3.47 and 5.23 times compared to the Cd CO₃ treatment,respectively.After 10 days of drainage,it was found that the C_DGT-Cd content in CdS treatment increased by 5.43 and 11.5 times compared to Cd CO₃ treatment,respectively,and Cd S was more prone to oxidation during the drainage phase than Cd CO₃.In order to further explore the influencing factors of the change in Cd S activity,a mixed metal sulfide（Cd S+Cu S）addition experiment was designed based on the thermodynamic competitive order.The results show that during the flooding stage,Cu competes with Cd for S2-produced by sulfate reduction,while Cu generates sulfide before Cd.After drainage,the Cd concentration in Cu S-treated soil is 3.68 times higher than that in Cd S-treated soil,and the presence of Cu will significantly promote the dissolution of Cd S.（4）Further,the mechanism of the influence of Fe-S coupling on Cd activity in HN paddy soil was explored through the experiment of external addition of ferrihydrite sulfate coprecipitation.Under long-term flooding（>20 days）,the reduction of S and Fe released from coprecipitation promotes the production of sulfur-containing secondary iron ore（Fe S/Fe S₂）and Cd S precipitation,resulting in a decrease in soil Cd activity.Compared to the addition of ferrihydrite,increasing the S/Fe ratio can improve the fixation effect of ferrihydrite on soil Cd because sulfate reduces the crystallinity of ferrihydrite and increases the adsorption of Cd.After drainage,the activity of Cd increased,and the coprecipitation treatment of ferrihydrite sulfate delayed the release of Cd,which decreased by 28.9%-44.1%compared to CK.The formation of Fe S/Fe S₂ in the environment after adding ferrihydrite sulfate coprecipitation is not the main adsorbent for immobilizing Cd,and preferential oxidation in the drainage stage delays the oxidative release of Cd S.In summary,the decrease in Cd activity during the flooding stage is attributed to Cd S precipitation and the formation of secondary minerals such as Fe S/Fe S₂.In sulfur-deficient soils,long-term（120d）application of sulfur promoted the precipitation of Cd S and the formation of Fe S/Fe S₂,thereby reducing the activity of Cd.In contrast,short-term（5d）application of sulfur promoted the dissolution of Fe/Mn,thereby increasing the activity of Cd.After drainage,Cd S is increased by the activity of oxidized Cd,and the release process is affected by the content of Cu in the soil.Exogenous addition of ferrihydrite sulfate coprecipitation can delay the degree of Cd release after soil drainage.