Synchronized Passivation Of Cd And As In Paddy Soil By Fe-based Desulfurization Material Effect And Mechanism

Cadmium（Cd）and arsenic（As）are highly toxic,widely distributed and prone to compound pollution.They are also toxic elements that seriously threaten the safety of agricultural products,human health and the sustainable development of agriculture and rural areas.Cd and As are two elements with different properties.There are great differences in the influencing factors and regulation paths of their effectiveness,which also brings difficulties to the remediation and regulation of compound polluted soil.The availability of As and Cd in paddy soil is affected by p H,Eh and other factors,and the regulation ways are different.It is very challenging to reduce its availability and the absorption of rice at the same time.Using natural or artificial modified materials to passivate As and Cd in soil is a common method to reduce the activity of heavy metals and other elements in soil.Screening low-cost and efficient passivation materials has been an important direction for researchers for a long time.In this study,a natural Fe-based desulfurization material,which is an industrial by-product with low background value of heavy metals and rich content of plant nutrients,was selected in a steel plant in Hunan.Based on the fact that the material is rich in nutrients such as iron（Fe）,silicon（Si）,manganese（Mn）,sulfur（S）and calcium（Ca）and has high safety,we assume that the material has high efficiency of simultaneous adsorption of As and Cd in the environment and reducing the effectiveness of As and Cd in the soil rice system,And applied to the potential of Contaminated Farmland restoration.Through material screening experiment,pot experiment,solution culture experiment and related mechanism research,the following conclusions are drawn:1.Select suitable test materials from the perspective of environmental safety and the adsorption capacity of materials for Cd and AsFour kinds of industrial by-products from different process stages of steelmaking/ironmaking were collected and analyzed.Four kinds of materials（HLF-1,HLF-2,HLF-3 and HLF-4）belong to class II general industrial solid waste and do not belong to the category of hazardous waste.According to the ecological indicators of As,Cd,Pb,Cr and Hg in fertilizers（GB/T 23349-2020）,combined with the detection results of relevant elements in natural Fe-based desulfurization materials,no heavy metals in materials HLF-3 and HLF-4 exceed the standard,Meet the basic requirements for the application of agricultural materials.The adsorption test results show that the adsorption capacity of Fe-based desulfurization material（HLF-3）for Cd and As is stronger than that of blast furnace slag（HLF-4）,which can be used in the next research.2.Inhibition and control effect of Fe-based desulfurization materials on Cd and As absorption and accumulation in riceThe Fe-based desulfurization material had an obvious resistance and control effect on the absorption and accumulation of Cd and As in rice.The dry weight of rice stems,leaves and grains in each treatment increased significantly,and the material had a significant effect on the increase of grain yield.After the application of materials,the soil p H value increased by 2.4units,and the content of Fe（II）in the soil also increased significantly.Soil available Cd(0.09-0.10 mg kg^-1)decreased by 88.0-89.6%,and soil available As decreased from 29.4 mg kg^-1（CK）to 8.8 mg kg^-1（T3）,which decreased by 69.9%.The contents of Cd in brown rice and Cd in roots,stems and leaves of different treatments were significantly lower than those of CK without materials.Grain Cd decreased by 26.4-51.6%,and grain inorganic As decreased by37.3-42.7%.Fe and Mn in root surface iron film increased by 44.2%and 178.6%,which were conducive to the reduction of Cd and i As.The enrichment coefficients of Cd and As（BAFCd,BAFAs）of each treatment decreased with the increase of addition,and there was significant difference between treatments.Soil ferrous was negatively correlated with grain Cd,grain As,available Cd and available As.Soil p H value is the main factor affecting the concentration of soil available Cd and As and the concentration of grain Cd and As.The concentration of grain Cd and As is positively correlated with the concentration of soil available Cd and As.The transfer coefficients of Cd and As from stem to grain and from leaf to grain were less than 1.3.Mechanism of root soil interface process of Fe-based desulfurization materials inhibiting and controlling Cd and As absorption by riceThe morphology of Cd and As in paddy soil changed greatly after application of Fe-based desulfurization materials.In general,acid extraction of Cd and As in paddy soil decreased and residue increased.Compared with CK soil,acid extraction of Cd in other treatments decreased by 62.79-74.42%(0.22-0.32 mg kg^-1).Compared with CK(28.23 mg kg^-1),the acid extracted As(8.18-16.37 mg kg^-1)of other treatments decreased by 42.01-71.02%.The application of Fe-based desulfurization materials decreased the activities of catalase（S-CAT）,urease（S-UE）,acid phosphatase（S-ACP）and alkaline phosphatase（S-AKP）in paddy soil.The morphology and structure of rice root tip changed greatly compared with CK.The root tip fibrosis of rice treated with CK was serious,and the root tip morphology was damaged,especially the root crown was seriously poisoned by heavy metals.The root system of rice treated with Fe-based desulfurization material was complete,the surface of root cap,root tip separation area and elongation area was smooth and the growth condition was normal.Compared with CK,the organelles in each treatment were more complete and abundant.At the same time,the cell wall was relatively thin and the cytoplasm was more.The root soil interface process mechanism of Fe-based desulfurization material preventing and controlling cadmium and arsenic absorption by rice is as follows:（1）Mechanism of soil chemical precipitation and organic matter complexation and chelationThe high p H value of the added material helps to reduce the bioavailability of Cd.The properties of rich Ca and Fe in the material lead to the formation of complex between Ca-As and Fe-As in the soil and high adsorption capacity for As,so As to reduce the content of available As in the soil.Fe-As and Ca-As complexes significantly promote the reduction of the total amount of Cd and i As in rice grains.Therefore,the residual As gradually increases with the increase of the amount of material added.（2）Adsorption mechanism of iron film on root surfaceThe contents of Cd,As,Fe and Mn in iron film on the surface of rice roots increased with the increase of material addition.Fe-based desulfurization materials have strong adsorption capacity for Cd and As,and their large specific surface area provides the required sites for adsorption.With the addition of Fe-based desulfurization materials,the contents of Fe and Mn in the iron film deposited on the root surface increased,so that As and Cd were isolated in the iron film,thus preventing their transportation from the root to the rice grain.The enrichment of Fe（5.9%）and Mn（1.7%）in Fe-based desulfurization materials may be the reason for the increase of Fe and Mn content on the root surface.In particular,the addition of Fe-based desulfurization materials can increase the content of Fe（II）in the treated soil.The content of Fe in the iron film on the root surface was significantly positively correlated with the content of As,while the contents of Fe and Mn were significantly positively correlated with the content of Cd.4.Adsorption of Cd and As in solution by Fe-based desulfurization materials and its mechanismBased on the results of the effects of different initial p H values on the adsorption of Cd and As,when the initial p H value of the solution is 6,the material has a high adsorption capacity of Cd and As at the same time.The adsorption of Cd by materials includes both physical adsorption and chemical adsorption,and the adsorption of As is mainly chemical adsorption.The maximum adsorption capacity（Qmax）of the material for Cd is 526.31 mg g^-1and that for as is 2.67 mg g^-1.The adsorption mechanism of Fe-based desulfurization materials for Cd and As:1)coprecipitation:XRD analysis showed that a new diffraction peak was formed after adsorption,while XPS results showed that Cd（OH）2 was formed in the adsorption process.The mechanism of these changes is the same as that of Cd adsorption on Ca-Si materials.High p H and a large number of functional groups are the two main mechanisms of Cd adsorption.2)Complex surface functional groups:the infrared spectrum results show that functional groups such as-COOH and-OH participate in the adsorption process.After adsorbing As,the material forms Fe-As and Ca-As complexes,so As to reduce the concentration of As in the solution.3)Ion exchange:Cd²⁺is exchanged with the iron ion released by the material,and the Fe content decreases after adsorption from EDS analysis.However,it remains to be confirmed whether the ion exchange between Cd²⁺and other metal ions occurs.Through this study,it can be seen that the industrial by-product Fe-based desulfurization material obtained in this paper has the effect of simultaneous adsorption of Cd and As in water,that is,the adsorption capacity of Cd is greater than that of As,and has passivation effect on Cd and As in farmland soil.It is a better natural adsorption material.Because it is an industrial by-product,its economic benefit is higher than that of other synthetic materials.However,due to the high p H value of the material itself,it is suitable for acid soil in South China by properly controlling its addition proportion in farmland restoration.Fe-based desulfurization materials combine the characteristics of safety,low cost and high availability.They have broad application prospects in the passivation and safe utilization of As and Cd contaminated paddy soil.In general,the materials reported in this paper have greater application potential than the reported natural materials or modified materials,which can provide material and technical reference for the passivation and remediation of As Cd composite contaminated soil.