Passivation Of Cadmium And Arsenic In Paddy Soil By Iron-modified Clay Minerals And Its Mechanism

With the acceleration of industrial development and urbanization,the problem of heavy metal pollution in the environment has become increasingly serious.Rice is one of the maincrops in China.Cd and As contamination in paddy soils often results in Cd and As in brown rice exceeding the limit value of national food safety standard.The effects of soil pH?Eh on the bioavailability of Cd and As in soil have opposite effects.Therefore,how to repair the soil polluted by cadmium and arsenic togetherand control the content of Cd and As in rice is an urgent problem to be solved.In this paper,the effects of different clay minerals on the adsorption of Cd and As before and after iron modification were studied,and the modified materials with high adsorption of Cd and As were screened out.On this base,the kinetics and thermodynamics of Cd and As adsorption by the modified naterials and their influencing factors were further invetigated.Then the passivation effect and mechanism of different iron-modified materials on paddy soil polluted by Cd and As were studied by a pot experiment.The obiective of the paper is to provide a reference for remediation of paddy fields contaminated by Cd and As.The main research results are as follows:1.The effects of different proportions of iron-modified clay minerals?sepiolite,bentonite,diatomaceous earth?on the adsorption of Cd and As were studied.The results show that 10%iron modified sepiolite has the highest adsorption efficiency for Cd and As in solution.2.The adsorption of Cd²⁺and AsO₃^3-on natural sepiolite before and after iron modification and its influencing factors were studied.The results showed that iron-modified sepiolite formed a new hydroxyl bond and its surface roughness increased.Under the condition of 1:100 solid-liquid ratio,the adsorption efficiency of Asbolite iron modified to As????20mg/L?and Cd²⁺?10mg/L?increased from 34%and 55%to 98%and 95%,respectively.Compared with natural sepiolite,the maximum adsorption capacity of iron-modified sepiolite for Cd and As increased by about 2.5 times and 9.0times,respectively.pH has little effect on the adsorption of cadmium and arsenic by the two materials,but has the best effect at pH 6.As the solid-liquid ratio increases,the adsorption efficiency of Cd²⁺increases with the two materials,but has little effect on the adsorption efficiency of As???.Iron-modified sepiolite is a potential material for the treatment of combined pollution of Cd and As.3.A pot experiment was conducted to investigate effects of natural sepiolite?1%,2.5%?,natural bentonite?1%,2.5%?,iron-modified sepiolite?1%,2.5%?,iron-modified bentonite?1%,2.5%?and iron-based biochar?1%?on plant growth,Cd and As accumulation of rice,and Cd and As species in soil.The results showed that different passivation treatments had no significant effect on rice yield?p>0.05?.Natural sepiolite,2.5%iron-modified sepiolite and 2.5%iron-modified bentonite significantly increased soil pH value?p<0.05?,while other treatments had no significant effect?p>0.05?.Natural sepiolite,2.5%iron-modified sepiolite and 1%iron-based biochar treatment reduced the ratio of exchangeable Cd in soil and the uptake of Cd by rice.The contents of As in rice straw,husk and brown rice were significantly negatively correlated with the content of Fe in brown rice.The As in the soil is mainly in the residual state,and the iron-modified sepiolite and iron-modified bentonite could reduce the proportion of exchangeable As in the soil.Treatments of 1%sepiolite,2.5%iron-modified sepiolite and 2.5%iron-modified bentonite could simultaneously reduce the content of Cd and As in brown rice,which were 30.8%,33.3%,30.8%?Cd?and 19.2%,60.0%,61.7%?As?lower than CK,respectively.