E_h-coupled Dynamic Release,Transformation And Phytoavailability Of Arsenic And Cadmium In Contaminated Soils

Soil pollution by heavy metals and the subsequent ecological environment and food safety problems are one of the serious environmental problems in China in recent years.China is a large rice-producing country.Paddy soil is one of the main types of arable land in China.The pollution of arable land led to the accumulation of cadmium and arsenic in rice-producing areas,and the exceeding rate of heavy metals in rice grain in some areas reached 10%.Since rice is mostly in a flooded environment in the early growth stage,and undergos a unique flooding-drying process due to drainage.Previous studies indicated the formation of CdS precipitate after the flooding treatment,which significantly reduced the bioavailability of Cd in the soil.However,the flooding promoted the reduction of As(V)to As(?)and improve the bioavailability of As.Recent study showed proper regulation of rice flooding-drying time can reduce the uptake of Cd and As by rice,but the corresponding mechanism remained largely unclear.In this thesis,the coupled soil Cd/As and related elements(Fe/Al/Ca/S/P/DOC)release characteristics were studied by simulating the Eh gradient change process using a high-precision soil microreactor.Synchrotron-based X-ray fluorescence(?-XRF)spectroscopy was explored to reveal the release mechanism of As from paddy soils under different Eh conditions Moveover,soil column experiments were used to simulate the transformation process of As and Cd in the flooding-drying process of rice fields.Finally,the effects of different combinations of flooding-drying time on the uptake of heavy metals by crops were studied through pot experiments.The results are as follows(1)With the increase of Eh,the pH and DOC values of the soil solution both decreased,while the concentrations of Cd,S,and Al in the soil solution showed similar growth trends,and the concentrations of As,P,and Fe showed similar growth trends.This indirectly indicates that Cd is mainly controlled by the formation of metal sulfides,and As is mainly affected by iron manganese oxides or iron sulfides.Synchrotron-based ?-XRF analysis further found that under low Eh conditions,the high-content point signals of As,Fe,and S on the soil particles coincide and have a high correlation,indicating that iron metal sulfides are the main sorbing phase of As(2)Regardless of flooding or drying,the proportions of different forms of As in the soil were:specific adsorbed fraction>crystalline iron-manganese combined As>residual fraction>amorphous combined fraction>non-specific adsorbed fraction.Crystalline iron-manganese-bound As transformed to specifically adsorbed and non-specific As in the early stage of flooding,and the specifically adsorbed As transformed to crystalline iron-manganese-bound As in the later stage of flooding.The effect of flooding-drying conditions on the As fraction was mainly on the change of specifically adsorbed As fraction.Under flooded conditions,the percentage of different forms of Cd to total Cd was:reducible Cd>oxidizable Cd>acid-soluble Cd>residual Cd.Under dry conditions,acid-soluble and oxidizable Cd and the content of Cd in the reducible fraction were insignificant changed.When flooded for 10 days,the acid-soluble and reducible Cd content was the highest,indicating relatively high environmental risk Under the dry condition,the transformation of Cd fractions completed within 5 days(3)Under flooded conditions,the contents of acid-soluble and reducible Cd in the Int-F6D6 treatment group increased by 14%and 5%,respectively,compared with the Flooded treatment group,thus significantly improving the mobility of Cd.However,in the dry conditions,the Cd content of each fraction of the two treatment groups was almost equal.These results indicated the mobility of Cd could be regulated significantly by proper water management.Moreover,the biomass of rice also improved through water management.Although the bioaccumulation coefficient of Cd in this experiment is low,compared with other Cd-accumulating plants,rice had large biomass and easy planting,thus being with significant advantages.After calculation,the theoretical value of Cd removal in one season of rice cultivation in the Int-F6D6 treatment group was around 405.52mg,which was promising for the remediation of Cd-contaminated soil.