The Efficiency Of Cd Immobilization By Silicate In Paddy Soil

A large amount of cadmium?Cd?is discharged into the environment during industrialization and urbanization,resulting in serious Cd contamination problems in paddy soils in some areas of China.Cd has strong mobility and beneficiation ability from environment to enter the food chain,and threatening the safety of agricultural products and human health.Taking the above analysis into account,a novel Ca-Si composite mineral?CS?,sepiolite?SP?and foliar water-soluble silicon fertilizer?YS?are systematically studied to investigate the efficiency and mechanism toward Cd in rice-soil system.In order to provide a theoretical basis for the safe and effective production of rice by Cd-contaminated paddy soil.The main result are as follows:?1?With initial Cd at 250 mg/L,the final equilibrium of Cd sorption by CS was reached after 8 h with Cd removal rate approaching 100%and the maximum Cd sorption capacity of CS was 72.2 mg/g.The ion exchange and complexation are the main mechanisms for CS adsorption of Cd.In addition,Compared with the pristine CS,Cd precipitation as Cd₂?OH?₃Cl,Cd?OH?₂ as well as a minor formation of CH₆BrCdN was detected on the post-adsorption CS.?2?With the CS amendment,bioavailable Cd in rice rhizosphere was reduced by 92-100%.EDX line scan illustrated much more intensified Si deposition along root cross-section in the CS treatment,which resulted in1.5-2.1-fold higher Cd sequestration in the CS-amended root than control.As a direct result,the root-to-shoots Cd translocation was reduced significantly by 42%-51%with the CS treatment relative to control.The CS amendment increase Si,Ca,and Zn in brown rice and simultaneously reduced Fe,Mn,Mg and Cu in brown rice.?3?The maximum Cd sorption capacity of sepiolite to Cd in aqueous solution is 4.2 mg/g with initial Cd at 50 mg/L,mainly through ion exchange,surface hydroxyl?-OH?complexation and production of CdCO₃,CdCl₂,CdO and Cd?OH?₂.precipitation.?4?Pot experiments showed that SP and CS amendment can decreased CaCl₂-Cd by 19.5%-34.0%and simultaneously increased the bioavailable Si,Ca and Se in rice rhizosphere,promote the exchangeable Cd to Carbonates-bound and Organic-bound Cd transitions to reduce the absorption of Cd by rice and Se transform from poorly soluble state to bioavailable state.In the SPC and SCY treatments,the majority of Cd in rice plant was retained in roots,with Cd translocation factor(TF_shoots/root)being as low as 0.2 compared to 6.2 in the control.In the SCY treatment,the 93.5%of Cd in brown rice was significantly decreased and simultaneously the Si in brown rice was significantly increased.In addition,a sligthtly less significant affect the absorption of various mineral elements?Se,Ca,Cu,Fe,Zn,Mg,and Mn?by brown rice with SCY the treatment.