The Soil Chemistry Mechanisms Of Influences On Cadmium Chemical Speciation And Bioavailability With Exogenous Silicon

Cadmium ranks the top in soil inorganic pollution.Soil cadmium pollution has become a major hidden danger that threatens food security and restricts sustainable agricultural development.The area of farm lands polluted by cadmium?Cd?has reached 2.786×10³ km²,and the reduction amount of rice production is up to 10¹00 kg,the amount of Cd-rich rice production from the Cd-contaminated farm lands is up to 1.2×10¹00 kg.Therefore,the study on the remediation and utilization of cadmium-contaminated soil has certain theoretical and practical significance for the food security and prevention and control of soil cadmium pollution.The effects of silicon on reducing the bioavailability of cadmium in soil were demonstrated,but there were few studies on how silicon affected soil cadmium chemistry behavior and speciation.In response to previous studies there are certain disadvantages that silicon-containing materials change the pH of the soil system and introduce accompanying ions.Based on eliminating the alkalinity of Na₂SiO₃ and the distinction of Na⁺in different treatments,according to the results of rice pot experiments and soil cadmium adsorption,the soil and rice was considered as a whole system,the chemical behavior in liquid,speciation transformation in solid phases of cadmium in soil and the cadmium bioavailability were systematicly researched.The combination of Field Donnan membrane technology?Field-DMT?,infrared spectroscopy technique?FT-IR?,Raman spectroscopy technique?Raman?,X-ray diffraction technique?XRD?and X-ray photoelectron spectroscopy technique?XPS?were utilized for providing the Cd speciation information in soil liquid phase and the chemical behavior on solid phase with Fe_xO_y.The main conclusions are as follows:1.Exogenous silicon alleviated the toxicity of cadmium on the growth and development of rice seedlings reduced the cadmium content of various parts of the plant and increased rice yield.Exogenous silicon enhanced the antioxidant enzyme activities and photosynthetic pigment content of rice seedlings under cadmium stress,Si improved the ultrastructure of organelles and reduced the cadmium content in the plants.Under cadmium stress,silicon application increasing SOD and CAT activities,reduced POD activity and MDA content;increasing the content of chlorophyll?A,B,A+B?and carotenoids in rice seedling leaves which increasing photosynthetic efficiency of rice seedling leaves;recovering the function and structure of leaf cell,chloroplast and mitochondria,tending to be more complete and orderly,reducing numbers and size of starch grains and osmium particles after adds silicon.Silicon application significantly increased biomass and yield;reduced cadmium content in rice roots,stems,leaves,glume and brown rice.When the soil cadmium content and exogenous silicon addition were 5 mg kg^-11 and 120 kg^-11 respectively,the cadmium content of brown rice meets the national food safety standards(?0.02 mg kg^-1).2.Silicon changes the proportion of different forms of cadmium in the 3 soils,exogenous silica promoted the transformation of cadmium speciation into inactive state in soil solid phase.Reducing the ratio of acid-extractable cadmium?P<0.01?and reduced cadmium?P<0.01?,however the ratio of oxidizable cadmium?P<0.05?and residual cadmium ratio were increased.3.Silicon affected the chemical behavior of cadmium at the solid-liquid interface of soil.The silicon application suppressed the adsorption of cadmium in alkaline soil and promoted the adsorption of cadmium in neutral and acid soil.The amount of residual adsorption cadmium increased with the increasing silicon addition,which means silicon enhances the fixation ability of soil for cadmium.The R-P,Toth and Freundlich equation are the optimum to describe the adsorption characteristics of Cd in different pH soils under different silicon concentrations.The cadmium adsorption on the three kinds of soils were spontaneous reactions with endothermic and entropy increasing.4.Exogenous silicon affects the chemical speciation of cadmium in soil solution,During the growth period of rice,exogenous silicon increased the pH and Eh values of three soil solutions;decreased the soluble and free Cd concentrations in acidic and neutral soil solutions;and increased the concentration of soluble cadmium in alkaline soils,however the free-state cadmium concentration is reduced.5.Exogenous silicon affects the chemical form of cadmium.In alkaline soil?solutions?,the soil chemistry mainly reflected in soluble silicon-cadmium complexes with fluorescent properties which formed by the bonding of Cd²⁺and SiO₃^-through oxygen bridges,The above soil chemical mechanism promoted to reduce the bioavailability of cadmium in alkaline soil;The formation of Fe_xO_y-Si-Cd complexes with unit structural of Fe-O-Si-O-Cd was the main soil chemical mechanism for the reduction of the bioavailability of cadmium in acidic and neutral soils.Under the combined effects of the above soil chemistry,exogenous silicon reduced the bioavailability of cadmium.