Research On Remediation And Removal Of Cadmium In Contaminated Farmland Soil By Superparamagnetic Nano-Fe₃O₄@SiO₂ Functionalized Materials

The problem of heavy metal pollution in soil has become a major agricultural ecological and environmental problem that has become increasingly prominent in China,posing a serious threat to the sustainable development of the economy,society and modern agriculture.How to overcome the current shortcomings of soil passivation and plant restoration in the field of heavy metal restoration in farmland soil?It is imminent to develop low-cost,high-efficiency,no-pollution farmland heavy metal control and repair technologies.Magnetic nano-functionalized materials have received extensive attention in the field of water pollution control.The significant advantage is that some magnetic functionalized materials have excellent adsorption properties for heavy metals,and can be quickly separated under an external magnetic field to achieve the recovery of heavy metals.Based on the occurrence form of heavy metals in soil,it is a method with development potential to mount and enrich heavy metals on some specific magnetic functional materials,combine agricultural engineering technology to recover these materials,and achieve in-situ removal of heavy metal pollution in farmland.In this paper,nano-Fe₃O₄ is used as the magnetic core,and the surface is coated with inorganic materials such as silicate to modify,and a new type of superparamagnetic micro-nano Fe₃O₄@Si O₂ functionalized material（hereinafter referred to as MFS）,The efficiency and influencing factors of cadmium removal by MFS on actual farmland contaminated soil were discussed,which laid the foundation for the development and application of new superparamagnetic micro-nano functionalized materials in soil pollution remediation.Repair has great significance and practical application value.The main results are as follows:（1）Preparation and adsorption kinetics/thermodynamics of new superparamagnetic micro-nano MFS functionalized materials:using iron salt,ferrous salt and ethyl orthosilicate as main reagents,nano-MFS composite materials were prepared by co-precipitation method.With cadmium as the target pollutant,a static adsorption test is carried out,Tests show that MFS has a good adsorption effect on Cd in solution,and it can reach an equilibrium state within 2 hours of adsorption.The adsorption process conforms to the quasi-second-order kinetic equation,and is controlled in two stages:internal diffusion and external diffusion;The Langmuir and Frundlich models are generally used to describe the isothermal adsorption characteristics.At temperatures of 298K,308K,and 318K,respectively,the maximum adsorption capacity of MFS for Cd²⁺is 59.42 mg·g^-1,66.58 mg·g^-1,71.59 mg·g^-1,temperature increase can promote the reaction;With the increase of the initial p H,the adsorption of Cd²⁺by MFS increases,and the optimal initial p H is 6～7;The interference ions in the solution have a certain inhibitory effect on the adsorption reaction,the interference anions are Cl^->NO₃^->SO₄^2-,and the interference cations are Mg²⁺>Na⁺>Ca²⁺;The thermodynamic parametersΔG,ΔH,ΔS indicate that the adsorption of Cd by MFS material is a spontaneous,endothermic and entropy increasing process.（2）Structural characterization and mechanism analysis:The structural characterization of BET,XRD and FTIR showed that Si O₂ was successfully modified on the surface of Fe₃O₄,and the specific surface area after modification was increased to 72.86m²·g^-1;Cd²⁺is successfully adsorbed on the surface of MFS without destroying the main structure of MFS material;The SEM image confirmed that the MFS dispersion is better and the morphology is more complete.The protruding particles on the surface are more regular spheres with a diameter of about 38.70nm,XRD,EDS spectrum and XPS spectrum revealed that Cd²⁺was successfully adsorbed on the material and coordinated with-OH on the surface of the material.After adsorption,the MFS nanoparticles and cadmium reunite together,and the internal and external surface area decreases;Hysteresis loop analysis（VSM）analysis shows that although the saturation magnetization of MFS is reduced after Cd²⁺adsorption,MFS magnetic materials before and after Cd²⁺adsorption and after regeneration show typical superparamagnetic behavior.Under the action of external magnetic field,MFS can be quickly separated from the solution;The structural characterization of BET,XRD,FTIR,and SEM showed that the MFS specific surface area,saturation magnetic strength,and morphological characteristics after regeneration were basically restored,and they have the potential for reuse.（3）The remediation test of cadmium in contaminated soil by MFS shows:With the increase of MFS dosage（0～1.5%）,the removal rate of soil cadmium increased,the optimal removal rate of total cadmium was 23.20%,and the optimal reduction rate of available cadmium was 35.94%;The greater the removal rate of cadmium,the maximum removal rate is reached after 30 days of repair;the removal rate of cadmium changes to the acid-soluble and reducible state after the addition of the adsorbent material,followed by the oxidizable state,and the residual state Large,respectively converted 32.56%,30.85%,20.90%,1.32%.The removal rates of cadmium by materials with different degrees of pollution are 16.27%,17.58%,19.62%,22.47%,21.63%,and 21.52%,that is,with the increase of the degree of pollution,the removal rate of cadmium by materials gradually increases and tends to balance;Soil p H,ionic strength,and organic matter content have different effects on the adsorption process of cadmium by MFS materials.With the increase of soil p H,the removal rate of total cadmium and effective cadmium decreases;the increase of Na⁺ionic strength and organic matter content of soil solution will inhibit Adsorption of cadmium by materials.（4）Organic acid activator combined with MFS to repair cadmium contaminated soil:Adding activators tartaric acid and citric acid to the actual contaminated soil increases the removal rate of cadmium,the removal rate of total cadmium in soil by MFS was 22.36%and21.97%respectively.After citric acid activated cadmium contaminated soil,the decrease rate of acid-soluble,reducible and oxidizable states increased,which converted 47.00%,28.45%,17.00%,1.93%,the tartaric acid activation effect is the same as citric acid,but the citric acid activation treatment effect is slightly better than tartaric acid,and its total cadmium removal rate is higher.The material in the soil was recovered with a magnetic bar,and the recovery rate was measured to be 79.47%～88.5%after washing and freeze-drying.