Simultaneous Immobilization Of Lead,cadmium And Arsenic In The Contaminated Soil By Iron-doped Hydroxyapatite

The production process of my country’s non-ferrous smelting,chemical and other industries produces serious lead,cadmium,arsenic and other complex pollution to the surrounding soil,which eventually seriously damages the ecological environment and endangers human health.Its control has become the top priority of current environmental pollution control.Chemical solidification/stabilization remediation has the advantages of low treatment cost,short cycle,and wide adaptability,and is widely used in the remediation of heavy metal-contaminated soils.However,lead,cadmium and arsenic have completely opposite chemical properties,and it is difficult to stabilize lead,cadmium and arsenic simultaneously.Aiming at how to solve the problem of co-contamination of lead,cadmium and arsenic composite heavy metals,it is proposed to control and modify the surface groups of hydroxyapatite materials,in order to obtain an iron-doped hydroxyapatite composite that can efficiently stabilize lead,cadmium and arsenic.To study the mechanism of its interaction with lead,cadmium,and arsenic,and to evaluate the effect and mechanism of the restored soil.The main research contents and results are as follows:（1）Fe Cl₃·6H₂O was added during the preparation of hydroxyapatite by ultrasonic-microwave-assisted hydrothermal method,and a weakly crystalline iron-doped hydroxyapatite（Fe-HAP）material with the removal ability of Lead（Pb）,cadmium（Cd）and arsenic（As）was obtained.The structure and properties of Fe-HAP were studied by X-ray diffraction,X-ray photoelectron spectroscopy,and specific surface area analysis.Through SEM,XRD,XPS analysis,Fe-HAP surface iron elements mainly exist in the form of Fe₂O₃,Fe OOH and a small amount of Fe₃O₄.And the doping of Fe³⁺will decrease the crystallinity of HAP;by BET analysis of Fe³⁺doped samples,it is found that the specific surface area,pore volume and pore size are optimized:compared with ordinary HAP,the specific surface area of Fe-HAP increased from 90.59m2/g to 191.22 m2/g,and the pore volume increased from 0.318 m3/g to0.938 m³/g.Considering the adsorption and removal rates of As（Ⅴ）,Pb²⁺,and Cd²⁺,the optimal preparation process conditions of Fe-HAP were obtained through orthogonal experiments as follows:iron content of 40%,synthesis p H value of 10.5,ultrasonic power of 200 W,The ultrasonic-microwave time was 15 min.（2）The optimum process conditions for stabilization of Pb,Cd and As in soil by Fe-HAP were determined,and the speciation transformation law of Pb,Cd and As in soil after remediation was also explored.With Fe-HAP（Fe/（Fe+Ca）=0.4）as soil remediation agent,the addition amount is 5%,after 60 days of remediation,the removal rates of bioavailable Pb,Cd and As in the soil are 79.77%and 51.3%,respectively.and 37.5%.Tessier method and Wenze method were used to extract and analyze the forms of lead,cadmium and arsenic in soil before and after remediation,after 60 days of Fe-HAP passivation,the occurrence forms of lead,cadmium and arsenic were significantly changed compared with that of hydroxyapatite（HAP）,and the residue state has increased significantly.The residual Pb and Cdin soil increased from 5.5%and 3.5%to 86.9%and 15.8%.The stable state of arsenic（residue state,Fe-Al oxide combination state）increased from 77%to 83%.It can be seen that the mobility and bioavailability of lead,cadmium and arsenic are effectively reduced after Fe-HAP stabilization.（3）The mechanism of adsorption-isomorphous replacement stabilization for Pb,Cd and As by Fe-HAP was revealed.In the solution system,through batch experiments,the adsorption kinetics of Pb,Cd and As on Fe-HAP conformed to the pseudo-second-order model,indicating that the adsorption was controlled by chemisorption;After fitting the Langmuir adsorption isotherm,the maximum adsorption capacities of Fe-HAP for Cd²⁺and As（V）were calculated to be 476.2 mg g^-1and195.69 mg g^-1,respectively,while Pb²⁺was more in line with the Freundlich model.XRD,SEM and XPS analysis indicated that Fe-HAP formed stable Pb₅（PO₄）₃OH,Ca_5.5Pb_4.5（PO₄）₆（OH）₂,Cd₃（PO₄）₂·4H₂O,Cd（OH）₂and Fe₃（As O₄）₂·6H₂O minerals with Pb,Cd and As.In conclusion,the simple and efficient removal performance of Fe-HAP indicates that it has broad application prospects in the comprehensive remediation of lead,cadmium and arsenic co-contaminated soils.