Study On Passivation And Remediation Effect And Mechanism Of New Fly Ash Modified Materials On Soil Cadmium

China’s soil is facing serious heavy metal pollution problems,in which the Cd pollutant point exceeds the standard rate of 7.0%,and heavy metals are difficult to degrade,high toxicity and accumulation,which makes soil environmental problems more and more serious.The current soil remediation mainly uses in-situ passivation repair technology,but the choice of passivating agent becomes a major difficulty,and the high cost of the passivating agent increases the difficulty of soil remediation.Therefore,this study aimed to study a low-cost,high-efficiency,passivating agent that is easy to use in the field.In this study,a new type of solid waste coal ash was synthesized to simulate the soil water environment,and explore the adsorption mechanism of heavy metals in synthetic materials,the passivation effect of synthetic materials on heavy metals in soil was studied by pot and field experiment,in order to pollute the farmland with heavy metals,provide a scientific theoretical basis.1.Intermediate material（IP）was synthesized from fly ash（FA）by low temperature alkali fusion method.IP was characterized by scanning electron microscope coupled with energy dispersive spectrometry（SEM-EDS）,X-ray diffraction（XRD）,X-ray fluorescence（XRF）,X-ray photoelectron spectroscopy（XPS）,Fourier transform infrared spectroscopy（FTIR）,and Brunauer–Emmett–Teller（BET）.The adsorption properties and mechanism of synthetic zeolite for Cd²⁺were investigated by static adsorption experiment.The results showed that the Si—O—Si and Si—O—Al bonds in FA fractured to form[SiO₄]^4-and[AlO₆]^5-monomers after NaOH modification.The surface area of ZE increased to 21.54m²·g^-1.The equilibrium adsorption capacity of the Cd²⁺on IP was 49.38 mg·g^-1 under the conditions of the IP dosage of 0.1 g,Cd²⁺initial concentration of 100 mg·L^-1 and adsorption time of 30 min,which more than 57.29%FA reaction.IP adsorption of the Cd²⁺removal efficiency was more 90%at pH=4⁹.At 35℃,the adsorption kinetics of IP for Cd²⁺followed the pseudo-two-order kinetic equation model with rate constant of 0.0586g·（mg·min）^-1.The Cd²⁺adsorption isotherm is accorded with Langmiur model,1/n was 0.4324.The thermodynamic experiments revealed that the Gibbs free energy change△G was between-20⁰ kJ·mol^-1,indicated that the adsorption was a spontaneous and endothermic process.From the D-R isotherm model,the adsorption energy was more than 16 KJ·mol^-1,which demonstrated that the adsorption of Cd²⁺was a combination of physical adsorption and chemical adsorption.2.The effects of three passivates agents（FA,IP and ZE）on soil pH,effective cadmium（Cd）,Brassica juncea（Brassica juncea）biomass and Cd content in different parts were studied in pot experiment.The possible immobilization mechanism of Cd was elucidated.The results showed that application of the three immobilizing agents resulted in reduction of the content of bioavailable Cd in soils and a decrease in the amount of Cd in mustard.Moreover,the available Cd decreased by 48.35%and46.19%,respectively,after 1.5%IP and ZE passivators were added in the soils,and the content of Cd in the upper part of India mustard reduced by 37.51%and 41.06%,respectively.The soils pH increased about 0.59 and 0.62 units,respectively,after 1.5%IP and ZE passivators were added in the soils,and the biomass of mustard increased 20%.Thus,the IP and ZE passivating agents effectively reduced the available Cd and promoted growth of the crops.3.The effects of three passivating agents on the biomass of rice,the contents of heavy metals in different parts,the soil pH and the contents of Cu,Zn,Mn,Cd,Pb and Ni in soil were studied by field experiments.It was found that IP and ZE could effectively alleviate the toxic effect of heavy metals in paddy fields,and improve the biomass and yield of rice.Compared with blank rice,fresh weight increased by 16.34%and 15.81%,respectively,and soil pH value was improved.After the remediation and passivation of the soil by the passivating agent,the metal elements in the rice reached the national limit standard.The modified IP had the most significant passivating effect on Cd and Ni,with the heavy metal content decreasing by more than 30%,followed by the passivating effect on Cu and Mn,with the content remaining above 20%,and the metal passivating effect of Zn and Pb reaching more than 9%.In summary,the low-temperature alkali fusion of coal ash could effectively adsorb Cd²⁺,increased the soil pH,promoted the crop growth,reduced the soil available Cd and plant edible part Cd content,and provide new ideas for farmland soil remediation.