Preparation Of Oyster Shell Passivator And Its Application In The Remediation Of Heavy Metal Contaminated Farmland Soil

According to the survey by the Ministry of Land and Resources,more than 10%of Chinese arable land has been polluted by heavy metals to varying degrees,such as lead（Pb）,cadmium（Cd）,mercury（Hg）,arsenic（As）,and chromium（Cr）.Severe and mostly compound pollution.In-situ passivation remediation technology is a remediation technology aimed at reducing pollution risks.Due to its low cost,simple operation,and high remediation efficiency,it has good application prospects for the remediation of moderately and slightly polluted soils.There are many kinds of passivation agents,and different passivation processes and reaction mechanisms directly affect the effect of passivation.From the research and practice at home and abroad,the research objects of in-situ passivation remediation technology are mostly acidic soils,and the passivation effects on weakly alkaline soils polluted by heavy metals are quite different.Therefore,the development of high-efficiency passivation agents is the key to the successful application of passivation technology to weakly alkaline soils contaminated by heavy metals.Based on evaluating the risk of heavy metal pollution in farmland soil in Anxin County,Xiong’an New Area,oyster shell powder was selected as the core material of this study for H₃PO₄wet modification.The oyster shell powder was modified by grinding and calcination with K₂HPO₄,and the morphology,structure,chemical composition,and other properties of the oyster shell powder before and after modification were analyzed.The oyster shell powder adsorption mechanism of heavy metals with different surface properties was revealed.Through adsorption kinetic experiments and adsorption isotherm experiments,the hydroxyapatite（HAP）,reed biomass charcoal（RBC）,oyster shell powder（OS）,H₃PO₄wet,modified oyster shell powder（MP-OS）,and Adsorption effect of K₂HPO₄grinding and calcining modified oyster shell powder（MC-OS）on heavy metal ions.With the help of modern instrumental analysis methods,the interaction mechanism of different passivators and heavy metals was revealed from the microstructure,and the difference in the binding mechanism of different passivators and heavy metals was determined.Hydroxyapatite,reed biochar,oyster shell powder,and modified oyster shell powder were selected as passivating agents,cinnamon soil in Anxin County was used as the test soil,ryegrass（Lolium perenne L）was used as the test plant,and heavy metal Cd and Pb are the remediation objects.Through pot experiments,different passivating agents were applied to the remediation of heavy metals in soil,and the feasibility and environmental effects of modified oyster shell powder in the remediation of heavy metal polluted farmland were clarified.The safe application of husk powder in soil remediation of polluted farmland provides a practical method and theoretical basis.The main conclusions are as follows:（1）The soil in the study area is weakly alkaline,and the contents of Cu,Zn,and Ni in the soil are all lower than the risk screening values for soil contamination of agricultural land.In comparison,the contents of Cd and Pb are higher than the risk screening values for soil contamination of agricultural land and lower than the risk intervention value for soil contamination of agricultural land.The proportions of the points of the values are 64.58%and16.67%,respectively.The farmland with high Cd and Pb content is mainly distributed near roads and factories and concentrated primarily on 0-20cm topsoil.The contents of Cd and Pb in wheat grains vary from 0.02 to 0.09 mg·kg^-1and 0.02 to 1.03 mg·kg^-1,respectively.According to the National Food Safety Standard for Contaminant Limits in Foods（GB2762-2017）,the Cd content in wheat grains meets the standard,but 4.17%of the samples are close to 0.1 mg·kg^-1(more than0.09 mg·kg^-1).The Pb content of 50%of the wheat grain samples exceeds the lead limit in the standard.There is a risk of Cd and Pb contamination in the study area.The potential ecological risk index in the study area is 288.83;the soil heavy metal pollution is a medium-high ecological risk level.The average value of Cd’s single-factor ecological risk index is 233.51,which belongs to the high ecological risk and is the main influencing factor,followed by Pb,Pb single-factor ecological risk index.The average value of the factor ecological risk index is 31.79,which is a low ecological risk.The soil heavy metal pollution risk in the study area is in the order of Cd>Pb>Cu>Zn>Ni.Cd and Pb in soil are significantly disturbed by the production activities of heavy metal processing enterprises around the farmland.It is speculated that there are two primary sources of soil heavy metal pollution in the study area.Cd,Pb,Zn,and Cu are mainly industrial and mobile sources,and Ni is primarily agricultural and natural sources.（2）The optimum preparation conditions of H₃PO₄wet,modified oyster shell powder（MP-OS）are as follows:H₃PO₄concentration is 0.5 mol·L^-1,modification temperature is 65℃,and the modification time is 120 min,The liquid-solid ratio of H₃PO₄/oyster shell powder is 7.5.The optimum preparation conditions of K₂HPO₄grinding and calcining modified oyster shell powder（MC-OS）were that the mass ratio of K₂HPO₄/oyster shell powder was 15%,the grinding time was 5 min,the calcining temperature was 400°C,and the calcining time was 2 h,the best adsorption effect on Cd²⁺and Pb²⁺could be achieved.（3）The pseudo-first-order kinetic model can better describe the kinetic characteristics of Cd²⁺and Pb²⁺by OS,indicating that the adsorption of Cd²⁺and Pb²⁺by OS is controlled by physical adsorption,and the concentration of Cd²⁺and Pb²⁺is the main factor affecting the adsorption rate.Both the pseudo-second-order kinetic model and the Elovich kinetic model can describe the whole process of adsorption of Cd²⁺and Pb²⁺by HAP,RBC,MP-OS,and MC-OS.The concentration and the amount of passivating agents are the main factors affecting the adsorption rate.The extramembrane diffusion model and intraparticle diffusion model show that the adsorption process of Cd²⁺and Pb²⁺by five passivators is affected by multiple steps,divided into three stages.In the initial stage of the reaction,the boundary layer diffuses rapidly,and Cd²⁺and Pb²⁺diffuse from the solution to the outer surface of the passivator for rapid adsorption.In the middle stage of the reaction,intraparticle diffusion occurs,and Cd²⁺and Pb²⁺diffuse into the passivator and occupy the internal active sites.The later stage of the reaction is the equilibrium stage,and the sites that can be combined with Cd²⁺and Pb²⁺in the passivating agent are reduced,and the adsorption equilibrium is gradually reached.The adsorption of Cd²⁺and Pb²⁺by the five passivators are all preferential adsorption,which the Langmuir model could fit.The adsorption of Cd²⁺and Pb²⁺belong to monolayer chemisorption.The maximum adsorption capacities of HAP for Cd²⁺and Pb²⁺are 150.04 mg·g^-1and 58.85 mg·g^-1,respectively;the maximum adsorption capacities of MP-OS for Cd²⁺and Pb²⁺are 139.59 mg·g^-1and 180.59 mg·g^-1,respectively 1;The maximum adsorption capacity of MC-OS for Cd²⁺and Pb²⁺is 182.52 mg·g^-1and 3494.09 mg·g^-1,respectively.The modified oyster shell has a better adsorption effect on Cd²⁺and Pb²⁺than HAP.Comprehensive analysis shows no secondary pollution problem in the preparation process of MC-OS,the economic cost is low,the adsorption effect is the best,and the economic benefit is the highest.（4）OS,MP-OS,and MC-OS all show different degrees of lamellar structure,and the surface is dominated by mesopores,all of which are negatively charged.The modified oyster shell powder has increased charge,electrical conductivity,pore volume,and average pore size,and the specific surface area increased from 0.341 m²·g^-1before being unmodified to 3.138m²·g^-1and 5.120 m²·g^-1.The formation of new pores after modification is the main reason for the improved adsorption performance of oyster shell powder.OS almost entirely shows the characteristic absorption peaks of Ca CO₃,MP-OS shows strong Ca HPO₄and Ca₃（PO₄）₂characteristic absorption peaks,and MC-OS shows more Ca（OH）₂and Ca O characteristics absorption peaks.Compared with OS,MP-OS and MC-OS have higher phosphorus content,the surface contains O-H symmetrical stretching vibration peak and P=O stretching vibration peak,the crystallinity in the crystal structure becomes smaller,and the bonding mode is changed.The modified oyster shell is similar to HAP,with more pores and a larger specific surface area,thus improving the adsorption performance.RBC mainly exists as mesopores.The pore volume and average pore size of OS,MP-OS,and MC-OS are more significant than RBC.The larger pore volume makes the modified oyster shells have a higher ability to adsorb heavy metals than RBC.Compared with the oyster shells before and after modification,HAP and RBC contain more C and less Ca and P.Compared with RBC,Ca-O symmetric stretching vibration absorption peaks exist in HAP,OS,MP-OS,and MC-OS is the main factor for better adsorption of Cd²⁺and Pb²⁺.（5）Different passivator treatments can effectively reduce the effectiveness of Cd and Pb in soil,and the reduction rates are 18.15%-58.24%and 23.92%-60.34%,respectively,and the passivation effect of modified oyster shell powder is better,second only to HAP.MC-OS has the best passivator treatment effect,reducing the content of soil available Cd and Pb by 48.66%and49.68%,respectively.Different passivator treatments can reduce the content of Cd and Pb in the upper part of ryegrass and the content of Cd in the lower part of the ryegrass by 45.16%-90.32%,1.63%-53.12%,and 39.18%-83.51%,respectively,but increase the content of Pb in the lower part of the ryegrass.The reason for the content of Pb in the underground may be that the phosphate precipitation in the lower part of the plant restricts the transport of Pb²⁺to the upper part of the plant.The accumulation of Cd and Pb in the lower part of the ryegrass is higher than that in the upper part,and the HAP and MC-OS treatment groups could better reduce the absorption of Cd and Pb in the polluted soil in the upper part of the ryegrass.The modified oyster shell powder can more effectively reduce the transport capacity of heavy metals from the underground to the shoot.The three treatment groups with smaller transport coefficients are HAP,MC-OS,and MP-OS,and the Cd transport coefficients are 0.20,0.21,and 0.27,and the Pb transport coefficients are 0.34,0.34,and 0.47,respectively.With careful consideration,MC-OS can be used as the optimal formula for practical application in heavy metal-contaminated farmland.