Remediation Effect And Mechanism Of Fe-Zn Composite Modified Biochar On Alkaline Soil Contaminated Cadmium

Northern China is the main producing area of wheat.In recent years,the soil in the area has been seriously contaminated with cadmium（Cd）,which has caused the wheat to be poisoned by Cd,and the accumulation of Cd in wheat grains exceeds the standard.Wheat is the main food consumed by the population in northern China,thus ensuring the safe production of wheat plays an important role in ensuring human health.The p H value of soil in northern China is greater than 7.0.And it is difficult to achieve a remediation effect in alkaline soil by increasing soil p H value and water management measures to reduce the availability of Cd.Conventional passivation materials have no obvious passivation effect on Cd-contaminated alkaline soil,such as sepiolite,palygorskite,bentonite,etc.Therefore,it is of great significance to develop a new functional material suitable for alkaline soil remediation,which can effectively reduce the Cd content of wheat grains.In this research,a new type of iron-zinc composite modified biochar was prepared to investigate the remediation effect and mechanism of Cd in alkaline soil and wheat through batch adsorption,soil incubation,column leaching,and wheat pot experiments.The main results of this research are shown in the following four aspects:（1）Novel iron-zinc oxide composite modified biochar（Fe/Zn-RBC,Fe/Zn-DBC,and Fe/Zn-YBC）were successfully prepared using Robinia pseudoacacia branches,durian shells,and corn stalks as biomass.The characterization results of SEM,XRD,and FTIR indicated that the iron and zinc were successfully loaded onto the biochar.The adsorption data of Cd（II）on Fe/Zn-BC conformed to the models of pseudo-second-order kinetic,Langmuir isothermal,and Redlich-Paterson.According to the results of batch experiments,the maximum sorption capacity of Fe/Zn-RBC,Fe/Zn-DBC,and Fe/Zn-YBC for Cd（II）in water was increased by about 5,3,and 3 times compared with RBC,DBC,and YBC,respectively.The main mechanism is the precipitation of Cd（II）and carbonate,the complexation of Cd（II）with oxygen-containing functional groups and metal-oxygen groups（Fe-O and Zn-O）,and finally adsorbing of Cd（II）in the form of Cd CO₃,Cd（OH）₂,and Cd O.（2）The soil incubation results of iron-zinc oxide composite modified corn stover biochar（Fe/Zn-YBC）on Cd-contaminated alkaline soil showed that adding Fe/Zn-YBC increased soil p H value,DOC,and EC content.After 42 d of immobilization,the addition of 0.5%-3%Fe/Zn-YBC reduced DTPA-Cd、Ca Cl₂-Cd,and TCLP-Cd contents in alkaline soils by 23.73%-52.50%,30.85%-44.63%,and 37.41%-49.59%,respectively.Fe/Zn-YBC treatments promoted the conversion of the exchangeable fraction into the Fe/Mn oxide-bounded fraction of Cd and increased the abundance and diversity of soil bacterial communities.Furthermore,the SEM-EDS,XRD,and FTIR results for Fe/Zn-YBC separated from the test soils showed that the distribution of Cd adsorbed on Fe/Zn-YBC was positively correlated with Fe,Zn,and O.Additionally,the Cd complexes（Cd CO₃,Cd Zn Fe₂O₄,and Cd O）detected on Fe/Zn-YBC indicated that the immobilization mechanism of Fe/Zn-YBC in alkaline soil was the complexation and precipitation.（3）The leaching results of Cd-contaminated alkaline soil with particle sizes of 0.85-2 mm,0.15-0.85mm,0.075-0.15 mm and<0.075 mm showed that the highest content of Cd extracted by DTPA was obtained at a solid-liquid ratio of 1:5 and a leaching time of 1440 min.In the Fe/Zn-YBC treatments,the leaching rate of Cd in soils with particle sizes of 0.85-2 mm and 0.15-0.85 mm was higher than those in soils with particle sizes of 0.075-0.15 mm and<0.075 mm,indicating that Fe/Zn-YBC has a significant remediation effect on soils with small particle sizes.Adding Fe/Zn-YBC increased the content of free Fe/Mn oxide and amorphous Fe/Mn oxide-bound Cd in four particle size soils,and promoted the conversion of the exchangeable fraction to the Fe/Mn oxide-bounded fraction of Cd.Moreover,the addition of Fe/Zn-YBC increased the specific surface area and total pore volume of the four-particle size soils,while promoting the movement of the 0.85-2 mm and 0.15-0.85 mm soils to the small particle size distribution,as well as promoting the 0.075-0.15 mm soils and<0.075 mm soils moved toward larger particle size distributions.（4）The results of the pot experiment showed that Fe/Zn-YBC reduced the Cd content of JM22 and AK58 wheat roots by 50.37%-67.57%and 50.90%-73.25%,respectively.In the treatments supplemented with 0.5%-3%Fe/Zn-YBC,the content of Cd in JM22 and AK58 grains decreased by 60.56%-78.33%and 58.31%-82.67%,respectively.And the restored wheat grains of JM22 and AK58 were all below 0.1mg/kg,meeting the requirements of national food safety standards.Fe/Zn-YBC inhibited the accumulation of Cd in the grains by reducing the transfer coefficient of Cd from roots to straw and grains.Furthermore,adding Fe/Zn-YBC increased the Zn,Fe,and Mn contents,and decreased the Pb content in grains.The correlation analysis showed that the grain Cd content was negatively correlated with Zn,Fe,Cu,and Mn contents of the grains and roots,and positively correlated with the contents of Pb in grains and roots.The addition of Fe/Zn-YBC promoted the development of wheat roots and increased the grain yield.