Adsorption Characteristics And Mechanism Of Biochar/Attapulgite Composite For Heavy Metals Lead And Cadmium Pollution

In recent years,soil heavy metal pollution has become one of the most important environmental problems to be solved.Solidification/stabilization technology has been widely used in remediation of soil heavy metal pollution due to its advantages of high speed,good effect and low cost.The key of this technology is to choose the stabilization material with good passivation effect,low cost and no secondary pollution.The pyrolysis of crop straw to prepare biochar can realize the resource utilization of agricultural and forestry resources,adsorb and fix pollutants,and realize carbon sequestration and emission reduction.Clay minerals are low in cost and abundant in source.Because of their special layered structure,high specific surface area and high ion exchange capacity,clay minerals have become one of the research hotspots of soil heavy metal pollution control.Therefore,the research and development of biochar composite clay mineral materials can reduce the cost and improve the adsorption capacity of heavy metals.In this paper,crop straw and attapulgite were used as raw materials to prepare biochar and its composite attapulgite by pyrolysis without oxygen.The effects of different raw materials and pyrolysis temperature on the physicochemical properties of biochar and its composites were investigated,and the adsorption mechanism of Cd or Pb in solution under different preparation conditions was quantitatively analyzed.Through batch adsorption experiments combined with characterization analysis and indoor soil culture experiments,the competitive adsorption mechanism of Cd and Pb by modified composite materials was systematically analyzed,and the adsorption mechanism of heavy metals and the passivation effect and mechanism of Cd and Pb composite polluted soil were further studied.The main conclusions are as follows:Using rice and wheat straw as raw materials,biochar was prepared by anaerobic pyrolysis in a tubular furnace at different temperatures,and batch adsorption experiments of Cd and Pb were carried out to explore the effects of raw material types and pyrolysis temperature on biochar adsorption properties.The results showed that the pyrolysis temperature greatly affected the physicochemical properties of biochar,such as yield,p H and electrical conductivity,and the biochar prepared at 600°C had good adsorption performance.The preparation of biochar/attapulgite composites at this temperature was continued,and the batch adsorption experiments of Cd and Pb were carried out to study the effects of different adsorption conditions on the adsorption properties of the composites.In addition,SEM,XRD,FTIR and other methods were used to characterize and analyze the composite materials before and after adsorption.The mechanism of action was analyzed from qualitative and quantitative perspectives to clarify the leading adsorption mechanism.The results showed that the specific surface area of the composite was significantly higher than that of the original biochar.When the ratio of rice straw to attapulgite was 5:1,the RABC5-1 composite had the best adsorption effect on Cd²⁺and Pb²⁺,and the maximum adsorption capacity reached 132 mg/g and 220 mg/g.The quasi-second-order kinetics and Langmuir isothermal model were more consistent with the adsorption process of Cd²⁺and Pb²⁺.The adsorption mechanisms of Cd²⁺and Pb²⁺mainly include precipitation,functional group complexation,ion exchange and cation-πinteraction.Among them,precipitation plays a leading role in heavy metal adsorption process.In order to further improve the adsorption performance of the composite,the optimized rice straw composite（BA）was modified with phosphoric acid,sodium hydroxide and iron and manganese oxides.The adsorption behavior of composite materials in Cd,Pb single and binary-polluted systems was investigated by carrying out adsorption kinetics,isothermal adsorption and competitive desorption experiments.and the adsorption mechanism of the modified composites on the heavy metals Cd and Pb in solution was analyzed in depth by combining with BET,SEM,FTIR and XPS characterization methods.The adsorption experiment after modification showed that the removal efficiency of Cd²⁺and Pb²⁺by Na OH modified composite（NBA）increased by 17.52%and 22.28%,respectively,compared with the original composite（BA）.The adsorption experiments show that the quasi-second-order kinetic model and Langmuir isothermal model can better simulate the adsorption process in Cd and Pb binary metals,and the adsorption capacity increases with the increase of p H value.The competitive desorption experiments show that Cd²⁺and Pb²⁺compete for the active adsorption sites on the adsorbent surface under the combined pollution condition.Pb²⁺not only has a competitive advantage on the adsorption sites,but also can replace the adsorbed Cd²⁺to be re-adsorbed.Characterization analysis of NBA showed that the pore structure of the modified composites was significantly improved.FTIR and XPS analysis showed that the adsorption mechanism of heavy metals was mainly the complexation between hydroxyl,carboxyl and other oxygen-containing functional groups and heavy metals.The modified composite materials were applied to Cd and Pb contaminated soil to carry out indoor soil culture experiment.In this study,the effects of modification methods and addition amount of composite materials on soil physical and chemical properties,available state and speciation of heavy metals in soil were studied,and the passivation mechanism of composite materials on heavy metals in soil was further explored.The results showed that the composite material could effectively improve the soil p H value,and the soil p H value increased by 16.02%after NBA treatment.The soil electrical conductivity and organic matter content increased after the application of composite materials.At the same time,after the application of composite materials,the effective contents of Lead and cadmium extracted by DTPA decreased significantly compared with the control group.When the application amount was 0.6%,the effective Cd and effective Pb after NBA treatment decreased52.30%and 67.25%respectively compared with the control group.The application of composite materials can promote the transformation of Pb and Cd in soil from unstable state（acid soluble state）to stable state（residue state）.The precipitation between Na OH modified composite materials and heavy metal ions may be the main mechanism of NBA passivation of Cd and Pb in soil.