| According to National Soil Pollution Survey Bulletin,more than 34%of the mining soil in China has suffered serious heavy metal pollution due to human activities such as mining and industrial waste disposal.Among them,arsenic(As),lead(Pb),and cadmium(Cd)are the main pollutants.The heavy metals in soil can cause serious threat to human life and health through direct or indirect contact with human body.Therefore,efficient methods for the simultaneous remediation of three heavy metals are urgently needed.In this paper,magnesium-aluminum modified biochars(MABs)were prepared by a one-step synthesis method and applied to the simultaneous remediation of As,Pb and Cd in the soil of the mining area in Gansu.The effects of synthesis temperature,rainfall and aging time on the remediation properties of As,Pb,and Cd were explored,and their synchronous repair mechanism was successfully revealed.The results of this study proved that MABs have good application prospects,and also provided a theoretical basis for the simultaneous restoration of As,Pb,and Cd in mining soil,the main conclusions are as follows:(1)MABs were prepared by one-step synthesis method at different pyrolysis temperatures(300℃,500℃ and 700℃).Scanning electron microscopy-Energy dispersive spectroscopy(SEM-EDS)results showed that Mg and Al elements were successfully attached to the surface of biochar.X-ray diffraction(XRD)results showed that its main forms were magnesium/aluminum hydroxide(Mg/Al-OH)and magnesium/aluminum oxide(Mg/Al-O),and Mg/Al-OH gradually transformed into Mg/Al-O with the increase of pyrolysis temperature.N2 adsorption-desorption results indicate that the increase of calcination temperature leads to the increase of specific surface area and pore volume of MABs.(2)By optimizing the material synthesis conditions,the optimal ratio of biomass to deionized water was determined to be 1:60(m:v);the optimal dosage of the material was 2%(wt);The comprehensive remediation effect of As,Pb and Cd by MABs prepared at 500℃(MAB 500)was the best,and the remediation efficiencies were about 50%,97%and 66%,respectively.Oxalic acid inhibited the remediation of As,Pb and Cd;Humic acid also inhibited the remediation of Pb,promoted the remediation of Cd,and had no effect on the remediation of As.The single and combined pollution remediation effects showed that As-Pb,Pb-Cd exhibited mutual promotion,and As-Cd exhibited mutual inhibition.The kinetics and characterization results revealed that the remediation mechanisms for As,Pb and Cd mainly include electrostatic attraction,precipitation/co-precipitation,surface functional group complexation and ion exchange.(3)Through simulated rainfall leaching(pH=4,7,8)experiments,it was found that MAB 500 inhibited the vertical migration of As,Pb and Cd in soil.The concentrations of As and Cd in the effluents of the MAB 500 treatment groups decreased by 34-86%and 21-28%,respectively.The optimal model for the cumulative release of As in the effuents changed from first-order kinetic model to more complex double-constant rate model,indicating that MAB 500 limits the migration of As in soilIn addition,more As,Pb,and Cd in the treatment groups were remained in the topsoil compared with the control group.(4)By evaluating the remediation effect of MAB 500 on the three heavy metals,it was found that the leaching toxicity concentrations of As,Pb,and Cd all dropped below the standard limit after treatment,and changed from an exchangeable state to more stable carbonate-bound state and residual state.After 28 dry-wet cycles,the remediation effect of MAB 500 on As,Pb,and Cd decreased by about 24%compared with that after 7 cycles,while the remediation effect of MAB 500 on the three heavy metals after 28 freeze-thaw cycles was stronger than that after 7 cycles.11-18%.MAB 500 was oxidized during the above process and the crystal structure was transformed into layered double hydroxide structure. |
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