Sulfur-modified Biochar Supported Nanoscale Iron Sulfide Composite For Stabilizing Remediation Of Cadmium Contaminated Soil

Mining,metal smelting and sewage irrigation usually cause heavy metal pollution problems in water and soil environment,which is highly valued by the state and scholars.Stabilization/passivation technology is a relatively economical and environmentally friendly remediation method,but the choice of stabilizer/passivator is a key factor in this technology.In this study,cadmium-contaminated soil was used as the research object.Sulfur-modified biochar supported nanoscale iron sulfide composite(FeS@SBC)was successfully prepared in this study.The physicochemical properties of raw biochar(BC),sulfur-modified biochar(SBC)and FeS@SBC were analyzed by different characterization methods.The adsorption performance of cadmium in aqueous solution was verified by batch adsorption experiments.The effects of BC,SBC,and FeS@SBC with different application amounts on p H value,EC value,TCLP toxicity leaching and BCR morphology distribution of cadmium-contaminated soil were analyzed by soil incubation experiments to explore the stabilization mechanism.The changes of p H,EC value and morphological distribution of stabilized soil under acid rain conditions were simulated by leaching column experiments to verify the persistent stability.The growth and changes of biomass of water spinach were analyzed by pot experiments to elaborate the inhibiting mechanism of stabilizers on cadmium-contaminated soil.The research conclusions are summarized as follows:(1)Compared with the elemental composition of BC and SBC,the C content of FeS@SBC decreased,and the O,S and N increased.After loading iron sulfide,the BET specific surface area and total pore volume of FeS@SBC were 6.25 m~2/g and 0.033 cm~3/g,respectively,which belonged to typeⅢisotherm,providing more active sites for the adsorption of Cd(II).Meanwhile,SEM/TEM-EDS,XRD and FTIR characterization confirmed that iron sulfide was successfully loaded onto the surface of sulfur-modified biochar.The pseudo-second-order kinetic and Langmiur isothermal model can well describe the adsorption process of Cd(II)by FeS@SBC,and the maximum adsorption capacity is as high as 262.34 mg/g.Thermodynamic analysis indicated that the adsorption of Cd(II)by FeS@SBC was spontaneous and endothermic process.Through FTIR spectroscopy and XPS analysis,the adsorption mechanism of FeS@SBC on Cd(II)was mainly surface complexation,chemical precipitation,ion exchange and electrostatic attraction.(2)After 28 days of incubation,compared with the CK group,the p H value of soil could be improved by adding BC and SBC with different dosages,and it increased with the increase of the dosage,while CCS-FeS and FeS@SBC decreased slightly.The result of EC value is opposite.The 2%FeS@SBC could well reduce 90%of TCLP toxicity leaching amount of Cd in soil.After the four materials were applied to the contaminated soil,the occurrence form of cadmium was changed,and it was promoted to transform from the form with high bioavailability to the form with high non-bioavailability.The acid extractable content of cadmium was effectively reduced,and the stabilization effect was obvious.The XPS spectra of the soil stabilized by FeS@SBC showed changes in C1s,O1s,Fe2p,S2p and Cd3d,and the stabilization mechanism was mainly co-precipitation,supplemented by ion exchange and complexation.(3)The results showed that the leaching concentration and accumulation of Cd under BC,SBC and FeS@SBC treatments decreased with the increase of application amount during the 14-day leaching period.The peak leaching concentration and the cumulative leaching amount of Cd in the soil treated with 2%FeS@SBC were 70.9%and 71.6%,respectively,lower than that in the control group.After leaching,CK,BC,SBC and FeS@SBC treatments had significant differences in all four fractions,the same was that the acid extractable fraction decreased with the increase of application amount,and the residual fraction was opposite.(4)The application of 2%biochar materials had different degrees of effects on the growth of water spinach.Compared with CK,the application of BC,SBC,and FeS@SBC materials all effectively promoted the growth of water spinach,and their growth and biomass were higher than those of the CK group.Among them,the biomass of water spinach increased by 134.95%,BCF and TF values decreased by 62.25%and 40.39%,respectively,and soil available cadmium concentration decreased by 23.87%under FeS@SBC treatment.Bio-transmission electron microscopy and EDS characterization analysis showed that FeS@SBC remediation was beneficial to prevent the absorption and translocation of Cd by water spinach,which was due to the complexation and precipitation of FeS@SBC surface functional groups with Cd.