| As a major producer of chrome chemical industry,China has left a large number of sites with serious pollution after the relocation of chrome chemical enterprises.The soil in the site often contains high concentrations of Cr(Ⅵ),which seriously affects the development and reuse of the site and poses a huge threat to the surrounding environment.Among many remediation materials,nano-zero-valent iron(n ZⅥ)has been widely used in the remediation process of Cr(Ⅵ)contaminated sites due to its small particle size,large specific surface area and high reduction activity.However,n ZⅥ has problems such as easy oxidation,easy agglomeration,and poor electron selectivity,which seriously affect its application effect.Therefore,in order to improve the defects of n ZⅥ,in this study,phosphoric acid-modified biochar-supported sulfided nano-zero-valent iron functional material(PBC-Sn ZⅥ)was prepared by means of phosphoric acid-modified biochar loading combined with sulfurization modification.The soil remediation experiment explored the influencing factors of PBC-Sn ZⅥ on Cr(Ⅵ)removal,the stabilization effect and long-term stability of PBC-Sn ZⅥ on Cr(Ⅵ)in soil,and finally comprehensively analyzed the mechanism of PBC-Sn ZⅥ’s superior performance.Combined with XPS characterization results,the removal mechanism of Cr(Ⅵ)by PBC-Sn ZⅥ was analyzed.The specific results are as follows:(1)The effect of material synthesis parameters on the removal of Cr(Ⅵ)from PBC-Sn ZⅥ was explored,and the material was analyzed by SEM,FTIR,XRD,VSM.The results showed that when the mass ratio of carbon to iron was 1:10 and the mass ratio of sulfur to iron was 1:4,the maximum removal efficiency of Cr(Ⅵ)by PBC-SNZⅥ was 70.86%.In addition,phosphoric acid modification significantly increased the specific surface area of common biochar(BC)and the number of phosphorus-containing functional groups on the BC surface.The dispersity of Sn ZⅥ was significantly increased when PBC was used as the carrier compared with BC.In addition,the combined effect of PBC loading and vulcanization reduced the saturation magnetization of n ZⅥ,which was conducive to the reduction of the agglomeration of n ZⅥ.(2)The reasons for the stronger Cr(Ⅵ)removal efficiency of PBC-Sn ZⅥ were explored from multiple perspectives.The results showed that phosphoric acid modification increased the adsorption capacity of BC on Fe(II)in solution from 10.96 mg/g to 30.68 mg/g,promoting better dispersion of n ZⅥ on PBC.In addition,compared with n ZⅥ,sulfide modification and PBC loading significantly reduced the corrosion potential of n ZⅥ(from-0.87 V to-0.99 V),indicating that the modification process improved the corrosion rate of n ZⅥ and the electron transport ability of the material system.The change trend of Fe(II)and total Fe concentration in the system during the reaction process showed that PBC-Sn ZⅥ can react with Cr(Ⅵ)rapidly,dissolve and produce Fe(II)and total iron,and when the system generated precipitation,it did not affect the release process of Fe(II)in the system.(3)The performance and influencing factors of PBC-Sn ZⅥ in removing Cr(Ⅵ)were investigated by aqueous test.The results showed that PBC-Sn ZⅥ had a strong removal effect on Cr(Ⅵ)under acidic conditions,the reaction reached equilibrium at 60 min and the maximum adsorption capacity was 335.55 mg/g.The adsorption process conformed to the Avrami adsorption kinetic model and the Langmuir and Sips isotherm adsorption models,indicating that the process was a multiple adsorption process dominated by chemical adsorption and was a monolayer adsorption with uniform distribution of active sites on the surface of the material.In addition,the adsorption process was an endothermic spontaneous process and common anions and humic acid had no effect on the adsorption process,indicating that PBC-Sn ZⅥ had a certain stability.(4)The influence factors and stabilization effects of PBC-Sn ZⅥ on the removal of Cr(Ⅵ)in soil were explored through soil remediation experiments.The results showed that when the dosage of PBC-Sn ZⅥ was 2%,the Cr(Ⅵ)content in the restored soil decreased by 78.38% after42 d,which was 2.03 times of the removal effect of n ZⅥ,indicating that PBC-Sn ZⅥ had a strong removal effect on Cr(Ⅵ)in the soil.The increase of PBC-Sn ZⅥ dosages and soil water contents promoted the removal of Cr(Ⅵ).The removal rate of Cr(Ⅵ)increased firstly and then decreased with the increase of humic acid dosages.In addition,three leaching experiments(DTPA,SBET and TCLP)proved that n ZⅥ,Sn ZⅥ,BC-Sn ZⅥ and PBC-Sn ZⅥ treatments could significantly reduce the extractable concentration of Cr(Ⅵ)in soil,among which PBC-Sn ZⅥ has the best effect,indicating that PBC-Sn ZⅥ has a better stabilization effect on Cr(Ⅵ)in soil.Tessier continuous extraction test showed that the contents of exchangeable and carbonate bound Cr decreased by16.46% and 5.72%,respectively,after the treatment of PBC-Sn ZⅥ.The contents of iron and manganese oxide combined state,organic combined state and residue state increased by 15.26%,1.10% and 5.81%,respectively.After remediation,the mobility factors of Cr in the soil decreased from 0.2385 in the control group to 0.0167,indicating that the active Cr in the soil transformed to the stable state after PBC-Sn ZⅥ treatment,and the mobility of Cr decreased significantly.In addition,Cr(Ⅵ)content in the repaired soil decreased to a certain extent after 15 cycles of freeze-thaw cycle/dry-wet alternate process,indicating that the form of Cr in the repaired soil was relatively stable.(5)The removal mechanism of Cr(Ⅵ)by PBC-Sn ZⅥ was explored in detail by X-ray photoelectron spectroscopy(XPS)analysis.The results showed that part of Cr(Ⅵ)in the reaction system was reduced to Cr(III),which was adsorbed on the surface of the material.Among them,PBC-Sn ZⅥ was mainly reduced by n ZⅥ,reducing sulfur species and surface groups of biochar.In addition to reduction,the mechanisms include electrostatic attraction and co-precipitation. 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