Stabilization Effects For Cr(Ⅵ) In Soil By Modified NZVI And Its Migration Risk Assessment

Agricultural and urban soil in China have faced the serious heavy metals pollution,endangering food security and human health.Therefore,it is extremely urgent to carry out the efficient remediation for heavy metals in soil.In recent years,nanoscale zero-valent iron(NZVI)with low cost,strong reduction activity and simple synthesis process has been widely used in the removal of pollutants in soil and water environment.However,its application has been limited by its disadvantages such as easy agglomeration,microtoxicity and oxidation deactivation.Therefore,in this study,three kinds of safe and economical surfactants are selected to modify NZVI and applied to stabilize Cr(VI)in soil,in order to overcome the above shortcomings.The most suitable material is obtained by evaluation of removal effect,optimization by response surface methodology and simulation of soil column migration.The main results are as follows:(1)NZVI,cationic polyquaternium-7 modified NZVI(M550-NZVI),non-ionic macrogol 4000 modified NZVI(PEG-NZVI)and anionic sodium carboxymethyl cellulose modified NZVI(CMC-NZVI)are prepared by liquid phase reduction method.The particle sizes of NZVI,M550-NZVI,PEG-NZVI and CMC-NZVI are 141-206,42-170,70-191 and 66-200 nm,respectively.The main compositions of materials are Fe0 and Fe3C.At the dosage of 10 g/kg,PEG-NZVI and CMC-NZVI can almost achieve 100%removal efficiency for 300 mg/kg Cr-contaminated soil.The kinetics of Cr(VI)removal by four materials conforms to the pseudo second-order model(R2>0.99),and the maximum adsorption capacities of M550-NZVI,PEG-NZVI and CMC-NZVI are 14.08,18.18 and 19.01 mg/g,respectively.After stabilization for 28 days,Cr mainly exists in the soil as residue state.The reaction mechanism of Cr(VI)stabilized by NZVI includes extraction,reduction,adsorption and co-precipitation,among which the chemical reduction process plays a dominant role.(2)The optimal model by response surface methodology(RSM)for three factors(moisture content,culture time and pH)of NZVI,M550-NZVI,PEG-NZVI and CMC-NZVI are quadratic,with fitting coefficient(R2)of 0.9277,0.9033,0.9723 and 0.9646,respectively.NZVI and CMC-NZVI correspond to quadratic model with R2 of 0.9669 and 0.9289 for acetate ion,humic acid ion,and carbonate ion,respectively.However,M550-NZVI and PEG-NZVI fit the 2FI model with R2 of 0.7695 and 0.9235,respectively.Moreover,NZVI,PEG-NZVI and CMC-NZVI can achieve better removal efficiency at low pH,while M550-NZVI is more suitable for neutral conditions.Besides,acetic acid can promote the removal of Cr(VI),while humic acid and carbonic acid can consume and block Fe0 and decrease the removal rate.In conclusion,CMC-NZVI has better Cr(Ⅵ)removal effect and anti-interference performance compared with the other three kinds of materials,being recommended for the RSM model.(3)The maximum adsorption capacity of Cr(Ⅵ)for topsoil is 53.9 μg/g,while that for deep soil is only 5.7 μg/g.The adsorption capacity of Cr(Ⅵ)in topsoil is obviously higher than that in deep soil.The Cr(Ⅵ)solution can penetrate the soil column quickly,showing strong migration.After 15 days of leaching with deionized water,the concentrations of Cr(Ⅵ)in the soil column leachate filled with PEG-NZVI and CMC-NZVI are 0.002 mg/L and 0.005 mg/L respectively,which are far below the standard limit of class Ⅲ groundwater(0.05 mg/L).The Cr(Ⅵ)concentrations of exchangeable(EX)and carbonate-bound(CB)in the soil column are lower than 0.5 and 0.2 mg/kg,respectively,and there is no obvious cumulative effect in the middle and bottom layer of the soil column.The Cr(Ⅵ)in soil column are mainly converted into residue state.After stabilized by 10 g/kg NZVI,the migration risk of Cr(Ⅵ)is low.