Experimental Study On Remediation Of Cr(Ⅵ),Cd,and Pb Compound Contaminated Soil Using FeS NPs

The soil contamination of heavy metals is becoming more and more serious.In the fields of metal smelting,chemical manufacturing and electroplating,the compound contamination of cadmium（Cd）,lead（Pb ）and chromium（Cr）cannot be ignored.According to the research progress of soil remediation,the commonly used remediation materials including carbonate,phosphate and Fe（Ⅱ） have deficiencies in simultaneously stabilizing Cr（Ⅵ）,Cd and Pb in soil,so that new remediation materials need to be further developed.FeS NPs could achieve good efficiencies in the remediation of single Cr（Ⅵ）,Cd and Pb contaminated aqueous solutions and soils,but there is little research on the simultaneous stabilization of Cr（Ⅵ）,Cd and Pb in soil.FeS NPs were used as a remediation material to study the simultaneous removal performances of Cr（Ⅵ）,Cd and Pb in aqueous solutions.The potential mechanism of simultaneous removal of Cr（Ⅵ）,Cd and Pb in aqueous solution by FeS NPs was explored by Visual MINTEQ theoretical model,solid phase product characterization and chemical precipitation theoretical calculation.FeS NPs were used to stabilize Cr（Ⅵ）,Cd and Pb in compound contaminated soil.The stabilization kinetics,the effects of FeS NPs dosage,phosphate and oxygen on the simultaneous stabilization of heavy metals were investigated.The existing forms of heavy metals in soil,the physicochemical properties of soil,and the mineral phase composition of soil before and after remediation were analyzed.The mechanism of simultaneous stabilization of Cr（Ⅵ）,Cd,and Pb in soil by FeS NPs was constructed.The research conclusions are as follows:（1）FeS NPs could simultaneously remove Cr（Ⅵ）,Cd and Pb from aqueous solutions.The effect of FeS NPs suspension was significantly better than the washed FeS NPs.The efficiency increased with increasing of FeS NPs addition ratio,the removal efficiencies of Cr（Ⅵ）,Cd and Pb could achieve 95%,99%,and 99%respectively.The acidic conditions significantly favored the removal of aqueous Cr（Ⅵ）,while the alkaline conditions favored the removal of Cd and Pb .Oxygen would slightly inhibit the removal of Cr（Ⅵ）,and there was no significant influence on that of Cd and Pb .（2）A potential mechanism was proposed for the simultaneous removal of Cr（Ⅵ）,Cd and Pb by using FeS NPs.Cr（Ⅵ）may be absorbed by FeS NPs and reduced to Cr（Ⅲ）,then precipitated to Cr（Ⅲ）-Fe（Ⅲ）or Cr（Ⅲ）（oxy）hydroxides.Cd reacts with FeS NPs to form the solids of Cd S and Cd-Fe-hydroxides.Pb is released from PbCrO₄ and precipitated to PbS by FeS NPs.Interactions of the three heavy metals involves the cationic bridging effect of Cd to promote the adsorption of Cr（Ⅵ）,the enhanced adsorption effect to Cd by[Cr,Fe]（OH）₃.In addition,SO₄^2- as the product of the reaction of Cr（Ⅵ）and FeS NPs could precipitate Pb to PbSO₄.Cr（Ⅵ）and Pb could generate PbCrO₄ precipitates,then PbCrO₄ is converted to PbS by FeS NPs.（3）The combined addition of FeS NPs and phosphate could achieve simultaneous stabilization of Cr（Ⅵ）,Cd and Pb in the soil.In the soil with initial Cd,Pb ,Cr（Ⅵ）content of 82,922,78 mg/kg,the Cr（Ⅵ）content was reduced to 5.39 mg/kg,and the leaching concentration of Cd and Pb were reduced to 0.14 and 0.12 mg/L with the molar ratio of FeS-to-metal of 2:1 and phosphate-to-Pb of 8:1.Oxygen had no significant effect on the stabilization of Cr（Ⅵ）,Cd and Pb .In the soil with initial Cd,Pb ,Cr（Ⅵ）content of 401,4638,457 mg/kg,the Cr（Ⅵ）content was reduced to 19.26 mg/kg,and the leaching concentration of Cd and Pb were reduced to 1.6 and 1.2 mg/L with the molar ratio of FeS-to-metal of 3:1 and phosphate-to-Pb of 4:1.Oxygen could promote the reduction of Cr（Ⅵ）,and slightly inhibit the stabilization of Cd and Pb .Within 240 days of stabilization,the leaching concentration of Cr showed a decrease,and the leaching concentration of Pb and Cd both showed an increase within a change of 10%.（4）A potential mechanism was proposed for the simultaneous stabilization of Cr（Ⅵ）,Cd and Pb by using FeS NPs.Fe（Ⅱ）and S（-Ⅱ）could reduce Cr（Ⅵ）to Cr（Ⅲ）,and generated Cr（Ⅲ）-Fe（Ⅲ）（oxy）hydroxides adsorbed on the surface of soil particles.Meanwhile,the reduction of CrO₄^2-destroyed the balance of PbCrO₄ precipitation and dissolution to result the release of Pb .A part of Pb could be adsorbed by FeS NPs and generated PbS with S（-Ⅱ）.Cd competed with Pb for the adsorption sites of FeS NPs and Pb had an apparent competitive advantage.A small part of Cd was adsorbed by FeS NPs and formed Cd S with S（-Ⅱ）.Then phosphate was added to the soil,negative charge on the surface of soil particles increased,and free Pb and Cd would be adsorbed on the surface of soil particles.Cd and Pb would react with phosphate to form Pb₃（PO₄）₂ and Cd₃（PO₄）2.Excess FeS NPs reacted with phosphate to form Fe₃（PO₄）₂ to fix phosphorus,thus effectively alleviating the secondary pollution caused by phosphorus leaching.