Evaluation Of The Long-term Efficacy Of Carboxymethylcellulose Modified Iron Carbon Materials For Remediation Of Cr(Ⅵ) Contaminated Site

Industrialisation has led to the discharge of large amounts of Cr-containing wastewater flowing into water and soil,and Cr（VI）can easily migrate in soil due to its special properties.In recent years,nano zero-valent iron（n ZVI）has been widely used in the remediation of Cr（VI）contaminated soils due to its high reactivity.However,the disadvantages of easy agglomeration and oxidation limit the application of n ZVI.Therefore,in this study,a CMC-n ZVI/BC material with high oxidation resistance was prepared using sodium carboxymethyl cellulose（hereafter referred to as CMC）and biochar（hereafter referred to as BC）as the surface modifier and carrier of n ZVI particles,respectively,by liquid phase reduction.It was applied to the remediation of Cr-contaminated soils at different levels to investigate the stabilization effect of the material on Cr and the changes of soil physicochemical properties during the remediation process.On the basis of this,the stability of Cr in the remediated soil was investigated through the natural ageing process（wet-dry cycles and freeze-thaw cycles）to reveal the remediation mechanism;CMC-n ZVI/BC was used as a filler material for the permeable reactive barrier（PRB）technology,and its removal performance and transport pattern of Cr（VI）under continuous operation were investigated with numerical simulations.The main findings are as follows:（1）The performance of different CMC to Fe mass ratios on aqueous solutions of soil containing Cr（VI）was investigated during the synthesis of CMC-n ZVI/BC materials,and the results showed that the best results were obtained for CMC/Fe=0.4,with 99.8%removal rate.Material characterization revealed that the CMC in CMC-n ZVI/BC bounded to the surface of n ZVI particles in a bidentate chelate manner and that the modification of CMC and BC significantly inhibited the agglomeration behavior of n ZVI particles,enriched the oxygen-containing functional groups of the material and caused a shift from a polycrystalline to an amorphous structure of Fe⁰in the material,and it also reduces the leaching of iron ions.Air ageing experiments demonstrated the enhanced antioxidant capacity of CMC-n ZVI/BC and the removal efficiency of Cr（VI）from soil was still 92.6%after 10 weeks.（2）The CMC-n ZVI/BC material showed good remediation results for the CS chromium salt plant contaminated soil at a high concentration（Cr=5620mg/kg）and the BJ soil at a low concentration（Cr=525 mg/kg）.The toxicity characteristic leaching procedure（TCLP）,simulated acid rain leaching procedure（SPLP）and simple bioaccessibility extraction test（SBET）were used to evaluate the remediation effect of CMC-n ZVI/BC added to the two soils within 90 days.For CS soil with a material dosage of 30 g/kg.After 90days of remediation,the concentrations of TCLP-Cr（VI）,SPLP-Cr（VI）and SBET-Cr（VI）decreased by 96.6%,94.3%and 98.9%,respectively.For BJ soil with a dosage of 20 g/kg,the corresponding Cr（VI）concentration decreased by 97.0%,90.0%and 98.8%,respectively.CMC-n ZVI/BC also significantly reduced the proportion of HOAc-Extractable Cr in the two soils,greatly reducing the environmental risk.The characterization analysis showed that the main mechanism of Cr stabilization by CMC-n ZVI/BC was adsorption,reduction and coprecipitation,and a more stable Fe Cr₂O₄mineral phase was formed on the surface.（3）The stability of Cr in the remediated soil was evaluated using wet-dry cycles and freeze-thaw cycles.The results showed that the TCLP-Cr（VI）in the BJ soil were 70.0%and 65.0%lower than before the wet-dry cycles and freeze-thaw cycles respectively for the two soils applied with CMC-n ZVI/BC,while the TCLP-Cr（VI）in the CS soil remained low and showed no signs of rebound.In addition,with the wet-dry and freeze-thaw cycling processes,some of the amorphous iron oxides in the soil are converted to crystalline iron oxides,which was conducive to the stabilization of Cr,and the process further caused the redox reactions of Fe⁰and Fe（II）on the surface of CMC-n ZVI/BC to reduce Cr（VI）to Cr（III）,forming a more crystalline Fe Cr₂O₄mineral phase.（4）CMC-n ZVI/BC was applied as a filler in the design of the PRB dynamic column and combined with the use of COMSOL simulation software to construct a model of the PRB dynamic column,and the simulated values were compared with the measured values.The results showed that the measured values had a good correlation with the simulated trends,further indicating that the model could be applied to the transport pattern of Cr（VI）under the PRB dynamic column under laboratory conditions,and the difference between the measured and simulated values was mainly related to the by-reaction water.In summary,the CMC-n ZVI/BC modified with CMC and BC had a good and long-term remediation effect on two different levels of Cr-contaminated soils,and the Cr in the soil remained extremely stable even under wet-dry/freeze-thaw cycles;and the material could effectively and long-term remove Cr（VI）from simulated groundwater as PRB reaction medium.The Fe-biochar nanomaterials prepared in this study had a good ability to remove Cr（VI）from soil and simulated groundwater in different contamination scenarios,which provided theoretical basis and data support for the remediation of Cr-contaminated sites,agricultural fields and groundwater.It is of great significance for the remediation of heavy metal contaminated soils.