Study On In Situ Remediation Of Hexavalent Chromium Contaminated Groundwater By Biochar Supported Nano Zero-Valent Iron Stabilized With Carboxymethyl Cellulose

Chromium(Cr)is widely used in industries such as dyestuff,electroplating and petroleum refining,and its largest use is in the production of metal alloys,such as stainless steel and protective metal coatings.Due to improper disposal and other reasons,it is usually discharged directly into the environment with industrial wastewater and solid waste,leading to soil and groundwater contamination.Hexavalent chromium(Cr(Ⅵ))is considered to be a toxic,mutagenic and carcinogenic metal with high solubility and high migration.Therefore,there is a need to develop simple and effective technologies to remove Cr(Ⅵ)from groundwater.Nano zero-valent iron(nZVI)is generally considered promising as an in situ remediation agent for groundwater injection and has already been applied at sites.Compared with other remediation agents,nZVI has the advantages of large specific surface and high reactivity.However,nZVI is prone to agglomeration and passivation during application,which leads to poor electron transfer and difficult migration in aquifers.Biochar(BC)as a carrier material has the advantages of low cost,environmental friendliness,richness in various functional groups,and improved electron transfer efficiency of nZVI.However,BC is inherently less stable and difficult to migrate in the subsurface.In this paper,we consider the use of ball-milled to reduce the particle size of BC and then supporting with nZVI.In order to stabilize the migration in the subsurface to maintain the long-term ability to remove contaminants,the stability of the material should be further improved to improve the settling phenomenon.Carboxymethyl cellulose(CMC)was chosen as a stabilizer to improve the stability and thus the mobility of the material.In this paper,a low-cost and green approach was used to modify nZVI,which simultaneously improves reactivity,stability and mobility.The nZVI@BC reaction system was constructed with ball-milled BC as a carrier supporting with nZVI,and then nZVI@BC was stabilized by CMC to synthesize a new efficient,anti-passivation nanoscale repair material,CMC-nZVI@BC.The material before and after modification was characterized to analyze its morphology,structure and material composition.The sedimentation properties of nZVI before and after modification were investigated and the stabilization mechanism was elucidated.Using Cr(Ⅵ)as the target pollutant,the reactivity of CMC-nZVI@BC was studied and the removal mechanism was analyzed.The migration distribution characteristics of CMC-nZVI@BC in porous media and the remediation effect on Cr(Ⅵ)contaminated aquifers were investigated,which provided some theoretical support for the practical application of nZVI.The specific research results are as follows:(1)CMC-nZVI@BC,a highly efficient and passivation-resistant nanoscale repair material,was developed,successfully synthesized and characterized.nZVI@BC had a specific surface area 14.4 times that of nZVI.The specific surface area of CMCnZVI@BC was 2.74 times higher than that of nZVI,and modification of BC and stabilization of CMC increased the specific surface area of nZVI and effectively improve the dispersibility of nZVI.The mechanism of CMC binding to nZVI@BC was monodentate chelation and intermolecular hydrogen bonding.(2)In terms of stability,the modification of both BC and CMC improved the stability of nZVI to varying degrees,and CMC improved the stability of nZVI more significantly.5g L-1 CMC concentration could ensure that CMC-nZVI@BC did not settle within 7 days.The negatively charged CMC could be adsorbed on nZVI@BC,which had a significant effect on the improvement of electrostatic stability of CMCnZVI@BC.The adsorbed phase and the dispersed phase CMC jointly resist the settling of nZVI@BC,and the material stability can be improved by increasing the content of the dispersed phase CMC.(3)In terms of reactivity,the removal of Cr(Ⅵ)by nZVI@BC at Fe/C = 2.0 was47.6% higher than that by nZVI(same mass Fe concentration),and the BC accelerated the removal of Cr(Ⅵ)mainly by increasing the electron transfer rate of nZVI and forming iron-carbon microelectrolysis.The mechanism of Cr(Ⅵ)removal by CMCnZVI@BC was mainly summarized as chemisorption,reduction reaction and precipitation complexation.When only individual influences were considered,Cl-had a slight contribution to CMC-nZVI@BC,SO42-had little effect on the removal of Cr(Ⅵ)by CMC-nZVI@BC,while NO3-,HCO3-,Ca2+ and Mg2+ had a significant effect on the removal of Cr(Ⅵ)by CMC and HA had a significant inhibitory effect.(4)In terms of mobility and repairability,the media size had a significant effect on the migration distribution of CMC-nZVI@BC,and the larger the media size,the wider the reaction zone formed by CMC-nZVI@BC.The migration behavior of CMCnZVI@BC in saturated porous media was migration-retention-desorption-remigration,and strain was the main mechanism for particle retention in the media.The effective remediation period of CMC-nZVI@BC in coarse sand and medium sand was 4 and 6days,respectively,and the remediation effect in medium sand was better than that in coarse sand,which was not applicable to media with particle size less than 0.25 mm.CMC-nZVI@BC provided a favorable reduction environment for Cr(Ⅵ)removal over a long period of time.No significant adverse effects on the groundwater chemical environment were observed during the operation of the reaction zone.