Experimental Study On In Situ Reactive Zone Remediation Of Cr(Ⅵ) Contaminated Groundwater By Sulfidated Micron Zero Valent Iron Stabilized With Xanthan Gum

Chromium（Cr）is widely used in many developing industries such as metallurgy,electroplating,etc.However,large amount of Cr enters the groundwater system^[1]due to the lack of prevention and monitoring awareness of the industry,causing serious environmental pollution and posing threat to people.As one of the most dangerous valence states of Cr.Hexavalent chromium（Cr（VI））has carcinogenicity,teratogenicity,and mutagenicity,and has been listed as the first human carcinogen.Thus it is necessary to develop effective methods to eliminate Cr（VI）in contaminated groundwater.Micron zero valent iron（mZVI）was an underground remediation material,which had great application potential to replace nano zero valent iron（n ZVI）from the perspective of economic and health benefits.However,mZVI was highly prone to gravitational settling,which limited its wide application forin situ remediation of contaminated groundwater.This paper was devoted to develop an efficient and economical groundwater remediation material based on mZVI,which should possess excellent stability,reactivity,and transportability.Therefore,in this paper,XG was selected to improve the stability and transportability of mZVI,and sodium dithionite（Na₂S₂O₄）was selected to improve the reactivity of mZVI,and XG stabilized and Na₂S₂O₄ sulfidated mZVI（XG-S-mZVI）was prepared and characterized.The properties of XG-S-mZVI was explored from the aspects of stability,reactivity,and transportability,respectively,and the applicable formation conditions of XG-S-mZVI was also determined.Provide theoretical support and guidance for the in situ reaction zone repair of mZVI.The specific research results are as follows:（1）Preparation and characterization of XG-S-mZVI.The mean diameter of XG-S-mZVI was 12.037μm,and the specific surface area was 2.977 m² g^-1.Ferric sulfides（Fe S and Fe S₂）were attached to the surface of mZVI particles.XG existed in XG-S-mZVI in two forms:one was covered on the surface of S-mZVI particles,called the adsorbed XG（XG_A）;the other was dispersed between S-mZVI particles,called the dispersed XG（XG_D）.The adsorption configuration of XG_A with microparticles were hydrogen bonding and monodentate chelation.（2）In terms of stability:when the concentration of XG was 2.0 g L^-1,the stability of the XG-S-mZVI system reached the best state.At this time,the zeta potential of XG-S-mZVI was-55.0±1.3 m V.XG_A generating repulsive force and XG_D generating viscous network worked together to resist the sedimentation of S-mZVI,so that S-mZVI particles maintained good suspension stability for at least one week,and the effect of XG_D was greater than that of XG_A.By fitting with the power law equation,it showed that XG-S-mZVI possessed good rheological property.（3）In terms of reactivity:sulfidation enhanced the electron transfer rate and electron selectivity of XG-S-mZVI,thereby improved the reactivity of XG-S-mZVI for reducing Cr（VI）.Cr(VI removal rate constant by XG-S-mZVI was determined to be832.4 times than bare mZVI.The contribution of Fe⁰ and Fe²⁺to the reduction of Cr（VI）accounted for 70.3%and 23.2%,respectively,and S^2-and S₂^2-contributed the rest.The fitting equation Y=2.207X+25.49 of the minimum dosage of XG-S-mZVI and the initial Cr（VI）concentrations was obtained without the consideration of the influence factors,where X referred to the initial concentration of Cr（VI）(mg L^-1),Y referred to the minimum dosage of XG-S-mZVI(m L L^-1).The fitting equation was applicable to the removal of Cr（VI）by XG-S-mZVI under ideal conditions.In view of the fact that there are many unfavorable factors affecting the remediation effect in the actual site,the calculated minimum dosage of XG-S-mZVI could be expanded（such as 1.2 times or 1.5 times）.This was taken as the final implantation amount of XG-S-mZVI.In the case of only considering the individual influencing factors,we found that Cl^-and SO₄^2-had little effect on the reduction of Cr（VI）by XG-S-mZVI,while NO₃^-,HCO₃^-,Ca²⁺,Mg²⁺and HA had adverse effect on the reduction of Cr（VI）by XG-S-mZVI.（4）In terms of transportability and repairability:when the particle size of the media increased from 0.25-0.50 mm（medium sand）to 0.50-1.00 mm（coarse sand）,the transport distance of XG-S-mZVI increased from～50 cm～80 cm.According to the analysis of the breakthrough curves and retention profiles of XG-S-mZVI,straining effect was the main mechanism for the retention of XG-S-mZVI in coarse sand and medium sand.61.4%and 72.2%of Cr（VI）was reduced and intercepted by XG-S-mZVI in situ reactive zone,respectively,in coarse sand and medium sand,and finally existed in the form of stable(Cr_xFe_1-x)（OH）₃（s）.The XG-S-mZVI in situ reactive zone for remediation Cr（VI）-contaminated groundwater was only suitable for media with a grain size larger than 0.25 mm.