Using Clay To Modify NZVI Apply For Chromium Remediation And Its Co-transport Mechanism

Chromium（Cr）is one of the common trace metals that cause serious global environmental pollution.It mainly exists in the environment in the form of metallic chromium,trivalent chromium（Cr（III））and hexavalent chromium（Cr（VI））.Hexavalent chromium is the most harmful and toxic to the environment.Cr（VI）can be accumulated in soil and groundwater and transferred to humans through the food chain.Therefore,it is essential and important to remove Cr（VI）in the environment.As an effective soil remediation material,nano-zero-valent iron（nZVI）with a typical core-shell structure can react with various environmental pollutants.However,nZVI is easily oxidized and aggregated and hinders the reactivity of nZVI,so it is necessary to modify nZVI.In recent years,green synthesis methods using plant extracts and other renewable biological resources have received more and more attention.Among these plant extracts,green tea extract has become a relatively high-quality reducing agent and stabilizer in green synthesis because it is rich in active polyphenols and caffeine.At present,limited research focuses on the green synthesis of nZVI by tea leaves.In this experiment,nZVI is supported by adding clay minerals（fly ash,CFA,sepiolite）,which can improve the dispersibility,stability,transportability,and reactivity of nZVI.The effect of different environmental factors（dosage,flow rate and clays）on clay-nZVI remediation Cr（VI）and the effect of co-transport.The experimental conclusions are as follows:（1）The transportability of fly ash-loaded nano-zero-valent iron（CFA-nZVI）in different electrolyte solutions and the reaction mechanism of repairing Cr（VI）were explored.The loading of CFA greatly improved the reactivity of nZVI,increased the distance between nZVI particles,and improved the dispersion performance of nZVI.The reaction kinetics between CFA-nZVI and Cr（VI）was explored.CFA-nZVI remediation Cr（VI）was divided into two parts:adsorption and reduction.The reaction reached equilibrium at 180 min.At the same time,the effect of CFA-nZVI with different loading ratios on the remediation and removal of Cr（VI）was explored.In this explore,CFA-nZVI showed a stronger ability to remediation heavy metals than the nZVI by green synthesis.When Fe:CFA=10:1,CFA-nZVI showed the best remediation effect on Cr（VI）.The maximum remediation capacity reach306.5 mg·g^-1,and the corresponding reduction capacity was 190.45 mg·g^-1.The adsorption capacity wass 116.12 mg·g^-1,and reduction plays a major role.The transportability of CFA-nZVI in the porous medium is also analyzed.With the increase of ionic strength,the electric double layer between the nanoparticles was compressed,the repulsive force was reduced,and the particles were easily aggregation.Thus,the transport of CFA-nZVI in porous media was inhibited.（2）The influence of environmental factors on the transport of CFA-nZVI in porous media and the remediation of Cr（VI）were explored.The increase in the dosage of CFA-nZVI reduced the porosity in the porous medium,increased the concentration of CFA-nZVI in the porous medium,promoted the aggregation of nanoparticles in the porous medium,and inhibited the transport of CFA-nZVI in the porous medium.Due to the increased of the dosage,the contact reaction time and contact probability of the nano-particles and Cr（VI）are increased,thereby improving the effect of CFA-nZVI remediation Cr（VI）in the porous medium.The increase in flow rate during transportation reduces the attractive force between CFA-nZVI nanoparticles and porous media,thereby promoted the transport of CFA-nZVI,but reduced the reaction time of CFA-nZVI and Cr（VI）,promoted the effect of CFA-nZVI to remediation Cr（VI）.After adding clay minerals,the porosity in the porous medium was reduced and the attractive force between the nanoparticles was increased,the transport of CFA-nZVI was inhibited.The inhibitory effect was kaolin>montmorillonite>diatomite.The layered structure of montmorillonite and kaolin reduced the possibility of contact reaction between CFA-nZVI and Cr（VI）,and inhibited the remediation effect of CFA-nZVI on Cr（VI）in porous media.The addition increased the effective contact area of CFA-nZVI with Cr（VI）,promoting the remediation of Cr（VI）by CFA-nZVI.（3）Sepiolite loaded nano-zero-valent iron（SPT-nZVI）was synthesized by green synthesis,which improved the reactivity and dispersion performance of nano-zero-valent iron.The load ratio of nZVI to sepiolite was changed,and it was explored that when the optimal load ratio is Fe:SPT=10:1,SPT-nZVI has the best repair effect on Cr（VI）.In the whole process,The reduction effect of SPT-nZVI on Cr（VI）is greater than the adsorption effect.The effect of clay minerals on the restoration of Cr（VI）by SPT-nZVI was studied.In Na Cl electrolyte,clay minerals had no effect on the restoration of Cr（VI）.In the electrolyte of Ca Cl₂,the addition of montmorillonite had no effect on the restoration of Cr（VI）by SPT-nZVI.VI)It has a promoting effect.Kaolin and diatomite have no significant effect on the restoration of Cr（VI）.In addition,clay minerals improved the suspension stability of SPT-nZVI.In the Na Cl electrolyte,montmorillonite has a covering effect on SPT-nZVI in porous media due to its special lamellar structure,inhibiting the co-transport of SPT-nZVI and Cr（VI）,and also inhibiting SPT-nZVI can remediation Cr（VI）,while diatomite and kaolin have no effect on the co-transport of SPT-nZVI and Cr（VI）,which promoted the repair of Cr（VI）in porous media by SPT-nZVI.In the Ca Cl₂ electrolyte,montmorillonite and kaolin have no obvious effect on the co-transport of SPT-nZVI and the remediation of Cr（VI）.The addition of diatomite earth will inhibit the co-transport of SPT-nZVI and the repair of Cr（VI）.