Remediation Mechanism Of Hexavalent Chromium Contaminated Groundwater Using Sulfidated Nano Zero-valent Iron

Groundwater is seen as important strategic resources and its pollution has attracted more and more attention,and is severe especially in chromium contaminated sites.The chromium in groundwater mostly exists in the form of Cr（III）and Cr（Ⅵ）,where Cr（Ⅵ）with carcinogenicity,teratogenicity and high toxicity is the main fact causing groundwater pollution on the site.Currently,converting the highly toxic Cr（Ⅵ）to Cr（III）and reducing its mobility in aquifer is the main approach for Cr（Ⅵ）-contaminated groundwater remediation.PRB is a preferred technology for Cr（Ⅵ）-contaminated groundwater remediation.The most common reactive material based on PRB and other remediation technologies is（nano）zero-valent iron.However,its application has certain limitations,such as easy passivation and deactivation,poor selectivity to target pollutants,etc.Recent studies have shown that sulfidated nano-sized zero-valent iron（S-nZVI）has a better repair performance in overcoming the aforementioned limitations.Therefore,based on the investigation of Cr（Ⅵ）-contaminated site and the improvement of traditional remediation materials,this paper prepared S-nZVI materials that can effectively remove Cr（Ⅵ）from groundwater,analyzed the detailed mechanism,explored the impact of S-nZVI preparation methods on the materials’characteristics and remediation performance for Cr（Ⅵ）,successfully constructed the S-nZVI-PRB reactor whose operation effect was verified by the numerical simulation.The specific research contents and results are as follows:1.The actual contaminated site was investigated and sampled.The Cr（Ⅵ）pollution in the groundwater of the site is serious,where the maximum concentration can reach to 293.0 mg/L and there exists co-pollution by Cr（Ⅵ）-NO₃?in the groundwater.Cr（Ⅵ）in the groundwater mainly exists in the form of Cr O?2?,HCr O??and Cr?O?2^-.The original PRB reactor on the site has poor remediation effect on the Cr（Ⅵ）-contaminated groundwater,and the highest concentration of Cr（Ⅵ）downstream of PRB can exceed 100 mg/L.2.The improved traditional PRB packing has a limited removal ability for Cr（Ⅵ）in groundwater.From the seven traditional materials such as fly ash,zeolite powder,powdered activated carbon,iron filings,columnar activated carbon,zeolite,and iron sand,iron filings and activated carbon were screened out to have a relatively good ability on Cr（Ⅵ）removal from groundwater,and the best ratio is 5:1.Low pH and the addition of organic acids are beneficial to the removal of Cr（Ⅵ）.The maximum removal capacity of the best mixed material for Cr（Ⅵ）in actual groundwater is only4.4 mg/g.Therefore,it is urgent to develop new materials and innovative technologies for Cr（Ⅵ）-contaminated groundwater remediation.3.The effective removal of Cr（Ⅵ）by S-nZVI prepared with sodium sulfide as a precursor benefits from the Fe S shell structure,and its removal effect is mainly affected by the pH and ion types of groundwater.Low pH and Ca²⁺/Mg²⁺type groundwater can significantly enhance the removal of Cr（Ⅵ）by S-nZVI,while alkaline conditions and CO₃^2-/HPO₄^2-can significantly inhibit the remediation effect.The Fe S on the surface of S-nZVI provides electron donors such as Fe²⁺and S^2-,and thus,the inner core Fe（0）transfers electrons to the outer Cr（Ⅵ）,which directly enhance the reduction of Cr（Ⅵ）.The process mechanism of Cr（Ⅵ）removal by S-nZVI includes simultaneous adsorption,reduction and co-precipitation.At the molecular level,Cr（Ⅵ）is reduced step by step with the participation of electrons and hydrogen ions,accompanied by energy reduction,until the precipitated product of Cr（III）molecules is generated.This remediation process will cause a significant increase in groundwater pH and a significant decrease in ORP,which may further affect the long-term removal efficiency.4.The preparation method has a significant impact on the physical and chemical characteristics of S-nZVI materials and the efficiency for Cr（Ⅵ）-contaminated groundwater remediation.The study shows that the two-step preparation of S-nZVI is easy to form a typical"core-shell"structure,while the crystal structure of Fe（0）in the one-step preparation of S-nZVI is more obvious.S-nZVI prepared in different ways can improve the selectivity of nZVI to Cr（Ⅵ）,but the sensitivity to oxygen is different.Under the same experimental conditions,S-nZVI materials prepared by two-step method using three sulfides（Na₂S₂O₄,Na₂S₂O₃,Na₂S）as precursor or one-step method using Na₂S as precursor have a better removal efficiency on Cr（Ⅵ）in groundwater.However,two kinds of S-nZVI materials prepared by one-step method using Na₂S₂O₄or Na₂S₂O₃ as precursors have a poor removal efficiency on Cr（Ⅵ）in groundwater and are not suitable for Cr（Ⅵ）-contaminated groundwater remediation.5.The in-situ remediation efficiency of PRB reactors constructed with S-nZVI as a new material for Cr（Ⅵ）-contaminated groundwater is affected by factors such as material filling methods,groundwater pH and ion types,and pollution patterns.Compared with the layered filling method,the mixed filling of S-nZVI and quartz sand can greatly improve the utilization rate of S-nZVI.During remediation,the water chemistry changes from an oxidizing environment with a low pH value to a reducing environment with a high pH value.Ca²⁺,Mg²⁺or lower pH in groundwater are beneficial to improve the remediation efficiency of Cr（Ⅵ）,while CO₃^2-,NO₃?or alkaline conditions are inhibitory.With the increase of pollution times,the remediation efficiency of S-nZVI-PRB reactor to Cr（Ⅵ）weakens,and the first remediation process makes a greatest contribution to the overall remediation efficiency.6.A numerical model is used to simulate the migration process of the Cr（Ⅵ）plume through the S-nZVI-PRB reactor.It is shown that the basic pollution hydrogeological conditions and Cr（Ⅵ）pollution status of the actual site are the main factors for the in-situ remediation when designing the PRB reactor.Simulation and prediction of S-nZVI-PRB operation effect of the actual site showed that the S-nZVI-PRB reactor is effective for Cr（Ⅵ）-contaminated groundwater remediation.