Mechanism Study On The Polysulfide Modified Nanoscale Zero-valent Iron For In-situ Solidification Of Cr(Ⅵ) In Groundwater

Chromium salt is a significant chemical raw material widely utilized in the metallurgical and leather industries to support China’s industrial modernization.However,China,being the world’s largest producer and consumer of chromium salts,faces a critical environmental challenge concerning the large amounts of untreated chromium slag that has accumulated in open-air storage.As a result,high concentrations of hexavalent chromium（Cr（Ⅵ））continue to infiltrate the environment,resulting in severe soil,farmland,and groundwater pollution.In various parts of the country,including Chongqing,Changsha,and Yunnan,serious chromium pollution incidents have occurred,with Cr（Ⅵ）levels in polluted water bodies exceeding the standard by 200 to 3000 times and remediation costs running into hundreds of millions.The presence of Cr（Ⅵ）in groundwater poses a significant threat to human health and the safety of the ecological environment as it is a highly carcinogenic,teratogenic,and mutagenic toxic substance.As an ideal and efficient agent,nanoscale zero iron（nZVI）can be widely used for in-situ remediation of heavy metals.However,there are some problems in practical application,such as corrosion,inactivation,agglomeration,etc.,which limit their development.Therefore,it is of great significance for practical application to improve the performance and reduce the cost of nano-iron based materials.In this study,the novel polysulfide modified nanoscale zero valent iron（PS-nZVI）was synthesized based on the method of sulfide modified nanoscale zero valent iron（S-nZVI）,to repair Cr（Ⅵ）pollution in groundwater and to improve the defect of nano-ferri-based materials as fillers for permeable reaction walls（PRBs）.The research can provide technical support for the prevention and control of groundwater Cr（Ⅵ）pollution on the site.The main research contents and conclusions are as follows:（1）Preparation of novel zero-valent iron sulfide and its mechanism with Cr（Ⅵ）in groundwater.Based on the traditional S-nZVI preparation method,four types of S-nZVI were prepared by traditional sulfur source including Na₂S,Na₂S₂O₃,Na₂S₂O₄ and new sulfur source CaS_x,to discuss the effect and mechanism of Cr（Ⅵ）solidification.The results showed that the removal rate of 200 mg/L Cr（Ⅵ）from groundwater by the CaS_x-nZVI is about 1.2-1.5 times than that of traditional materials.Combined with the material microstructure and structure composition,the new CaS_x-nZVI has a more typical FeSn structure,which can inhibit the oxidation corrosion of internal Fe0 and improve the electron transfer efficiency,thus improving the removal efficiency of Cr（Ⅵ）.The doping of sulfur increases the reduction performance of the material,forming FeS,FeS₂ and other sulfur complexes,which can promote the repair of Cr（Ⅵ）,and help to inhibit the contact between internal Fe⁰ and oxygen,thus improving the service life of the material.When the sulfur content of materials is too high,a thicker layer of sulfur iron will be formed due to surface deposition,which will affect the removal effect of Cr（Ⅵ）in groundwater.When the sulfur-iron ratio is 1/5,the synthetic CaS_x-nZVI has the best repair effect on Cr（Ⅵ）.In this case,the reaction between the material and Cr（Ⅵ）conforms to the quasi-first order and quasi-second order kinetic models,which proves the adsorption and reduction.The isothermal adsorption experiment accords with the Langmuir model,which demonstrated the monolayer adsorption.The lamellar structure of sulfur and iron complexes（FeS_n）on the surface of the material would become the lamellar structure of Fe,Cr and S appeared on the surface of the mineral,in which Cr mainly existed in the form of Cr（Ⅲ）.The removal mechanism of Cr（Ⅵ）by CaS_x-nZVI is as follows:Using surface components such as FeS_n（n>2）,FeS₂ and FeS,a large amount of Cr（Ⅵ）in the system is reduced to Cr（Ⅲ）which is more easily adsorbed.Then,Cr（Ⅲ）is fixed by adsorption and co-precipitation on the solid phase surface to achieve the purpose of stable removal.（2）Study on the effects of environmental factors on CaS_x-nZVI for groundwater Cr（Ⅵ）restoration.The effects of environmental factors on Cr（Ⅵ）restoration by CaS_x-nZVI were studied in two aspects including physicochemical properties of groundwater and aquifer medium to better realize the application of CaSx-nZVI to Cr（Ⅵ）remediation in groundwater.The reaction between CaSx-nZVI and Cr（Ⅵ）is endothermic and can occur spontaneously with the increase of temperature.When the initial p H is less than 3,Fe⁰inside the material is easier to release reducing electrons and promote the conversion of Cr（Ⅵ）to low price Cr（Ⅲ）.When the p H increases to 7,the gradual conversion of HCr O₄^-to Cr O₄^2-reduced the adsorption free energy to inhibit the reaction.At the same time,Fe（OH）₃,Fe₃O₄ and other iron oxides and hydroxide precipitates were formed on the surface of the material,which had a passivating effect on Cr（Ⅵ）removal.Cations such as Ca²⁺and Mg²⁺are beneficial to improve the repair of Cr（Ⅵ）through ion exchange and a small amount of flocculation precipitation.The presence of CO₃^2-will hinder the reaction of CaS_x-nZVI with Cr（Ⅵ）because of the hydrolysis and the increase of the p H value of the system and the complex with iron.The existence of NO^3-shows the phenomenon of first inhibiting and then promoting the reaction because of the competitive adsorption and promoting the Fe⁰ corrosion.As for aquifer medium,the presence of quartz sand has no effect on the repair process of the material.Iron-manganese oxide can promote the effect of the system due to its adsorption performance,but will inhibit the reduction performance of part of CaS_x-nZVI.The presence of clay minerals will slightly inhibit the Cr（Ⅵ）removal process.（3）Study on the mechanism of Cr（Ⅵ）restoration by zero-valent iron polysulfide nanoparticles.To shed light on the interaction mechanism of sulfur,iron and chromium during the removal of Cr（Ⅵ）by CaS_x-nZVI,PS-nZVI was prepared to reveal the microscopic mechanism of the reaction with Cr（Ⅵ）and clarify the mechanism of S_n^2-components in the solidification of Cr（Ⅵ）.The study found that PS-nZVI greatly improved the remediation effect on Cr（Ⅵ）in groundwater when compared with the traditional S-nZVI,and the reaction process was in line with the quasi-second-order reaction kinetic model,indicating the chemisorption.With the polysulfide doping,the Fe⁰ chain-like bone chain gradually takes on a flower-like structure of iron polysulfide because of the formation of FeS_n（n≥1）and oxygen（hydroxide）compounds of iron.A small amount of FeS_n can promote electron transport and improve the reduction efficiency,while excessive deposition would lead to the occupation of adsorption sites and reduce adsorption pores and inhibit the repair of the system.Combined with XPS semi-quantitative analysis of the key elements of PS-nZVI in the process of reaction,using the software THOUGHREACT to simulate the change process of the key components in the process of reaction,the mechanism of Cr（Ⅵ）in PS-nZVI solidified groundwater and the interaction mechanism of key elements such as sulfur,iron and chromium have been elucidated.Under the initial acidic condition,H⁺in the system is first consumed.Under the action of FeS and FeS₂structures formed on PS-nZVI surface,Cr（Ⅵ）is reduced to Cr（Ⅲ）,accompanied by the formation of Cr（OH）₃,Fe（OH）₃ precipitates.With the stability of p H,the chrome-containing minerals on the surface of the material began to increase,and the sulfur and iron complexes were gradually consumed,producing a large amount of Fe²⁺.S_n^2-（n>2）plays the role of intermediates,promotes the conversion between S^2-,S₂^2-and S⁰,improves the proportion of FeS₂ on the material surface,facilitates the generation of more electrons and adsorption sites,and is the key to significantly better repair effect than traditional S-nZVI.At the end of the reaction,the system tends to be stable,Cr（Ⅵ）and Cr（Ⅲ）in the liquid phase are completely removed,while the surface of the solid phase material mainly contains Cr（OH）₃,Cr₂O₃,FeS₂,S⁰,Fe₂O₃,Fe₃O₄,Fe（OH）₃ and other sulfur-Fe-chromium components.（4）Experimental study on in situ remediation of Cr（Ⅵ）in groundwater by PS-nZVI in a one-dimensional simulated column.The effects of PS-nZVI addition,influent flow rate,initial p H,and coexisting ions on Cr（Ⅵ）removal from groundwater were investigated based on the one-dimensional simulated column experiment.When the filling ratio of PS/Q was increased from 1/150to 1/100,the overall removal amount arosed from 34.5 mg/g to 172.9 mg/g,but further increasing the filling ratio had little effect on the removal effect.When the influent flow rate is 3 m L/min,the hydraulic retention time is short due to the faster flow rate,so the removal rate is about 15%lower than that at 2 m L/min.When the initial p H<3,it is conducive to the conversion of hexvalent chromium to trivalent chromium.However,the adsorption ability rather than the reduction effect of PS-nZVI on Cr（Ⅵ）at high p H is the main control mechanism,so the overall removal amount is reduced to 50 mg/g.The influence of background ions mainly plays a role by changing the p H of the system.HCO₃^-inhibits the repair effect of the simulated column due to its alkaline hydrolysis,and Ca²⁺has a slight inhibition effect due to competitive adsorption.The classical model fitting results of the above one-dimensional simulation columns show that Yoon-Nelson’s fittted better than that of the Thomas model under almost all conditions,which indicates that complex physicochemical changes occur in the reaction system.Combined with the penetration curve of Br^-tracer and Cr（Ⅵ）,the coupling process of"convection+dispersion+adsorption"occurs in the reaction simulation column could be concluded.The morphology distribution of Fe at different heights after the reaction indicates that the middle part of the simulated column is the main location of the reaction due to the effect of gravity and water flow,and the occurrence of Fe is mainly from carbonate bound state to Fe oxides bound state.Based on the aforementioned research,the utilization of polysulfides for modifying nano zero-valent iron（nZVI）can generate stable novel repair materials,which can effectively diminish the elevated levels of Cr（Ⅵ）in groundwater and transform it into steady Cr（Ⅲ）to consolidate on the surface of the material.This action lowers the possibility of the substance returning to the natural environment and causing harm.The exterior of PS-nZVI features a distinct flower-like structure made of FeSn,while the interior contains complete Fe⁰.By means of the reduction of S and Fe,Cr（Ⅵ）in the liquid phase experiences electron transfer and other mechanisms to ultimately become Cr（Ⅲ）.Following this,through adsorption and coprecipitation,among other processes,sulfur iron chromium components such as Cr（OH）₃,FeS₂,Fe₃O₄,Fe（OH）₃ are formed on the surface of the material,thus further solidifying Cr（Ⅵ）in groundwater in situ as a Permeable Reactive Barrier（PRB）filler,thus endowing it with significant practical application potential.