Mechanism Analysis Of In Situ Remediation Of Cr(Ⅵ) Contaminated Aquifer By Fe(Ⅲ)-Strengthening Plant Extracts

The pollution of groundwater and soil caused by chromium（CR）is the most common and serious problem.Because Cr（Ⅵ）has stronger toxicity and migration ability than Cr（Ⅲ）,the traditional remediation of Cr（Ⅵ）contaminated groundwater mostly reduces Cr（Ⅵ）to Cr（Ⅲ）,and then makes it precipitate by adsorption or adjusting pH to remove total chromium.However,it is often necessary to add exogenous alkaline substances to adjust the pH value of the underground environment when removing total chromium,which undoubtedly increases the operation cost of the remediation process;and some reducing agents will have a certain impact on the underground environment,such as agglomeration and sedimentation of nano zero valent iron in the process of use,or the formation of mineralized substances to block the pores,etc.The above problems hinder the practical application of in-situ chemical reduction remediation of Cr（Ⅵ）contaminated groundwater.In contrast,plant extracts are more environmentally friendly and can be degraded by indigenous microorganisms in soil and groundwater,which is a more environmentally friendly reductive remediation reagent.However,under conventional conditions,the reduction efficiency of Cr（Ⅵ）by reducing organic compounds is poor,and the half-life of the reaction is usually several weeks,months or even years.This also leads to Cr（Ⅵ）concentration exceeding the standard in wetlands and swamps where the content of reducing organic matter is high.Therefore,in order to improve the remediation efficiency of Cr（Ⅵ）contaminated groundwater by reducing organic compounds,it is necessary to add strengthening reagents to enhance the efficiency of electron transfer.In this study,corncob extract was used as electron donor,Fe³⁺was used as strengthening reagent,and the system of simultaneous reduction of Cr（Ⅵ）and precipitation of Cr（Ⅲ）was constructed.In this study,the main research results obtained are as follows:（1）In this paper,tannic acid,Fe（Ⅲ）and tannic acid+Fe（Ⅲ）control systems were used to repair Cr（Ⅵ）contaminated groundwater respectively,and the feasibility of rapid reduction of Cr（Ⅵ）by tannic acid enhanced by Fe（Ⅲ）under weak acid conditions was verified.According to the results of batch experiments,the best remediation effect can be achieved when the concentration ratio of Cr（Ⅵ）to tannic acid is 3/1（n/N）,and the concentration ratio of Fe（Ⅲ）to tannic acid is 5/3（n/N）.In addition,it is proved that the Fe（Ⅲ）/tannic acid system can precipitate Cr（Ⅲ）from groundwater in the form of precipitation without adding exogenous alkaline substances,so as to achieve the purpose of simultaneous reduction of Cr（Ⅵ）in groundwater.（2）In this paper,the corn cob rich in plant polyphenols was selected as the source of plant extract.It was proved that Fe（Ⅲ）could effectively enhance the reduction of Cr（Ⅵ）by CE.With the increase of pH,the recovery efficiency of Cr（Ⅵ）in the reaction system decreased significantly.When pH is less than 4,the reduction effect is the best（99.99%）.At the same time,CE+Fe（Ⅲ）system can effectively fix Cr（Ⅲ）,and the process does not need to add alkaline substances to regulate pH.（3）The mechanism of simultaneous reduction of Cr（Ⅵ）in groundwater by CE+Fe（Ⅲ）system was explored in this paper.By using a variety of characterization methods,it was found that Fe（II）and n ZⅥ produced by CE and Fe（Ⅲ）had a better reduction effect on Cr（Ⅵ）,while Fe OCl had a better adsorption effect on Cr（Ⅲ）.The functional groups of CE as electron donors mainly include aldehyde group,ester group,hydroxyl group and amino group.Some small molecule organic compounds in corncob extract are coated on the surface of nanoparticles to prevent NZⅥ from being oxidized by dissolved oxygen in water,reduce the sedimentation and agglomeration of particles,and enhance the migration ability of remediation agents in aquifer.（4）The one-dimensional simulation column experiment shows that CE+Fe（Ⅲ）system can maintain a better Cr（Ⅵ）reduction capacity in the presence of medium.The reduction efficiency of Cr（Ⅵ）and the removal efficiency of total chromium will be improved obviously under the condition of increasing the injection amount of repair reagent,or in hydrogeological conditions with small medium size and slow groundwater flow rate.