Study On The Migration Mechanism Of GT-nZVI In Groundwater Aquifer And Its Remediation Efficiency Of Cr(?) Pollution

In recent years,hexavalent chromium pollution in groundwater is increasingly serious.How to remove hexavalent chromium in groundwater efficiently has become a major aim of environmental problems.Among groundwater remediation technologies,the in-situ reaction zone technology has become a research hotspot due to its advantages of simple construction operation and remediation of deep aquifer pollution.However,it requires that remediation materials have high reactivity and good migration ability towards pollutants.Nano iron reduced by plant has the advantages of environmental protection,good stability,low cost and good reaction performance,it has a broad application prospect in remediation of heavy metal pollution in groundwater.In this paper,nine kinds of rich polyphenol plants were selected,which were wide distributed.After preparation and characterization of nano iron,the nano iron(GT-nZVI)made from green tea extract was selected in the end.Based on one-dimensional sand column experiment,the migration and distribution of GT-nZVI in aquifer was studied,and the influence of various factors on the migration performance of GT-nZVI was investigated;The removal effect of GT-nZVI on Cr(?)in groundwater was studied under different reaction conditions.Through simulating measured value,the kinetics of Cr(?)removal by GT-nZVI was analyzed,and the mechanism of the reaction was discussed;The removal efficiency and hydrochemistry of Cr(?)in porous media by GT-nZVI were simulated in a two-dimensional simulation tank.The results are as follows:(1)Comparing with the oxidation resistance property of nine plant leaves extracts.They top three plants,including green tea nano-zero-valent iron(GT-nZVI),pomegranate leaf nano-zero-valent iron(PG-nZVI)and black tea nano-zero-valent iron(BT-nZVI),were selected to produce green nano-iron.The characterization of three green nano iron and their stability suspension were detected by UV-vis,SEM,FT-IR,XRD and TG.The reaction activities of three green nano iron for removal Cr(?)was investigated.The experimental results showed that the GT-nZVI with the smallest particle size(about 88 nm)has best suspension stability than the other two green nanoirons.The surface of GT-nZVI was covered with more organic compounds,such as C=C group.In the same reaction condition,compared with the other two kinds of green nano iron,GT-nZVI had better removal effect on Cr(VI)and stronger oxidation resistance.(2)One dimensional quartz sand column was used to invistigate GT-nZVI migration in porous medium according to the adjustment of flow velocity and medium particle size and concentration.The results indicate larger velocity can reduce the collision probability between GT-nZVI and medium surface.The increase of water flow velocity makes the deposited GT-nZVI particles on medium surface return to water flow because it can improve the shear force,which contribute to reduce retention and increase migration distance.The size of the medium has a great influence on the migration and distribution of GT-nZVI,but it can still maintain a good migration in different medium size.The injection concentration has little effect on GT-nZVI migration property.The organic matter on the surface of GT-nZVI increases the steric hindrance and slows down the aggradation rate Therefore,the medium has little effect on the retention of GT-nZVI with the injection concentration increases.(3)GT-nZVI had a good removal effect on Cr(VI)in groundwater.The removal process had both reduction reaction and adsorption,and the reduction reaction dominate removal process.The kinetic model could well describe the reaction kinetic behavior.Changing the pH value of solution,reaction temperature,initial concentration of pollutants and the concentration of GT-nZVI can impact the Cr(VI)removal significantly.The reaction rate is positive correlation to the concentration of GT-nZVI and negative correlation to the concentration of Cr(VI).Lower pH help the removal of Cr(VI)by GT-nZVI,while opposite in the alkaline condition.(4)The GT-nZVI reaction zone formed in the simulation experiment can affect the chemical properties within a radius of 30 cm around the injection well in 12 hours,and form an effective repair range of 10?20 cm.The polluted groundwater flowing through the reaction zone has been repaired to some extent,and the combination between Cr(?)and Fe(?)have been removed in the form of precipitation.The injection of GT-nZVI will cause the decrease of pH and ORP value forming a partial acid environment.With the reaction proceeding,the pH and ORP value will gradually increase.After GT-nZVI react complete the reaction zone disappeared and the chemical properties of groundwater environment gradually returned to the original level.