Preparation Of Green Nano-zero-valent Iron Materials And Its Application In The Restoration Of Cr(?) In Groundwater

Groundwater is an important source of freshwater that humans rely on for production and living.However,with the development of industrialization,some industries have caused serious pollution due to the unreasonable discharge of Cr(?)-containing waste water and waste slag.Because of the slow flow rate of groundwater and the fact that heavy metals themselves are not easily degraded,the pollution of water bodies causes lasting damage to the surrounding environment,organisms and human health.Therefore,removing Cr(?)pollution from groundwater becomes an environmental problem.Recent studies have shown that nanoscale zero-valent iron as a material with strong reduction,high activity and large specific surface area can effectively remove Cr(?)pollution in water.However,the common chemically synthesized nano-zero-valent iron is easily agglomerated in the synthesis process,and is easily oxidized in the air and poor in stability.In addition,the reducing agent that synthesizes nano-zero-valent iron itself easily causes secondary pollution to the environment.The application of nano zero-valent iron poses challenges.In response to the above problems,Ginkgo biloba leaves were used as the raw material,and the extraction solution obtained by ultrasonication was used as a reducing agent and stabilizer to synthesize Ginkgo biloba nano-zero-valent iron(GB-nZ?).Because the raw material itself was biodegradable,secondary pollution caused by the use of chemical reagents can be avoided,and the stability of GB-nZ? surface is enhanced due to the presence of organic materials.In this study,the composition,morphology and structural characteristics of the material were characterized by SEM,FTIR and XRD.The GB-nZ? was used to remove Cr(?)contaminants in the simulated groundwater.The effects of different factors such as dosing amount,initial concentration of Cr(?),initial reaction pH and reaction temperature on the removal efficiency and reaction kinetics of Cr(?)in groundwater was studied,to provide a reference for the future application of green nano zero-valent iron in the environment.The experimental result is:(1)The GB-nZ? particles synthesized with Ginkgo biloba extract are uniform and have a regular globular structure.The surface is covered with organic substances of the Ginkgo biloba extract.The material before the reaction is slightly agglomerated and the structure is loose.After reaction,GB-nZ? particle surface was corroded and the overall structure becomes more compact.This is due to the presence of iron oxide,iron hydroxide and iron oxyhydroxide on the surface of the material after the reaction,indicating that during the reaction Fe0 was Oxidized and the oxidation products were deposited on the GB-nZ? surface.(2)The removal mechanism of Cr(?)by GB-n Z? is divided into two stages:reduction and adsorption.First,Cr(?)was reduced by Fe~0 and produce Fe(III),Fe(II)and Cr(III).In the reaction process,the concentration of OH~-in the reaction environment is increased due to the consumption of a large amount of H~+.The redox products formed in the first stage continue to react with OH~-to form iron-chromium complexes.The reaction precipitation products have good adsorption characteristics and can remove chromium contaminants from the water through adsorption.(3)Removal of Cr(?)pollution in water environment with GB-nZ?.When the initial Cr(?)solution concentration was 5 mg/L,the dosage of GB-nZ? was 0.4 g/L,the reaction pH=5 and the reaction temperature was 303K,the adsorption capacity of GB-nZ? material was 10.21 mg/g after 180 min,and the Cr(?)removal rate was 81.24%.GB-nZ? has a good removal effect on Cr(?)with lower initial concentration.Increasing the dosage of GB-nZ?,lowering the pH value and raising the reaction temperature are all beneficial to improve the Cr(?)degradation rate of GB-nZ?.Neutral and alkaline conditions will passivate the surface of GB-nZ?,which is not conducive to the degradation of Cr(?)by GB-nZ?.(4)The removal of Cr(?)by GB-n Z? was more in line with the Lagergren pseudo secondary adsorption kinetics,that is,the adsorption process is controlled by chemical reaction,but the first-order reduction kinetics of Langumir-Hinshelwood can also be used to describe the degradation of Cr(?)by GB-n Z? under low-pH acidic conditions.(5)The reaction thermodynamics of Cr(?)removal by GB-nZ? shows that the reaction is endothermic and spontaneous,and the reaction process is the process of entropy increase.Calculated using the formula of Arrhenius,the reaction activation energy Ea was 33.00 KJ/mol,indicating that the reaction process is easy to occur.