Research On Removal Of Nitrate In Groundwater By Green Tea Extracts Synthesized Nanoscale Zero Valent Iron-Copper Bimetal

The situation of nitrate pollution to water bodies has becomeincreasingly severe in recent years.Excessive nitrates in water will not only cause water eutrophication and destroy aquatic ecological environment,but nitrate will also be converted into carcinogenic nitrite,which pose a serious threat to human health,and even death.The treatment of nitrate-contaminated water bodies and soils by nanoscale zero-valent iron and its bimetallic material is a research direction that has attracted extensive attention in recent 20 years and has achieved remarkable results.According to the research results obtained so far,it has been found that although the nanoscale zero-valent iron and its bimetal have good removal efficiency for nitrate degradation under experimental conditions,they still face the disadvantages that the nanoscale zero-valent iron materials prepared freshly are easy to reunite,oxidation and activity reduction at the condition of long-term storage.In addition,the preparation of materials is mostly made by the liquid phase reduction method of sodium borohydride,and the preparation process uses toxic reducing agents and organic solvents.It is easy to cause secondary pollution to the environment.These above disadvantages make the nanoscale zero-valent iron materials have serious limitations in the actual treatment of nitrate in groundwater,especially in situ groundwater remediation.Therefore,it is necessary to improve the preparation and properties of traditional nanoscale zero-valent iron materials.In view of the above defects of nanoscale zero-valent iron,the greentea was used as the raw material to extract bioreductive substances,such as tea polyphenols,vitamins and caffeine.The green tea extract was used as the green reducing agent and stabilizing agent for the reduction of the mixed solution of ferrous ions and copper ions into spherical nanoscale zero-valent iron-copper bimetallic?GT-nZVI/Cu?particles with core-shell structure.Toxic chemical reducing agents sodium borohydride and organic solvents were not used throughout the preparation process,thereby reducing the risk of secondary pollution,while improving the oxidation resistance and dispersibility of the material and avoiding agglomeration and oxidation.In this paper,nanoscale zero-valent iron-copper bimetallic?GT-nZVI/Cu?materials was synthesized using the prepared green tea extract for studing its effect on remediation of nitrate in the simulated groundwater environment under different factors.The main influencing factors including reaction time,initial concentration of nitrate,initial pH values,and the volume ratio of green tea extracts and metal ion mixture solution.And compared with the treatment of nitrate with nano-zero-valent iron?nZVI?prepared by the traditional method.The optimum reaction conditions for nitrate removal by nanoscale zero-valent iron-copper in groundwater were discussed.It provided a theoretical reference for the in-situ remediation of nitrate in groundwater.The main conclusions of this paper are as follows:?1?The results of characterization by scanning electron microscopy?SEM?,X-ray diffraction?XRD?,Fourier transform infrared spectroscopy?FTIR?and X-ray photoelectron spectroscopy?XPS?showed that the nanocrystalline zero-valent iron-copper bimetallic?GT-nZVI/Cu?synthesized by green tea extract had a regular spherical structure and a little agglomeration among the particles,but relatively dispersed relative to nano-zero-valent iron?nZVI?.The particle size was distributed in the range of 60¹00 nm.After reaction with nitrate,the surface of the nanoscale zero-valent iron-copper particles synthesized by the green tea extract was partially corroded.However,the morphology and size of the particles did not change obviously before and after the reaction.It fully demonstrated that the nanoscale zero-valent iron-copper bimetallic materials synthesized by the green tea extract have good stability and dispersibility.?2?The results of batch experiments showed that the nanoscalezero-valent iron-copper synthesized by green tea extract has good removal efficiency on nitrate in groundwater.When the volume ratio of green tea extract and metal ion mixture was 1:2,the initial pH value of groundwater sample was 5,the reaction temperature was 303 K,the initial nitrate concentration was 20 mg?L^-1,and GT-nZVI/Cu dosage was 2.0 g?L^-1,the removal efficiency of nitrate in groundwater was 79.3%after 75 min of reaction.Changing the initial pH of the solution,the reaction temperature and the reaction time will have a significant effect on the removal of nitrate.Decreasing the initial pH,increasing the reaction temperature and prolonging the reaction time were beneficial to the removal of nitrate in the solution.?3?The results of adsorption-reduction kinetics study showed that theprocess of nitrate removal by GT-nZVI/Cu under different influencing factors was in good agreement with pseudo-second-order adsorption kinetic model and pseudo-first-order reduction kinetic model.Compared with pseudo-first-order reduction kinetic model,it was better to accord with pseudo-second-order adsorption kinetic model,which showed that the adsorption played a leading role in the process of nitrate removal,but the reduction effect cannot be ignored.It was proved that the process of removing nitrate by GT-nZVI/Cu was the result of the combination of adsorption and reduction.?4?According to the results of thermodynamic study,it is shown thatGibbs free energy changes?G⁰<0,enthalpy change?H⁰>0 and entropy change?S⁰>0 in the process of nitrate removal under different reaction temperature conditions.It was fully shown that the process of removing nitrate by GT-nZVI/Cu was an endothermic and spontaneous reaction process.It is proved theoretically that the process of removing the nitrate by GT-nZVI/Cu was scientific feasible.?5?The results of comparative experiments showed that GT-nZVI/Cuhad lower nitrate removal efficiency than the traditional nanoscale zero-valent iron.Moreover,the removal efficiency of nanoscale zero-valent iron to nitrate was close to 100%.But the latter material had obviously agglomerates before and after the reaction,and had obvious corrosion after the reaction.It is further explained that the material prepared by the green synthesis method has stability and corrosion resistance,and is more suitable for application in complex environment.