Research On Modification Of Nano-zero-valent Iron Based On Mango Peel And Removal Of Cr(Ⅵ) From Water

At present,the pollution of the water environment has aroused the close concern of researchers.Cr（Ⅵ）in the water is highly toxic and can be passed to human step by step through the food chain,bringing potential hazards to human health.As a new material,nano-zero-valent iron（n ZVI）has achieved remarkable achievements in the treatment of Cr（Ⅵ）in water for its large specific surface area and high reduction activity.However,the traditional synthesis method of n ZVI requires expensive equipment,and the required chemical reagents may cause secondary pollution to the environment.In addition,n ZVI tends to agglomerate into large particles due to magnetism and intermolecular forces,which greatly reduces its activity.The application of n ZVI in environmental treatment has greatly limited by these defects.In response to the existing problems,this research used mango peel extract as a reducing agent and mango peel modified by citric acid as a support to greenly synthesize the nano-zero-valent iron supported by citric acid modified mango peel（MP-n ZVI/CP）and used it to remove Cr（Ⅵ）in water.First,the polyphenol content in mango peel extract was optimized by single factor method.We obtained extract at the extraction temperature was 60℃,the liquid-to-mass ratio was 25:1,and the volume fraction of ethanol was 50%.The removal efficiency of MP-n ZVI on Cr（Ⅵ）was much higher than extracts and Fe²⁺solution.The three processes of removing Cr（Ⅵ）were in accordance with the pseudo second-order kinetic process,indicated that the process was dominated by chemical action.These characterizations showed that the synthesized MP-n ZVI mainly exists in amorphous form and the Fe⁰ in the center coated by organic compounds.The Fe⁰ particles size in the center were about 50 nm,and the aggregates’diameter was about 290 nm.Secondly,Modification of MP-n ZVI was necessary as it tended to aggregate easily.The results indicated that the removal effect of MP-n ZVI/CP on Cr（Ⅵ）was better than single MP-n ZVI,manifested that there was a synergistic effect between MP-n ZVI and modified mango peel.The removal efficiency of Cr（Ⅵ）decreased with the increase of p H,the increase of the dosage,the decrease of the initial concentration and the increase of temperature.The characterization indicated that the original mango peel had a smooth surface.The surface roughness was significantly increased,the number of-COOH and-OH functional groups on the surface also increased after citric acid modifying.Nano-zero-valent iron was uniformly dispersed on the modified mango peel,and the specific surface area and the number of mesopores of the material is significantly increased.After reacting with Cr（Ⅵ）,the surface of the material collapsed,the corrosion phenomenon was more obvious,and the amount of iron oxide and iron hydroxide increased.Thirdly,the results shown that the pseudo second-order model performed the best among the four Kinetic model.There were three stages in the removal process:fast reaction-slow reaction-equilibrium,suggesting the removal process involved various mechanisms such as intra-particlen diffusion and membrane diffusion.The results indicated that the Freundlich model conformed to the best among the five adsorption isotherm model.Combined with several other model parameters,we learned that the process was easy to react and involved strong chemical effects.The△H₀>0,△S₀>0,△G₀<0 of the process indicated that the process was a thermodynamic spontaneous and irreversible endothermic reaction.The XPS characterization shown that both Cr（Ⅵ）and Cr（Ⅲ）existed on the material after the reaction,while the content of Cr（Ⅲ）was significantly higher than that of Cr（Ⅵ）,indicating that the removal of Cr（Ⅵ）by composite materials involved adsorption and reduction,and reduction was dominant.Cr（Ⅲ）-Fe（Ⅲ）hydroxides confirmed the existence of co-precipitation.In addition,the functional groups on the metal surface may also be ion-exchanged with Cr（Ⅵ）.Finally,the removal efficiency of the composite material was still higher than 70%after being stored in the air for 75 days.0.1M HCl had the best regeneration effect,and the removal capacity after four uses was 20.28 mg g^-1.when different concentrations of ions coexist,Cl^-,HCO₃^-,NO₃^-had a little effect,H₂PO₄^-will strongly inhibited the removal of Cr（Ⅵ）,while SO₄^2-could promoted the removal of Cr（Ⅵ）.When applied to different types water,the removal effect of tap water was greatly reduced.However,Cr（Ⅵ）had excellent removal effect in the range of 2.5～25 mg L^-1 in seawater.The water spinach’s rooting rate,germination rate and chlorophyll content of the leaves all increased,and the total chromium content decreased after being treated by the MP-n ZVI/CP.