Enhanced Removal Of Nitrate From Groundwater Using Reduced Graphene Oxide-supported Nanoscale Zerovalent Iron

The pollution of nitrate in groundwater has become one of the universal global environmental problem.Nitrate nitrogen has no direct harm to the human body.But the nitrites,produced by metabolism,and secondary pollutants,generated by interaction with organic compounds containing nitrogen in the environment,pose a huge potential threat to human health and the ecological environment.Recently,a chemical reduction method using zero-valent iron(ZVI)has been widely studied for contaminated wastewater treatment.Because of its low cost,high efficiency and no secondary pollution,this technology is considered as one of the most promising methods for reducing nitrate nitrogen.However,due to the high surface energy of NZVI,its surface is easily passivated resulting in the reduction of NZVI activity,which limits its application.In order to solve this problem,it is an effective method to load NZVI on the surface of another substance.In this study,nanoscale zero-valent iron was loaded on graphene oxide in situ by liquid phase reduction method.The preparation conditions were optimized by investigating the effects of the loading capacity(in terms of rGO content)and the concentration of the reducing agent NaBH4 on the denitrification capacity and the antioxidation abilities of rGO-NZVI.The transmission electron microscope(TEM),Energy dispersive X-ray analysis(EDS),X-ray diffraction(XRD)and Fourier transform infrared(FTIR)were used to characterize the rGO-NZVI.And rGO-NZVI was applied to the study of denitrification.The removal effect of reduced iron powder,nZVI and rGO-nZVI on nitrate nitrogen and their product composition were compared.The effects on the denitrification of rGO-NZVI and the reaction kinetics under different influencing factors were investigated.Furthermore,the denitrification process and reaction mechanism of nitrate nitrogen were discussed.Finally,it is used to remove nitrate nitrogen in the simulated groundwater environment,which provides a theoretical basis for the nitrate remediation of groundwater.The experimental results of optimization of preparation conditions show that rGO-NZVI has good removal of nitrate nitrogen and guarantees better oxidation resistance when the content of rGO is 7.5%and the concentration of NaBH4 is 0.2 mol/L.Characterization results show that the dispersion of NZVI increases after loading,and the NZVI is combined with the oxygen-containing group on rGO.Compared with NZVI and reduced iron powder,rGO-NZVI showed higher reduction activity for nitrate nitrogen,and the generation rate of nitrogen was higher than that of NZVI.With the same dosage of rGO-NZVI,the effects of initial concentration of nitrate nitrogen,initial pH value,reaction temperature and coexisting ions on the denitrification were investigated.The results showed that the removal rate of nitrate nitrogen decreased with the increase of initial concentration.The rGO-NZVI can retain high reactivity at initial pH of 9,which predicted that the loaded NZVI can effectively widen the pH application range.And With the increase of pH,the content of N2 in denitrification products has increased.The denitrification process of rGO-NZVI is an endothermic reaction.HCO3-and PO43-have a great negative influence on the removal of nitrate nitrogen,while Cl-and SO42-have a slight influence and the degree of influence of SO42-is greater than Cl-.Kinetic studies show that rGO-NZVI,with rGO content of 7.5%,has a reaction order of 1.85 for the reduction of nitrate nitrogen indicating that the reaction is a complex process consisting of adsorption/desorption and redox reactions.Denitrification process and electron production by NZVI oxidation occured on the surface of rGO-NZVI.The main reaction product is ammonia nitrogen,with the production of a small amount of N2.And as the reaction proceeds,the pH of the reaction system increases.By simulating groundwater conditions,the effects of nitrate initial concentration and coexisting ions(Cl" and SO42-)on denitrification were investigated.Studies have shown that the initial concentration of nitrate has a great influence on the removal rate and reaction rate of nitrate nitrogen.With the increase of initial concentration,more time is needed to ensure that the nitrate nitrogen concentration reaches the standard limit.The presence of the coexisting ions(Cl-and SO42-)reduces the reaction rate and increased the time to reach the concentration limit.