Removal Of Cr(?) And P-Chloronitrobenzene With Oxalate Modified Zero-valent Iron

The rapid development of social economy and the continuous improvement of modernization have brought convenience to people's lives.Meanwhile,excessive development has brought great pressure to the natural environment that we rely on for survival.Among them,the problem of water pollution is particularly serious in China.Due to the discharge of sewage from chemical plants,heavy metal pollution and toxic organic pollution destroy the water resources.Environmental workers at home and abroad are committed to developing various water restoration technologies.Traditional water restoration technologies have improved the water body environment to a certain extent,but there are still some problems such as unsatisfactory treatment effect high cost and easy rebound.In recent years,zero-valent iron(ZVI)has been widely used in wastewater treatment due to its advantages of cheap and easy availability and environmental friendliness.However,ZVI is easily oxidized by air during storage and transportation,the surface of ZVI is covered by oxide layer,which will block the transfer of electrons outward,resulting in low activity.Therefore,it is necessary to find effective means to modify ZVI and improve the activity of ZVI to remove pollutants.In this paper,we use sodium oxalate to modify the surface functional group of ZVI.Through the modification of functional groups on the surface of oxide layer,the purpose of enhancing the removal activity of ZVI to target pollutants without removing the oxide layer was achieved.First,we treated commercial ZVI with sodium oxalate solution to obtain oxalate-modified ZVI(OA-ZVI),which was applied to the removal of hexavalent chromium(Cr(?)).The experimental results showed that the reaction rate of removing Cr(?)by OA-ZVI was about 30 times higher than that by ZVI.The results showed that this oxalate modification favored a more stable adsorption of Cr(?)on the ZVI surface with a lower adsorption energy,and also accelerated electron transfer from the iron core to the surface,which could produce more surface-bound Fe(?)to easily reduce Cr(?)in a single electron transfer path,greatly promoting the reduction removal of Cr(?).The composition of the surface elements and the change of valence state were analyzed by X-ray photoelectron spectroscopy(XPS).The electron transfer capacity of the material was analyzed by Tafel curve.In situ attenuated total reflection Fourier transform infrared(ATR-FTIR)and density functional theory calculation(DFT calculation)were used to study the coordination of functional groups on the surface of OA-ZVI.The possible charge transfer in the system is simulated by differential charge calculation.Finally,we proposed a possible mechanism of oxalate modification promoting Cr(?)removal with ZVI.Next,OA-ZVI was applied to remove p-chloronitrobenzene(p-CNB)in the absence of oxygen.The benzene ring of p-CNB has two electron-absorbing bases,so p-CNB is very stable and cannot be directly degraded by free radicals.Therefore,p-CNB can only be reduced to p-chloroaniline(p-CAN).Experiments showed that OA-ZVI could achieve more efficient reduction removal of p-CNB and generate more reduction products p-CAN.We calculated the electron effective utilization ratio of two materials in reducing p-CNB,5.5%of the electrons consumed by ZVI were used to restore p-CNB,28.8%of the electrons consumed by OA-ZVI were used to restore p-CNB,and the electron selectivity of OA-ZVI was greatly improved.This study found that when p-CNB was adsorbed on the surface of ZVI,it was mainly an unstable outer-sphere adsorption,while when p-CNB was adsorbed on the surface of OA-ZVI,it formed a stable inner-sphere adsorption.Therefore,the enhanced electronic selectivity of OA-ZVI was caused by the enhanced molecular adsorption of p-CNB.Besides,it is known from the previous work that the presence of oxalates induces the transfer of ZVI electrons outward,producing more electrons for p-CNB reduction.The composition of ZVI and OA-ZVI before and after the reaction were analyzed by Raman spectroscopy and extended X-ray absorption fine structure spectroscopy(XAFS).The bonding coordination of p-CNB on two zero-valent iron surfaces was studied by ATR-FTIR.The reduction of H20 to H2 in two zero-valent iron systems was analyzed by hydrogen radical determination and hydrogen production experiments.Finally,we proposed a possible mechanism of oxalate modification promoting p-CNB reduction with ZVI.This study provides an environmental-benignity surface modification method of ZVI.and also sheds light on the importance of surface functional groups on the Cr(VI)removal performance of ZVI.