Electrochemically Regulated Reduction Of Nitrate On NZVI@D201:Properties And Mechanism

Nitrate contamination of water resource is a global issue due to the rapid progress of agricultural and industrial activities.Excessive nitrate in water tends to form eutrophication and algae bloom.It's crucial to remove nitrate from water and the ideal treatment is to convert it to harmless nitrogen.Currently,various nitrate removal technologies have been developed,including physicochemical technology,biological denitrification,chemical/catalytic reduction and electrocatalytic reduction.With the advantages of no chemical input,high catalytic efficiency,ambient operating conditions and minimal sludge generation,electrocatalytic reduction has attracted more attention.At present,the research on electrocatalytic reduction of nitrate mainly focused on electrode materials based on precious metals including Cu,Ag,Ni,Pd,Co.While precious metals were not suitable for engineering applications due to their high costs.Zero-valent iron(ZVI)is a well-known inexpensive material with strong reduction ability for water decontamination,including nitrate reduction.So far,most literatures published has focused on Fe(0)as a reductant to remove nitrate.Resultantly,the ZVI would be consumed quickly as the reducing agent and ammonia was found to be the main species in the final product,which was not favorite.Recently,few reports have pointed out the possibility that ZVI act as the electrochemical catalyst for the nitrate electrochemical reduction.It showed excellent removal performance and nitrogen selectivity for nitrate electrochemical reduction,which suggested a new approach for fast and eco-friendly nitrate reduction and a novel nZVI application.During the selective electrochemical reduction of nitrate by nZVI,the nitrate could be firstly reduced to ammonia at the nZVI cathode,and the co-existing chlorine was oxidized at the anode and then reacted with water to form hypochlorous acid(HClO).Finally,the hypochlorite ions can oxidize ammonia to nitrogen effectively.Although electrochemical regulation enhancing the reduction of nZVI to nitrateexhibited significant nitrogen selectivity,it was not clear whether nZVI as a catalyst or a reductant.It was necessary to deeply explore the detailed mechanism.If we can clarify the actual role that zero-valent iron played in the nitrate electrochemical reduction,it is conducive to further development of Fe-based electrocatalytic reduction materials and technology.Herein,the present study is first to investigate the performance of electrochemical reduction of nitrate by the nanocomposite nZVI@D201.A columnar electrochemical reaction device was constructed with graphite column as the cathode and ruthenium-iridium plated titanium rod as the anode.nZVI@D201 was filled into hollow graphite column.Firstly,we compared the difference of the removal capacity for nitrate and the selectivity for nitrogen with and without electricity.By additional voltage,the removal efficiency up to 80%can be kept in 60 h of experiments and nitrogen selectivity of nitrate reduction was approach 95%.Furtherly,the mechanism of enhancement for nitrate reduction by nanocomposite nZVI@D201 was studied in detail.A series of experiments were designed to investigate the role of generated H2/[H]in cathode,the role of electrons,the role of Fe(0)and its corrosion products.Importantly,we clarified that e-and[H]were the specific factors of nitrates reduction on nZVI via electrochemical regulation.Combined with the XPS and XRD characterization for nZVI@D201 at the different reaction stages,the related mechanism of electrochemical regulation of nZVI for the removal of nitrates was revealed at last.The nitrate removal on nZVI@D201 via electrochemical regulation may attribute to the chemical reduction with nZVI and the produced highly reactive structural-Fe(?)as well as nanocomposites of Fe(0)@FexOy-Fe(?)system.Fe(0)@FexOy-Fe(?)turned into Fe(0)@FeXOy-Fe(?)after the reduction of nitrate,the produced Fe(?)could be converted effectively to Fe(?)in situ and form Fe(?)/Fe(?)couple by e-or[H]regeneration from electrochemical regulation to remove nitrate continuously.It provides scientific guide to developing well nitrate reduction by nZVI coupling with electrochemistry.