Study On The Reduction Of Nitrate By Zero Valent Iron (ZVI) -Based Materials Combined With Carbon Dioxide Radicals

Nitrate is a common contaminant in surface water and groundwater,which poses great threat to the balance of ecological system and human health.Chemical denitrification using zero valent iron-based（ZVI-based）materials have gained much attention due to its high efficiency,abundance and environmentally friendly characters.However,two key issues,that is,the iron oxides produced during the reduction will inhibit the continue reaction of iron and the major products after reduction is ammonium,which hinder the application of this technology.To address these issues,a strategy that can transform nitrate into nitrogen with high efficiency and high selectivity is proposed.HCOO^-was used to be a ligand to complex iron ions to overcome the surface passivation on Fe⁰,at the same time,HCOO^-is the precursor of carbon dioxide radicals（CO2·^-）which can be decomposed into CO2·^-at the presence of UV and Fe³⁺.With the help of CO2·^-,nitrite,the intermediate product in the process of nitrate reduction by Fe⁰-based materials,can be reduced into nitrogen.In this study,firstly,the feasibility of reducing NO3^-by Fe⁰ based materials combined with CO2·^-was analyzed by combining the experiment with DFT theoretical calculation.Then,Fe⁰/HCOO^-/Fe³⁺/UV system was established by combining Fe⁰ with CO2·^-.The efficiency and mechanism of the reduction of NO₃^-by Fe⁰/HCOO^-/Fe³⁺/UV system were discussed.After that,two kinds of Fe⁰based materials（Fe⁰-Cu⁰ and Fe⁰-Cu⁰-Cu2O）were synthesized via modification of the surface of Fe⁰.The as-prepared materials were characterized by XRD,SEM,EDS and XPS.The effects of synthesized conditions and other conditional parameters on nitrate removal were discussed.Finally,two novel reductive systems(Fe⁰-Cu⁰/HCOO^-/Fe³⁺/UV and Fe⁰-Cu⁰-Cu2O/HCOO^-/UV)were established and the efficiency and mechanism of nitrate removal in the above two systems were discussed respectively.The main conclusions are as follows:（1）It was proved by DFT theoretical calculation combined with experimental results that HCOO^-and Fe³⁺could effectively produce CO₂·^-through photo-induced electron transfer process and then CO2·^-could efficiently reduce NO2^-to N2.At the same time,NO2^-is the intermediate product of two-step reduction of NO3^-thus it is a feasible strategy to combine Fe⁰ based materials with CO2·^-to reduce NO3^-to N2 with high efficiency and selectivity.（2）Nitrate reduction by Fe⁰ alone needed acidic condition and lower p H facilitated the reaction,in which the main products were ammonium.With the addition of UV,HCOO^-and Fe³⁺,nitrate reduction rates were speeded up but the selectivity towards nitrogen remained low and the ammonia was still the main end product.（3）In the system of Fe⁰-Cu⁰/HCOO^-/Fe³⁺/UV,the removal efficiency of NO₃^-by Fe⁰-Cu⁰was sharply enhanced compared with that Fe⁰alone and 10%Cu loading showed the highest removal efficiency.Compared with one-step method(NO3^-was directly reduced in the Fe⁰-Cu⁰/HCOO^-/Fe³⁺/UV system),two-step method（NO₃^-was firstly reduced to NO₂^-in the Fe⁰-Cu⁰/HCOO^-/UV system,then the accumulated nitrite in the effluent from the first-step reduction could be further reduced into nitrogen by CO2·^-radicals）had the higher nitrogen selectivity.100%nitrate removal efficiency and 98%N2selectivity were achieved through two-step process at initial NO3^-concentration of 50mg（N）/L,initial p H of 6.47 and Fe⁰-Cu⁰ dosage of 12 g/L.Almost no formate residual was found in the effluent due to the action of photo-induced electron transfer of Fe³⁺.（4）In the system of Fe⁰-Cu⁰-Cu₂O/HCOO^-/UV,the reduction activity of Fe⁰-Cu⁰-Cu2O composite for nitrate in the presence of HCOOH and UV was significantly dependent on the copper loading content.The Fe⁰-Cu⁰-Cu2O composite with 1.6%copper loading content had the highest reduction activity for nitrate.The 100%removal efficiency of nitrate and 95.4%of N2 selectivity were achieved at following conditions:Fe⁰-Cu⁰-Cu2O composite dosage of 50 g/L,initial p H of 2.5,HCOOH dosage of 15.0mmol/L,initial nitrate concentration of 50 mg N/L,and reaction time of 60 min.The Fe⁰-Cu⁰-Cu2O/HCOO^-/UV system had good stability for nitrate reduction because the reduction efficiency of nitrate decreased only 10%after Fe⁰-Cu⁰-Cu2O composite was used for four times.In Fe⁰-Cu⁰-Cu₂O/HCOO^-/UV system,e^-from galvanic-type galvanic cell consisting of Fe⁰ and Cu⁰,e^-_cb on the surface of Cu₂O and the CO₂·^-,which was produced mainly by the reaction between HCOOH and UV,played an important role in the reduction of nitrate to nitrite.It was confirmed that the high N2 selectivity was due to the selective reduction of nitrite by CO2·^-radicals.In this study,the composites Fe⁰-Cu⁰ and Fe⁰-Cu⁰-Cu2O were easy to get and synthesize.The iron-based materials were combined with a green reductant,carbon dioxide radicals,to establish the novel systems(Fe⁰-Cu⁰/HCOO^-/Fe³⁺/UV and Fe⁰-Cu⁰-Cu2O/HCOO^-/UV).Efficient removal of nitrate with high N2 selectivity was achieved in this study and a new way to reduce nitrate was provided.