Construction and Mechanism of Highly Efficient Electrochemical Reduction System for Nitrate in Groundwater

Author:Kuang Zuo

Supervisor:feng chuan ping chen nan


Degree Year:2019





Nitrate contamination in groundwater became an increasingly serious environmental problem due to the increased fertilizers application in agriculture and improper treatment for industrial waste or municipal sewage.Elevated nitrate concentration directly affects the safety of residential drinking water.Electrochemical technology has attracted a large number of researchers,due to its high efficiency,small area occupied and environmental respectability.However,the nitrate reduction efficiency of traditional electrochemical system is relatively lower because of the less reactive sites in the system.Meanwhile,by-products accumulation,high energy consumption and secondary pollution in the electrolyte also limit the application of the system.This research is aimed to improve the electrochemical nitrate reduction performance by using polarized iron net,adjusting surface area ratio of anode to cathode(A/C),combining electrolysis and adsorption technology and modifying boron doping diamond(BDD)electrode.The system construction,electrodes behavior and the mechanism of nitrate and by-products removal are investigated.This study provides the theoretical foundation for improving electrochemical technology of nitrate reduction.This study constructed an intensified system by introducing three pieces of iron net between the three-dimensional(3-D)perforated iron cathode and Ti/IrO2-Pt anode,in which the 3-D perforated iron cathode constructed by two pieces of iron net series connection.The developed 3-D cathode has larger specific surface area and lower impedance for enhancing nitrate reduction efficiency to 90%.In addition,the introduced iron net is polarized as bipolar electrodes that combine the effects of electrochemical and chemical reduction,thus increasing reactive sites for nitrate reduction.The system increases the nitrate reduction efficiency to 99%and decreases the energy consumption to 34.82 kWh/n-nitrate-N.Cu-Zn and Ti/IrO2-Pt are selected as cathode and anode to construct the electrochemical system,and the A/C of the system is adjusted to 1.00,0.46,0.23 and0.12.Results showed that decreasing A/C could enhance the direct electrochemical oxidation capacity for by-products removal.Meanwhile,the smaller A/C decreases reactive sites of anode,and thus decreases the active chloride production from 398.5to 132.5 mg-Cl/L in the presence of 1.0 g/L NaCl.All the by-products are removed though less active chloride accumulation,which inhibits the negative effect of active chloride on nitrate reduction.It could be found that decreasing A/C could improve nitrate reduction performance.An electrochemical-adsorption(ECA)system was constructed by packing iron particles and zeolites between electrodes,combining the effects of electrolysis and adsorption.Response surface methodology(RSM)with Box–Behnken design(BBD)was applied to investigate the effects of independent variables(iron particle,zeolite and current density)and their interaction on the system performance,also determining its optimum working state.The optimal conditions for the amount of iron particles(19.74 g)and zeolites(28.19 g)as well as the current density(18.72 mA/cm2)result in a high nitrate removal efficiency of 95%(initial concentration of 50 mg-N/L),with less ammonia and nitrite accumulation.Synergistic effects of electrolysis and zeolite adsorption accelerate ammonium removal by increasing the rate of ion directional migration.The system avoids the problem of adding NaCl for by-products removal in the traditional system.This study is among the first to systematically study the electrochemical nitrate reduction behavior on boron-doped diamond(BDD)films with different surface terminations and boron-doping levels.The highest nitrogen gas selectivity of 44.5%could achieve using a BDD electrode with hydrogen-terminated surface and B/C ratio of 1.0%.C-H bonds on CR-BDD serve as the anchor points for attracting NO3-anions close to the electrode surface,thus accelerating the formation of NO-3(ads).Therefore,hydrogen-terminated BDD exhibits higher electrochemical reactivity for reducing nitrate.Meanwhile,the hydrophobicity of the hydrogen-terminated BDD inhibits water electrolysis and subsequent adsorption of atomic hydrogen,retarding ammonia formation and increasing the selectivity for nitrogen gas formation.Different kinds electrochemical system developed in this study could efficiently remove nitrate and by-products.This research provides a new idea for developing the electrochemical nitrate removal technology.