Electrochemical method to remove nitrate in high-salt water

Abstract

Nitrate removal is an important target for the treatment of high-salt water such as marine aquaculture tail water or reverse osmosis concentrated water. The purpose and requirements of high salt water to remove nitrate are different from groundwater or domestic sewage. The salinity of the marine aquaculture tail water is about 3.5%, and the carbon and nitrogen are relatively low (less than 10:1), marine aquaculture tail water needs to be recycled, so it is necessary to remove the nitrate while retaining the other salt ions. Reverse osmosis concentrated water generally achieves solid-liquid separation by evaporation. The separated salt residue includes sulfate, chloride and nitrate. If sulfate or chloride with higher purity can be separated, it can be used as a resource. At present, nanofiltration technology can be used to separate divalent salts from monovalent salts, but nitrate and chloride are both monovalent salts, and it is difficult to separate further. So these special requirements make the nitrate removal technology for groundwater or domestic sewage not suitable for high-salt water. Ion exchange, reverse osmosis and electrodialysis only separate nitrate from waste water but not removing it, meanwhile reverse osmosis and electrodialysis technology will remove nitrate and other ions (such as Cl−) from the water together. The biological denitrification process cannot adapt to the high salt environment. Electrochemical reduction denitrification can achieve the conversion of nitrate to nitrogen through electron transfer, and high salt environment can provide the higher conductivity required by this process. Electrochemical methods are expected to meet the special requirement of high-salt water removal for nitrate. Electrode material is the core of electrochemical reduction of nitrate which will directly affect the N2 selectivity, current efficiency and reaction kinetics of electrochemical reduction of nitrate. This review briefly introduces the cathode materials used in electrochemical reduction nitrate including metal materials, carbon materials, and modified electrode materials. The factors of pH and current density that affect the performance of cathode reduction of nitrate to produce nitrogen, the effects of pH on the rate of nitrate reduction vary with electrode materials, and acidic conditions are conducive to the adsorption of hydrogen on the electrode surface, so that the electrode surface is in a reduced state, which is conducive to the formation of N−H bonds and can promote the formation of NH4+, alkaline conditions are more conducive to the accumulation of NO2−. With the increase of current density, the removal rate and reduction rate of nitrate increase. But if the current density is too high, the energy consumption increases more, the side reaction of brine electrolysis increases, and the current efficiency decreases. Therefore, it is necessary to find a balance between reducing energy consumption and NO3− treatment requirements in order to obtain highly impressive treatment performance. This paper outlines the technical characteristics of total nitrogen removal with the coordination of cathode and anode. At present, most studies only use unipolar to remove nitrate, the products of electrocatalytic reduction of nitrate are mainly ammonia and nitrite, and the N2 selectivity is still low. In order to solve this problem, the researchers used the cathode and anode coordination method, by adding chloride ions in the electrolyte which generating HClO, Cl· and ClO· to oxidize NH4+ to N2, sodium chloride is generally selected as the added chloride salt. The review summarizes the research process of electrochemical removal of nitrate and gives suggestions for the application of electrochemical technology in high-salt water denitrification at last.