Electrochemically-induced reduction of nitrate in aqueous solution

Electrochemical Oxidation of Nitrogen towards Direct Nitrate Production on Spinel Oxides

Nitrates are widely used as fertilizer and oxidizing agents. Commercial nitrate production from nitrogen involves high-temperature-high-pressure multi-step processes. Therefore, an alternative nitrate production method under ambient environment is of importance. Herein, an electrochemical nitrogen oxidation reaction (NOR) approach is developed to produce nitrate catalyzed by ZnFe_x Co_2-x O₄ spinel oxides. Theoretical and experimental results show Fe aids the formation of the first N-O bond on the *N site, while high oxidation state Co assists in stabilizing the absorbed OH^- for the generation of the second and third N-O bonds. Owing to the concerted catalysis, the ZnFe_0.4 Co_1.6 O₄ oxide demonstrates the highest nitrate production rate of 130±12 μmol h^-1  g_MO ^-1 at an applied potential of 1.6 V versus the reversible hydrogen electrode (RHE).

Electrochemical nitrate removal with simultaneous magnesium recovery from a mimicked RO brine assisted by in situ chloride ions

Electrochemical reduction is effective to remove nitrate but byproducts such as ammonia and nitrite would need chloride addition for indirect oxidation to nitrogen gas. Herein, electrochemical nitrate reduction was investigated to remove nitrate from a mimicked reverse osmosis (RO) brine containing chloride that eliminates the need for external chloride addition. Both Cu/Zn and Ti nano cathodes exhibited the best performance of nitrate removal with >97 % removal in either Na₂SO₄ or NaCl electrolyte, although with different products. Complete nitrate reduction to nitrogen gas was realized in the RO brine whose complex composition decreased the electrode efficiency, for example from 71.4 ± 0.2%-49.4 ± 0.3 % with the Cu/Zn cathode after 5 cycles of operation. Magnesium was recovered at the same time of nitrate removal and the purity of Mg(II) could reach 96.8 ± 2.0 % after proper pH pre-treatment. In a preliminary adsorption study, a key byproduct - chlorate was reduced by 49.8 ± 2.7 % after 3-h adsorption by 100 g L^-1 activated carbon. These results have demonstrated the simultaneous electrochemical nitrate removal and resource recovery from a complex water like a RO brine and provided new information such as byproduct management and electrode deterioration.

The share of electrochemical reduction, hydrogenotrophic and heterotrophic denitrification in nitrogen removal in rotating electrobiological contactor (REBC) treating wastewater from soilless cultivation systems

There is a growing global environmental problem of agricultural wastewater from soilless plant cultivation systems. In most countries dominate open fertilization systems, in which excess of nutrient solution is discharged in an uncontrolled way into the ground inside greenhouses or adjacent areas. Wastewater from such systems is characterized by a very high concentration of nitrogen and phosphorus compounds and their discharge into the environment causes significant pollution of the water and soil environment. The goal of the research was to determine the contribution of electrochemical reduction of nitrogen, hydrogenotrophic and heterotrophic denitrification in the process of nitrogen removal in a rotating electrobiological contactor (REBC) depending on hydraulic retention time (HRT) and electric current density (J). Synthetic sewage with characteristics corresponding to wastewater from soilless cultivation of tomatoes was the subject of the research. The first part of the experiment included determination of the effect of HRT on the effectiveness of bio-processes of nutrients removal in a rotating biological contactor (RBC). The second concerned the effect of HRT and J on the effectiveness of nutrients removal in a rotating electrochemical contactor (RECC), while the third part - the effect of HRT and J on the effectiveness of nutrients removal in REBC. RBC was characterized by low efficiency of denitrification (6.2 to 9.2%). The effectiveness of nitrogen removal in RECC was determined by both electric current density and hydraulic retention time. The highest efficiency was 53.4%. REBC nitrogen removal effectiveness was higher than in RBC and in RECC. The nitrogen removal efficiency increased along with increasing values of HRT, reaching the maximum value of 68.6% for J=10.0A/m² and HRT=24h. The contribution of hydrogenotrophic denitrification in total nitrogen removal increased with the increase of electric current density.

Electrolytic control of hydrogen peroxide release from calcium peroxide in aqueous solution

The in situ generation of hydrogen peroxide (H₂O₂) for water treatment is more practical than the use of liquid H₂O₂, which is costly to store and transport. Calcium peroxide (CaO₂), a solid carrier of H₂O₂, can release H₂O₂ on dissolution in water. However, the constant H₂O₂ release rate of CaO₂ has been a bottleneck constraining its wider application. In this study, a practical electrochemical method using a divided cell is developed to control the rate of release of H₂O₂ from CaO₂. The results show that the rate of H₂O₂ release from CaO₂ is enhanced in the anolyte. The increase in H₂O₂ release is positively correlated with the current. Under a current of 100 mA, the H₂O₂ concentration was 2.5 times higher after 30 min of electrolysis than in the control experiment in which no current was applied. Water electrolysis in the anodic compartment generates protons that not only: (i) en-hance dissolution of CaO₂ and release of H₂O₂, but also (ii) neutralize the alkaline pH resulting from CaO₂ dissolution, thus providing new advantages for the use of CaO₂. This effective technique may be suitable for the sophisticated control of H₂O₂ release in environmental applications.