Studies On Removal Of Ammonia In Wastewater By Electrochemical Oxidation

Electrochemical oxidation has attracted extensive attention in the field of wastewater treatment due to its high efficiency,environmental friendliness and easy operation.As one of the important parameters to be considered in wastewater treatment,how to remove ammonia nitrogen from wastewater efficiently and quickly has become the focus of many researchers.In this paper,the factors influencing the removal of ammonia from wastewater by a two-dimensional electrode system were studied.The free chlorine,hydroxyl radical（HO·）and by-products（nitrate,nitrite and chloramine）generated during electrolysis were measured.In addition,particle electrode（Sn O₂-Sb₂O₃/GAC）was prepared by modifying tin-antimony metal oxides on coconut shell granular activated carbon（GAC）.Based on the two-dimensional electrode system,a three-dimensional electrode system was constructed by filling the Sn O₂-Sb₂O₃/GAC particle electrode between the two electrode plates to improve the removal of ammonia and reduce the pollution caused during the treatment process.Finally,relevant mechanism and products related to the electrochemical oxidation of ammonia nitrogen were analyzed.The main results of this paper are as follows:（1）The factors affecting the removal of ammonia from wastewater by the two-dimensional electrode system were investigated,including current density,initial chloride ion concentration,initial p H,plate spacing and initial ammonia concentration.The results showed that current density and initial chloride ion concentration were the main factors influencing the electrochemical oxidation of ammonia.The higher the current density and initial chloride ion concentration,the faster the removal rate of ammonia.The initial p H,plate spacing and initial ammonia concentration had little effect on the electrochemical oxidation of ammonia.Weak alkaline conditions（p H=8and 9）were more favorable to the removal of ammonia.The appropriate conditions for treating wastewater with ammonia concentration of 50 mg/L were:current density of 5.0 m A/cm²,initial chloride ion concentration of 500 mg/L,plate spacing of 4 cm,and weakly alkaline condition.Under these conditions,80%of the ammonia could be removed by electrolysis in 60 min.（2）The direct oxidation of ammonia in the two-dimensional electrode system was bad and there was basically no HO·generation during the electrolysis.The removal of ammonia in the two-dimensional electrode system was mainly due to the oxidation of free chlorine and the concentration of free chlorine produced increases with the increase of current density.The two-dimensional electrode system mainly oxidized ammonia nitrogen by free chlorine generated by the electrode.The main products were N₂（45.15 mg/L,90.32%）,nitrate（2.38 mg/L,4.76%）,nitrite（0.01mg/L,0.02%）and chloramine（2.45 mg/L,4.90%）.The content of nitrite was very low and therefore negligible.（3）SnO₂-Sb₂O₃/GAC particle electrode was prepared through impregnation-calcination method.XRD results showed that the Sn O₂-Sb₂O₃/GAC particle electrode was successfully loaded on the GAC and SEM images of the Sn O₂-Sb₂O₃/GAC particle electrode showed that the GAC surface was covered with multiple layers of dense metal oxides.The Sn O₂-Sb₂O₃/GAC particle electrode was filled between the electrode plates to construct a three-dimensional electrode system to treat ammonia wastewater.The results showed that the addition of the Sn O₂-Sb₂O₃/GAC particle electrode significantly increased the ammonia removal efficiency and had good long-term stability.The HO·and free chlorine generated by the three-dimensional electrode system were detected.It was found that the HO·generated by the Sn O₂-Sb₂O₃/GAC particle electrode was the main reason for its contribution to the removal of ammonia and a small amount of chlorine could also be generated on its surface.The three-dimensional electrode system mainly oxidized ammonia by free chlorine and HO·generated by the particle electrode.The main products were N₂（46.34 mg/L,92.09%）,nitrate（2.00 mg/L,3.97%）,nitrite（0.07mg/L,0.14%）and chloramine（1.91 mg/L,3.80%）.The nitrite was also negligible.Compared with the two-dimensional electrode system,the three-dimensional electrode system could increase the proportion of N₂ and reduce the proportion of by-products such as chloramine and nitrate.（4）The removal of ammonia by four different electrochemical oxidation systems were investigated.The results showed that:in the two-dimensional chlorine-free system,all ammonia was removed by direct oxidation of electrode plates,which was very limited;in the two-dimensional chlorine-containing system,the removal of ammonia was mainly achieved by free chlorine;in the three-dimensional chlorine-free system,HO·played a major role in the removal of ammonia;in the three-dimensional chlorine-containing system,free chlorine,HO·and direct oxidation together played a role in the removal of ammonia,among which,free chlorine removed most ammonia.Chloride ions had a inhibitory effect on the generation of HO·to some extent.According to the inhibiting experiments the degradation of ammonia by HO·mainly happened on the surface near the particle electrode rather than in the aqueous solution.（5）The electrochemical oxidation system was used to treat phenol-containing ammonia wastewater to simulate the treatment of coking wastewater.It was found that there was a competitive relationship between the removal of COD and ammonia.At a current density of 15 m A/cm²,ammonia concentration of 50 mg/L and COD of300 mg/L,the three-dimensional electrode system could remove 92.02%of ammonia and 68.95% of COD within 120 min.