Application Of Boron-doped Diamond Anode In Phenolic Wastewater Containing Nitrite Salt

Phenolic pollutants are the most important raw materials in the synthesis industry,and considerable phenolic coumpounds would be discharged with the wastewater which would cause damage to the environment.China and the United States Environmental Protection Agency have listed phenolic pollutants as the priority control pollutant.In recent years,the boron-doped diamond anode electrochemical oxidation process has attracted widespread attention due to its outstanding degradation efficiency,such as controllable electrolytic process,non-selective oxidation ability,and cleanness without secondary pollution.This technology is gradually gain great processes in the treatment of phenolic wastewater.Meanwhile,the actual industrial wastewater often contains high concentrations of nitrite salt,because it is also extensive used in the field of medicine,construction,smelting,pesticides and so on.However,the behavior of the nitrite in the BDD treatment of phenolic wastewater has not been reported.This content intends to select a series of phenolic pollutants as model pollutants(phenol,1-naphthol,2-naphthol and bisphenol A)and nitrite as electrolytes to investigate whether nitrated side reaction would take place in the BDD anode electrochemical oxidation system.In addition,the influence between each factor and nitrated by-products would be taken into further consideration.This article will be expanded in the following three areas:(1)Taking the simplest compound—phenol as the simulated pollutant,the nitrite concentration and current density were selected as the factors to be examined.The experimental results show that sodium nitrite can be regarded as an active electrolyte during electrolysis,and can quickly react with hydroxyl radicals to generate nitrogen dioxide radicals.Electrophilic nitrogen dioxide radicals easily undergo substitution and coupling reactions with the electron-rich phenol structure,leading to the formation of a series of nitrophenol by-products.These by-products mainly include nitrating monomers,dimers and trimers.In contrast,in a comparative experiment with sodium nitrate,it was found that a medium concentration(10 mM)of nitrate only conducts electricity during the electrolysis process,and at a higher concentration(100 mM),a small amount of nitrate ions is at the cathode The surface is reduced to nitrite ions,which in turn produces a small amount of simple nitrated phenol by-products.(2)1-naphthol and 2-naphthol,which are isomers of each other,were used as simulated pollutants,and nitrite was used as the electrolyte for electrochemical oxidation experiments.Based on the results of the phenol experiment,process factors such as flow rate and initial pH were added to investigate,and the structure-activity relationship between the difference in molecular structure and the side reaction of nitrification was examined.The experiment result showed that both naphthols had higher electrochemical activity than phenol,and 1-naphthol had higher activity than 2-naphthol.This result can explain the experimental phenomenon that the varieties of by-products of 1-naphthol are less than those of 2-naphthol.In addition,in the 2-naphthol experiment,nitrated 2-naphthol coupling phenol by-products were identified,and this type of by-product was not found in the 1-naphthol experiment.This result also illustrates the subtle molecular structure.The difference still has a certain effect on the electrochemical activity(3)One of the highest yielding compounds in the world—bisphenol A was used as the model pollutant.The response surface method based on the central composite design was used to optimize the experiment,and the influences of current density,nitrite concentration,flow rate,and initial pH on the mineralization effect of bisphenol A and the rule of nitrated side reactions were particularly investigated.Among them,the initial pH value and the nitrite concentration have the most important influence on the mineralization efficiency.The removal rate of bisphenol A is favor at low initial pH and low electrolyte concentration.In addition,the type of nitrated by-products mainly depends on the initial concentration of nitrite.With low concentrations,the degree of nitrification and polymerization of the by-products is low and stop at the state of trimers;while at high concentrations,the degree of nitrification and polymerization both increase significantly,and nitrated tetramers,pentamers,and nitrated bisphenol A coupling phenol by-products could be identified.As is known to all,nitrophenols are highly biotoxic and difficult to degrade.The formation of nitrophenolic by-products not only reduces the mineralization of phenolic wastewater,but also increases the toxicity of the treated wastewater and the difficulty of subsequent treatment.Therefore,in order to suppress the occurrence of such nitration side reactions,it is necessary to focus on the influence of process factors on the side reaction when using boron-doped diamond electrochemical oxidation process.In addition,the nitrite ion can be removed or oxidized to nitrite ion in advance before the electrochemical process.