Detection Of Hydroxyl Radical In Electrochemical Process And Research Of The Mechanism Of Pyridine Degradation

Pyridine is a refractory nitrogen-containing heterocyclic compound（NHCs）and its related wastewater can not be effectively treated by a single treatment technology.Electrical advanced oxidation processes（EAOPs）is considered to be an effective pyridine wastewater treatment technology for its environmental friendliness,applicability to various pollutants,high energy efficiency and simple control process.The core of EAOPs is the production of a large number of hydroxyl radicals（HO·）,but currently there is a lack of efficient and simple HO·detection technology.A spectrophotometric HO·detection method is established to study the generation and influence mechanism of HO·during EAOPs by choosing a suitable,selective and sensitive capture agent,which is of great significance for studying the electrochemical oxidation mechanism of organic compounds.In this paper,two HO·capturing agents were selected,salicylic acid（SA）and terephthalic acid（TA）,to capture HO·produced under the same electrochemical conditions.At the same concentration,the electrochemical oxidation reaction of SA and TA（the first 10 min of the reaction）with HO·are consistent with the first-order reaction kinetics.The reaction rate constants of SA and TA are 2.90×10^-4 s^-1 and 1.35×10^-3 s^-1（the first 10 min of the reaction）,respectively.Compared with SA,TA has a higher capture efficiency for HO·.In this electrochemical research system,TA was selected as the capture agent for HO·.TA was used as a scavenger to detect the HO·generated by the electrochemical system by UV-visible spectrophotometry,and the optimal electrochemical conditions for HO·generation were studied.The results show that the electrochemical properties of the three electrode materials（foam nickel,foam iron,graphite）are not much different,but under the same electrochemical conditions,the voltage required for the foam nickel electrode is the smallest.With the increase of electrolyte Na₂SO₄ concentration,the migration rate of electrons in the solution increases,and in this process,the capture rate of HO·increases greatly.The alkaline condition is more favorable for the formation of HO·,the pH is in the range of 3～9,and the HO·accumulated capture amount increases as the pH value of the solution increases.After the pH is increased to 9,the amount of HO·produced by the electrochemical system tends to be stable.The current density has a great influence on the formation rate of HO·.Increasing the current density under the same electrochemical oxidation conditions can accelerate the generation of free radicals,the hydroxylation reaction rate of TA increases,and the free radical capture efficiency increases.The experimental design was carried out by RSM（Box-Behnken）design method,and the related electrochemical conditions generated by HO·were optimized.The influence degree of each factor on HO·generation was ranked as follows:electrolyte concentration c（Na₂SO₄）>current density>pH.The degradation mechanism of pyridine in this electrochemical system under different pH conditions was investigated.According to the cyclic voltammetric scanning analysis before the reaction of pyridine solution under different pH conditions,the overall redox peak was not obvious,indicating that pyridine did not directly react with the electrode.After the electrochemical reaction of the pyridine solution,obvious redox peaks appeared.The reason was that in the electrochemical system,pyridine decomposed to form small molecular organic matter,which causes the solution to be more susceptible to redox reactions.The GC-MS analysis of the pyridine degradation products showed that the degradation products of pyridine were different under different pH conditions.The main reason was that the pH conditions affected the opening mode of HO·to pyridine.