Electrochemical Oxidation Of Nitrogen-Heterocyclic Compounds And Toxicity Evolution Of Wastewater

Nitrogen heterocyclic compound is difficult to be degraded in the natural environment because of its stable structure, biological toxicity, potential carcinogenicity, teratogenicity and mutagenicity, resulting in posing a major problem for the environment as well as a threat to human health. The biodegradation extent of nitrogen heterocyclic compounds is highly affected by the size of nitrogen heterocyclic rings and the fused degree among rings. Furthermore, the toxicity and potential mutagenicity of nitrogen heterocyclic compounds increased with the increase of ring number and nitrogen (N) atom. Quinoline and nicotine were selected as the representative nitrogen heterocyclic compounds with two rings. In this study, electrochemical oxidation of quinoline and nicotine was studied in a reactor using a modified β-PbO2and stainless steel net as anode and cathode, respectively. Sodium sulfate (Na2SO4) was used as supporting electrolyte. The effects of current density, pH, dose of Na2SO4and initial quinoline or nicotine concentration were investigated. The intermediates formed during electrochemical processes were detected by high performance liquid chromatography (HPLC), ion chromatography (IC) and gas chromatography-mass spectrometry (GC-MS). On the basis of analysis results, the electrochemical oxidation mechanism of quinoline and nicotine were speculated. The toxicity evolution of synthetic quinoline and nicotine wastewater during electrochemical oxidation was tested through duckweed growth inhibition method, and the factors of pH and added nutrients towards toxicity were also investigated. The results provide a theory basis for biological treatment when electrochemical technology was used as a pretreatment method.(1) It was showed that quinoline could be degraded effectively by electrochemical oxidation. The removal of quinoline was mainly dominated by current density, while Na2SO4concentration and initial pH under study had little effect on it.99.6%of quinoline could be degraded within60min under the appropriate conditions:150mg/L of quinoline, initial pH6.2,2g/L of Na2SO4and50mA/cm2of current density.Under the appropriage conditions, the mechanism of electrochemical oxidation of quinoline was proposed as follows:under the attack of continuously in-situ produced OH on the surface of β-PbO2anode, hydroxylation at carbon8and2were the first steps in the pathway and this was followed by an oxidative decarboxylation to yield pyridine-2-carboxaldehyde, nicotinic acid and acetophenone, then were further degraded into biodegradable organic acid like formic acid, acetic acid, propionic acid, oxalic acid and fumaric acid. In addition, the N atom of quinoline was transformed to nitrate ion (NO3-) in solution.The quinoline solutions used to culture duckweed were oxidized by electrochemical oxidation under the following conditions:150mg/L of quinoline, initial pH6.9,2g/L of Na2SO4and30mA/cm2of current density. Duckweed was cultured in quinoline solution after treated for20,60,100,120,160,200and240min, and2g/L of Na2SO4solution was used as the control. The results showed that untreated quinoline (0min) could be absorbed or utilized by duckweed and promotes the growth of duckweed. Whereas treated quinoline solution inhibited the growth of duckweed, and the inhibition increased with the increase of reaction time. When exposed to100min or120min-treated solution for1day, duckweed was totally dead. Further increasing the reaction time, the inhibition of treated solutions was gradually reduced as the degradation of intermediates. The quinoline solution after oxidized for240min was no longer restraining for the growth of duckweed and the content of7d-chllorophyll a was17%more than the value of control group. When the pH of treated quinoline solutions was adjusted to neutral, the whole inhibition to duckweed was remarkably relieved. But the intermediates formed in the early stage inhibited the growth of duckweed, the chlorophyll a content of duckweed cultured in60min-quinoline solution for7days was43.2%less than the control group. With the increase of reaction time, the inhibition was gradually eliminated, and quinoline solutions treated for above120min promoted the growth of duckweed and the content of7-d chlorophyll a was within the range of68-201%, which was more than that of the control value. When the pH of treated quinoline solutions was adjusted to neutral and the nutrients were added, the growth condition of duckweed was better and the dry weight of duckweed cultured for7days increased about50%. But the inhibition trend to duckweed was almost the same as the pH adjustment only protocol. Quinoline solution treated less than100min was inhibitive to duckweed, while oxidizing for above120min, its inhibition to duckweed was basically removed. Duckweed growth inhibition test results showed that the growth of duckweed was strikingly affected by pH of treated quinoline solutions and the toxicity of generated intermediates. All in all, the treated quinoline solution within100min was toxic and60min-solution was the most toxic one. When reaction time exceeded120min, the electrochemical treated quinoline solution had no more than biological toxicity. It indicated that the intermediates generated in the preliminary stage of quinoline’s electrochemical oxidation were toxic, and it would be totally converted to biodegradable organic acids after120min, thus the toxicity was eliminated.(2) The results showed that nicotine could be effectively removed by electrochemical oxidation. The removal of nicotine was mainly governed by current density, while the impact of Na2SO4concentration and initial pH under study on the removal of nicotine was small. More than99%of nicotine could be degraded within90min under the appropriate conditions:100mg/L of nicotine, initial pH6.0,1.5g/L of Na2SO4and40mA/cm2of current density. Electrochemical oxidation techniques could be successfully applied to actual tobacco sheet wastewater treatment, after30min of reaction time, nicotine in the wastewater was completely removed, and the color of wastewater turned from deep brown to colorless, meanwhile, the removals of chemical oxygen demand (COD), suspended particulate matter (SS) and turbidity were60.7%,70.4%and59.6%, respectively.Under the appropriate conditions, the mechanism of electrochemical oxidation of nicotine was speculated as follows:under the attack of continuously in-situ produced OH, nicotine was transformed to myosmine and cotinine at the first stage. Then myosmine and cotinine were further degraded into3-pyridinecarboxaldehyde,1-methyl-2-pyrrolidinone,1-(3-pyridinyl)-1-propanone,4-(methylamino) butyric acid and1-(3-pyridinyl)-ethanone, and then were converted into biodegradable organic acids like formic acid, acetic acid, propionic acid, oxalic acid and fumaric acid. In addition, the N atom of nicotine was transformed to NO3-in solution.200mg/L of nicotine was oxidized by electrochemical oxidation under the following conditions.1.5g/L of Na2SO4was used as the control group culture solution.The duckweed was cultured in treated nicotine solution for4days. Compared with the value of control, the untreated nicotine had no obvious inhibitory effect on the growth of duckweed, but the frond number and chlorophyll a content of duckweed cultured in treated nicotine solutions of15min for4days increased100%and87%, while inhibited about90%and55%in nicotine solutions treated for above60min, respectively. When adjusting the pH of treated nicotine solutions in appropriate range or adding nutrients in it, the results showed that there’s no obvious inhibitory effect was observed on duckweed. Therefore, the inhibition of nicotine solutions treated after60min during electrochemical oxidation towards duckweed was mainly come from the low pH value of solutions caused by the formation of organic acids. So there’s no biological toxicity of nicotine during electrochemical oxidation.In summary, quinoline and nicotine could be degraded effectively by electrochemical oxidation. The removal was mainly controlled by current density, while the effect of the Na2SO4dosage and initial pH was not obvious. The common oxidation mechanism of quinoline and nicotine was as follows. Firstly, one ring was opened and mostly transformed to pyridine derivatives, subsequently cleaved to organic acids such as formic acid, acetic acid and propionic acid and so on. The final form of N atom was NO3-. The toxicity of quinoline and nicotine solution during electrochemical oxidation process was highly affected by the pH of the treated solutions. After adjusting the pH to neutral, the toxicity of treated quinoline solution was increased firstly and then decreased and was eventually elimimated after120min of oxidation. Nevertheless, the oxidized nicotine solutions expressed no biological inhibition after pH adjustment. These results could provide a basis guideline for subsequent biological treatment.