Electrochemical Oxidation Behavior And Mechanism Of Heterocyclic Compounds

Heterocyclic compounds containing nitrogen, sulfur, or oxygen atom were highly toxic to microorganism through inhibiting cytochrome P450s mediated oxidation, resulting in that biodegradation is unsatisfactory for effective abatement of these compounds in wastewater. Once the heterocyclic ring was opened or cleaved, the toxicity to microorganisms could be avoided and the detoxification of wastewater was achieved. However, the heterocyclic ring was difficult to cleave using general oxidant such as Cl2or using H2O2directly, sometimes, more toxic intermediates were produced. Hydroxyl radical (·OH) is a powerful oxidant with reactivity second only to fluorine and has the advantanges of short lifetime, clean, safety and no residue effect. When the heterocyclic compounds were converted to biodegradable organic acids, the toxicity of heterocyclic compounds was reduced. Fenton process and photocatalytic oxidation could generate OH by employing catalyst, but this will result in second pollution. Direct in situ generation of-OH through electrochemical oxdidation without the addition of chemicals will be the promising alternative.In this paper, pyridine, thiophene, and tetrahydrofuran were selected as the typical heterocyclic compounds containing nitrogen, sulfur, and oxygen atom. Simutaneously, in order to compare the difference between the oxidation of sulfur heterocyclic compound and alkanethiol, ethanethiol was also chosen as model pollutant. The electrochemical oxidation behaviors of them were investigated using linear sweep voltammetry (LSV) with an electrochemical workstation (CHI660C), a small P-PbO2electrode (10mm x6mm), Pt wire electrode and saturated calomel electrode (SCE) were set as the working, counter and reference electrodes, respectively. Electrochemical oxidation experiments were carried out under galvanostatic conditions using β-PbO2as anode and a stainless steel net as cathode. The evolution of selected pollutants and intermediates during electrochemical oxidation were monitored and quantified. According to the analysis results, the electrochemical oxidation mechanisms of heterocyclic or heteroatomic compounds were separately proposed. The main results were as follows:1) LSV voltammograms showed that pyridine can be easily oxidized on the surface of P-PbO2and the peak oxidation potential was1.06V vs. SCE at pH3.5and 25mV/s of scan rate, the reaction was controlled not only by adsorption but also by diffusion. Pyridine was efficiently oxidized by employing OH electrogenerated on the surface of β-PbO2anode and completely eliminated after90min of reaction under160mA/cm2of current density,5g/L of Na2SO4and pH3.5. The analysis of high performance liquid chromatography (HPLC) showed that fumaric acid, oxalic acid and formic acid were the main intermediates, implying that reactions between OH and pyridine were proved to be the direct cleavage of pyridine ring to biodegradable, nontoxic carboxylic acids. The anlaysis of IC showed that nitrite ions (NO2-) and nitrate ions (NO3-) were completely converted from the decomposition of pyridine-nitrogen within the first60min, this further proved that the detoxification and the improvement of biodegradability of nitrogen-containing heterocyclic compounds could be realized via direct cleavage of the heterocyclic ring under mild conditions.2) Tetrahydrofuran (THF), as typical cyclic ether widely used in the bulk chemical and pharmaceutical industries, is an inhibitor of cytochrome P450-dependent enzymes and biorefractory. LSV displayed that THF could be easily oxidized on the surface of p-PbO2in NaCl and Na2SO4medium, the oxidation was controlled by diffusion process and the oxidation peak potentials were1.30V and1.21V vs. SCE, respectively. Results of bulk electrolysis of THF wastewater showed that the chemical oxygen demand (COD) removal was around97%after300min of reaction under the following conditions:pH3.0,11.2mA/cm2of current density,205mg/L of THF and10g/L of NaCl. Compared with that of in the Na2SO4medium, the existence of NaCl in wastewater was beneficial to enhance the electrochemical performance due to the indirect oxidation of chlorine producing on the surface of anode. The degradation of THF in the NaCl medium obeyed the pseudo-first order kinetics. HPLC analysis showed that succinic acid was the main organic acid during the oxidation of THF.4-hydroxybutyrate and y-butyrolactone were not detected by gas chromatography mass spectrometry (GC-MS), implying that the oxidation pathway of THF under the attack of OH was starting via the cleavage of the ring to yield succinic acid. The BOD5/COD was increased from0.12to0.55-0.71after120min of reaction and under60mA/cm2of current density, suggesting that the biodegraditability of wastewater was significantly improved. Additionally, the electrochemical oxidation was applied to treat the high salinity pharmaceutical wastewater containing tetrahydrofuran (THF) generated from the eluting process at11.2mA/cm2. The results showed that electrochemical oxidation could make use of the salinity and acidity to treat this wastewater and the COD was decread from24500mg/L to16639mg/L,32.1%of COD removal was obtained after8h treatment. Therefore, electrochemical oxidation offers the potential and clean alternatives to treat high salinity, high acidity organic wastewater without dilution and nutralization, which has the significant and practical meaning for treatment of industial effulent.3) Thiophene is the typical sulfur-containing heterocyclic compund and is biorefractory due to the inhibition of CYP450oxidation, exhibiting the toxic, carcinogenic and mutagenic effects. LSV analysis showed that thiophene can be easily oxidized on the surface of P-PbO2in Na2SO4medium and phosphate buffer at pH6.0and25mV/s of scan rate, the peak oxidation potentials were1.15and1.24V vs. SCE, respectively. Thiophene was completely eliminated by-OH electrogenerated on the surface of β-PbO2anode after25min of reaction under100mA/cm2of current density and pH6.0. The analysis of HPLC showed that fumaric acid, maleic acid and oxalic acid were the main organic intermediates. Simultaneously, according to the balance of sulfur, thiophene sulfur was completely converted into SO42". Therefore, the pathway was the direct cleavage of thiophene ring to form nontoxic and biodegradable intermediates rather than the addition of OH on thiophene ring to form thiophene sulfoxide, epoxide and sulfone, which are toxic to mammals and microorganisms. Thus, electrochemical generation of OH takes full advantage in a continuous way to break down heteroaromatic ring and to achieve the purpose for detoxification of harmful compounds cleanly and rapidly under mild conditions.4) Ethanethiol is one of the typical malodorous compounds due to its lower threshold value. The typical voltammgrams of LSV demonstrated that ethanethiol could be oxidized on the β-PbO2electrode in Na2SO4, KC1medium and phosphate buffer at very positive potentials, the peak oxidation potentials were1.36,1.25and1.24V vs. SCE, respectively. Results of bulk electrolysis showed that ethanethiol could be thoroughly destructed and eliminated within35min in phosphate buffer (pH6.0) under60mA/cm2of current density due to the successive attack of·OH electrogenerated on the surface of β-PbO2anode, COD was decread from960mg/L to450mg/L. HPLC analysis showed that acetic acid was the main intermediate and can be further mineralized to CO2and H2O, while sulfur atom in ethanethiol was finally converted to nonodorous sulfate ions (SO42-), implying that high deodorization performance could be achieved. According to the sulfur balance, it could be concluded that the scission of ethanethiol was occurred via starting with OH attack and followed by C-S bond cleavage. This study highlights a promising technology for the clean and safe removal of odorous compounds from industrial effluents.In a summary, the rings of heterocyclic compounds containing nitrogen, sulfur and oxygen atoms could be effectively opened by hydroxyl radicals (·OH) electrochemical generated and were converted to nontoxic and biodegradable organic acids. Similarly, the bond of C-S in ethanethiol can also be cleaved by·OH, thus deodorization was achieved. Electrochemical oxidation offers a clean and environmental friendly technology for detoxification of heterocyclic copounds and deodorization of alkanethiol.