【作者】 马香娟；

【导师】 吴祖成；

【作者基本信息】 浙江大学 ， 环境工程， 2013， 博士

【摘要】 含氮、含硫和含氧杂环化合物会通过抑制细胞色素P450酶的氧化反应而对微生物产生毒性,致使生化法难以处理该类化合物废水。然而,杂环化合物中的杂环一旦破坏,则可避免其对微生物的毒性,达到脱毒的目的。使用一般的氧化剂如C12或直接使用H202,不仅很难破环,甚至还会生成一些毒性更强的中间产物。羟基自由基(·OH)是仅次于氟的强氧化剂,具有寿命短、绿色和安全等优点。若能利用·OH攻击杂环化合物使其破环,生成易被微生物降解的有机酸,则杂环化合物的毒性就降低了。Fenton氧化和光催化氧化需通过添加催化剂产生·OH,易导致二次污染的问题。不用添加化学药剂且可原位连续地产生-OH的电化学技术将是比较理想的选择。本文选择吡啶、四氢呋喃和噻吩作为含氮、含氧、含硫杂环化合物的代表性污染物,同时,选择乙硫醇为烷基硫醇类的代表性污染物,以比较含硫杂环化合物与烷基硫醇类化合物之间的氧化行为与降解机理差异。以Pt为对电极,饱和甘汞电极(SCE)为参比电极,利用线性扫描伏安法(LSV)在电化学工作站(CHI660C)上考察了上述物质在氟树脂改性的β-PbO2工作电极(10mm×6mm)上的电化学氧化行为和主要影响因素。在此基础上,以β-PbO2电极为阳极,不锈钢网为阴极,采用电化学氧化吡啶、四氢呋喃、噻吩和乙硫醇水溶液,通过分析和检测污染物浓度变化和中间产物的生成动态,推测出不同杂原子化合物的氧化机理以及杂原子的最终形态,探讨了不同杂原子化合物的电化学降解共性规律,为电化学用于含杂原子化合物的脱毒或脱臭提供了理论依据。主要结果如下：1)线性扫描伏安(LSV)结果表明,吡啶易于在β-PbO2电极表面氧化,在pH3.5,扫描速率为25mV/s的条件下,吡啶在1.06V vs. SCE位置出现氧化峰,且吡啶在P-PbO2电极表面的氧化是一个受表面控制且不可逆的反应过程。在pH3.5, Na2SO4用量为5g/L,电流密度为160mA/cm2的条件下,反应90min后,100mg/L的吡啶可完全去除。高效液相色谱(HPLC)分析表明,反丁烯二酸、草酸和甲酸是氧化过程中生成的有机酸。离子色谱(IC)分析结果显示,吡啶环上的氮杂原子转化为亚硝酸根(NO2-)和硝酸根(NO3-)。据此推测吡啶的电化学降解机理为：在羟基自由基(·OH)的作用下,吡啶环首先在C-N键处,直接发生开环反应,生成反丁烯二酸、草酸和甲酸。吡啶杂环破环后,就避免了吡啶对微生物的毒性和抑制,废水更易于生化处理,同时也去除了吡啶的恶臭味。通过平衡反应过程中吡啶的去除率与NO2-、NO3-的生成量之间的关系发现,反应开始的60min内,去除掉的吡啶氮杂原子全部转化为NO2-和NO3-,进一步证实了在电化学持续产生的·OH作用下,可直接将吡啶开环生成低毒、易于生化的有机酸和无机离子。2)四氢呋喃是一种广泛应用于医药和化工产品生产的环状脂肪族醚,也是细胞色素P450酶的抑制剂。LSV结果表明,四氢呋喃在NaCl和Na2SO4体系中均易于在β-PbO2电极上发生氧化反应,对应的氧化峰电位分别为1.30V和1.21Vvs. SCE,氧化过程主要受表面扩散控制。在pH为3.0,电流密度为11.2mA/cm2, NaCl用量为10g/L的条件下,反应300min后,初始浓度为205mg/L的四氢呋喃废水,其化学需氧量(COD)去除率可达97%。与Na2SO4体系相比,由于电极表面产生的氯气的间接氧化作用,NaCl电解质能增强电化学氧化四氢呋喃的效果,降解过程遵循拟一级动力学规律。HPLC分析表明,丁二酸是电化学氧化四氢呋喃过程中生成的唯一有机酸,气质联用仪(GC-MS)未检测到2-羟基四氢呋喃和γ-内丁酯,表明·OH能将四氢呋喃中的氧杂环打开,生成易生化降解的丁二酸。在电流密度为60mA/cm2条件下,电化学氧化四氢呋喃120min后,废水的BOD5/COD比由0.12增加到0.55-0.71。将电化学技术直接用于处理含四氢呋喃的实际医药废水,结果表明,电化学可直接利用废水的酸度和NaCl为电解质来处理该废水,不需进行稀释和中和,在电流密度为11.2mA/cm2的条件下,反应8h后,废水的COD从24500mg/L降到16639mg/L,COD去除率达到32.1%。可见,电化学氧化为高盐、高酸度医药废水处理提供了一种清洁的前处理技术,具有非常重要的现实意义。3)噻吩是典型的含硫杂原子化合物,会通过抑制细胞色素P450酶的氧化而对微生物产生毒性、致癌性和致突变性。在pH6.0,扫描速率为25mV/s的条件下,LSV结果表明,噻吩在Na2SO4和磷酸盐缓冲溶液体系中,均能在β-PbO2电极表面发生氧化反应,对应的氧化峰电位分别为1.15V和1.24V vs. SCE,且噻吩在β-PbO2电极表面氧化是一个受表面控制且不可逆的反应过程。在电流密度为100mA/cm2和pH6.0的条件下,电化学氧化500mg/L的噻吩溶液,反应25min后,噻吩的去除率达到近100%,且噻吩中的硫全部转化为硫酸根(SO42-). HPLC检测分析发现,反丁烯二酸、顺丁烯二酸和草酸为反应过程中生成的主要有机酸,未检测到对哺乳动物和微生物产生毒性的噻吩加成物,如噻吩亚砜、噻吩环氧化物和砜等。表明在β-PbO2电极表面产生的·OH直接攻击下,噻吩中的C-S键发生直接断裂,生成了无毒且易生化降解的有机酸。4)乙硫醇是嗅阈值最低的典型恶臭污染物。LSV结果表明,乙硫醇在Na2SO4、KCl和磷酸盐缓冲溶液体系中均能在β-PbO2电极上发生电化学氧化反应,对应的氧化峰电位分别为1.36、1.25和1.59V vs. SCE.以β-PbO2为阳极,不锈钢网为阴极,在电流密度为60mA/cm2和含1007mg/L乙硫醇的磷酸盐缓冲溶液(pH6.0)中,反应35min后,乙硫醇的去除率近100%,COD从960mg/L下降到450mg/L.通过HPLC分析发现,乙酸为乙硫醇分子中C-S健断裂后生成的主要中间产物。乙硫醇中的硫原子转化为无臭味的SO42-。通过硫平衡计算,乙硫醇中的硫全部转化为SO42-。可见,电化学对烷基硫醇类污染物同样具有较好的脱臭效果。综上所述,电化学产生的羟基自由基(·OH)能有效地使含氮、含氧和含硫杂环化合物破环,并使杂环化合物转化为易于生化降解的有机酸,较好地实现了脱毒的目的。同样地,·OH还能使乙硫醇的C-S键断裂,起到了较好的脱臭效果。因此,电化学为杂环化合物的脱毒和烷基硫醇类的脱臭提供了一种清洁和环境友好的技术。更多还原

【Abstract】 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.更多还原