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随着我国经济的迅速发展,能源消耗持续增加,工业挥发性有机化合物(VOCs)排放加剧了细颗粒物(PM2.5)和臭氧的形成,导致区域大气复合污染问题日趋严峻,给人体健康和生态环境造成了极大危害[1-3]. 我国生态环境部在“十四五”规划中,明确将VOCs纳入环境保护税征收范围. 苯系物作为工业源VOCs中的一类典型污染物,其工业排放源涉及有机化工、石油化工、包装印刷、涂料等多个重点行业,其光化学活性高且毒性强,排放浓度限制低,治理难度大[4-5]. 以低浓度为主要特征的苯系VOCs治理是污染控制领域的重点和难点.
为了满足日益严格的大气污染物排放标准,学术界和产业界开发了吸附法、催化燃烧法、吸收法、生物法、光催化氧化法、过硫酸盐氧化法等多种VOCs控制技术[6-8]. 然而,这些技术存在催化剂失活、成本高、二次污染等问题. 虽然湿法洗涤工艺被认为是一种方便经济的VOCs去除技术,但这种物理方法不能使VOCs矿化,而湿法洗涤工艺结合特定的高级氧化工艺(AOP)常用于低浓度VOCs的去除[9-10]. 耦合工艺利用了AOP技术具有能产生高氧化还原电位的活性物质,可促进VOCs在水相中氧化为H2O、CO2和其他无机物[11]. 近年来,基于真空紫外(VUV)协同过一硫酸盐(PMS)的高级氧化技术因其效率高、反应条件温和、投资成本低等优点而被广泛用于有机污染物的去除[12-13]. 真空紫外灯能发射约占总能量8.0%左右的185 nm高能光子(185 nm波长相当于6.7 eV的光子能量),不仅可以打破大多数物质的化学键,还能与空气中的O2和H2O作用生成O(1D)、O(3P)、·OH等活性物质(式1—2)[14-16]. 其中,O(3P)和O2反应生成O3,O3和H2O在254 nm的光子作用下生H2O2(式3—4). 此外,254 nm的光子可在水存在的情况下分解O3和H2O2,产生超氧自由基(
${\rm{O}}_2^{\cdot -} $ )和羟基过氧自由基(·HO2)活性氧物质(式5—9)[17]. 基于过一硫酸盐或者过二硫酸盐(PS)活化产生硫酸根自由基(${\rm{SO}}_4^{\cdot -} $ )的高级氧化技术因其催化效率高、较宽的pH适应范围、高的矿化程度及对环境更加友好等特点,在环境修复领域受到越来越多的关注[18],PMS与PS活化特性相比,PMS中的O—O键更容易被激活,更有利于水中有机污染物降解. 研究学者发现,采用PMS联合VUV新型高级氧化体系,可以进一步提高污染物去除效率和减少副产物的产生[19-20]. Xie等[11]采用紫外(UV)活化PMS在自制反应装置中去除水中气相乙酸乙酯和甲苯,发现在UV/PMS最优体系下,98.3%的乙酸乙酯和96.5%的甲苯可以被高效去除. Amiri等[21]采用VUV/PMS工艺处理天然有机物(NOM)和细菌消毒,在20 min内将TOC浓度从3.83 mg·L−1降至0.15 mg·L−1,实现完全消毒. 总的来说,VUV/PMS呈现出设计和操作相对简单的优势,在去除水中污染物方面具有很大潜在的应用前景. 然而,如何根据真空紫外及PMS的特性选择合适的氧化剂或催化剂从而提高污染物的降解效率是VUV/PMS体系研究的重点之一.ZSM-5分子筛具有比表面积大、优异的择形催化性和较高的水热稳定性等特点,作为催化剂或载体被广泛应用于VOCs的催化降解[22-23]. Aziz等[24]通过构建Fe-ZSM-5/UV体系去除不同苯系VOCs(苯、甲苯、乙苯和二甲苯),发现Fe-ZSM-5/UV体系具有较好的催化性能和化学稳定性,且可以有效减少有毒副产物的产生. 而通过构建Fe-ZSM-5/VUV/PMS体系消除低浓度苯系VOCs还有待深入研究.
因此,本研究针对单一VUV或PMS技术消除苯系VOCs过程中产物降解不彻底和效率不高的问题,通过构建Fe-ZSM-5系列催化剂,并将其耦合VUV/PMS湿法去除低浓度甲苯污染物. 借助比表面积和孔隙分析仪(BET)、X射线衍射(XRD)、氨程序升温脱附(NH3-TPD)、吡啶红外(Py-FTIR)等表征手段分析了催化剂的物化结构特征. 同时,利用电子顺磁共振(EPR)和液相色谱质谱(LC-MS)检测VUV/PMS湿法体系降解甲苯过程中主要活性物种及产生的中间产物,阐明甲苯降解机理,为VUV/PMS湿法处理工业低浓度苯系VOCs工艺的开发提供新思路.
Fe-ZSM-5协同真空紫外和过硫酸盐催化氧化甲苯的强化工艺与机制
A study on enhanced process and mechanism for toluene remocal by combining Fe-ZSM-5 with vacuum ultraviolet and persulfate
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摘要: 近年来,真空紫外(VUV)协同过硫酸盐的高级氧化工艺已被逐渐开发用于处理低浓度挥发性有机污染物(VOCs). 然而,如何根据VUV及PMS的特性选择合适的催化剂从而提高工业低浓度苯系VOCs的降解和矿化效率成为当前的挑战. 因此,本研究从改性ZSM-5的酸性入手,通过浸渍湿法成功制备了Fe-ZSM-5催化剂. 利用Fe-ZSM-5耦合真空紫外和过硫酸盐的高级氧化体系在湿法条件下降解低浓度甲苯污染物,并对其催化性能和降解机制进行了探讨. 结果表明,VUV+PMS+Fe-ZSM-5体系在240 min内仍能保持接近100.0%的甲苯去除率,矿化率高达67.0%. 采用氨程序升温脱附(NH3-TPD)和吡啶红外(Py-FTIR)等表征对样品酸类型和酸量进行定性和定量分析,结果表明Fe改性减少了ZSM-5催化剂Brønsted酸的量,而显著增加了Lewis酸的量,有利于提高反应的活性和稳定性. 电子顺磁共振谱(EPR)结果表明,在VUV+PMS+Fe-ZSM-5体系下,硫酸根自由基(
${\rm{SO}}_4^{\cdot -} $ )和羟基自由基(·OH)作为主要活性物种参与了甲苯的降解. 液相色谱-质谱(LC-MS)检测到苯甲醇、苯甲醛、苯甲酸、醋酸、丙酸、2-戊酮等多种中间产物,而气相色谱仪(GC)没有检测到出口气体中的气态中间产物,表明该工艺能有效避免空气的二次污染并将中间产物转移到液相进行下一步处理. 研究结果为VUV/PMS湿法处理工业低浓度苯系VOCs工艺的开发提供新思路.Abstract: In recent years, the advanced oxidation process of vacuum ultraviolet (VUV) synergistic peroxymonosulfate has been gradually developed for the treatment of low concentration of volatile organic pollutants (VOCs). However, it is urgent to select an appropriate catalyst for coupling with VUV/PMS system to further enhance the degradation efficiency and mineralization efficiency of industrial low-concentration benzene series of VOCs. Therefore, based on the acidity characteristics of ZSM-5, this study aims to fabricate Fe-ZSM-5 catalyst through wet impregnation method, and evaluates the catalytic activity and degradation mechanism of VUV+PMS+Fe-ZSM-5 system on toluene removal under wet condition. The results showed that nearly 100.0% of toluene was degraded within 240 min in VUV+PMS+Fe-ZSM-5 system, and the mineralization rate was as high as 67.0%. Ammonia temperature programmed desorption (NH3-TPD) and pyridine infrared (FTIR) were utilized to analyze the type and amount of acid species of the catalysts. It can be achieved that Fe modification reduces the amount of Brønsted acid in ZSM-5 catalyst, but increases the amount of Lewis acid, which may be beneficial to improve the activity and stability of the reaction. Electron paramagnetic resonance (EPR) spectrum demonstrated that sulfate radicals ($ {\rm{SO}}_4^{\cdot -}$ ) and hydroxyl radicals (·OH) are the main active species in the VUV+PMS+Fe-ZSM-5 for toluene removal. Several intermediate products including benzyl alcohol, benzaldehyde, benzoic acid, acetic acid, propionic acid, 2-pentanone were detected during the degradation process by liquid chromatography-mass spectrometry (LC-MS), while no gaseous intermediates were found in the outlet by gas chromatographic (GC), indicating that this process can effectively avoid secondary air pollution and transfer intermediates to the liquid phase for further treatment. The study provides a new idea for low-concentration benzene series of VOCs control by catalyst+VUV/PMS wet process.-
Key words:
- vacuum ultraviolet /
- peroxymonosulfate /
- photocatalysis /
- Fe-ZSM-5 /
- toluene
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表 1 300 ℃下ZSM-5和Fe-ZSM-5催化剂的酸度
Table 1. Acidity of ZSM-5 and Fe-ZSM-5 catalysts at 300 ℃
催化剂
CatalystBrønsted位置/cm−1
Brønsted sitesLewis位置/cm−1
Lewis sitesBrønsted/
(μmol·g−1 cat)Lewis/
(μmol·g−1 cat)IB/IL ZSM-5 1545 1448 26.00 7.48 3.48 Fe-ZSM-5 1540 1446 18.55 16.38 1.13 表 2 VUV+PMS+Fe-ZSM-5体系在氧化过程中产生的中间产物
Table 2. Intermediate products produced in the oxidation process of VUV+PMS+Fe-ZSM-5 system
质荷比
m/z化学式
Chemical formula名称
Name结构
Structure60 C2H4O2 醋酸 74 C3H6O2 丙酸 86 C5H10O 2-戊酮 108 C7H8O 苯甲醇 108 C7H8O 2-羟基甲苯 108 C7H8O 4-羟基甲苯 107 C7H6O 苯甲醛 122 C7H6O2 苯甲酸 -
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