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挥发性有机物(volatile organic compounds,VOCs)大多数具有剧毒、致癌和危险性[1],同时也是区域臭氧和二次有机气溶胶形成的主要原因[2],严重影响人类健康和大气环境。我国工业源排放的VOCs占人为排放总量的60%左右[3],其中化工、工业涂装、印刷和家具制造占中国总排放量的50%,十四五规划纲要建议强化大气污染综合治理和联防联控,对环境空气质量提出更高要求,因此,应当对工业源VOCs排放加强综合控制。
印刷业VOCs排放具有风量大、浓度低等特点,主要通过吸附法进行末端处理。工业应用中常见的吸附剂主要有活性炭、分子筛、树脂等[4–6],活性炭因具有比表面积大、孔隙率高以及稳定性好等特点被广泛应用[7]。在吸附法治理VOCs废气中,废气由多组分VOCs组成,吸附净化过程中会产生竞争吸附现象。HUANG等[8]发现饱和蒸气压较低的VOCs会取代饱和蒸气压较高的VOCs。YAO等[9]发现CAR-AC吸附二元VOCs时,乙酸乙酯对丙酮和乙醛产生竞争现象,导致丙酮和乙醛产生“驼峰”,使其在活性炭上的吸附容量降低。有研究表明,VOCs吸附容量与活性炭的活性位点有关[10],目前增加活性位点的方法分为物理改性和化学改性,物理改性主要是在高温下活化条件下增加比表面积和孔体积[11]。化学改性有酸改性、碱改性、盐改性、杂原子改性和掺杂金属氧化物等[12-14]。相较于其他方法,掺杂金属氧化物方法具有环境污染小、对特定的VOCs亲和力高等特点。ZHOU等[15]通过浸渍法将Mg、Zn、Cu和Zr掺杂进活性炭中,发现AC/ZnO对丙酮和甲醇的吸附性能最好;金春江等[16]通过对山桃核基活性炭掺杂Fe(NO3)3制备的AC-3对乙酸乙酯的吸附容量从498.07 mg·g−1提升至973.04 mg·g−1,YANG[17]在生物质炭中掺杂氧化镁以增强对极性VOCs的吸附性能。BAUR等[18]通过向活性炭纤维中掺杂碱性金属氧化物(La2O3、CaO、MgO、ZnO、Al2O3)为乙醛吸附提供了更多的活性位点。目前改性活性炭提高对极性VOCs的吸附研究多集中在单一组分,二元吸附研究较少。
本研究根据彩板印刷排放特征,选取甲苯、乙酸乙酯和异丙醇为特征污染物,采用等体积浸渍法制备出针对极性VOCs吸附容量强的载镁活性炭,并通过实验进行二元吸附性能测试,结合表征和密度泛函理论从微观层面分析竞争吸附机制,为活性炭吸附VOCs的工业应用提供参考。
氧化镁掺杂改性活性炭的制备及其对彩板印刷VOCs废气的吸附性能及机理
Preparation of magnesium oxide doped modified activated carbon and its adsorption properties and mechanism toward color plates printing VOCs
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摘要: 针对实际工业应用中活性炭吸附VOCs存在的竞争吸附导致吸附量减少的问题,以彩板印刷过程产生的甲苯、乙酸乙酯、异丙醇为代表,采用浸渍法对煤基活性炭进行掺杂氧化镁改性,提高活性炭在实际应用中多组分吸附体系的吸附容量。结果表明,氧化镁的最佳掺杂比和焙烧温度分别为1%和450 ℃,相比原始活性炭,AC/MgO-1%在单组分吸附实验中对甲苯、乙酸乙酯、异丙醇的吸附容量分别提高了42%、18%、25%,在二元吸附实验中,弱吸附质在AC/MgO-1%的被取代量由51%、55%、38%下降至24%、44%、33%,二元竞争作用减弱,吸附容量提高34%~80%;经五次吸脱附循环实验后,AC/MgO-1%的吸附容量仍维持在92.68%以上。DFT计算结果表明,甲苯、乙酸乙酯、异丙醇在AC上吸附的过程中,甲苯为强吸附质,在氧化镁上吸附时异丙醇为强吸附质,氧化镁掺杂可在一定程度上减少二元竞争作用;SEM、BET、FTIR及XRD表征结果表明,掺杂氧化镁虽会减少比表面积,但会增加其极性官能团和活性位点,提高吸附容量。Abstract: Aiming at the adsorption capacity reduction of VOCs on activated carbon due to the competitive adsorption in industrial applications, coal-based activated carbon was modified with magnesium oxide doped by impregnation to improve the adsorption capacity for multicomponent adsorption, taking toluene, ethyl acetate, and isopropanol as the representatives of VOCs in color plates printing. The results showed that the optimal doping ratio and calcination temperature of magnesium oxide were 1% and 450 ℃, respectively. Compared with the original activated carbon, the adsorption capacity of modified activated carbon to toluene, ethyl acetate and isopropanol in single-component adsorption experiments increased by 42%, 18% and 25%, respectively, the substitution amount of weak adsorbates in the binary adsorption experiments decreased from 51%, 55% and 38% to 24%, 44% and 33%, respectively, and the adsorption capacity increased by 34%~80%, indicating that the binary competition was weakened. The adsorption capacity of AC/MgO-1% remained above 92.68% after five regeneration cycles. DFT calculations showed that the adsorption process of toluene, ethyl acetate, and isopropanol, toluene was a type of strong adsorbate adsorbed on AC, while on magnesium oxide, isopropanol was a type of strong adsorbate, and magnesium oxide doping could reduce the binary competition effect to a certain extent. SEM, BET, FTIR, and XRD characterization results indicated that doping of magnesium oxide decreased the specific surface area, but it increased the polar functional groups and active sites, and improved the adsorption capacity.
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Key words:
- activated carbon /
- modified /
- adsorption /
- VOCs /
- DFT
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表 1 VOCs的物性参数
Table 1. Physical parameters of VOCs
VOCs 分子式 摩尔质量/(g·mol−1) 密度/(g·cm−3) 沸点/℃ 极性指数 动力学直径/nm 甲苯 C7H8 92.14 0.867 110.6 2.4 0.67 乙酸乙酯 C4H8O2 88.11 0.902 77.06 4.3 0.52 异丙醇 C3H8O 60.06 0.785 82.45 4.3 0.47 表 2 AC和AC/MgO的结构参数
Table 2. Structure parameters of AC and AC/MgO
样品 比表面积/
(m2·g−1)微孔比表面积/
(m2·g−1)介孔比表面积/
(m2·g−1)总孔体积/
(cm3·g−1)微孔体积/
(cm3·g−1)介孔体积/
(cm3·g−1)AC 763 433 94 0.614 0.205 0.310 AC/MgO-0 620 375 53 0.591 0.17 0.336 AC/MgO-0.5% 563 382 59 0.553 0.180 0.331 AC/MgO-1% 693 450 71 0.556 0.211 0.268 AC/MgO-1.5% 607 376 63 0.508 0.176 0.262 AC/MgO-2% 528 363 48 0.519 0.171 0.298 表 3 VOCs在活性炭和氧化镁上的吸附能和吸附键长
Table 3. Adsorption energy and bond length of VOCs on activated carbon and magnesium oxide
吸附分子 活性炭模型
吸附能/
(kJ·mol−1)活性炭模型
吸附键长/
nmMgO模型
吸附能/
(kJ·mol−1)MgO模型
吸附键长/
nm甲苯 −21.98 0.350 −6.21 0.29 乙酸乙酯 −7.37 0.316 −12.81 0.25 异丙醇 −3.09 0.275 −23.70 0.22 -
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