School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, United States.
Environ Sci Technol. 2011 Nov 15;45(22):9726-34. doi: 10.1021/es202271z. Epub 2011 Oct 24.
An electrochemical carbon nanotube filter has been reported to be effective for the removal and electrooxidation of aqueous chemicals and microorganisms. Here, we investigate how carbon nanotube (CNT) chemical surface treatments including calcination to remove amorphous carbon, acid treatment to remove internal residual metal oxide, formation of surficial oxy-functional groups, and addition of Sb-doped SnO(2) particles affect the electrooxidative filter performance. The various CNT samples are characterized by scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS) and electrochemically evaluated by cyclic voltammetry, open circuit potential versus time analysis, and electrochemical impedance spectroscopy. Voltammetry results indicate that the near CNT surface pH is at least two units lower than the bulk pH. The electrooxidative performance of the various CNT samples is evaluated with 1 mM of methyl orange (MO) in 100 mM sodium sulfate at a flow rate of 1.5 mL min(-1). At both 2 and 3 V, the efficacy of electrochemical filtration is observed to be function of CNT surface chemistry. The samples with the greatest electrooxidation were the calcinated then HCl-treated CNTs, i.e., the CNTs with the most surficial sp(2)-bonded carbon, and the Sb-SnO(2)-coated CNTs, i.e., the CNTs with the most electrocatalytic surface area. At 3 V applied voltage, these CNT samples are able to oxidize 95% of the influent MO within the liquid residence time of <1.2 s. The broader applicability of electrochemical filtration is evaluated by challenging the C-CNT-HCl and C-CNT-HNO(3) networks with various organics including methylene blue, phenol, methanol, and formaldehyde. At 3 V applied voltage, both CNTs are able to degrade a fraction of all the organics with the extent organic degradation dependent on both CNT and organic properties. The C-CNT-HCl network generally had the better oxidative performance than the C-CNT-HNO(3) network with an exception being the positively charged methylene blue. The extent of MO degradation, steady-state current, anode potential, effluent pH, and back pressure are also measured as a function of applied voltage (1-3 V) and CNT surface chemistry. Mass spectrometry of electrochemical CNT filter effluent at 2 and 3 V is utilized to evaluate plausible electrooxidation products. Energy consumption as compared to state-of-the-art electrodes and strategies to tailor the CNT surface for a specific target molecule are discussed.
电化学碳纳米管过滤器已被报道可有效去除和电氧化水溶液中的化学物质和微生物。在这里，我们研究了碳纳米管（CNT）化学表面处理，包括煅烧以去除无定形碳、酸处理以去除内部残留的金属氧化物、形成表面氧官能团以及添加 Sb 掺杂的 SnO(2)颗粒，如何影响电氧化过滤性能。通过扫描电子显微镜（SEM）、热重分析（TGA）和 X 射线光电子能谱（XPS）对各种 CNT 样品进行了表征，并通过循环伏安法、开路电位随时间分析和电化学阻抗谱进行了电化学评估。伏安法结果表明，近 CNT 表面的 pH 值至少比本体 pH 值低两个单位。在 1.5 mL min(-1)的流速下，使用 1 mM 甲基橙（MO）在 100 mM 硫酸钠中评估了各种 CNT 样品的电氧化性能。在 2 和 3 V 下，观察到电化学过滤的功效与 CNT 表面化学性质有关。电氧化效果最大的是煅烧然后用 HCl 处理的 CNT，即表面 sp(2)键合碳最多的 CNT，以及 Sb-SnO(2)涂层的 CNT，即具有最大电催化表面积的 CNT。在 3 V 施加电压下，这些 CNT 样品能够在<1.2 s 的液体停留时间内氧化 95%的流入 MO。通过用各种有机物（包括亚甲蓝、苯酚、甲醇和甲醛）挑战 C-CNT-HCl 和 C-CNT-HNO(3)网络，评估了电化学过滤的更广泛适用性。在 3 V 施加电压下，两种 CNT 都能够降解一部分所有有机物，有机物降解程度取决于 CNT 和有机物的性质。C-CNT-HCl 网络通常比 C-CNT-HNO(3)网络具有更好的氧化性能，除了带正电荷的亚甲蓝。MO 降解程度、稳态电流、阳极电位、流出物 pH 值和背压也作为施加电压（1-3 V）和 CNT 表面化学的函数进行测量。在 2 和 3 V 下使用质谱法分析电化学 CNT 过滤器的流出物，以评估可能的电氧化产物。还讨论了与最先进的电极相比的能耗以及针对特定目标分子定制 CNT 表面的策略。
