Electrochemical Carbon Nanotube Filter Oxidative Performance as a Function of Surface Chemistry

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.