CNT-based Electrochemical Filter for Water Treatment: Mechanisms and Applications

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CNT-based Electrochemical Filter for Water Treatment: Mechanisms and Applications

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Title: CNT-based Electrochemical Filter for Water Treatment: Mechanisms and Applications
Author: Liu, Han
Citation: Liu, Han. 2015. CNT-based Electrochemical Filter for Water Treatment: Mechanisms and Applications. Doctoral dissertation, Harvard University, Graduate School of Arts & Sciences.
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Abstract: Water treatment is a critical global challenge requires innovated effective solution. Conventional treatment methods includes membrane filtration, adsorption and electrochemical oxidation of organics. This dissertation proposed a new hybrid of the three conventional technologies and investigated the mechanism and application potential for water treatment using carbon nanotube (CNT) based anodic filter. A qualitative reactive transport model consisting of mass transport, adsorption/desorption, electron transfer was developed to understand reaction mechanism and compare the filtration with the batch electrochemical system. The mass transport step is found to be the key advantage (>6-fold) of the electrochemical filtration over the batch system due to convective mass transport through the filter pores. This indicate synergy between the filtration and the electrochemisty. A quantitative model coupling the above three steps was also build to quantitatively simulate the reaction kinetics and probe information about reactive sites. The model simulation was successfully validated by experimental methyl orange oxidation data and was further used to identify kinetics from reactive sites. It was found that two types of reactive sites–the sidewall sites and oxy-defect sits are both reactive for ferrocyanide oxidation with a slightly different overpotential. Also, microscopic flux and steamline plot again verify the convective mass transport enhance reaction kinetics–synergy of the filtration and electrochemistry. To improve potential for organic oxidation applications, the CNT filter was coated with 3.9±1.5 nm bismuth-doped tin oxide (BTO) nanoparticles. In the oxalate oxidation experiments, the current efficiency increased by 1.5–3.5 times and TOC removal increased by 2–8 times after coating. The BTO-CNT anode energy consumption was 25.7 kWh kgCOD-1 at ~93% TOC removal and 8.6 kWh kgCOD-1 at ~50% TOC removal, comparable to state-of-the-art oxalate oxidation processes (22.5–81.7 kWh kgCOD-1). The anode stability also improved by extending the working potential range from 1.4 V of pure CNT to 2.2 V of BTO coated CNT. For inorganic ion removal applications, the CNT filter was coated by a 5.5 ± 2.7 nm layer of TiO2 for arsenic removal. Compared with the conventional adsorption column using granular adsorbents, the TiO2-CNT filter is a highly dispersed nano-system allows forced convective transport through the pores whereas diffusive mass transport dominates and limits kinetics for granular adsorbents. As a result, adsorption kinetics of the TiO2-CNT filter increased by two orders of magnitude due to structure improved site accessibility (20–30 fold), internal convection (4–6 fold) and electrosorption (0.15–2 fold). Groundwater samples containing 44 ppb As were treated by single-pass filtration, and 12500 bed volumes (residence time of 4.5 s; 127 L m-2 h-1; 5.8 mg m-2 h-1). TiO2 filter was successfully regenerated by 5 mM NaOH for both As(III) and As(V).
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