Carbon Nanotube Membrane Stack for Flow-through Sequential Regenerative Electro-Fenton Supporting Information Environmental Science & Technology Guandao Gao 1,2*, Qiaoying Zhang 2, Zhenwei Hao 1 and Chad D. Vecitis 2 Key Laboratory of Pollution Processes and Environmental Criteria (Ministry of Education), Tianjin Key Laboratory of Environmental Remediation and Pollution Control, College of Environmental Science and Engineering, Nankai University, Tianjin 300071, China 1 School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138 2 *Corresponding author. Tel/Fax: +0086-22-23501117 E-mail: Gaoguandao@nankai.edu.cn ( Guandao Gao) Address: College of Environmental Science and Engineering, Nankai University, Tianjin 300071, China s1
Chemicals. Hydrochloric acid (HCl; 36.5 38.0%), nitric acid (HNO 3 ; 69.8%), sulfuric acid (H 2 SO 4 ; 95.0 98.0%), phosphoric acid (H 3 PO 4 ; 85.0%), sodium hydroxide (NaOH), ethyl alcohol (EtOH; 95.0%), dimethylsulfoxide (DMSO; 99.9%), potassium hydrogen phthalate (KHP), sodium sulfate (Na 2 SO 4 ), sodium persulfate (Na 2 S 2 O 8 ), sodium bicarbonate (NaHCO 3 ), and sodium carbonate (Na 2 CO 3 ) were purchased from Sigma-Aldrich. All chemicals were reagent grade except the DMSO that was spectrophotometric grade. CNT Calcination. To remove any amorphous or other non-cnt carbon impurities, 1 1 g of as-received MWNTs first was calcinated in a tube furnace by increasing from room temperature to 400 o C at a rate of 5 o C per min and holding for 60 min at 400 o C (Thermolyne, 21100). If multiple CNT treatment steps were used, calcination was always completed first and the sample is given the C- prefix. CNT Acid Treatment. Two types of acid treatment were completed depending on whether the goal was only to remove any residual metal catalyst impurities (conc. HCl) 1 or if the oxidative formation of surface functional groups such as hydroxyl and carboxyl groups was also desired (conc. HNO 3 ). 1, 2 Both acid treatments were completed as follows, 0.5 g of CNT was placed into 0.5 L of acid and heated to 70 o C in a round-bottom flask with stirring and a condenser for at least 12 hours (overnight). After heating, the sample was cooled to room temperature and vacuum filtered through a 5-µm PTFE membrane (Omnipore, Millipore) to collect the CNTs. The CNTs were then washed with MilliQ deionized water (DI) until the filter effluent ph is near DI s ph, at least 500 ml. The sample was then oven dried at 100 o C before use. Materials treated with HCl are labeled with the HCl suffix and materials treated with HNO 3 are labeled with the HNO 3 suffix unless coated with metal oxide nanoparticles, see following section. Electrochemical CNT Filter Preparation. The CNT filters were produced by first dispersing the CNTs in DMSO at 0.5 mg/ml and probe sonicating (Branson, Sonifier S450) for 15 min at an applied power of 400 W/L. Then, 30 ml of the sonicated CNTs in DMSO were vacuum filtered onto a 5-µm PTFE membrane (Millipore, Omnipore, JMWP), resulting in filter loadings of 1.5 to 1.6 mg/cm 2. The CNT filters were washed with 100 ml EtOH, 100 ml 1:1 DI-H 2 O:EtOH, and 250 ml DI-H 2 O to remove DMSO before use. Finally, the prepared filter was loaded into a filtration casing modified for electrochemistry, as described in our previous studies, Figure S1. 3, 4 Briefly, the CNT filter is operated anodically and is electrically connected via a titanium ring and wire to the DC power supply. A perforated stainless steel sheet is operated as the cathode, with an insulating silicone rubber o-ring separating the electrodes. The electrochemically active elements are incorporated into a modified polycarbonate 47-mm filter casing (Whatman). Before every experiment, the titanium ring was polished with sandpaper to optimize the s2
electrical connectivity between the titanium and the CNTs. SEM Analysis. Scanning electron microscopy was completed in Harvard s Center for Nanoscale Systems on a Zeiss FESEM Supra55VP. Micrographs were analyzed with ImageJ software to determine aerial pore size that was an average of at least 100 measurements. XPS Analysis. X-ray photoelectron spectroscopy was completed on an ESCA SSX-100 in Harvard s Center for Nanoscale Systems. For all samples, a survey spectrum (0-1,000 ev), an C-1s (274-294 ev), an O-1s (522-542 ev), and an Fe-2p3 (700-720 ev) scans were completed. For the C-CNT-SS, Sn-3d5 (476-496 ev) and Sb-3d5 (520-540 ev) were also completed. Data was analyzed using CasaXPS. s3
Figure S1. Depiction and Images of the Lab-Scale E-Fenton Apparatus. The appropriate influent solution was aerated by O 2, meanwhile peristaltically pumped (Masterflex) through the CNT filter stacks at the flow rate of 1.6 ml min -1, and CNT filter was assembled in the modified commercial polycarbonate filtration casing contacted with DC power. s4
Figure S2. Electrochemical Filtration Apparatus and Sandwiched Electro-Fenton System Based on Carbon Nanotube Membrane Stacks. A) Design of the modified commercial polycarbonate filtration casing, the perforated titanium plate is pressed into the carbon nanotube filter as anode or cathode, B) the images of the open cell, C) images of the unfolded sandwich membrane stacks including four layers, D) SEM images of the MWNT network in-plane and cross-section, and E) schematic of main roles of every layer in Membrane Stacks. s5
Figure S3. Schematics of Three Kinds of Oxidation Processes and Electrochemical generation of H 2 O 2. A) E-Fenton, B) Fenton, C) Electrochemistry and D) Electrochemical production of H 2 O 2 process. s6
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Figure S4. Properties of Electro-generation of H 2 O 2 with CNT Filter as Cathode. A) Effluent DO (mg/l), B) effluent ph value, C) i-e relation and D) V-E relation. Effluent DO (mg/l) 18 16 14 12 10 8 C-CNT, ph5.99; ph3.25; ph8.73 0.0-0.2-0.4-0.6-0.8-1.0-1.2-1.4 A) Cathode Potential vs Ag/AgCl (V) Effluent ph Value 9 8 7 6 5 4 C-CNT, ph5.99; ph3.25; ph8.73 B) 3 0.0-0.2-0.4-0.6-0.8-1.0-1.2-1.4 Cathode Potential vs Ag/AgCl (V) Current (ma) 140 120 100 80 60 40 20 C) Applied Voltage (V) -15-14 -13-12 -4-3 -2 D) 0 0.0-0.2-0.4-0.6-0.8-1.0-1.2-1.4 Cathode Potential vs Ag/AgCl (V) -1 0.0-0.2-0.4-0.6-0.8-1.0-1.2-1.4 Cathode Potential vs Ag/AgCl (V) s8
Figure S5. Reduction of Fe 3+ at a Series of Cathode Potential @ -0.5 V, -0.8 V, -1.0v vs Ag/Agcl. Charge (C) 20 16 12 8 @ -1.2 V @ -1.0 V 4 @ -0.8 V @ -0.5 V 0 0 200 400 600 800 1000 Time (s) s9
Figure S6. X-ray Photoelectron Spectrum of the Various CNT Samples in Various Binding Energy Ranges: C1s, O1s, and Fe2p for All Samples. A) CNT-COO, B) CNT-COO exchanged Fe 3+ (CNT-COOFe 3+ ), C) CNT-COO-exchanged Fe 3+ then electro-reduced (CNT-COOFe 3+ -R), D) CNT-COOFe 3+ -R then flow H 2 O 2, E) CNT-COOFe 3+ -R flow OXA +H 2 O 2. Full scan C-1s O-1s Fe-2p A B C D E s10
Flow-through Fenton Chemistry Modeling. In model I, only the Fenton reaction (eq. 1) was considered and to exclude the effect of CNT COOH oxidation an experiment was completed in the absence of CNTs where [H 2 O 2 ] in = 14 mg L -1 was flowed (J = 1.6 ml min -1 ) into a beaker containing 0.56 mg Fe 2+ (similar to Fe 2+ content of CNT-Fe 2+ ). In model II, reaction (6) is also included to account for CNT surface carboxylate oxidation that would reduce ability to bind Fe: OH + CNT-COO - Fe 2+ CNT + CO 2 +H 2 O + Fe 2+ (1) The flow-through E-Fenton reactor was modeled as a plug-flow reactor (PFR) with a hydraulic residence time of ~2 s, J = 1.6 ml min -1, and a bed volume of 0.053 ml for CNT network. The [H 2 O 2 ] in = 14 mg L -1, and filter Fe = 0.47 mg Fe 3+ assuming that 1 Fe is sorbed for every 2 surface O i.e., XPS O/C ratio = 0.041 before reaction and all surface O assumed to be carboxylate based for convenience. 5 Simulation of the Flow-through Fenton Reaction. Imagination and experiments in figure 3 and 4 was also simulated and verified by the model PFR (plug flow reactor) in figure 5. In model I, only the reaction (1) (k 1 = 63 M -1 s -1 ) is considered, simulation results indicate that H 2 O 2 will breakthrough at ~ 600 s meaning inactivation of Fe 2+ (figure 5A). the special experiment designed, excluding effect of the COOH reaction with OH on Fenton, was explored over time and displayed in figure 5B, and its breakthrough time is also near 600 s (10 min) and match well the simulation data in figure 5A. In model II, it simulates effect of CNT-COOFe oxidation on the subsequent Fenton reaction. The loss of Fe 2+ is assumed to be equal to the loss of COO- due to reaction (6) and Fe 2+ oxidation to Fe 3+ in reaction (1). Based on the above analysis and definition, the simulation results in figure 5C display that H 2 O 2 will breakthrough at ~ 300 s, and the real experiment in figure 5D inactivated at ~ 300 s (5min) matchs well the corresponding simulation. Generally, the experimental data and the related simulation explain well why CNT-Fe 2+ filter can catalysis well H 2 O 2 and that it will lose its activity due to Fe 2+ oxidation in reaction (1) without the regeneration as well as Fe n+ loss resulted from the oxidation of COOFe on CNT attacked by OH from Fenton in absence of pollutants. s11
Figure S7. Kinetic Model Description and Simulation of Fenton Reaction in figure 3 and 4. A), B) and C), D) are effluent H 2 O 2 ratio in model I and experiment, respectively, as well as in model II and experiment, respectively. This reaction kinetic model is based on PFR (plug flow reactor) to illustrate Fenton reaction on the oxidized CNT filter. In model I, only the reaction of (1) (the second order rate constant is k 1 = 63 M -1 s -1 ) is considered. In model II, besides reaction (1), the reaction of (2) (k 2 = 4.3*10 9 M -1 s -1 ) is included in the system. s12
Table S1. Comparison with the Peer Works on E-Fenton. Cell configuration (batch/flow) EF; GDE(-) / BDD(+); membrane-divided cell; Batch EF; Carbon-felt (-)/ Pt (+) undivided cylindrical cell; batch Sono-EF; Carbon-felt (-)/ Pt (+); undivided cylindrical cell; batch; EF; Iron (-) / iron (+); undivided cell; batch Photo-EF; Iron (-) / iron (+); undivided tank; Batch EF; Carbon fiber or black (-)/(+); cylindrical cell; Flow EF; CNT filter (-) / CNT(+); Membrane stack; Flow cell Fe Source, loss and regeneration 150 mg/l Fe 2+ ; further additions of 100 mg/l of iron every 150 min Operation condition ph 1.5; j=1 ma cm -2 ; 0.5 mm Fe 2+ ph 3.0 I= 200 ma 0.1 mm Fe 3+ ph 3.0 ultrasounds at 20 W Fe 2+ from sacrificial iron anode; Fe 2+ from sacrificial iron anode; Iron (Fe2+/Fe3+) ; impregnated 3-D carbon electrodes CNT-Fe 2+ (0.47 mg/15 mg on CNT); electro-regenerat ed in-situ; ph4.0 j=60 ma cm -2 ph 3.0; I=2.5 A; 4W UV with 1.4 Wm 2 at 253.7 nm ph 3.0; -1.3 V J=5.2 ma cm -2 Neutral ph; -0.5 V/ 2.2V; j=0.52 ma cm -2 EC and CE Pollutant Ref. n/a n/a n/a 530 kwh/kg COD; 1753 kwh/kg TOC n/a CE (H2O2): 0-70% 45.8 kwh/kg TOC; CE (H2O2): 10-45% Flame retardant Olive oil mill Herbicides 2,4-D, DNOC, azo dye AB Alcohol distillery Landfill leachate, n/a Oxalate 6 7 8 9 10 11 This paper s13
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