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Configuration and operation of magnetic BESs
Single-chamber microbial fuel cells (MFCs) (cylindri- cal chamber, 28 mL) were constructed, as previously described [33]. The anode was stainless steel mesh (SSM, type SUS304) coated with Fe3O4@N-mC (5 mg/cm2). The cathode rolled the activated carbon-PTFE air cathode [34]. The effective area of each cathode and anode was 7 cm2, respectively, which were connected by titanium wires through an external resistance of 1000 Ω. The wires on the electromagnetic launcher were intertwined on the external walls of the anode chamber in order to obtain a pulse electromagnetic field (symmetric periodic square wave) with the frequency of 100 Hz. A magnetic inten- sity of 5 uT was applied on the reactors as they were 5 cm away from electric magnetic field analyzer (EHP-200A, Narda, Inc., Italy). Activated sludge from a wastewater treatment plant (Harbin, China) was used to inoculate the magnetic MFCs (MMFCs). MMFCs were operated in both the presence (PEMF-MMFCs) and absence of PEMFs (PEMF-OFF-MMFCs). The cultured solution contained (per liter) 2 g sodium acetate, 50 mM phos- phate buffer solution (PBS) (11.55 g/L Na2HPO4·12H2O, 2.77 g/L NaH2PO4·2H2O, 0.31 g/L NH4Cl and 0.13 g/L KCl), 10 mL mineral solution, and 10 mL vitamin solu- tion [35]. For each test, there was a duplicate MMFC in operation in the fed-batch mode at a constant tempera- ture (25 ± 2 °C).
For the pure culture test, single-chamber microbial electrolysis cells (MECs) were constructed using a 250- mL anaerobic bottle with a liquid volume of 100 mL. The anode was carbon paper and the cathode rolled activated carbon-PTFE. The effective area of each cathode and anode was 5.4 cm2 (1.8 cm × 3 cm), respectively. A fixed voltage of 0.6 V was applied to MECs by a programma- ble power source (3645A, Array, Inc.). All reactors were inoculated with Geobacter sulfurreducens PCA (ATCC 51573) purchased from American Type Culture Collec- tion (ATCC). MECs were filled with the medium contain- ing (per liter, pH 6.8): 0.82 g sodium acetate, 0.1 g KCl, 1.5 g NH4Cl, 0.6 g NaH2PO4, 2 g NaHCO3, 10 mL min- eral solution, and 10 mL vitamin solution. Prior to use, the medium was flushed with N2-CO2 (80:20). All MECs were operated at a constant temperature (35 ± 2 °C). The pulse electromagnetic fields were applied on MECs by a sequential “ON” and “OFF” process to analyze the instan- taneous effect on the current generation by MECs. The current in the circuit was conducted by measuring the voltage across a high-precision resistor (10 Ω).
Analytical and electrochemical techniques
MMFC voltage over the external resistance was auto- matically recorded every 5 min by a multi-channel data acquisition system (Model 2700 with 7702 module,
Keithley Instruments Inc., USA). Both linear sweep vol- tammetry (LSV) and electrochemical impedance spec- troscopy (EIS) were measured on the two-electrode mode. This analyzed the entire cell with the anode as the working electrode and the cathode as the reference electrode by using Autolab Potentiostat/Galvanostat (Autolab PGSTAT 128 N, MetrohmAutolab Inc., Nether- lands). Polarization and power density curves were plot- ted based on linear sweep voltammetry (LSV), which was conducted from open-circuit voltage (OCV) to 0 V with a scan rate of 0.1 mV/s. Electrochemical impedance spec- troscopy (EIS) was conducted with a frequency range of 100 kHz–10 mHz at the OCV of MFCs. The Nyquist plots were analyzed using NOVA software. The imped- ance spectra were analyzed by fitting to the equivalent circuit (EC): R (Q [RW]). The EC contained Rs, Rp, W, and CPE four parts. Rs represents ohmic resistance; Rp represents activation resistance; constant phase element (CPE) represents the electrical double-layer capacitor; and Warburg (W) represents diffusion resistance [36].
X-ray diffraction (XRD) characterization was obtained on a Bruker AXS D8-Advanced diffractometer with CuKα radiation (λ = 1.5418 Å). Raman experiments were per- formed on a LabRAM XploRA Raman microscope using a 532 nm excitation line from an argon-ion laser with a power of 0.15 mW. The magnetization curve was carried out on Quantum Design MPMS-7 SQUID magnetom- eter at 300 K under varying magnetic fields. The samples were analyzed by Fourier transform infrared spectroscopy (FTIR) on Perkin Elmer 100 spectrometer from 400 to 4000 cm−1 and X-ray photoelectron spectroscopy (XPS; PH1-5700 ESCA system, US) using a hemispherical ana- lyzer and an aluminum anode (monochromatic Al Ka, 1486.6 eV) as the source (at 12–14 kV and 10–20 mA). The microstructures of samples were observed by scan- ning electron microscope (SEM; Helios Nanolab600i, FEI, USA) and transmission electron microscope (TEM; JEM- 2100 electron microscope, JEOL, Japan) [37].
Illumina sequencing of 16S rRNA gene amplicons
After 2 months of operation, the anodes of MMFCs were cut and fragmented using sterile scissors [38]. Genomic DNA of the anode biofilms were extracted using the PowerSoil DNA Isolation Kit (Mo Bio Laboratories, Inc., Carlsbad, CA) according to the manufacturer’s instruc- tions. DNA from two pieces of an electrode were iso- lated and mixed equally for PCR amplification after DNA concentration was determined using Qubit fluorometer (Thermo Fisher Scientific). V4–V5 regions of bacterial and archaeal 16S rRNA genes were amplified using universal primers: 515F (5′-GTGCCAGCMGCCGCGGTAA-3′) and 907R (5′-CCGTCAATTCCTTTGAGTTT-3′); U519F (5′-CAGYMGCCRCGGKAAHACC-3′) and 806R
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