2.2. Analytical Methods
In order to identify the impact of in situ formation of AgNPs on the TFC–FO membrane properties, the pristine and modified TFC–FO membranes were characterized in terms of morphological observation, surface hydrophilicity, surface roughness, surface charge, anti-microbial properties and intrinsic separation characteristics. Specifically, a scanning electron microscope (SEM, S-4800, Hitachi, Minato-Ku, Japan) and an energy dispersive X-ray spectrometer (EDX, Falcon, EDAX Inc., Philadelphia, PA, USA) were applied for observing the membrane surface and identifying the AgNPs on the modified membrane surface, respectively. The surface hydrophilicity and the surface roughness were determined by a surface contact angle analyzer (OCA-15EC, Dataphysics, Stuttgart, Germany) and by an atomic force microscope (AFM, Bruker MuLtimode 8, Karlsruhe, Germany), respectively. In addition, the surface charge was analyzed by a SurPASS solid surface zeta potential analyzer (Anton Paar Co., Ltd., Glaz, Austria). The specific procedures for the SEM, EDX, contact angle analyzer, AFM and zeta potential analyzer can be found in previous literature [27
]. The transport properties of the TFC–FO membranes were determined using a bench-scale FO system (8.5 cm × 3.9 cm) with DI water as the feed solution and 1 M NaCl as the draw solution [29
]. The pure water permeability flux and the salt permeability coefficient were measured according to the method described in previous literature [28
To quantify the AgNPs loaded onto the TFC–FO membrane surface, the modified membrane samples (1 cm × 1 cm) were firstly digested by 7% nitric acid at 105 °C for 2 h to promote dissolution of the silver from the membrane [32
], and then the filtrate obtained by a 0.45 μm filter membrane was used for Ag analyses by an inductively coupled plasma mass spectrometer (ICP-MS, 720ES, Agilent, Santa Clara, CA, USA) [16
]. In order to examine the residual silver loading on the membrane after the dissolution experiment, 4 cm2
coupons of in situ AgNP modified membranes were placed in 10 mL of 5 mM NaHCO3
solution (pH value of 8.3). After five days dissolution, the dissolved silver concentration in the solution was quantified with ICP-MS.
The antimicrobial property of the modified TFC–FO membrane was assessed using E. coli
as the model bacteria [33
]. An overnight-cultured bacteria in Luria Bertani (LB) medium was diluted to 108
colony-forming units (CFU)/mL before using. A bacterial viability assay was carried out according to previous literature [36
]. Specifically, the pristine and modified FO membranes were contacted with 1 mL bacteria solution, and then the membranes were gently rinsed and the remaining liquid was removed after 24 h incubation. A confocal laser scanning microscope (CLSM, LSM 710, ZEISS, Jena, Germany) was used for observing the distributions of dead and living cells on the pristine and AgNP modified membrane samples. In addition, the cytometry method was used to quantify the number of live bacteria attached to the FO membrane samples.
Water flux through the FO membrane was measured by the volume change of the draw solution versus time [8
], and the reverse salt flux was calculated based on the conductivity change of the anolyte [37
]. Silver ion concentrations of the anolyte were monitored at the end of experiment using ICP-MS (720ES, Agilent) after digesting by 7% nitric acid at 105 °C for 2 h, and the total organic carbon (TOC) was determined using a TOC analyzer (Shimadzu TOC-Vcsh, Kyoto, Japan). The measurements and calculations of TOC, ammonia nitrogen (NH4+
–N), total nitrogen (TN), and total phosphorus (TP) concentrations in the FO permeate and their rejections by the FO membrane have been shown in our previous study [7
]. In addition, the removal efficiencies of contaminants by the combination of microorganisms and FO membrane and only by microorganisms were also referred to in the previous study [7
The foulants on the FO membrane surface were collected by ultrasonic (25 °C, 500 W, 20 kHz) for 30 min, and then their mixed liquor suspended solids (MLSS) and mixed liquor volatile suspended solids (MLVSS) concentrations were determined according to Chinese NEPA standard methods [38
]. An EDX (Falcon, EDAX Inc., Philadelphia, PA, USA) and an SEM (S-4800, Hitachi, Minato-Ku, Japan) were applied for analyzing the element compositions and capturing the surface images of the fouled FO membranes, respectively. A CLSM (LSM 710, ZEISS, Jena, Germany) was used for observing the distributions of biofoulants, including microorganisms, proteins and polysaccharides on the fouled FO membrane samples. The specific methods of SEM, EDX and CLSM analyses have been reported in previous literature [17
The OsMFCs voltages were recorded every 5 min by a data acquisition system (RBH8221, Ruibohua Co., Beijing, China). The polarization curve and internal resistance were obtained by using a series of different external resistances [40
]. The volumetric densities of power and current were calculated based on the anode liquid volume [41
All measurements were conducted at least three times, and the data were given with the mean value and the standard deviation.
2.3. Set-up and Operating Conditions
In order to compare the performance between the pristine and modified TFC-FO membranes, two identical laboratory-scale OsMFC set-ups (denoted as control OsMFC and AgNP-OsMFC, respectively) were operated in parallel. The schematic diagram of the set-up is shown in Figure S1
. It consisted of an anode chamber and a cathode chamber (each with an effective volume of 144 mL), and the FO membrane was located between the two chambers. A carbon brush and a carbon cloth coated with Pt (0.3 mg/cm2
) were pretreated as anode and cathode electrodes, respectively [7
]. The pristine (supplied by Hydration Technologies Innovations, Albany, GA, USA) and the AgNP modified (made in house) TFC–FO membranes with an effective membrane area of 48 cm2
and an orientation of active layer facing the feed solution (AL-FS) were applied in the control OsMFC and the AgNP-OsMFC, respectively. The operation of both set-ups was stopped when their voltage was lower than 50 mV and then the solution in anode and cathode chambers would be replaced with fresh wastewater and 0.5 NaCl solution, respectively [7
Both OsMFCs were operated under closed circuit conditions with 500 Ω external resistance at room temperature of 30 ± 0.5 °C during the whole experiment. The synthetic domestic wastewater was fed into the anode chamber, and its concentrations of TOC, NH4+
–N, TN, and TP were 118.4 ± 4.0, 28.7 ± 0.5, 34.9 ± 1.5, and 2.91 ± 0.11 mg/L, respectively. The composition of the synthetic wastewater could be found in Table S1 (Supplementary Material)
. The 0.5 M NaCl solution was used as the draw solution. The anode and cathode chambers were circulated with a buffer tank at a cross-flow velocity of 0.03 cm/s. The seeded sludge in the anode chambers was collected from a local domestic wastewater treatment plant (Taihu Xincheng Wastewater Treatment Plant, Wuxi, China). The initial MLSS and MLVSS of the sludge in both OsMFCs were 3.2 and 2.4 g/L, respectively.