Article Cite This: Environ. Sci. Technol. 2019, 53, 7504−7512
pubs.acs.org/est
Silver Nanowire-Modified Filter with Controllable Silver Ion Release for Point-of-Use Disinfection Wensi Chen,† Jinyue Jiang,†,‡ Wenlong Zhang,† Ting Wang,† Jianfeng Zhou,† Ching-Hua Huang,† and Xing Xie*,† †
School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States School of Environment, Tsinghua University, Beijing 100084, People’s Republic of China
‡
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S Supporting Information *
ABSTRACT: Waterborne diseases related to unsafe water are still major threats to public health in some developing countries and rural areas. Providing affordable and safe drinking water globally remains a great challenge in the coming decades. In this study, we develop a high-throughput and conductive silver nanowire (AgNW)-modified composite filter via depositing thin and ultralong AgNWs on a macroporous substrate. An electrochemical filtration cell (EFC) equipped with the composite filter achieves controllable Ag+ release at a μg L−1 level and superior bacterial inactivation performance (>6log inactivation efficiency) with an operation voltage of only 1 V at a high flux of 100 m3 h−1 m−2. Under such operation conditions, each composite filter (effective area: 0.79 cm2) can treat at least 750 mL of the bacterial suspension (∼107 CFU mL−1 of Escherichia coli) with a low effluent Ag+ concentration below 50 μg L−1 and almost negligible energy consumption of only ∼70 J m−3.
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INTRODUCTION In 2015, about 29% of the global population (2.1 billion people) still did not have convenient access to safe and affordable drinking water.1 Waterborne diseases due to consumption of pathogen-contaminated water, such as cholera, salmonellosis, giardiasis, and cryptosporidiosis, remain a great threat to human health and result in ∼2000 deaths every day.2−5 Inactivating the pathogens in water by an effective water disinfection process is critical to the protection of public health.6 However, most of the currently available disinfection methods (e.g., chlorination, ozone oxidation, and ultraviolet irradiation) are not suitable in resource-limited settings (e.g., rural areas, remote islands, and some developing countries) because of high capital cost, intensive energy consumption, the need of chemical supplies, and/or requirement of trained operators.7−9 In many developing and developed communities/countries, even though centralized drinking water treatment and distribution systems as improved drinking water sources have been established in urban regions, they are not universally or consistently free of microbial contamination.10,11 Thus, there is an urgent need for a low-cost, effective, reliable, and easily applicable point-of-use water disinfection method. Promising alternative disinfection technologies have been enabled by functional nanomaterials, e.g., carbon nanotubes, titanium dioxide nanoparticles, and silver nanoparticles (AgNPs), due to their unique physical, chemical, and biological properties, e.g., conductivity, photocatalytic activity, and biotoxicity.12−16 For example, Liu and co-workers developed vertically aligned MoS2 nanofilms to harvest the whole spectrum of visible light and achieve fast and efficient © 2019 American Chemical Society
water disinfection via generation of reactive oxygen species (ROS).17 Plazas-Tuttle and co-workers synthesized nanohybrids of carbon nanotubes and erbium oxide to produce ROS with microwave irradiation and successfully inactivate pathogens such as Pseudomonas aeruginosa.18 Huo and coworkers fabricated polydopamine-protected Cu3P nanowire electrodes for electroporation-disinfection in water, which enable high efficient bacterial inactivation at a flux of 2 m3 h−1 m−2 for 15 days.19 Among these nanomaterials, AgNPs are particularly attractive for their large specific surface area, high interfacial reactivity, low-toxicity to human cells, and broadspectrum antimicrobial activity without generating carcinogenic disinfection byproducts.20,21 For water disinfection, researchers have embedded AgNPs into organic or inorganic filters made of various materials such as paper, synthetic polymer, ceramic, and alumina.22−29However, such practical applications include problems as follows: (i) some silver surfaces connected to the substrate or covered by binding materials are inaccessible for pathogen inactivation,30 (ii) loosely bound AgNPs may easily detach from filters and result in not only the loss of filter integrity but also an excess of silver in the treated water,31,32 and (iii) the Ag+ release is uncontrolled and highly affected by the properties (e.g., pH and ion species) of the water being treated.33 Received: Revised: Accepted: Published: 7504
March 19, 2019 May 30, 2019 June 11, 2019 June 11, 2019 DOI: 10.1021/acs.est.9b01678 Environ. Sci. Technol. 2019, 53, 7504−7512
Article
Environmental Science & Technology
of reaction conditions (details are described in Supporting Information).38 The utilization of high molecular weight poly(vinylpyrrolidone) (PVP) and adequate Br− ions helped to passivate the Ag(100) surface and restrain the NWs from lateral growth.49,50 The precursor injection rate, magnetic stirring rate, and reaction time were optimized to produce high-quality and high yield AgNWs with an average diameter below 35 nm and lengths up to 70 μm (optimization process described in Supporting Information). Composite Filter Fabrication and Electrochemical Filtration Cell (EFC) Device Construction. The mixed cellulose ester (MCE) substrates with an average pore size of 8 μm (MilliporeSigma, Burlington, MA) were selected due to their high porosity (84%), hydrophilic surface, and high permeate flux (as high as 372 m3 h−1 m−2). The as-synthesized AgNW suspension (5 to 30 mL) was diluted into 100 mL by deionized (DI) water and filtered through an MCE substrate by vacuum filtration (Cole-Parmer, Vernon Hills, IL). During the rapid vacuum filtration, the well-dispersed AgNWs in the AgNW suspension self-assemble to a multilayer interconnected network bound to the porous structure. After drying in air, the PVP-wrapped AgNW-modified filter was obtained. Subsequently, sodium borohydride (NaBH4), a strong inorganic reducing agent, was applied to remove the PVP ligands wrapped around the AgNWs and to expose the Ag surfaces. The as-prepared filter was immersed in a 1.0 M NaBH4 water solution for 1 min, soaked in water for 15 min, and dried in a N2 flow to make the surface-cleaned AgNW-modified filter. The composite filter was cut into a square sheet (2.54 cm × 2.54 cm) by a paper punch and then fitted between two titanium mesh (Alfa Aesar, Tewksbury, MA) in a self-made plexiglass electrode holder to construct an EFC device (effective area: 0.79 cm2). Material Characterization. Imaging and analysis of the AgNWs and the AgNW-modified composite filters were performed with a scanning electron microscope (SEM, Hitachi SU8230, Tokyo, Japan) at 5 kV housed at the Institute for Electronic and Nanotechnology Material Characterization Facility at Georgia Tech. The specimens were coated with gold for 30 s (20 mA) using a sputter coater (Quorum Q150T ES, Lewes, United Kingdom) prior to imaging. The sheet resistance of the composite filters was measured by a fourpoint surface resistivity meter (EDTM, Toledo, OH). Cyclic voltammetry was performed in three-electrode mode with an Ag/AgCl reference electrode (Thermo Scientific, Waltham, MA) and a titanium mesh counter electrode using a potentiostat (Bio-Logic, Knoxville, TN). Silver loading on the composite filter was quantified using an inductively coupled plasma atomic optical emission spectrometer (ICPOES) by PerkinElmer (Waltham, MA) after acid digestion (the method detection limit is 0.6 μg L−1 according to the instrument manual). In brief, a piece of 1 cm2 composite filter was reacted with 2 mL of 70% HNO3 (Sigma-Aldrich, St. Louis, MO) under 40 °C for 30 min in a water bath (VWR, Radnor, PA) until the filter was completely disintegrated. Then, 8 mL of water was added to the reaction mixture, and the insoluble substance was separated by centrifugation at 4000 rpm for 15 min. An aliquot of 100 μL of the supernatant was diluted 100 times with DI water and analyzed by the ICPOES. All silver samples were prepared in 2% HNO3 matrix for the ICP-OES analysis. The standards were prepared by dilution and acidification of a silver standard solution (1 mg L−1) purchased from Sigma-Aldrich (St. Louis, MO).
Silver nanowires (AgNWs), or one-dimensional silver nanostructures, have very different physicochemical properties compared with AgNPs.34−38 Films made of interconnected AgNWs are the most promising next-generation transparent electrodes replacing indium tin oxide films because of their outstanding electrical, optical, and mechanical properties.39−41 Recently, researchers have opened more possibilities for AgNWs in biomedical products such as functional textiles, surface coated medical devices, and drug delivery systems.42−45 Since AgNWs have excellent conductivity and remarkable antimicrobial performance, a few trials have introduced AgNWs into water sterilization and successfully demonstrated the effectiveness of AgNWs in bacterial inactivation.46−48 Nevertheless, their applications are still limited by low efficiency, high energy consumption, long treatment time, and/or filter clogging problems. For example, Wen and coworkers fabricated AgNW-modified nanofiber membranes for electrochemical disinfection and achieved over a 5 log inactivation efficiency for both Escherichia coli (E. coli) and Staphylococcus aureus at a flow rate of 1 mL min−1 under a voltage of 3 V.48 However, since AgNWs were embedded in the nanofibers, lots of AgNWs were not available for bacterial inactivation. In addition, the AgNWs as the Ag source for the Ag+ release could also be replaced by AgNPs, and the small filter pore size (6 logs) with only 1 V of applied voltage at a high flux of 100 m3 h−1 m−2. The energy consumption for the treatment of 1 m3 of the contaminated water was reduced to only ∼70 J. Although the bactericidal performance of the EFC remains to be investigated in field applications, the reported results show that the EFC can potentially provide an alternative for point-of-use drinking water treatment.
and therefore may contribute to complete electrochemical oxidation of AgNWs.63−65 Impacts of Water Properties on EFC Operations. Previous studies on AgNPs have reported that water chemistry (e.g., ion strength, inorganic ligands, and organic matters) strongly affects the dissolution kinetics and bacterial toxicity of AgNPs.66−68 For example, chloride ion (Cl−), which is universally present in natural water, has been found to cause surface passivation and AgNP aggregation.69,70 To test the impacts of water properties on the AgNW-modified composite filters, river water dosed with E. coli (∼107 CFU mL−1) was treated with the EFC. The general properties of the water sample are listed in Table S2, and the disinfection performance is shown in Figure 6. When the water flux is 100 m3 h−1 m−2,
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.9b01678.
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Experimental methods and optimization results for the AgNW synthesis, more optical and SEM images of the substrate and composite filters, E. coli passing ratios, CV curve, EFC device configuration, electrochemical inhibition of the Ag+ release, electric voltages required during the EFC operations, residual Ag content analysis, energy consumption calculation, and general information on the water samples (PDF)
AUTHOR INFORMATION
Corresponding Author
Figure 6. Bacterial inactivation performance of the EFC for E. coli in river water at a water flux of 100 m3 h−1 m−2. Dashed black lines indicate that no live bacteria are detected in agar plates, and dashed blue lines indicate that Ag concentrations are below the method detection limit.
*E-mail:
[email protected]. ORCID
Wensi Chen: 0000-0002-6094-2935 Ting Wang: 0000-0002-4658-7789 Jianfeng Zhou: 0000-0002-4167-9887 Ching-Hua Huang: 0000-0002-3786-094X
the bacterial inactivation efficiency is only 0.3 log without electrochemical assistance and the released Ag+ concentration in the treated water is below the detection limit (
