Article pubs.acs.org/est
Aquatic Biofouling Prevention by Electrically Charged Nanocomposite Polymer Thin Film Membranes Charles-François de Lannoy,∥,† David Jassby,§,†,* Katie Gloe,§ Alexander D. Gordon,⊥ and Mark R. Wiesner∥ ∥
Department of Civil and Environmental Engineering, Duke University, Durham, North Carolina 27708, United States Department of Chemistry and §Department of Chemical and Environmental Engineering, University of California − Riverside, Riverside, California 92521, United States
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S Supporting Information *
ABSTRACT: Electrically conductive polymer-nanocomposite (ECPNC) tight nanofiltration (NF) thin film membranes were demonstrated to have biofilm-preventing capabilities under extreme bacteria and organic material loadings. A simple route to the creation and application of these polyamide-carbon nanotube thin films is also reported. These thin films were characterized with SEM and TEM as well as FTIR to demonstrate that the carbon nanotubes are embedded within the polyamide and form ester bonds with trimesoyl chloride, one of the monomers of polyamide. These polymer nanocomposite thin film materials boast high electrical conductivity (∼400 S/m), good NaCl rejection (>95%), and high water permeability. To demonstrate these membranes’ biofouling capabilities, we designed a cross-flow water filtration vessel with insulated electrical leads connecting the ECPNC membranes to an arbitrary waveform generator. In all experiments, conducted in highly bacterially contaminated LB media, flux tests were run until fluxes decreased by 45 ± 3% over initial flux. Biofilm-induced, nonreversible flux decline was observed in all control experiments and a cross-flow rinse with the feed solution failed to induce flux recovery. In contrast, flux decrease for the ECPNC membranes with an electric potential applied to their surface was only caused by deposition of bacteria rather than bacterial attachment, and flux was fully recoverable following a short rinse with the feed solution and no added cleaning agents. The prevention of biofilm formation on the ECPNC membranes was a long-term effect, did not decrease with use, and was highly reproducible. costs.5,6 Polyamide (PA) thin films are currently the state-ofthe-art material for desalination and wastewater reclamation.7 A solution to biofouling on PA thin films could revolutionize the entire RO process train by reducing costs associated with pretreatment, energy for high recoveries,8 and capital; by reducing the plant foot print through minimized pretreatment; and by increasing membrane lifetime through reduced chemical use and biofouling suppression. Finally, eliminating biofouling on these polymeric thin films would ease the environmental impact of RO by lessening carbon emissions and minimizing chemical waste.9 The development of novel strategies to overcome biofouling on membranes is a long-active area in membrane research.7 It has been suggested that small, applied electrical potentials on electrically conductive surfaces can prevent the growth and proliferation of biofilms.10,11 It has been further hypothesized that applying a positive bias creates an oxidizing environment
1. INTRODUCTION Biofouling of polymer surfaces is a ubiquitous and costly problem. Biofouling occurs when bacteria grow on surfaces known as biofilms, which secrete a thick sticky layer composed of extra-polymeric substances (EPS).1,2 Biofouling afflicts fields as widespread as medical equipment (prosthetics, IV tubing, and dialysis membranes), commercial and private boating (hulls and propellers), and industrial and municipal liquid treatment (orange juice concentration, protein separation, desalination, and wastewater treatment). The applications most beleaguered by biofilms are membrane-based desalination and wastewater treatment processes. These processes are challenged with conditions that are ideal for the rapid proliferation of bacterial colonies and the establishment of robust biofilms on the membrane surface. The confluence of high pressures concentrating an ideal nutrient balance onto rough organic surfaces contaminated with extremely high bacterial populations represents ideal biofouling conditions.2,3 Biofouling in membrane separation processes necessitates extensive pretreatment that incurs additional costs.4 Pretreatment includes chemical treatment (e.g., flocculation) and ultrafiltration (UF), which limit process efficiency and increase © XXXX American Chemical Society
Received: November 5, 2012 Revised: February 11, 2013 Accepted: February 15, 2013
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dx.doi.org/10.1021/es3045168 | Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Environmental Science & Technology
Article
Figure 1. (Left) A schematic of the pressurized vessel showing the path of fluid flow. (Right) The cross-flow cell with insulated electrodes that connect the membrane to the voltage source.
for the bacteria, which increases bacterial surface mobility and prevents bacteria from attaching. A negative bias creates a repulsive electrostatic force between the similarly charged bacteria and the surface. Applying an alternating electrical potential to a surface efficiently prevents bacteria from forming a biofilm.12 Thus, an electrically conducting membrane has the potential of solving the biofouling challenge. The application of this technique to RO and nanofiltration (NF) membrane systems is hindered by the inherent insulating nature of polymers used as membranes. Over the years, attempts have been made to use electrically conducting polymers to form water treatment membranes.13−16 However, traditional conducting polymers, such as polypyrrole, are notoriously difficult to process, and membranes made from these materials suffer from low selectivity, low flux, and often, low conductivity.13,17,14−16 Recently, it has been reported that electrically conducting carbon nanotubes (CNTs), deposited on a support layer in a dead-end configuration, are capable of bacterial inactivation once an electric bias is applied.18,19 However, RO and NF membrane processes rely on a cross-flow, where feedwater flows in parallel to the membrane surface. Nanomaterials that are simply deposited on a membrane surface will quickly wash away in the flow. To the best of our knowledge, there is no report of any electrically conductive RO or NF membranes, let alone the application of these membranes to the successful prevention of biofilm formation. We report here on the prevention of long-term biofilm growth achieved using a novel, highly electrically conductive polymer nanocomposite (ECPNC) tight NF membrane. This is made possible with the development of new polymer nanocomposite membranes 20 and a modification of the traditional cross-flow system that, taken together, eliminates the need for aquatic bacterial disinfection. We have created a novel modification of PA thin film composites through crosslinking with carboxylated multiwalled carbon nanotubes that imbues these thin films with electrical conductivity 20 orders of
magnitude greater than the base polymer while maintaining the requisite flux and salt rejection characteristics of traditional PA membranes. In long-term filtration of bacterially contaminated waters, these membranes were able to prevent the growth of biofilms when an electrical potential was applied across their surface.
2. MATERIAL AND METHODS 2.1. Electrode Modified Cross-Flow Cell. A polycarbonate, custom built, cross-flow filtration unit was used to measure salt rejection and membrane permeability, and was employed for all biofouling experiments including all control experiments. The filtration unit was designed with built-in insulated steel electrodes capable of delivering an electric charge to the membrane thin film surface (effective membrane surface area was 21.6 cm2) and to a counter unreactive platinum electrode located at the top of the channel, 5 mm above the membrane surface, as shown in Figure 1. During operation, a pressurized feed stream was passed over the membrane. This feed stream was separated into a permeate and a retentate stream. The former was collected to measure flux and rejection, while the latter was recycled to the feed stream. 2.2. Membrane Fabrication. ECPNC membranes were developed, characterized and then tested in bacterially contaminated waters to demonstrate their biofilm prevention capabilities. These ECPNC tight NF membranes were fabricated from carboxylated multiwalled carbon nanotubes (CNTs) reacted with polyamide to form a highly salt-rejecting thin film. CNTs (0.1 mg/mL) (cheaptubes.com, Brattleboro, VT;
