Cellulose Derived Graphenic Fibers for Capacitive Desalination of Brackish Water Nalenthiran Pugazhenthiran, † Soujit Sen Gupta, † Anupama Prabhath, † Muthu Manikandan, † Jakka Ravindran Swathy, † V. Kalyan Raman, ‡ and Thalappil Pradeep* ,† † DST Unit of Nanoscience (DST UNS) and Thematic Unit of Excellence (TUE), Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India ‡ Centre of Excellence (Biotechnology) & Water and Wastewater Technology, Thermax Limited, Pune 411019, India * S Supporting Information ABSTRACT: We describe a simple and inexpensive cellulose-derived and layer-by-layer stacked carbon fiber network electrode for capacitive deionization (CDI) of brackish water. The microstructure and chemical composition were characterized using spectroscopic and microscopic techniques; electrochemical/electrical performance was evaluated by cyclic voltammetry and 4-probe electrical conductivity and surface area by Brunauer-Emmett-Teller analysis, respectively. The desalination performance was investigated using a laboratory batch model CDI unit, under fixed applied voltage and varying salt concentrations. Electro-adsorption of NaCl on the graphite reinforced-cellulose (GrC) electrode reached equilibrium quickly (within 90 min) and the adsorbed salts were released swiftly (in 40 min) back into the solution, during reversal of applied potential. X-ray photoelectron spectroscopic studies clearly illustrate that sodium and chloride ions were physisorbed on the negative and positive electrodes, respectively during electro-adsorption. This GrC electrode showed an electro-adsorption capacity of 13.1 mg/ g of the electrode at a cell potential of 1.2 V, with excellent recyclability and complete regeneration. The electrode has a high tendency for removal of specific anions, such as fluoride, nitrate, chloride, and sulfate from water in the following order: Cl - > NO 3 - >F - > SO 4 2- . GrC electrodes also showed resistance to biofouling with negligible biofilm formation even after 5 days of incubation in Pseudomonas putida bacterial culture. Our unique cost-effective methodology of layer-by-layer stacking of carbon nanofibers and concurrent reinforcement using graphite provides uniform conductivity throughout the electrode with fast electro-adsorption, rapid desorption, and extended reuse, making the electrode affordable for capacitive desalination of brackish water. KEYWORDS: capacitive deionization, graphene, nanofiber, adsorption, water purification ■ INTRODUCTION Shortage of clean water is the most exigent problem faced by several communities in the developing world. Access to safe and potable water is threatened by population growth, climate change, and contamination of existing fresh water sources. 1-3 The need for clean water for domestic, agricultural, and industrial processes has resulted in intense search for alternate sources of water supply, such as brackish groundwater and seawater. 1,3 Reverse osmosis (RO), ultrafiltration (UF), and distillation processes are the most widely used treatment technologies for water today. 4 However, excessive energy requirements and the need for skilled personnel to maintain such facilities are limiting their large scale deployment in resource-limited settings. 5 Capacitive deionization (CDI) is increasingly being consid- ered as a viable solution for water desalination that is more energy efficient than the above-mentioned processes. This technology fundamentally involves adsorption of oppositely charged ions from the electrical double layer region over an electrode upon application of a potential leading to desalination. Subsequent desorption of the adsorbed ions when the potential is reversed leads to regeneration of the electrodes. 5-9 Although CDI is associated with high theoretical efficiency, cost effectiveness, and point-of-use (POU) utility, its practical applications for desalination are yet to be realized fully. 6-10 The existing mainstream CDI materials with their inherent limitations in stability and resistance to biofouling confine such electrodes for larger scale operations. Various methods are used to solve these issues and commercial products are available; although capital costs are higher than RO. An ideal CDI material should exhibit the following characteristics: high specific surface area, high conductivity, fast adsorption/desorption rates, electrochemical stability, resist- ance to biofilm formation, and easy processability. 6,11,12 The salt removal efficiency of various forms of carbon used as an electrode for CDI is generally reported in terms of salt (NaCl) adsorbed per gram of carbon. Li et al. and Kim et al. reported the capacity to be 0.275 and 3.7 mg/g, respectively for Received: June 21, 2015 Accepted: August 25, 2015 Published: August 25, 2015 Research Article www.acsami.org © 2015 American Chemical Society 20156 DOI: 10.1021/acsami.5b05510 ACS Appl. Mater. Interfaces 2015, 7, 20156-20163