Abstract
Membrane capacitive deionization (MCDI) for water desalination is an innovative technique that could help to solve the global water scarcity problem. However, the development of the MCDI field is hindered by the limited choice of ion-exchange membranes. Desalination by MCDI removes the salt (solute) from the water (solvent); this can drastically reduce energy consumption compared to traditional desalination practices such as distillation. Herein, we outline the fabrication and characterization of quaternized anion-exchange membranes (AEMs) based on polymer blends of polyethylenimine (PEI) and polybenzimidazole (PBI) that provides an efficient membrane for MCDI. Flat sheet polymer membranes were prepared by solution casting, heat treatment, and phase inversion, followed by modification to impart anion-exchange character. Scanning electron microscopy (SEM), atomic force microscopy (AFM), nuclear magnetic resonance (NMR), and Fourier-transform infrared (FTIR) spectroscopy were used to characterize the morphology and chemical composition of the membranes. The as-prepared membranes displayed high ion-exchange capacity (IEC), hydrophilicity, permselectivity and low area resistance. Due to the addition of PEI, the high density of quaternary ammonium groups increased the IEC and permselectivity of the membranes, while reducing the area resistance relative to pristine PBI AEMs. Our PEI/PBI membranes were successfully employed in asymmetric MCDI for brackish water desalination and exhibited an increase in both salt adsorption capacity (>3×) and charge efficiency (>2×) relative to membrane-free CDI. The use of quaternized polymer blend membranes could help to achieve greater realization of industrial scale MCDI.
Highlights
Capacitive deionization (CDI) has recently emerged as an environmentally friendly and energy efficient route to desalination of brackish water supplies.[4]
The surface morphologies of the polymer blend membranes were characterized by scanning electron microscopy (SEM) and Atomic force microscopy (AFM) imaging
Solution casting of membranes and immediate annealing at 60 °C for 24 h resulted in fixed PEI chains confined in the aromatic PBI
Summary
Capacitive deionization (CDI) has recently emerged as an environmentally friendly and energy efficient route to desalination of brackish water supplies.[4]. AEMs for MCDI were fabricated by polymerization of 4-(chloromethyl)styrene (CMS) monomer in a porous polyethylene substrate These AEMs achieved a high salt adsorption capacity (16.1 mg g−1) when utilized in MCDI combined with a commercial (CMX) cation-exchange membrane.[20] Cross-linked quaternized poly(vinyl alcohol). IEC.[23] PEI has been employed as a membrane filler in poly(ether sulfone) (PES) for the removal of organic dyes in water treatment by nanofiltration,[24] as well as being used as an additive to carbon nanotube (CNT) electrodes in CDI This results in improved performance over conventional CDI and that of commercial anion- and cation-exchange membranes.[25]. The weights of dry membrane pieces were recorded before and after immersion and the percentage weight loss in the electrolyte were calculated by a highly quaternized polymer blend based on PBI/PEI and its application as an AEM in MCDI for water desalination. We quantified the MCDI performance in terms of salt adsorption capacity and charge efficiency and compared our results with conventional CDI (without membranes) and relevant MCDI studies
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