In this review we discuss recent developments in the emerging research field of miniaturized desalination. Traditionally desalination is performed to convert salt water into potable water and research is focused on improving performance of large-scale desalination plants. Microfluidic desalination offers several new opportunities in comparison to macro-scale desalination, such as providing a platform to increase fundamental knowledge of ion transport on the nano- and microfluidic scale and new microfluidic sample preparation methods. This approach has also lead to the development of new desalination techniques, based on micro/nanofluidic ion-transport phenomena, which are potential candidates for up-scaling to (portable) drinking water devices. This review assesses microfluidic desalination techniques on their applications and is meant to contribute to further implementation of microfluidic desalination techniques in the lab-on-chip community.

Desalination as a sample preparation step is essential for noise reduction and reproducibility of mass spectrometry measurements. A specific example is the analysis of proteins for medical research and clinical applications. Salts and buffers that are present in samples need to be removed before analysis to improve the signal-to-noise ratio. Capacitive deionization is an electrostatic desalination (CDI) technique which uses two porous electrodes facing each other to remove ions from a solution. Upon the application of a potential of 0.5 V ions migrate to the electrodes and are stored in the electrical double layer. In this article we demonstrate CDI on a chip, and desalinate a solution by the removal of 23% of Na(+) and Cl(-) ions, while the concentration of a larger molecule (FITC-dextran) remains unchanged. For the first time impedance spectroscopy is introduced to monitor the salt concentration in situ in real-time in between the two desalination electrodes.

Novel energy-efficient desalination techniques, such as capacitive deionization (CDI), are a key element for the future of the fresh water supply, which is increasingly under stress due to the ever-growing world population and ongoing climate changes. CDI is a desalination technique where salt ions are removed from a flow channel by the application of an electrical potential difference across this channel and are stored in electrical double layers. The aim of this work is to visualize and explain the charging process of CDI using a new microfluidic approach. Namely, we implement the geometry of CDI on a chip and visualize the ion distributions in the channel using fluorescence microscopy. In contrast to normal CDI, our system was operated in the absence of flow, using non-porous electrodes. By using two pH-sensitive fluorescence dyes, we found the formation of pH waves across the channel, even though the system is operated at low potential differences in order to suppress Faradaic reactions, such as water splitting. From simulations of the transport process, we found that a small current density in the order of 0.1 A m −2 can trigger the formation of such pH waves. CDI generally benefits from large electrode areas relative to the channel cross section. However, this large area ratio will also increase the magnitude of these waves, which might lead to a reduction in desalination efficiency.