Periodic Reporting for period 4 - E-motion (Electro-motion for the sustainable recovery of high-value nutrients from waste water) Reporting period: 2021-05-01 to 2022-10-31 Summary of the context and overall objectives of the project Current water treatment technologies are mainly aimed to improve the quality of water. High-value nutrients, like nitrate and phosphate ions, often remain present in waste streams. Electro-driven separation processes offer a sustainable way to recover these nutrients. Ion-selective polymer membranes and intercalation materials are strong candidates to achieve ion selectivity.The main aim of E-motion is to modify carbon-based electrodes and ion-exchange membranes with thin films to introduce selectivity in electro-driven separation processes. Ultrathin ion-selective films will be designed, synthesized and characterized. The films will be mostly made by successively adsorbing polycations and polyanions onto the electrodes. Additional selectivity will be introduced by the incorporation of ion-selective receptors. Next to that, intercalation materials will be studied for their use in ion-selective separations. The adsorbed multilayer films as well as intercalation materials will be studied in detail regarding their stability, selectivity and ion transport properties under varying experimental conditions of salinity, pH and applied electrical field, both under adsorption and desorption conditions.The first main challenge related to the multilayers to optimize and to understand its architecture in terms of 1) stability towards an electrical field, 2) ability to facilitate ion transport. Also the influence of ion charge and ion size on the transport dynamics will be addressed. The focus of E-motion is set on phosphate ions, which is rather complex due to their large size, pH-dependent speciation and on the development of phosphate-selective materials. It is also aimed to develop a theory of water desalination with intercalation materials and to study such materials within the context of ion separation.E-motion represents a major step forward in the selective recovery of ions (including nutrients like phosphate) from water in a cost-effective, chemical-free way at high removal efficiency. The proposed electrode materials, surface modification strategies and the increased understanding of ion transport and ionic interactions in membrane media offer also applications in the areas of batteries, fuel cells and solar fuel devices. Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far Initially, carbon-based electrodes were combined with surface modifications to address (and tune) ion selectivity. In most cases ion-exchange membranes were added to a (membrane) capacitive deionization (CDI) system (MCDI). Polyelectrolyte (PE) multilayers (PEMs) on a standard grade cation-exchange membrane (CXM) were investigated in MCDI for the selective separation of Na+ and Mg2+. The Na+/Mg2+ selectivity was found to change from 0.5 to 2.8 upon the addition of a PEM and be stable during a 40-cycle experiment. Also, different pairs of (functionalized) PEs and different operational CDI conditions have been investigated to improve and/or impart selectivity.Furthermore, CDI electrodes made from carbon and conductive polymers showed an increased anion removal capacity (up to 65 mg Cl/g_anode) as well as an almost full selectivity towards chloride removal in binary anion solutions, with either monovalent phosphate or sulphate. Phosphate selectivity in CDI via such a rejection mode was further studied with various anion-exchange membranes (AXMs).In addition, intercalating CDI electrodes have been explored to study the selectivity to cations. Experimentally, the focus was on transition metal hexacyanoferrates. The salt removal capacity of nickel hexacyanoferrate electrodes was found to be ~35 mg/g (at 20 mM NaCl) and the PBA electrodes were found to consume 50% less energy than carbon electrodes for a similar degree of desalination. High inherent selectivities were obtained: Na+/Ca2+ ≈15 and Na+/Mg2+ ≈25. Interestingly, the preference of divalent ion was observed when using vanadium instead of nickel (Ca2+/Na+ ≈3.5). This opposite behavior was understood by density functional theory simulations. Furthermore, coating of the vanadium-based electrodes with a conducting polymer prevents contamination of the treated water following the electrode degradation.Furthermore, the use of multi-layered, polyelectrolyte coatings was also combined with intercalation materials. This enabled the simultaneous selectively of cations (by the electrodes) and anions (by the coated membranes).Crown-ethers (CE), organic compounds known to interact with cations selectively, were chemically bond to various polyelectrolytes from fossil- and bio-based origin. The resulting functionalized polymers were used to build multi-layered coatings. The build-up process, their thickness and viscoelasticity, as well as their interactions with various cations were studied with quartz crystal microbalance with dissipation monitoring (QCM-D). When changing from Cs+ to K+ and Na+ solutions, the QCM-D responses were found to be larger for the CE-containing coatings, indicating that these building blocks can facilitate the further development of ion separation applications.Finally, together with a strong, international team of CDI experts a review was written on recent advances in ion selectivity with capacitive deionization. Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far) E-motion was set to develop new materials and approaches to increase ion selectivity in e-driven separation processes. This ranges from monovalent/bivalent separations to the separation of equally charged ions and the separation (and recovery) of phosphate.A series of new (carbon-based and intercalation) electrode materials and polymer-coated electrodes/membranes are available and have been investigated in detail, not only in terms of their surface modification schemes, but also in terms of their further application in ion selectivity and ion recovery. While this is largely based on e-driven processes like capacitive deionization (CDI), it is anticipated that some these materials are also interesting in adsorption techniques that do not involve an electrical field. Furthermore, a series of newly functionalized (fossil- and bio-based) polyelectrolytes is available, including their characterization in terms of the molecular structure, characteristics when used in surface modifications and their (reversible) interactions with ions as studied with a quartz crystal microbalance (QCM).Phosphate adsorption (and selectivity) studies have been performed using a hybrid material made from activated carbon cloth impregnated with ferrous hydroxide. Also, phosphate rejection studies using ion-exchange membranes have been performed successfully.The stability of the electrode and membrane coatings was also addressed, and the related studies include information on the energy efficiencies of the ion separations. Ion separation properties of systems that involved new materials have been compared with standard carbon-based work.In terms of analytical tools and methods, the following results have been obtained: availability of a three-chamber CDI cell that enables one to monitor (and control) the electrode potential during the electrosorption and desorption process, including the acquisition of on-line pH and conductivity data. Use of ion-exchange membranes can completely isolate the outer chambers, which is the suitable design to study ion selectivity using (modified) membranes. Furthermore, different solutions can be pumped inside of the side chamber allowing to boost the capacitance of the electrodes, such as highly concentrated salt solutions. Project logo
