Project Se(L)ect(i)vity (PTDC/EQU-EQU/6193/2020)

The global transition to electric vehicles and renewable energy systems has heightened the demand for lithium-ion batteries (LIBs), creating an urgent need for sustainable battery recycling methods to recover critical raw materials, including lithium. Lithium-selective cation-exchange polymeric membranes are one of the emerging options to achieve such lithium recycling. To make this change even greener, instead of using traditional fossil-origin polymers to produce membranes, this research employed bacterial cellulose acetate (BCA), a bio-derived and eco-friendly polymer. By adding 5 wt% N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide (PP13–TFSI), an ionic liquid (IL) which is a plasticizer and lithium-ion conductor, and 20 wt% hydrogen manganese oxide (HMO), which is a lithium-selective inorganic filler, four BCA-based membranes (BCA, BCA-IL, BCA-HMO and BCA-IL-HMO) were prepared. The membranes were extensively characterized for their morphology, thermal stability, chemical, and mechanical properties. Subsequently, they were tested in diffusion cells (without applying any external driving force) for ionic conductivity, lithium selectivity, and lithium flux using binary salt mixtures and synthetic LIB leachate. The BCA-IL membrane outperformed other BCA-based membranes in terms of separation factors, achieving values of 10.50 (Li+/Mn2+), 11.75 (Li+/Ni2+), and 10.95 (Li+/Co2+) with a lithium flux of 0.12 mol m−2 h−1 when processing synthetic LIB leachate. Under the same conditions, the BCA-HMO membranes exhibited a higher lithium flux (0.51 mol m−2 h−1) but with lower separation factor values of 3.39 (Li+/Mn2+), 3.62 (Li+/Ni2+), and 3.36 (Li+/Co2+). The use of plant-derived cellulose acetate (CA) as an alternative to BCA was also assessed; however, despite promising ideal lithium selectivity values (for example, 112 for Li+/Ni2+ in the case of CA-HMO membrane), their conductivity was up to two orders of magnitude lower than that of BCA-based membranes. All these findings highlight the promising potential of BCA-based membranes for lithium recovery from lithium-ion battery leachates.

Flow electrode capacitive deionization (FCDI) is a desalination technology employing flowable carbon slurries to remove salt from an influent through the electro-sorption of ions at the surface of pores of activated carbon particles. This study presents an extensive morphological, electrochemical and rheological analysis of flow electrodes prepared using commercial (YP50F, YP80F, Norit, PAC) and lab-synthesized (KUA, PAC-OX) activated carbons. Simultaneous optimization of particle size, surface area, and surface chemistry of activated carbons is essential to enhance desalination efficiency in FCDI applications. The lab-made highly microporous activated carbon (KUA), prepared from a Spanish anthracite, exhibited a remarkably high specific surface area ( 2800 m2/g) but required first a particle size reduction through ball milling (from 56 μm to 12 μm) for the respective flow electrodes to achieve flowability. The slurry of milled fine KUA (designated as KUAF) shows a specific capacitance of 55 F/g, a 38-fold increase compared to its pristine form. The KUA-F flow electrode also achieved a maximum salt adsorption capacity of 185 mg/g, outperforming the leading commercial alternative (YP80F) by 26 %. The FCDI cell with the KUA-F flow electrode exhibited a desalination efficiency of 79 % at 15 wt% loading, surpassing YP80F by 29 %. In contrast, using PAC-OX (oxidized form of PAC), despite increasing oxygen functional groups and with relatively higher specific surface area, led only to a 2 % improvement in desalination performance, highlighting that oxidation alone at larger particle sizes and broader distribution is insufficient.

Mineral extraction from seawater brines has emerged as a viable solution to reduce Europe's reliance on imported Critical Raw Materials (CRM). However, the economic viability of this approach hinges on the local demand for sodium chloride, the primary product of such extraction processes. This study investigates the potential of residual brines, commonly known as "bitterns," generated during solar sea-salt extraction in traditional saltworks, as an alternative source of minerals. The Mediterranean region, encompassing South-European, North-African, Near East coasts, and parts of the Atlantic regions, is particularly conducive to exploring this prospect due to its extensive solar sea salt industry. Saltworks in the region, adopting various operational strategies based on feed quality or local climate conditions, produce different types of bitterns, each holding a latent resource potential that has remained largely unexplored. Within the framework of the EU-funded SEArcularMINE project, it was conducted an extensive analytical campaign to characterize bitterns collected from a diverse saltworks network. The analysis revealed the presence of sodium, potassium, magnesium, chloride, sulfate, and bromide in concentrations ranging from g/kg, while boron, calcium, lithium, rubidium, and strontium were found in the mg/kg range. Additionally, trace elements (TEs) such as cobalt, cesium, gallium, and germanium were detected at concentrations in the order of μg/kg. Detailed results on the composition of bitterns are presented, emphasizing the distinct characteristics observed at different sites. The estimated potential for mineral recovery from these bitterns is approximately 190 €/m3, considering the production capacity of about 9 Mm3 per year in the Mediterranean area. This finding underscores the significant contribution that mineral recovery from bitterns could make in securing access to CRMs for the European Union.

Salinity gradient energy is currently attracting growing attention among the scientific community as a renewable energy source. In particular, Reverse Electrodialysis (RED) is emerging as one of the most promising membrane-based technologies for renewable energy generation by mixing two solutions of different salinity. This work presents a critical review of the most significant achievements in RED, focusing on membrane development, stack design, fluid dynamics, process optimization, fouling and potential applications. Although RED technology is mainly investigated for energy generation from river water/seawater, the opportunities for the use of concentrated brine are considered as well, driven by benefits in terms of higher power density and mitigation of adverse environmental effects related to brine disposal. Interesting extensions of the applicability of RED for sustainable production of water and hydrogen when complemented by reverse osmosis, membrane distillation, bio-electrochemical systems and water electrolysis technologies are also discussed, along with the possibility to use it as an energy storage device. The main hurdles to market implementation, predominantly related to unavailability of high performance, stable and low-cost membrane materials, are outlined. A techno-economic analysis based on the available literature data is also performed and critical research directions to facilitate commercialization of RED are identified.