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

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.