The management of brines generated from seawater desalination has become a central issue in arid and semi-arid regions, particularly in Mediterranean basin countries and the Middle East. The rapid increase in desalination capacity, especially through reverse osmosis, has helped secure access to drinking water but has also generated a growing stream of hypersaline discharges. Global estimates indicate that worldwide brine production now exceeds 140 million m³/day, or more than 50 billion m³/year, with continuous growth driven by industrial desalination development [1].
One of the most widely studied pathways is the transformation of salts contained in brine into materials for thermal storage, particularly in the form of molten salts. Molten salts are already widely used in concentrated solar power (CSP) technologies as heat transfer fluids and thermal energy storage media. Commercial systems mainly use mixtures based on sodium and potassium nitrates, limited to approximately 565°C [2]. To exceed this limit, research is shifting toward molten chlorides and carbonates, which can operate above 700–800°C [3]. In this context, seawater brines appear as an abundant source of chlorides such as NaCl, KCl, and MgCl₂.
However, direct transformation of brine into functional molten salt is not possible without intermediate treatment steps. The main technical challenge is related to hydrated magnesium chloride, which at high temperature undergoes hydrolysis reactions producing corrosive HCl, leading to severe degradation of metallic materials [4].
The first valorization step consists of concentration and partial ion separation. Minimal liquid discharge (MLD) and zero liquid discharge (ZLD) technologies make it possible to achieve water recovery rates above 90–95% while crystallizing the remaining salts [5]. These systems are now considered a reference approach for converting liquid waste streams into recoverable solids.
Once salts are separated, purification and dehydration become essential. This step is particularly critical for MgCl₂, whose dehydration without hydrolysis represents a major industrial and energy-intensive challenge [6].
The resulting anhydrous salts can then be used to formulate eutectic mixtures. NaCl–KCl–MgCl₂ systems are being studied for next-generation CSP applications, with melting temperatures that can be reduced to around 400–450°C depending on composition [3]. However, their high corrosivity still limits large-scale industrial deployment.
These molten salts can be integrated into concentrated solar systems for heat storage. Modern CSP plants use these fluids to store thermal energy and generate electricity in a delayed manner. Their main advantage is their potentially low cost and abundance, but their implementation requires corrosion-resistant materials, particularly nickel-chromium alloys and advanced ceramics.
In parallel, molten carbonates represent another pathway. Systems based on Na₂CO₃ and K₂CO₃ are used in molten carbonate fuel cells and in certain thermal storage devices. Their production from brine requires additional chemical conversion steps [7].
Beyond energy applications, brine valorization is part of a broader strategy of recovering dissolved resources from seawater. Magnesium, present at about 1.3 g/L in seawater, is a strategic element, but its extraction remains energy-intensive [8]. Lithium, although present at very low concentrations (~0.17 mg/L), is also the subject of research, but its recovery is currently not economically competitive [9].
In this context, future desalination systems could evolve toward integrated biorefineries where water, energy, and minerals are co-produced. Hybrid models combining desalination, solar thermal energy, and salt recovery have been proposed to improve overall viability [1].
However, most brine valorization technologies remain at low to intermediate technology readiness levels (TRL 3–6), with limited industrial deployment. The main barriers remain high energy consumption, corrosion issues, and the lack of integrated value chains.
Ultimately, transforming salts contained in brine into molten chlorides, carbonates, or thermal fluids is scientifically feasible but relies on a complex treatment chain including concentration, separation, purification, and materials engineering. The real value of this approach depends primarily on its integration into coherent energy and industrial systems.
References
[1] Jones, E., Qadir, M., van Vliet, M. T. H., Smakhtin, V., & Kang, S.-M. (2019).The state of desalination and brine production: A global outlook.Science of The Total Environment, 657, 1343–1356.https://doi.org/10.1016/j.scitotenv.2018.12.076
[2] International Renewable Energy Agency (IRENA). (2012).Renewable Energy Technologies: Cost Analysis Series – Concentrating Solar Power.IRENA Working Paper, Volume 1, Issue 2.Abu Dhabi, UAE.ISBN: 978-92-9220-003-0
[3] Bradshaw, R. W., & Siegel, N. P. (2008).Molten nitrate salt development for thermal energy storage in parabolic trough solar power systems.Proceedings of ASME 2008 2nd International Conference on Energy Sustainability, Jacksonville, Florida, USA.https://doi.org/10.1115/ES2008-54175
[4] Sohal, M. S., Ebner, M. A., Sabharwall, P., & Sharpe, P. (2010).Engineering Database of Liquid Salt Thermophysical and Thermochemical Properties.Idaho National Laboratory (INL).INL/EXT-10-18297.
[5] Panagopoulos, A., Haralambous, K. J., & Loizidou, M. (2019).Desalination brine disposal methods and treatment technologies – A review.Science of The Total Environment, 693, 133545.https://doi.org/10.1016/j.scitotenv.2019.07.351
[6] Li, X., Yin, H., & Qiu, Y. (2020).Corrosion behaviors of alloys in molten chloride salts for thermal energy storage applications: A review.Solar Energy Materials and Solar Cells, 215, 110651.https://doi.org/10.1016/j.solmat.2020.110651
[7] Olivares, R. I. (2012).The thermal stability of molten nitrate salts for solar thermal energy storage in different atmospheres.Solar Energy, 86(9), 2576–2583.https://doi.org/10.1016/j.solener.2012.06.003
[8] United Nations Environment Programme (UNEP). (2020).Resource Recovery from Water: Principles and Case Studies.United Nations Environment Programme, Nairobi.
[9] Swain, B. (2017).Recovery and recycling of lithium: A review.Separation and Purification Technology, 172, 388–403.https://doi.org/10.1016/j.seppur.2016.08.031
