Waste Salt: Transforming an Environmental Burden into the Energy Transition’s Missing Link

Once considered an intractable byproduct of vital industrial processes, waste salt is rapidly emerging as a versatile and indispensable component in the global shift towards sustainable energy. Engineers worldwide are pioneering innovative methods to transform this environmental challenge into a suite of solutions, ranging from critical mineral extraction to advanced energy storage, positioning waste salt as a functional resource for a cleaner, more resilient energy future.

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The Unseen Challenge: The Growing Problem of Waste Salt

Industrial activities such as desalination, chemical manufacturing, mining, oil extraction, and food processing generate billions of tonnes of waste salt annually, often in the form of highly concentrated brines or solid mixed-salt cakes. Disposing of these high-salinity byproducts poses significant environmental threats, causing soil salinisation and destroying delicate aquatic ecosystems when released into oceans or freshwater systems.

The increasing demand for fresh water through seawater reverse osmosis (SWRO) desalination only exacerbates this issue. Globally, desalination plants produce over 140 million cubic metres of hyper-saline brine every single day.

Furthermore, the rise of Zero Liquid Discharge (ZLD) regulations in countries like China, India, and across Europe has forced industrial plants to evaporate wastewater completely, leaving behind millions of tonnes of mixed solid waste salt. Historically, this material has been sent to specialised landfills, representing a significant recurring liability and waste of resources.

Characterising Waste Salt Streams

Waste salt is rarely pure sodium chloride (NaCl). Depending on its industrial origin, it contains a complex mixture of salts, heavy metals, and organic residues. Understanding the composition of these streams is critical to unlocking their value.

To process these complex streams and integrate them into the circular economy, chemical engineers utilise advanced separation processes. The diagram below illustrates how raw industrial waste salt is systematically purified and redirected into high-value energy applications.

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Turning Brine into Bounty: Critical Mineral Extraction

The accelerating demand for critical minerals—essential for electric vehicles, renewable energy infrastructure, and electronic devices—has driven researchers to explore unconventional sources. Waste brines are proving to be a surprising treasure trove.

From Desalination Waste to Battery Gold

Brines from desalination plants, natural salt lakes, and geothermal energy waste streams contain valuable minerals such as lithium, magnesium, manganese, neodymium, and dysprosium. These elements are vital for manufacturing batteries, permanent magnets in wind turbines, and other green technologies. Extracting these minerals from concentrated waste streams can be more economical and sustainable than traditional terrestrial mining, potentially reducing recovery costs by over 50 per cent.

For lithium recovery, engineers are increasingly moving away from slow, weather-dependent solar evaporation ponds and towards Direct Lithium Extraction (DLE). DLE technologies utilise highly selective adsorption and ion-exchange materials to capture lithium ions ($L{i}^{+}$) from the brine, rejecting the background waste salt.

Innovations in Brine Processing

New technologies are revolutionising critical mineral extraction from brines. Researchers at the Pacific Northwest National Laboratory, for instance, have developed a saline solution-based method to selectively recover valuable elements like manganese, magnesium, neodymium, and dysprosium from e-waste and brines. These methods exploit the unique properties of metals to form solids at different rates, significantly speeding up separation processes from a typical 30 hours to just four hours in some cases.

Membrane-based separation processes are also being developed to efficiently process various minerals, including lithium and rubidium salts, from waste brine. These offer faster and more sustainable alternatives to methods like solar evaporation ponds, which contribute to water loss in sensitive ecosystems like the Great Salt Lake.

Additionally, mechanical vapour recompression (MVR) and fractional crystallisation are being paired to isolate pure sodium sulphate ($N{a}_{2}S{O}_4$) and sodium chloride (NaCl) from mixed industrial waste salts. Once separated, these purified salts bypass landfills and enter secondary markets.

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Salt's Role in Energy Storage: Thermal Batteries and Hydrogen Hubs

Effective energy storage is paramount for integrating intermittent renewable energy sources like solar and wind into the grid. Salt, in various forms, offers compelling solutions for both heat and gas storage.

Thermal Energy Storage (TES): Banking Heat with Salt

Waste salt has garnered significant interest as a low-cost, efficient medium for thermal energy storage (TES), effectively functioning as a "thermal battery". Companies like Waste Salt Technologies LLC are actively working on commercialising products that use repurposed waste salts to store thermal energy.

These systems can capture heat from various intermittent sources, such as solar thermal systems or curtailed grid power, and release it as process heat (with round-trip thermal efficiencies around 90 per cent) or convert it into electricity (around 40 per cent efficient). This technology addresses the mismatch between energy generation and demand, providing a stable and continuous heat source for industrial processes and contributing to a 100 per cent renewable power grid.

Molten salt storage systems, in particular, boast high heat storage density, low cost, and long lifespans. Traditional systems use solar-grade "solar salt" (a eutectic mixture of 60 per cent sodium nitrate, $NaN{O}_3$, and 40 per cent potassium nitrate, $KN{O}_3$). However, modern research is focused on utilising cheaper chloride-based waste salts (such as NaCl-$CaC{l}_2$-$MgC{l}_2$ blends). These molten waste chloride salts can withstand extreme temperatures exceeding 800 °C, unlocking higher thermodynamic efficiencies in next-generation concentrated solar power (CSP) and nuclear reactors.

Underground Caverns for Hydrogen and CO₂ Storage

Beyond thermal applications, extensive underground salt deposits offer immense potential for large-scale energy and emissions management. Research highlights that these geological formations can serve as secure holding tanks for hydrogen gas and sites for permanent carbon dioxide ($C{O}_2$) storage.

Salt caverns are created through solution mining, a process that pumps water underground to dissolve salt deposits, creating a large cavity and a corresponding brine stream. Because salt is naturally impermeable and self-healing under geological pressure, these caverns are ideal for storing highly diffusive gases like hydrogen. This capability is critical for expanding hydrogen as a clean energy carrier, as hydrogen requires significantly more storage space than natural gas for comparable energy output. Co-locating hydrogen production with $C{O}_2$ storage in surrounding porous rock formations, particularly in regions with abundant salt domes like the Texas Gulf Coast or the UK's East Yorkshire region, presents a synergistic approach to deep decarbonisation.

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Powering the Future: Green Hydrogen and Osmotic Energy

Engineers are also leveraging salt's inherent properties to generate electricity and produce clean fuels.

Saltwater Electrolysis for Clean Hydrogen

Producing "green hydrogen" from water electrolysis, powered by renewable electricity, is a key strategy for decarbonising energy systems. While traditional electrolysis requires ultrapure freshwater, researchers are making strides in direct saltwater electrolysis. This advancement is crucial given the scarcity of fresh water and the global abundance of seawater and saline industrial waste.

The primary chemical hurdle in saltwater electrolysis is the competitive reaction at the anode. Instead of the desired oxygen evolution reaction:

$$2H_{2}O\to O_2+4H^{+}+4e^{-}$$

the chloride ions ($C{l}^{-}$) present in saline water undergo the chlorine evolution reaction:

$$2C{l}^{-}\to C{l}_2+2e^{-}$$

The production of highly corrosive chlorine gas ($C{l}_2$) degrades standard anode materials rapidly and creates environmental hazards. To address this, engineers are developing highly selective catalyst coatings (such as nickel-iron hydroxide layered double hydroxides paired with a manganese oxide layer) that block chloride ions from reaching the active catalyst surface, thereby facilitating pure oxygen production.

Other approaches use forward osmosis and semi-permeable membranes to filter water in-situ before electrolysis. Electrochemical salt splitting is another emerging technology that uses electricity to separate salts, regenerating industrial acids and bases while producing clean water, thereby supporting circular economy principles in industrial wastewater treatment.

Harvesting Blue Energy from Salinity Gradients

Osmotic power, also known as "blue energy," harnesses the thermodynamic energy released when freshwater and saltwater mix across a semi-permeable membrane. The thermodynamic potential of this mixing is governed by the chemical potential difference between the two solutions. The osmotic pressure ($\Pi$) can be calculated using the van 't Hoff equation:

$$\Pi =iMRT$$

where:

• i is the van 't Hoff index of the solute (for NaCl, $i\approx 2$),
• $M$ is the molar concentration of the solution,
• $R$ is the ideal gas constant,
• $T$ is the absolute temperature in Kelvin.

This significant pressure difference can drive hydro-turbines to generate electricity without any combustion or carbon emissions.

Recent developments, such as Japan's first osmotic power plant in Fukuoka, integrate directly with existing infrastructure like desalination plants. By mixing the hyper-saline brine waste with treated municipal wastewater, these plants achieve a much larger concentration gradient than standard seawater-river setups, significantly boosting the power density of the membranes. This approach demonstrates the practical viability of osmotic energy, offering a stable, baseload, weather-independent renewable energy source with significant global potential.

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Beyond the Obvious: Other Salt-Enabled Energy Solutions

The versatility of salt extends even further, touching upon several other critical areas of the energy transition.

Nuclear Waste Transformation

In the nuclear sector, molten salt technologies are being developed to improve waste management efficiency. Companies like Moltex Energy are developing processes like "Waste to Stable Salt" (WATSS) designed to convert spent nuclear fuel into stable salt forms for use as fuel in Molten Salt Reactors (MSRs). This process reduces high-level waste volumes and extracts long-lived transuranic radioactive elements for power generation, offering an alternative to permanent geological repository disposal.

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The Road Ahead: Challenges and Opportunities in Salt Utilisation

While the potential of leveraging waste salt and natural salt deposits is immense, several technical and economic hurdles remain.

• Corrosion and Material Degradation: High-temperature molten salts and concentrated brines are highly corrosive. Developing cost-effective alloy materials and ceramic coatings capable of withstanding these aggressive environments over decades of service life is a primary focus of materials science.
• Separation Efficiency: Solid waste salts are highly complex mixtures. Achieving the fractional purity required to convert a landfill-bound mixed salt cake into industrial-grade reactants or battery-grade chemicals can be energy-intensive, requiring careful energy-balance optimisation.
• Capital Intensity: Building large-scale MVR evaporators, fractional crystallisers, and DLE infrastructure demands substantial upfront capital investment. Policy frameworks, carbon pricing, and circular economy mandates will play a vital role in de-risking these projects for early adopters.

Despite these hurdles, ongoing research and industrial scaling continue to expand the applications of waste salt. By repurposing these high-salinity industrial byproducts and leveraging their chemical and thermal properties, these technologies facilitate the transition to a circular economy and support the development of more resilient energy infrastructure, transforming an environmental liability into a functional industrial resource.