Reviewed by Lexie CornerJan 8 2025
An international group of researchers from the Massachusetts Institute of Technology has created a nanofiltration method that could decrease hazardous waste and increase the efficiency of aluminum production. The study was published in ACS Sustainable Chemistry and Engineering.
The researchers demonstrated the membrane’s performance in lab-scale experiments, pictured, using a novel membrane to filter various solutions that were similar in content to the waste streams produced by aluminum plants. Image Credit: Trent Lee
Aluminum is the second most produced metal globally, following steel, and is used in applications ranging from rocket boosters and circuit boards to soda cans and foil wrap. Global demand is projected to increase aluminum production by 40 % by the end of this decade, amplifying the environmental impact and pollution associated with its manufacturing waste.
Nanofiltration offers a potential solution by recovering aluminum ions from the waste streams of aluminum plants. These captured ions could be recycled and reintegrated into aluminum production, increasing yield while reducing waste.
The researchers tested a novel membrane designed to filter solutions similar to the waste streams generated by aluminum plants. In lab-scale experiments, the membrane selectively captured over 99 % of the aluminum ions in these solutions.
If scaled and implemented in existing production facilities, this membrane technology could significantly reduce aluminum waste and improve the environmental quality of industrial effluent.
This membrane technology not only cuts down on hazardous waste but also enables a circular economy for aluminum by reducing the need for new mining. This offers a promising solution to address environmental concerns while meeting the growing demand for aluminum.
John Lienhard, Department of Mechanical Engineering, Massachusetts Institute of Technology
Lienhard is also the Director of the Abdul Latif Jameel Water and Food Systems Lab (J-WAFS) at MIT.
Additional authors of the research include MIT Mechanical Engineering undergraduates Trent Lee and Vinn Nguyen, as well as Zi Hao Foo SM ’21, Ph.D. ’24, currently a postdoctoral researcher at the University of California, Berkeley.
A Recycling Niche
Lienhard's team at MIT develops membrane and filtration technologies to desalinate seawater and treat various wastewater sources. While exploring new applications for their work, the team identified an opportunity in aluminum manufacturing, specifically addressing the wastewater generated during the production process.
Aluminum production begins with the extraction of bauxite, a metal-rich ore, from open pits. The ore undergoes chemical processes to separate aluminum from the surrounding rock, producing aluminum oxide, or alumina, as a powder.
Most of this alumina is sent to refineries, where it is added to electrolysis vats containing molten cryolite, a mineral used to dissolve alumina. When a strong electric current is applied, cryolite facilitates the separation of aluminum and oxygen atoms by breaking their chemical bonds.
The resulting pure aluminum sinks to the bottom of the vat in liquid form, where it can be collected and shaped into various forms.
Cryolite’s solvent-like properties enable the efficient separation of alumina during the molten salt electrolysis process. However, impurities such as sodium, lithium, and potassium ions gradually accumulate, reducing the cryolite's ability to dissolve alumina.
Once these impurities reach a critical concentration, fresh cryolite must be added to the electrolyte to maintain process efficiency. The spent cryolite, a viscous sludge containing residual aluminum ions and contaminants, is then removed for disposal.
We learned that for a traditional aluminum plant, something like 2,800 tons of aluminum is wasted per year. We were looking at ways that the industry can be more efficient, and we found cryolite waste had not been well-researched in terms of recycling some of its waste products.
Trent Lee, Massachusetts Institute of Technology
A Charged Kick
The goal was to develop a membrane process capable of filtering cryolite waste and recovering aluminum ions from the waste stream. Specifically, the team aimed to selectively capture aluminum ions while allowing other ions, particularly sodium, which accumulates significantly in cryolite, to pass through.
The researchers hypothesized that extracting aluminum from cryolite waste could enable its reintegration into the electrolysis vat without adding excess sodium, which would otherwise impede the electrolysis process.
The design was based on adapting membranes commonly used in water treatment facilities. These membranes typically consist of a thin polymer layer with nanometer-scale pores.
Conventional membranes have a naturally negative surface charge, attracting positively charged ions while repelling negatively charged ones. However, the MIT team collaborated with the Japanese membrane manufacturer Nitto Denko to explore the use of commercial membranes tailored to repel and capture aluminum ions from cryolite wastewater while filtering other positively charged ions. Unlike sodium and other cations, which carry a +1 charge, aluminum ions have a stronger positive charge of +3.
Building on their previous work with membranes for lithium recovery from salt lakes and spent batteries, the team tested a novel Nitto Denko membrane. This membrane incorporates a thin, positively charged coating that strongly repels aluminum ions while allowing less positively charged ions to pass through.
The aluminum is the most positively charged of the ions, so most of it is kicked away from the membrane.
Zi Hao Foo, Department of Mechanical Engineering, Massachusetts Institute of Technology
To evaluate the membrane's performance, the team tested it with solutions containing ion balances similar to those found in cryolite waste. They observed that while sodium and other cations passed through the membrane, 99.5 % of aluminum ions were consistently captured.
The researchers also varied the pH levels of the solutions and found that the membrane remained effective, even after exposure to highly acidic conditions for several weeks.
Foo said, “A lot of this cryolite waste stream comes at different levels of acidity. And we found the membrane works really well, even within the harsh conditions that we would expect.”
The experimental membrane is currently about the size of a playing card. The researchers envision scaling it up to resemble the membranes used in desalination plants, where a long membrane is rolled into a spiral configuration. This design could be implemented to treat cryolite waste on an industrial scale in aluminum production facilities.
Lee said, “This paper shows the viability of membranes for innovations in circular economies. This membrane provides the dual benefit of upcycling aluminum while reducing hazardous waste.”
Journal Reference:
Lee, T. R., et al. (2025) Enhancing Resource Circularity in Aluminum Production through Nanofiltration of Waste Cryolite. ACS Sustainable Chemistry & Engineering. doi.org/10.1021/acssuschemeng.4c07268.