Selective Separation of Ni 2+ /Co 2+ and H + from MWCNT Purification Waste Acid by an Acid-Resistant Nanofiltration Membrane and Reuse Evaluation

Deep within the lithium-ion batteries that power electric vehicles and portable electronics lies a critical component: the carbon nanotube. These are microscopic, tube-shaped structures made of carbon atoms, renowned for their strength and ability to conduct electricity. To manufacture them, engineers often use tiny particles of metals like nickel and cobalt as catalysts to help the carbon grow into its perfect shape. However, once the tubes are formed, these metal particles remain trapped inside or stuck to the surface, acting as impurities that must be removed to make the material useful for high-performance batteries. The standard way to clean them involves boiling the tubes in strong sulfuric acid, a process that dissolves the unwanted metal particles into the liquid. This leaves behind a waste stream that is dangerously acidic and filled with dissolved metals. Traditionally, factories neutralize this waste with large amounts of chemicals, creating a sludge of hazardous waste and throwing away both the valuable metals and the acid, which is a costly and environmentally damaging cycle.

A team of researchers in China has found a way to break this cycle using a specialized filter. Instead of neutralizing the waste, they passed it through a membrane designed to withstand strong acid. This membrane acts like a very fine sieve that lets water and acid molecules pass through while holding back the heavier metal ions. The goal was to see if this simple separation could concentrate the valuable metals for reuse and return the clean acid to the factory to be used again. The researchers tested this method on two different types of waste: one containing cobalt and the other containing nickel. They found that the membrane could successfully separate the metals from the acid, enriching the metals to a level where they could be easily recovered, while producing a clean acid stream that was just as effective as fresh acid for the next round of cleaning.

The study focused on two specific waste streams generated during the purification of multi-walled carbon nanotubes. One stream came from tubes made with a cobalt catalyst, and the other from those made with a nickel catalyst. Both streams were highly acidic, with the nickel waste being slightly more aggressive than the cobalt waste. The researchers pumped these liquids through a commercial membrane module, applying pressure to force the liquid through the filter. As the process continued, they gradually concentrated the liquid, removing water and acid while keeping the metal ions behind. They monitored how well the membrane held onto the metals and how much acid passed through at different stages of concentration.

For the cobalt-containing waste, the membrane performed exceptionally well. As they concentrated the liquid twenty times, the amount of cobalt in the remaining liquid increased from a few hundred milligrams per liter to over four grams per liter. The membrane rejected more than eighty percent of the cobalt ions, meaning most of the metal stayed behind in the concentrated liquid. It also held onto other impurities like aluminum and magnesium. At the same time, the acid passed through the membrane with high efficiency. Even as the concentration of metals increased, the membrane continued to let the acid through, recovering more than ninety percent of the sulfuric acid in the total volume of liquid that passed through. The researchers also tested how changing the pressure affected the process. They found that increasing the pressure pushed more liquid through the filter, which actually helped the membrane hold onto the metals even better by reducing a buildup of particles on the surface of the filter.

The results for the nickel-containing waste were similarly promising, though the conditions were slightly different. The nickel waste was more acidic, which made the membrane work a bit differently. In this highly acidic environment, the membrane relied more on the physical size of the particles to do the separating rather than electrical forces. Despite this, the membrane still rejected more than ninety percent of the nickel ions throughout the entire process. By concentrating the liquid twenty-five times, they increased the nickel concentration from less than one gram per liter to nearly seventeen grams per liter. The acid recovery remained high, with over ninety-three percent of the sulfuric acid passing through the membrane. The higher acidity meant the liquid moved through the filter more slowly than the cobalt waste, but the separation remained stable and effective.

To prove that the recovered acid was truly useful, the researchers put it back to work. They took the clean acid that had passed through the membrane and used it to purify new batches of carbon nanotubes. They compared this recycled acid to fresh, brand-new sulfuric acid. The results were nearly identical. When they used the recycled acid to clean the cobalt-based tubes, the amount of leftover metal dropped from over six percent to less than half a percent. For the nickel-based tubes, the metal residue fell from over seven percent to half a percent. Fresh acid achieved almost the same low levels of residue. This confirmed that the acid, after being separated from the metals, was just as effective as new acid for the industrial cleaning process.

The study also examined the physical structure of the carbon nanotubes after they were cleaned with the recycled acid. Using powerful microscopes, the researchers looked at the tubes before and after purification. Before cleaning, the tubes were covered in dark specks of metal catalyst. After being treated with the recycled acid, these specks were gone, leaving the tubes clean and smooth. The structure of the tubes themselves remained intact, showing no signs of damage or breaking. This demonstrated that the recycling process did not harm the valuable material it was meant to clean.

This approach offers a significant improvement over the traditional method of neutralizing waste acid. The old method requires adding large amounts of chemicals to neutralize the acid, which creates a massive amount of toxic sludge that must be disposed of, and it destroys the value of the acid and metals. The new method uses a physical barrier to separate the components, avoiding the need for extra chemicals and preventing the creation of hazardous sludge. It allows the factory to keep the acid in a closed loop, using it again and again, while concentrating the valuable metals for recovery. While the researchers noted that they did not test how the membrane would hold up over many years of continuous use, their findings show that the process is technically feasible and effective for treating these specific industrial wastes. The study provides a clear path toward a more sustainable way of producing the materials needed for the next generation of energy storage.