A novel and environmentally friendly method for producing highly efficient filters for “forever chemicals” has been developed by a team of researchers. The new technique, which uses a ball mill to create nanoscale scaffolds, offers a promising solution for removing harmful per- and poly-fluoroalkyl substances (PFAS) from wastewater. This innovative process is not only effective but also more sustainable than current filtration methods, which are often laborious and energy-intensive.
The research, led by the German Federal Institute for Materials Research and Testing (BAM), focuses on the creation of covalent organic frameworks (COFs), which are porous materials with pores only a few nanometers wide. These tiny pores are perfectly sized to trap PFAS molecules, effectively filtering them out of water. What makes this method particularly noteworthy is its use of mechanochemistry, a process that relies on mechanical forces to drive chemical reactions. Crucial experiments to refine this technique were conducted at DESY’s X-ray source PETRA III, and the findings have been published in the journal Small.
Understanding the ‘Forever Chemical’ Problem
PFAS are a group of fluorinated compounds that are widely used in a variety of everyday products due to their desirable properties. They can be found in items such as non-stick cookware like Teflon pans, outdoor clothing, and firefighting foams. The same characteristics that make them so useful—durability, heat resistance, and dirt repellency—also make them incredibly persistent in the environment.
This stability is the root of the problem. PFAS compounds do not break down easily, earning them the moniker “forever chemicals.” As a result, they accumulate in soil, water, and even in living organisms. The potential harm these chemicals pose to human health has made their removal from the environment, particularly from wastewater, a significant challenge. Current filtration methods can be effective but are often complex and require a great deal of energy.
A Mechanochemical Approach to Filtration
The new filter material developed by the BAM-led team is based on an unconventional production technique known as mechanochemistry. This branch of chemical manufacturing initiates reactions through mechanical energy, such as friction and pressure, rather than relying on traditional methods that often involve solvents and heat. The process is more environmentally friendly, reducing both waste and energy consumption.
The Ball Mill Process
The heart of this new technique is a device called a ball mill. The specific mill used in the laboratory is a small plastic cylinder, comparable in size to a film canister. Inside this cylinder, a small amount of powder, a droplet of solvent, and two steel balls, each about the size of a peppercorn, are placed. A specialized device then shakes the ball mill back and forth more than 30 times per second.
This rapid agitation grinds the contents of the mill. Initially, the powder granules are broken down into smaller particles, which increases their surface area. After just a few minutes, the combination of frictional heat, increased pressure, and kinetic energy triggers a chemical reaction. The finely ground particles then begin to combine, forming the larger, scaffold-like structures of the COFs that can be used as a filter.
Optimizing the Process
To ensure the creation of the most effective filter material, the research team meticulously optimized the parameters of the ball mill process. They varied several factors, including the frequency of the ball mill’s agitation and the quantity of solvent used. Through these experiments, they determined that the best nanoscale scaffolds were produced at a frequency of 36 hertz. The ideal recipe also called for 266 milligrams of powder and the addition of 250 microliters of solvent, which is equivalent to just a few drops.
Advanced Analysis with X-ray Technology
A crucial element in the optimization of this new technique was the use of DESY’s PETRA III X-ray source. This powerful tool allowed the researchers to observe the chemical reaction as it happened inside the ball mill. By directing the X-ray beam into the operating mill, the team could monitor the crystalline transformations in real-time, down to the second.
The diffraction patterns generated by the X-rays provided a clear picture of the process. The researchers could see the signals from the initial starting materials diminishing while the signals of the target crystalline frameworks emerged. This live-action view of the reaction was instrumental in understanding the ideal conditions for creating the COFs and for ensuring the production of a high-quality filter material.
The Future of PFAS Filtration
The covalent organic frameworks produced through this ball mill technique represent a significant step forward in the effort to combat PFAS contamination. The nanoscale pores of these COFs are perfectly suited for trapping PFAS molecules, making them an effective filter material. Furthermore, unlike some other framework structures that have been used for filtration, this new material is free of heavy metals, making it a more environmentally friendly option.
Alternative Ball Milling Approaches
Other research has also explored the use of ball milling to address PFAS contamination, though with a different approach. One study, published in Environmental Science & Technology Letters, focused on the destruction of PFAS rather than filtration. In this method, researchers used a ball mill to grind PFAS with a piezoelectric additive called boron nitride. The mechanical forces generated by the milling process caused the boron nitride to create partial electrical charges, which helped to break the strong carbon-fluorine bonds of the PFAS molecules.
This process, which also operates at ambient temperature and pressure, was shown to be highly effective at destroying PFAS, both in their pure form and when present in contaminated soil. The researchers found that boron nitride was more efficient than other additives, such as potassium hydroxide, which can be corrosive and form problematic clumps. While this method is distinct from the filtration approach developed by the BAM team, it highlights the versatility of ball milling as a tool for environmental remediation.
