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According to the latest research, toxic chemicals that accumulate in groundwater and wastewater could be removed efficiently by ultrasound.
Technological development brings many environmental challenges, one of the most serious being pollution, whether from air pollution caused by exhaust fumes or soil and water pollution due to fertilizers and other chemical residues, among other substances. Although wastewater management is becoming more efficient in developed economies, data indicates the presence of so-called "forever chemicals" or PFASs in tap water. For instance, a recent study by the US Geological Survey found that 45% of tap water samples in the US contained traces of PFAS. But what is the problem with these substances, and why turn to ultrasound?
"Forever" chemicals, technically known as per- and polyfluoroalkyl substances or PFAS, are a popular term used to describe certain chemical compounds that are highly resistant to environmental degradation and can persist in the environment for long periods, ranging from hundreds to thousands of years. Common examples of everlasting chemicals include persistent organic compounds (POPs) like PCB, DDT, and brominated flame retardants. Additionally, they can be found in nonstick pans and stain-resistant fabrics. The US Environmental Protection Agency (EPA) has classified over 9,000 PFAS.
These chemicals carry certain risks because they can accumulate in ecosystems and the food chain, impacting human health and biodiversity once released into the environment. Several studies have linked them to cancer, reproductive issues, and problems with the immune system. So the question arises: how can we eliminate a virtually indestructible substance once it enters food chains and water systems?
Until now, attempts have been made to use oxidizing chemicals. However, scientists at Ohio State University in the US believe they have found a technique that could help solve the problem without relying on additives. Their study, published in The Journal of Physical Chemistry A, explores the potential of ultrasonic cleaning as a sustainable solution.
Researchers have turned to ultrasound emission at frequencies even lower than those used in medical imaging. This type of ultrasound compresses and stretches the liquid solution, generating accumulations of vapor that burst violently in a phenomenon known as cavitation bubbles.
These bubbles resemble small combustion chambers in which very high temperatures, reaching 10,000 degrees Kelvin, are achieved. Thus, the sudden heat decomposes the carbon-fluorine bonds of PFAS and generates harmless residues. Laboratory experiments for ultrasonic water purification were conducted with PFAS of three different sizes commonly found in compounds such as fire extinguisher foams. The result was that, within three hours, they could remove the smaller compounds, which are typically more problematic for other techniques.
The scientists behind this research acknowledge that their technology cannot be applied on a large scale, as it would require a significant amount of energy. However, an intriguing possibility exists – the development of domestic ultrasonic water purification systems. Moreover, as demonstrated in previous studies, this technology can also remove pharmaceuticals from tap water..
US scientists are not alone in their efforts to find ways to eliminate PFAS. In their case, it's a less flashy system but equally or even more effective, with potentially more practical applications. We're talking about the solution developed by a team at the University of British Columbia (UBC) based on an advanced silica filter that can absorb up to 99% of the PFAS present in water. The PFAS are then removed through electrochemical and photochemical processes developed by the same researchers.
The UBC team suggests that their system could be used to make water drinkable in remote and isolated areas or in domestic systems, as proposed for ultrasonic cleaning.
In recent years, we have explored various methods for obtaining drinking water. For example, there are large-scale technologies like fog harps or, on a domestic scale, systems like a machine that can generate up to ten liters of drinking water per day from the air. Additionally, there are reverse osmosis desalination technologies, which are the most widely used today for providing drinking water in arid coastal areas without river water or aquifers.