Professor of Wastewater Treatment, University of Massachusetts Dartmouth

PhD student at Dartmouth University, University of Massachusetts

The author does not work, consult, own shares, or accept funding for any company or organization that can benefit from this article, and has not disclosed any related affiliation beyond academic appointments.

The University of Massachusetts provides funding as a member of The Conversation US.

Humans have known since ancient times that silver can kill or prevent the growth of many microorganisms. It is said that Hippocrates, the father of medicine, used silver preparations to treat ulcers and heal wounds. Before the introduction of antibiotics in the 1940s, colloidal silver (tiny particles suspended in a liquid) was the main medicine used to treat burns, infected wounds, and ulcers. Today, silver is still used in wound dressings, creams and coatings for medical devices.

Since the 1990s, manufacturers have added silver nanoparticles to many consumer products to enhance their antibacterial and deodorant properties. Such as clothes, towels, underwear, socks, toothpaste and stuffed toys. Nanoparticles are ultra-small particles, ranging in diameter from 1 to 100 nanometers-they cannot be seen even with a microscope. According to a widely cited database, about a quarter of nano-material-based consumer products currently sold in the United States contain nano-silver.

A number of research reports claim that nano-silver is leached from textiles during washing. Studies have also shown that nano-silver may be toxic to humans and aquatic and marine life. Although it is widely used, little is known about its fate or long-term toxicity in the environment.

We are developing methods to turn this potential ecological crisis into an opportunity by recovering pure silver nanoparticles with many industrial applications from laundry wastewater. In a recently published study, we described a silver recycling technology and discussed key technical challenges. Our method solves this problem from the source-in this case, a separate washing machine. We believe that this strategy is expected to remove newly discovered pollutants from wastewater.

In the past decade, the use of nano-silver in consumer products has steadily increased. The market share of Yinji textiles increased from 9% in 2004 to 25% in 2011.

Several researchers measured the silver content of textiles and found that the silver content per kilogram of textiles ranged from 0.009 to 21,600 milligrams. Studies have shown that the amount of silver leached in the washing liquor depends on many factors, including the interaction between detergents and other chemicals and how the silver adheres to textiles.

In humans, exposure to silver can harm liver cells, skin and lungs. Prolonged exposure or exposure to large doses can cause a disease called Argyria, in which case the victim’s skin becomes a permanent blue-gray color.

Silver is toxic to many microorganisms and aquatic life, including zebrafish, rainbow trout, and zooplankton.

Once silver enters the sewer and eventually enters the wastewater treatment plant, it may damage the bacterial treatment process, reduce its efficiency, and cause the treatment equipment to become dirty. More than 90% of the silver nanoparticles released in wastewater will eventually remain in nutrient-rich biosolids at the end of wastewater treatment. These biosolids are usually used as agricultural fertilizers on the land.

This brings multiple risks. If plants absorb silver from the soil, they can concentrate it and introduce it into the food chain. It can also seep into groundwater or wash into rivers through heavy rain or erosion.

Our research shows that the most effective way to remove silver from wastewater is to treat it in a washing machine. At this point, the silver concentration is relatively high, and the silver is initially released from the treated clothing in a recyclable chemical form.

Once the laundry washing water is piped to the wastewater treatment plant and mixed with sewage and other sources of water, the silver concentration will be significantly reduced and can be converted into different chemical forms.

Some chemistry knowledge is helpful here. Our recycling method uses a widely used chemical process called ion exchange. Ions are charged atoms or molecules. In ion exchange, solids and liquids combine and exchange ions with each other.

For example, household soaps do not foam well in "hard" water that contains a lot of ions (such as magnesium and calcium). Many household water filters use ion exchange to "soften" the water, replacing these materials with other ions that do not affect its properties in the same way.

In order for this process to work, the ions in the switching position must be positively or negatively charged. Nano-silver is initially released from textiles in the form of silver ions, which are a cation-a positively charged ion (hence the plus sign in the chemical symbol is Ag).

Even at the source, removing silver from washing water can be challenging. Compared with other cations (such as calcium) that may interfere with the removal process, the concentration of silver in the washing solution is relatively low. Detergent chemistry further complicates the situation because certain detergent ingredients may interact with silver.

In order to recover silver without absorbing other chemicals, the recovery process must use materials that have a chemical affinity for silver. In a previous study, we described a potential solution: the use of ion exchange materials embedded with sulfur-based chemicals that preferentially bind to silver.

In our new research, we passed the wash water through an ion exchange resin column and analyzed how each major detergent component interacts with the silver in the water and affects the resin's ability to remove silver from the water. By controlling the process conditions such as pH, temperature, and concentration of non-silver cations, we can determine the conditions to maximize silver recovery.

We found that pH and calcium ion (Ca2) levels are key factors. Higher levels of hydrogen or calcium ions bind detergent ingredients and prevent them from interacting with silver ions, so ion exchange resins can remove silver from the solution. We also found that some detergent ingredients—especially bleach and water softeners—have reduced the efficiency of ion exchange resins. According to these conditions, we recovered 20% to 99% of the silver in the washing water.

Our findings can facilitate research on alternative detergent formulations that can increase silver recovery. They also showed that ion exchange technology can recover trace amounts of silver from washing water that contains a lot of detergent.

Today, wastewater is collected from multiple sources, such as homes and businesses, and transported to centralized wastewater treatment plants through long-distance pipelines. However, there is increasing evidence that these facilities cannot exclude newly discovered pollutants from the environment because they use a common treatment plan for many different waste streams.

We believe that the future will be decentralized systems that can treat different types of wastewater using specific technologies designed specifically for the materials they contain. If the wastewater from the laundromat contains different pollutants from the wastewater from the restaurant, why should they be treated in the same way?

Our method is more efficient and more effective to solve new environmental problems-it may only need to install a special water treatment filter in the washing machine as a simple step.

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