Chemists look to mine silver from laundry wastewater | Science News for Students

Chemists look to mine silver from laundry wastewater

Laundering can release the toxic metal, which often is added to fabrics to retard bacterial growth
Feb 12, 2018 — 6:30 am EST
washing machine
Laundered fabrics have become a notable source of silver, which can be lost to the environment. Now chemists are working to retrieve that precious metal from wash water.
Serenethos/iStockphoto

Believe it or not, laundry wash water can hold a mother lode of silver. And scientists are looking for ways to recover this precious metal. Their main goal is not to make big bucks selling it to people who make jewelry, coins or pricey flatware. Silver is toxic. So researchers want to catch it before it threatens wildlife. Now, two environmental engineers report preliminary success doing just that.

They have developed a way to extract silver from wash water. If the technique proves affordable and reliable, that silver could be recycled for a host of uses. The researchers have just shared their innovation in the January 2 ACS Sustainable Chemistry & Engineering.

For years, companies have added nano-sized bits of silver to all types of products — especially fabrics. Their main aim has been to fight the growth of odor-causing bacteria. That’s why this treatment has been especially popular for athletic wear, such as socks.

Studies soon showed, however, that nanosilver doesn’t stay put. It soon starts washing away in the laundry. Because that wash water ultimately makes its way into rivers, lakes and the ocean, so can the silver. And that could pose risks to wildlife.

pristine river
The same silver that can kill bacteria in sweaty socks, t-shirts, towels and other fabrics also can kill beneficial microbes in the environment. That’s why it’s important not to let costly silver escape during laundering.
simon_otngphoto/iStockphoto

“[Researchers] are walking a fine line between silver's desirable properties and its potential toxicity to the environment,” argues science journalist Silke Schmidt. “Products embedded with nano-silver,” she notes, “tend to lose some of their silver coating every time they're laundered.”

And stinky germs are not the only things that metal can poison. “Silver is harmful to humans, rats and aquatic species such as zebrafish and rainbow trout,” notes Tabish Nawaz. He’s an environmental engineer who works at the University of Massachusetts in Dartmouth. That same silver also can harm the growth of aquatic embryos, he notes, such as those of developing fish.

To protect all these creatures, he says, that silver needs to be removed from the laundry water before it spills out of a community’s waste-treatment plant. With that in mind, Nawaz teamed up with Sukalyan Sengupta, also at the University of Massachusetts. Together they are fine-tuning a technique to mine that nuisance silver from the laundry.

The trick: Turn to chemistry

Ions are electrically charged atoms or molecules. This means they’re missing electrons (making them positively charged) or have extra electrons (making them negatively charged). In the wash water, silver atoms exist as ions. These ions are positively charged.

Nawaz and Sengupta’s trick for harvesting silver is to trap those silver ions in a special type of resin. It, too, contains ions. That resin is made into beads and then packed into cylinders known as columns. Liquids, such as wash water, then get pumped through the columns.

When the liquid contacts the resin, something interesting happens. Ions in the liquid start swapping places with ions in the resin. As some ions are trapped, others are released. This process is known as ion exchange. (That’s why the resin is known as an ion-exchange resin.)

The idea is simple. Making it work, however, can be challenging.

The problem is that silver is not the only positive ion in the wash water. Laundry detergent, Nawaz points out, contains other types of positive ions, such as sodium ions. Any of these might bind to the resin in place of the silver.

What’s more, he notes, “several other detergent components react with silver.” They can do this through a range of other processes (known as complexation, precipitation, oxidation or reduction). The result? Those processes may make the silver unavailable to the resin.

But his team was able to overcome the challenge. How? Different ion-exchange resins contain different “functional groups.” These are particular group of atoms that perform particular roles in chemical reactions. Nawaz says that he and Sengupta used an ion-exchange resin with a functional group that targets silver. The functional group they chose is known as a thiol (THY-all).

To up their resin's performance, the engineers did not just add thiol. They also changed some other conditions (such as pH and temperature). Afterward, they were able to trap and remove 84 percent of the silver that had been added to water in the lab.

But how much silver can wash water deliver? After crunching a few numbers, Nawaz estimates that it could run to “about 2 grams [0.07 ounce] per person per day.” Multiply that by the population of the United States, for instance, and you’ll find that’s an estimated 652,000 kilograms (1,437,414 pounds) of silver!

Recovering that much before it enters the environment would be much better for wildlife than trying to catch it later.

Some doubts remain

Denise Mitrano is a geochemist in Dubendorf, Switzerland. She works at the Swiss Federal Institute of Aquatic Science and Technology. There, she studies how to measure nanopollutants in the environment. Mitrano questions whether it’s even necessary to trap silver from wash water. Many big cities run wastewater-treatment facilities. Their role is to clean up a community’s dirty water. And in many of those plants, she argues, “silver is effectively diverted to the sewage sludge.” That’s a special goopy type of waste separated from the water.

Silver may sit trapped in that sludge, Mitrano says. And that could prevent large amounts of it from getting  into the environment.

Cost is another issue to consider, she notes. Just because it’s possible to do something does not mean it can be done affordably or conveniently. Those are big issues here, she says. For instance, she asks, would the benefits of ion-exchange trapping outweigh the potential cost to the environment (and launderers) of letting this silver continue to wash away?

Those are questions engineers will have to explore. For now, Nawaz would like to see people play a bigger role in managing the wastes they release. “To be successful, effective waste management requires that everyone get on board,” he says. “We also need to inform people just how much our choices are affecting the environment.”

But if society is successful in getting rid of this toxic metal from waste water, the only question left may be what to do with all that silver.

Silver can kill bacteria. Ingesting lots of it, though, can turn your skin blue.
Risk Bites/YouTube

Power Words

(for more about Power Words, click here)

aquatic     An adjective that refers to water.

atom     The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and uncharged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.

bacteria     (singular: bacterium) Single-celled organisms. These dwell nearly everywhere on Earth, from the bottom of the sea to inside other living organisms (such as plants and animals).

chemical     A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.

chemistry     The field of science that deals with the composition, structure and properties of substances and how they interact. Scientists use this knowledge to study unfamiliar substances, to reproduce large quantities of useful substances or to design and create new and useful substances.

constituent     An ingredient or building block of some material.

detergent     A compound derived from petroleum products, often used for cleaning. Detergents work by breaking up and surrounding dirt particles or oily substances, so that they can be washed away with water.

electron     A negatively charged particle, usually found orbiting the outer regions of an atom; also, the carrier of electricity within solids.

embryo     The early stages of a developing organism, or animal with a backbone, consisting only one or a few cells. As an adjective, the term would be embryonic — and could be used to refer to the early stages or life of a system or technology.

engineer     A person who uses science to solve problems. As a verb, to engineer means to design a device, material or process that will solve some problem or unmet need.

environment     The sum of all of the things that exist around some organism or the process and the condition those things create. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature and humidity (or even the placement of components in some electronics system or product).

environmental engineer     A person who uses science to study and improve the natural environment.

extract     (v.) To separate one chemical (or component of something) from a complex mix. (noun) A substance, often in concentrated form, that has been removed from its natural source. Extracts are often taken from plants (such as spearmint or lavender), flowers and buds (such as roses and cloves), fruit (such as lemons and oranges) or seeds and nuts (such as almonds and pistachios).

fabric     Any flexible material that is woven, knitted or can be fused into a sheet by heat.

germ     Any one-celled microorganism, such as a bacterium or fungal species, or a virus particle. Some germs cause disease. Others can promote the health of more complex organisms, including birds and mammals. The health effects of most germs, however, remain unknown.

impair     (n. impairment) To damage or weaken in some way.

ion     (adj. ionized) An atom or molecule with an electric charge due to the loss or gain of one or more electrons. An ionized gas, or plasma, is where all of the electrons have been separated from their parent atoms.

metal     Something that conducts electricity well, tends to be shiny (reflective) and malleable (meaning it can be reshaped with heat and not too much force or pressure). 

molecule     An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

nano     A prefix indicating a billionth. In the metric system of measurements, it’s often used as an abbreviation to refer to objects that are a billionth of a meter long or in diameter.

population     (in biology) A group of individuals from the same species that lives in the same area.

resin     A sticky, sometimes aromatic substance, often secreted by plants. It may also be the viscous starting ingredient for some plastics that will harden when heated or treated with light.

risk     The chance or mathematical likelihood that some bad thing might happen. For instance, exposure to radiation poses a risk of cancer. Or the hazard — or peril — itself. (For instance: Among cancer risks that the people faced were radiation and drinking water tainted with arsenic.)

sewage sludge     A semi-liquid waste (with the consistency of pudding or wet mud) that is left when communities clean the water that flows down kitchen drains, toilets and sewers or out of manufacturing plants and streets. It often contains toxic metals. It may also contain many nutrients, which is why some farmers look to spread clean sewage sludge on their croplands.

sodium     A soft, silvery metallic element that will interact explosively when added to water. It is also a basic building block of table salt (a molecule of which consists of one atom of sodium and one atom of chlorine: NaCl). It is also found in sea salt.

solution     A liquid in which one chemical has been dissolved into another.

species     A group of similar organisms capable of producing offspring that can survive and reproduce.

thiol     An organic chemical that is similar to an alcohol, but instead of containing a hydroxyl group — an oxygen and hydrogen atom bound together — they have a sulfur atom bonded to the hydrogen. These chemicals often have a very strong and pungent — even repulsive — scent.

toxic     Poisonous or able to harm or kill cells, tissues or whole organisms. The measure of risk posed by such a poison is its toxicity.

waste     Any materials that are left over from biological or other systems that have no value, so they can be disposed of as trash or recycled for some new use.

wastewater     Any water that has been used for some purpose (such as cleaning) and no longer is clean or safe enough for use without some type of treatment. Examples include the water that goes down the kitchen sink or bathtub or water that has been used in manufacturing some product, such as a dyed fabric.

Citation

Journal: Tabish Nawaz and Sukalyan Sengupta. Silver recovery from laundry washwater: The role of detergent chemistry. ACS Sustainable Chemistry & Engineering, Vol. 6, January 2, 2018, p. 600. doi: 10.1021/acssuschemeng.7b02933.

Meeting: D. Mitrano and B. Nowack. Tracking nanomaterials through the laundry wash cycle: Release, dissolution, and complexation. American Chemical Society National Meeting, Denver, March 25, 2015.