Can concrete conquer air pollution? | Science News for Students

Can concrete conquer air pollution?

In the lab, powdered concrete sops up sulfur dioxide
Aug 18, 2017 — 7:00 am EST
pouring cement

Concrete, once hardened and then powdered, may be able to remove certain pollutants spewed by electric power plants.

tortoon/iStockphoto

From buildings and bridges to dams and driveways, people use billions of tons of concrete each year. Now this handy substance might find another use as well. Powdered concrete could collect harmful pollutants spewed by fossil-fueled power plants, a new study suggests.

Girish Ramakrishnan is a materials scientist at Stony Brook University in New York. Concrete can clean up some of these pollutants thanks to one of its ingredients — cement. That’s the stuff that slowly hardens and binds everything together.

In 2014, the Stony Brook researchers showed that powdered concrete can react with nitrogen oxides, often called NOx. These pollutants contribute to the formation of smog, a type of lung-choking pollution. NOx also can play a role in turning rainfall acidic. This can damage crops, buildings and more. But cement contains calcium compounds. And they will react with NOx, pulling them out of the air, Ramakrishnan and a colleague showed.

Curious, the team decided to see if concrete would do the same thing to another major air pollutant released by fossil fuel burning: sulfur dioxide.

Besides harming the environment, sulfur dioxide — or SO2 — can cause lung problems. In the air, sulfur dioxide helps create small particles of soot. As sulfur dioxide levels go up, so do cases of bronchitis, an inflammation of airways in the lungs. People with bronchitis often wheeze and cough.

To test the ability of concrete to pull SO2 from the air, Ramakrishnan’s team mixed up a batch of fresh concrete. Once it had hardened, they ground it into a fine powder. That increased the material’s surface area, explains Ramakrishnan. That extra surface area upped the number of sites where chemical reactions could take place between the concrete and air pollutants.

Each teeny gram of the team’s powdered concrete had a total surface area of between 8 and 10 square meters (86 and 108 square feet). That’s about the same as the floor area of a small room!

The scientists then packed about 3 grams (1 ounce) of this powder into a small tube to create a filter. This device was tiny — about the same size as a pencil eraser. Then the researchers pumped polluted air through it. Well, it was a gas meant to stand in for polluted air.

Normal air is mostly nitrogen mixed with oxygen. The researchers added SO2 but left that oxygen out (because otherwise it, too, might react with the pollutant they were studying).

The researchers then ran their mix of nitrogen and SO2 through the filter for about 8 hours. The concentration of SO2 was very low — only about 5 molecules of this for every million molecules of nitrogen. That may not sound like a lot, but it’s much higher than what is considered safe to breathe.

Afterward, the researchers looked at the filter under infrared light. If SO2 had reacted with the concrete, the interaction should have formed compounds called sulfates and sulfites. Those compounds absorb infrared light, Ramakrishnan explains. And the team’s instruments could monitor absorption to detect those compounds in the filter.

Those tests confirmed that SO2 indeed had been pulled out of the air and become bound to calcium-bearing compounds in the cement. Ramakrishnan and his team reported their results July 1 in Chemical Engineering Journal.

The team’s filter was very small, Ramakrishnan points out. This made it hard to precisely measure how much SO2 the powder-packed filter had removed from the gas. To get a better idea, the team is now testing larger filters. They hold 1 kilogram (2.2 pounds) of powdered concrete.

Outlook hazy

Not everyone has as rosy a view about the potential role of concrete in cleaning up pollution as Ramakrishnan does. Among these skeptics is Jeffery Roesler. He’s a civil engineer at the University of Illinois at Urbana-Champaign.

Most coal-burning power plants already remove a large fraction of the SO2 from their emissions. They do it using filters with relatively cheap calcium-containing substances, he notes. Those substances use the same type of chemical reactions as the concrete does. So it wouldn’t make sense, he argues, to make concrete just to grind it up into a more costly, less effective powder. No matter how well those concrete filters work in the lab, he says, they may cost too much to replace the older types already in use.

So why not recycle used concrete to cut the costs? Ramakrishnan says it might work, but not as effectively as new concrete.

Some nations, such as the United States, already set relatively strict limits on emissions of air pollutants. Concrete-based filters may not make sense in these countries, says Robert Farrauto. He’s an environmental engineer at Columbia University in New York City. Still, he adds, they might find use elsewhere. In other countries, where the levels of SO2 are higher, he says, using concrete might prove practical.

The team’s filters could offer another benefit as well. Powder particles in the used filters become covered with sulfates and sulfites. Those filters might themselves be recycled to make new concrete for roadways and other structures. Using them that way might reduce reactions with chlorine from chemicals like road salt, Ramakrishnan says. As he explains: If the calcium in the cement bound all of those sulfates and sulfites, then it won’t be available to react with chlorine. That, in turn, means road salt wouln’t damage roads or other concrete surfaces as much as it does today.

In the long run then, he argues, concrete-based filters might still save money.

Power Words

(for more about Power Words, click here)

acid rain    Any type of precipitation — rain, sleet, snow — that has picked up air pollutants (such as nitric acid or sulfuric acid) that have turned it acidic. Any pH value below 7 is acidic. Normal rain has a pH of about 5.6. Acid rain usually has a pH between 4.4 and 4.2. As it flows across the ground and into streams, this acid rain can harm plants and animals, such as insects and fish.

bronchitis    A disease caused when the airways that move oxygen to the lungs become irritated and inflamed. The germs that cause colds, flu and bacterial infections can sometimes trigger bronchitis. So can breathing in heavily polluted air, tobacco smoke or certain chemical fumes. Bronchitis may cause wheezing, too, and coughs that bring up thick mucus known as phlegm.

calcium     A chemical element which is common in minerals of the Earth’s crust and in sea salt. It is also found in bone mineral and teeth, and can play a role in the movement of certain substances into and out of cells.

cement     A finely ground material used to bind sand or bits of ground rock together in concrete. Cement typically starts out as a powder. But once wet, it becomes a mudlike sludge that hardens as it dries.

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.

chemical reaction     A process that involves the rearrangement of the molecules or structure of a substance, as opposed to a change in physical form (as from a solid to a gas).

chlorine     A chemical element with the scientific symbol Cl. It is sometimes used to kill germs in water. Compounds that contain chlorine are called chlorides.

civil engineer     An engineer who creates buildings, tunnels, water systems and other large projects that improve everyday life.

colleague     Someone who works with another; a co-worker or team member.

compound     (often used as a synonym for chemical) A compound is a substance formed when two or more chemical elements unite (bond) in fixed proportions. For example, water is a compound made of two hydrogen atoms bonded to one oxygen atom. Its chemical symbol is H2O.

concentration     (in chemistry) A measurement of how much of one substance has been dissolved into another.

concrete     To be solid and real. (in construction) A simple, two-part building material. One part is made of sand or ground-up bits of rock. The other is made of cement, which hardens and helps bind the grains of material together.

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.

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

filter     (in chemistry and environmental science) A device or system that allows some materials to pass through but not others, based on their size or some other feature. 

fossil fuel          Any fuel — such as coal, petroleum (crude oil) or natural gas — that has developed in the Earth over millions of years from the decayed remains of bacteria, plants or animals.

inflammation     The body’s response to cellular injury and obesity; it often involves swelling, redness, heat and pain. It also is an underlying feature responsible for the development and aggravation of many diseases, especially heart disease and diabetes.

infrared light     A type of electromagnetic radiation invisible to the human eye. The name incorporates a Latin term and means “below red.” Infrared light has wavelengths longer than those visible to humans. Other invisible wavelengths include X-rays, radio waves and microwaves. Infrared light tends to record the heat signature of an object or environment.

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).

nitrogen     A colorless, odorless and nonreactive gaseous element that forms about 78 percent of Earth's atmosphere. Its scientific symbol is N. Nitrogen is released in the form of nitrogen oxides as fossil fuels burn.

nitrogen oxides     Pollutants made up of nitrogen and oxygen that form when fossil fuels are burned. The scientific symbol for these chemicals is NOx (pronounced “knocks”). The principle ones are nitric oxide (NO) and nitrous oxide (NO2).

oxide     A compound made by combining one or more elements with oxygen. Rust is an oxide; so is water.

oxygen     A gas that makes up about 21 percent of Earth's atmosphere. All animals and many microorganisms need oxygen to fuel their growth (and metabolism).

particle     A minute amount of something.

pollutant     A substance that taints something — such as the air, water, our bodies or products. Some pollutants are chemicals, such as pesticides. Others may be radiation, including excess heat or light. Even weeds and other invasive species can be considered a type of biological pollution.

power plant     An industrial facility for generating electricity.

salt     A compound made by combining an acid with a base (in a reaction that also creates water).

smog     A kind of pollution that develops when chemicals react in the air. The word comes from a blend of “smoke” and “fog,” and was coined to describe pollution from burning fossil fuels on cold, damp days. Another kind of smog, which usually looks brown, develops when pollutants from cars react with sunlight in the atmosphere on hot days.

soot     Also known as black carbon, it's the sometimes oily residues of incompletely burned materials, from plastics, leaves and wood to coal, oil and other fossil fuels. Soot particles can be quite small — nanometers in diameter. If inhaled, they can end up deep within the lung.

sulfate     A family of chemical compounds that are related to sulfuric acid (H2SO4). Sulfates occur naturally in drinking water.

sulfur dioxide     A compound made of sulfur and oxygen. It is one of the pollutants that can form when a fossil fuel is burned. It’s also a gas naturally emitted during volcanic eruptions. Its scientific symbol is SO2.

surface area     The area of some material’s surface. In general, smaller materials and ones with rougher or more convoluted surfaces have a greater exterior surface area — per unit mass — than larger items or ones with smoother exteriors. That becomes important when chemical, biological or physical processes occur on a surface.

Citation

Journal:​ G. Ramakrishnan, et al. Reactions of SO2 on hydrated cement particle system for atmospheric pollution reduction: A DRIFTS and XANES study. Chemical Engineering Journal. Vol. 319, July 1, 2017. doi: 10.1016/j.cej.2017.02.135

Further Reading

To learn more about how concrete is mixed and manufactured, see the website of the Portland Cement Association.