Don't judge water by its clarity
Clear water is not always contaminant-free
Mikayla Armstrong, UNC-Chapel Hill, Department of Environmental Science and Engineering
Graduate trainee, UNC SRP Project 5
In 2020, Consumer Reports detected high levels of arsenic in several brands of bottled water. Their reporting highlighted an interesting (and concerning) aspect of water quality—clear and seemingly clean water can be deceptively contaminated.
Even when water is clear, it may need to be treated or filtered to ensure it does not contain contaminants that can be harmful to humans. As a student researcher for the UNC Superfund Research Program’s Project 5, I study membrane materials that remove contaminants from water.
How membranes work
Reverse osmosis is a water purification technology that filters out harmful contaminants from water that we cannot see with the human eye. I study the polymer materials, called membranes, used in reverse osmosis. In the reverse osmosis process, contaminated water is filtered through a membrane with pressure.
The membrane is designed to filter contaminants from water by:
- Size – membrane pores are less than a nanometer wide (100,000 times smaller than the diameter of a human hair)
- Charge – membranes have a negative charge
Most contaminants in water are larger than a nanometer wide and have a negative charge, so they have a harder time passing through the membrane than water. A water molecule is smaller than the membrane pores and has an overall neutral charge, so it passes through the membrane easily.
Arsenic: A tricky exception
Arsenic, a naturally occurring contaminant, is poisonous and has been associated with long-term health effects, even at low doses. It can be present in water in two forms: one is negatively charged, and the other is neutral. The neutral form is more harmful to humans and is problematic for reverse osmosis membranes because it is small—only 1.5 times the size of a water molecule—and neutral in charge, allowing it to pass through membranes easier than other larger, charged contaminants.
More than 43 million Americans rely on wells for drinking water and could be at risk of arsenic exposure.
In my research, I devise methods to make membrane pores small enough to keep the neutral form of arsenic from passing through, but not so small that fewer water molecules pass through the membrane.
This is a challenging task because I must make changes to the membrane at a scale I cannot see and then perform filtration experiments to determine if I was successful in making unseen changes to the membrane.
It is important to make reverse osmosis membranes better at removing arsenic because compared with other water purification technologies, reverse osmosis:
- uses fewer chemicals,
- has a smaller footprint,
- and requires fewer steps to implement.
What you can do
Hopefully, the outcome of my research will be a reverse osmosis-based filtration system that homeowners with arsenic contamination can use to effectively treat their well water.
In the meantime, the best way to protect your health is to find out what is in your water.
If you rely on well water, get it tested annually for bacteria. Testing should be done every two years for nitrates, nitrites, and heavy metals, like lead and copper.
It is always important to know with confidence what is in your drinking water because as some reports show, clear water is not always safe to drink.