When Flint, Mich., switched its water supply, a chemical cascade inside old pipes caused lead to leach into the city's drinking water, triggering a public health emergency.
When Flint, Mich., started using its own river as a water supply, the city’s drinking water became tainted with lead. This is the chemistry of how that happened.
This is Scientific American’s 60-Second Science. I’m Lydia Chain. Got a minute?
Normally, water managers add orthophosphate to the water to prevent corrosion. The orthophosphate bonds with lead in the pipes, making a protective coating between the lead and water. Where that shield is intact, corrosive chemicals like dissolved oxygen—O2—can’t interact with the lead.
This isn’t a one-time solution. Orthophosphate has to be added continually or the barrier breaks down. If it does break down, O2 combines with lead atoms, oxidizing them. O2 takes electrons from the lead, grabs a couple of hydrogen protons, and turns into H2O—water.
The result? Once oxidized, the lead dissolves into the water stream instead of sticking to the pipe. Flint’s processing plant did not add orthophosphates, so the pipes broke down.
That breakdown happened quickly for Flint. The river water had high levels of chloride, which accelerates corrosion. There were two possible sources of the chloride: ferric chloride used in conjunction with chlorine disinfection and road salt applied during tough Michigan winters.
The chemistry is hidden away in old, buried pipes, but the result flows right into a drinking glass.
Thanks for the minute! For Scientific American’s 60-Second Science, I’m Lydia Chain.
[The above text is a transcript of this video.]
Editors' Note: The reactions depicted in the animation are simplified for the purpose of illustrating a very complex process.
Normally, water managers add orthophosphate to the water to prevent corrosion. The compound bonds with metals and rusts in the pipes, making a protective coating between the metal and the water. Where that shield is intact, relatively low levels of lead will dissolve, or leach, into the water causing contamination. Orthophosphate can reduce the amount to levels well below the EPA’s recommended limit of 15 parts per billion (ppb). Without orthophosphate or similar anticorrosion additives, the levels can reach 500 ppb or more.
Orthophosphate coats the inside of pipes, but more importantly, it reacts in the water as a sort of chemical barrier. The chemical needs to be added continually or higher levels of lead will dissolve into the water. When Flint’s processing plant did not add orthophosphates, the layers of lead phosphate rust on the wall of the pipe broke down—some of that lead rust also fell off into the water at high levels.
Dissolved oxygen is only one way to corrode a metal. In reality, a complex brew of acids, salts, chlorine and many other chemicals are involved in oxidizing metal pipes.
For Flint, corrosion probably happened quickly, especially in the warmer summer months. The river water had high levels of chloride, which accelerates corrosion. There were three possible sources of the chloride: ferric chloride used to remove particles from water, chloride that was in the river naturally and road salt that accumulated after winter runoff.
Executive Producer: Eliene Augenbraun
Producer: Lydia Chain
Stock Audio Effects: AudioBlocks
Special Thanks: Daniel Giammar, Haizhou Liu, Amy Pruden, Min Tang, Marc Edwards