Lack of a corrosion inhibitor, high chloride levels, and other
factors cause the pipe passivation layer to dissolve and fall
o;, leading to increased corrosion in Flint’s pipes. As the
pipes corrode, chlorine disinfectant breaks down.
Phosphate corrosion inhibitor helps maintain a mineral
passivation layer on the inside of Flint’s pipes, protecting
them from corrosion. With little corrosion, chlorine
disinfectant levels remain stable.
Oxidants such as dissolved O2
corrode pipes and leach soluble metal.
Before: Treated Detroit water
After: Treated Flint River water
chlorine used as
Cl– Cl2 ;
Pb2+ O2 Pb2+ O2
O2 Fe2+ O2 Fe2+
Cities use various methods to prevent
lead pipes from leaching lead ions into
the water supply. One method is to create a protective layer on the inside of the
pipe. This is done by adding phosphate
ions (PO43–) to the water supply. Phosphate is also known as orthophosphate.
These negative polyatomic ions help to
create lead(II) phosphate (Pb3(PO4) 2), an
insoluble, mineral-like crust on the inside
of the pipes.
The addition of phosphate ions to the
water needs to be an ongoing, continual
process, and if the crust is maintained
over time, the water comes into less or
no contact with the lead pipe itself and,
as a result, fewer redox reactions take
place and less lead enters the water. If
the phosphate crust is not maintained, it
breaks down over time and lead can leach
into the water supply.
The water that Flint used to get from
nearby Detroit was treated with phosphate ions in this way and, as a result,
remained relatively free of lead ions.
When Flint changed its water supply to
river water, authorities failed to treat it
with phosphate ions, so the phosphate
crust broke down over time and lead ions
entered the water.
In addition to the loss of the phosphate
protective layer in the pipes, two other problems occurred, as well.
First, the Flint River water had some unusu-
ally high levels of chloride ions, which can
accelerate the corrosion of the pipes. In
part, these high chloride
levels came from salts
used to treat roads dur-
ing the cold and snowy
Michigan winters. Often,
chlorides enter rivers
as run-off from the
roads. This is actually
an example of connections between different
applications of chemistry: De-icing Michigan’s
slippery roads has some immediate and obvi-
ous benefits, but the run-off of chloride ions
can be an unintended consequence.
Second, the pH of the water from the
Flint water treatment plants was too low. By
maintaining a relatively high pH (around 10),
the solubility of another lead compound that
contributes to the protective layer, lead(II)
carbonate (PbCO3), decreases. This helps to
maintain the protective layer on the pipes. If
the pH is too low, then the protective layer
Data show that
many children have
elevated lead levels
in their blood.
Flint switches back
to using water from
January 2016: Michigan Gov.
Rick Snyder declares a state of
emergency in Flint. Then, President
Barack Obama signs an emergency declaration and orders federal aid for Flint.
may dissolve and fall off. We can explain
this by considering Le Châtelier’s principle,
which states that a system that is shifted
away from equilibrium acts to restore the
equilibrium by reacting in opposition to
Lead(II) carbonate is relatively insoluble
in water, but does dissolve a little, and in
the process sets up this equilibrium:
PbCO3 (s) ; Pb2+ (aq) + CO32– (aq)
The carbonate ions that are produced
can then react with hydrogen ions (acid)
present in the water supply, according to
CO32– (aq) + 2 H+ (aq)
➔ H2O (l) + CO2 (g)
The lower the pH, the more hydrogen
ions that are present in solution, and
the more the carbonate ions react. The
removal of carbonate ions from the equilibrium system of lead(II) carbonate in the
first equation results in an increase in the
concentration of lead ions. To restore the
equilibrium, the chemical reaction shifts
to the right, so that additional carbonate
ions (and lead ions) are produced.
To ensure that the number of lead and
carbonate ions is the same on the right
side of the equation, the reaction then shifts
to the left and to the right a few times until
a new equilibrium is reached. In this new
equilibrium, the number of lead ions is the
same as the number of carbonate ions, but
the number of lead ions is larger than what
it was during the original equilibrium (before
the carbonate ions were removed), resulting
in more lead ions at equilibrium than before.
Flint’s water was found to have pH values
between 7 and 8, which is not basic enough to
prevent lead carbonate from dissolving.