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 unusually 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 during 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 obvious 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 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 the shift.
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 any hydrogen ions (acid) present in the water supply, according to this equation:
CO32– (aq) + 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. The lack of a protective layer in the iron pipes can cause a similar oxidation reaction to the one occurring in the lead pipes:
Fe(s) → Fe2+ (aq) + 2 e–
The aqueous iron ions can cause the water to turn an unsightly rust color, but it can also have another troublesome side effect. This time, elemental chlorine (Cl2) can be reduced to chloride ions (Cl–). When the electrons are given up by the iron and picked up by the chlorine molecules, the following reduction reaction occurs:
2 e– + Cl2(aq) → 2 Cl–(aq)
The loss of elemental chlorine is a potentially huge problem for public health. Chlorine is added to the water supply to eliminate water-borne pathogens. The removal of elemental chlorine via reduction can mean that water-borne pathogens have a better chance of surviving, causing disease.