In April 2014, the city of Flint, MI switched its drinking water source from the city of Detroit to the Flint River in an effort to save millions of dollars annually. Immediately following the switch, residents began reporting symptoms of Pb poisoning. As of June 2017, 15 people have been criminally charged in relation to the Flint Water crisis – three of which were water quality chemists. Over three years later, we’re still unravelling what went wrong and who is at fault. One of the major concerns with older industrial city infrastructures (like Flint) is the use of Pb piping in service lines (this includes pipes, joints, and solder). In that case, what prevents the Pb(s) from dissolving into the water as Pb2+ over time? The primary treatment method to suppress Pb(s) dissolution into drinking water is to control its solubility by adding an anion like PO43- that has low solubility with Pb ions. Over many years, the Pb pipes have been coated with a thick layer of Pb3(PO4)2 scaling that prevents dissolution of Pb(s).
Imagine you are an analytical chemist consulting for the Flint Water Treatment Plant. The emergency financial manager decides to further cut costs by using the Flint River water without adding any corrosion inhibitors like orthophosphate (PO43-) to save the city an additional hundred thousand dollars a year (this actually happened). Your first task as consultant is to determine whether this is a safe choice.
1. With respect to solubility equilibria (Ksp), what are the key differences in Pb2+ solubility with and without orthophosphate added? How would the concentration of free Pb2+ be expected to change?
2. If you treat the water such that the system remains saturated with respect to the pipe deposits, how much free Pb2+ will be in the water? (In this first approximation, we’ll neglect interactions of Pb2+ or PO43- with water). First, write a balanced equation for the solubility of Pb3(PO4)2 in water:
3. Calculate the concentration of free Pb2+(aq) in an aqueous solution saturated with Pb3(PO4)2(s) assuming no ionic strength effects or reactions of either ion with water.
4. According to the EPA Lead and Copper Rule (LCR), the action level for Pb in drinking water (the level at which threat to human health requires public notification and action towards mitigation) is 15 ppb. If you were to add enough orthophosphate to the system saturated with respect to Pb3(PO4)2(s), would the [Pb2+] be below the action limit?