When it comes to lead in the +4 oxidation state there's a twist.
"If you have a strong oxidant, you can form species with lead in the +4 oxidation state," Giammar says. "These have very low solubility but they're only stable in the presence of a strong oxidant. As soon as the strong oxidant goes away, the lead +4 is no longer stable. It starts to come back to lead +2, and it can release the lead quite quickly."
The free chlorine the district had been using is a very strong oxidant. The chloramines they switched to are less strong.
"So when they switched to chloramines, the pipe scale that had formed over years of chlorine treatment began to release lead into the water," Giammar says. It was a classic example of an unintended consequence.
What the district case demonstrates, Giammar says, is that tap water is a manufactured product, not a natural resource. The water leaving the treatment plant can have essentially no lead in it, but by the time it reaches the faucet that could have changed.
The lead comes from the piping, but whether it is released depends on the chemistry of the water running through the distribution system.
Looking at water chemistry
When he read about the district problem, Giammar wrote a proposal to the Water Research Board offering to study the chemistry of the insides of pipes, particularly the dissolution rates of lead phosphates, lead carbonates and the lead +4 oxides, as a function of pH, added phosphate and disinfectants.
"That was our first entry into the field of lead and drinking water," he says.'/>"/>
|Contact: Diana Lutz|
Washington University in St. Louis