Evaluating Lead Chemistry Under Variable Water Quality Conditions
Lead has long been identified as a health issue in the United States. Government regulations limit the amount of lead allowed in distribution pipes, fittings and residential plumbing systems, but lead leaching problems persist as water quality changes or new technologies are implemented. In this work, experiments were developed to determine the effect of temperature, ionic strength, and carbonate concentrations on soluble lead concentrations over relevant pH ranges. Equilibrium models were developed to predict changes in soluble lead and the predominating solid(s) that form under these varied water quality conditions. Additional experiments were also performed to verify how effectively PbO2(s), a lead(IV) solid, could be measured using a colorimetric method. Results for the kinetic experiments tested over a period of 48 hours found that an increase in temperature from 25°C to 55°C brought a decrease in soluble lead (i.e. increased lead precipitation) across a pH range of 3.0-9.8. Increasing ionic strength (NaCl) as well as increasing the total carbonate concentrations in these solutions at 25°C resulted in higher measured soluble lead over 48 hours at pH 7.0 than the same kinds of experiments without these variables. In the solutions containing lead with and without NaCl and NaHCO3, no solid was modeled to predominate in equilibrium (6 week analysis). Since several of the 6-week experiments measured lower levels of soluble lead than originally added, the systems were unlikely to be at equilibrium. Additionally, the previously developed iodometric method proved to be a feasible method of quantifying PbO2(s) in water with 80-88% accuracy. These findings may have important implications towards how lead behaves in in-home water heaters or softeners.