Richard L. Valentine and Yi-Pin Lin

OBJECTIVES:

It is hypothesized that natural organic matter, free chlorine, and chloramines can act synergistically to either inhibit or accelerate the release of lead into drinking water, and that their involvement in the formation and dissolution of passivating lead oxide (PbO₂) may be primary factors governing this process. The overall objective of this study was to investigate these hypotheses to provide an improved understanding of the causes of excessive lead release into drinking water.

BACKGROUND:

Recently the practice of chloramination has come under scrutiny because it was implicated as a possible cause of an abrupt rise of lead levels when switching to this method of disinfection. However, little was actually known about the possible mechanisms and reactions involved. This project was proposed to fill critical knowledge gaps to allow an assessment of the potential for lead release under chloramination conditions.

HIGHLIGHTS:

Lead oxide is relatively unstable and is reduced to Pb(II) by water itself but is very slow at near neutral pH. More important was its reduction by natural organic matter (NOM). Pre-oxidation of the NOM with free chlorine significantly reduced the capacity of the NOM to reduce lead oxide but did not eliminate it. Monochloramine did not significantly oxidize Pb(II) to lead oxide as found for free chlorine, and unexpectedly was found to actually reduce lead oxide to Pb(II). Lead oxide could also oxidize iodide and formed iodoform in the additional presence of NOM.

APPROACH:

Batch kinetic experiments were conducted to investigate the reduction of commercial and laboratory synthesized PbO₂ in the presence of NOM, free chlorine, and monochloramine. These experiments were designed to reveal the individual roles of NOM, free chlorine, and monochloramine as well as the possible synergic effects of NOM/free chlorine and NOM/monochloramine on the reduction of PbO₂. The effects of important water chemistry parameters, which cover the normal water quality encountered in a water distribution system, were studied. Additional experiments using iodide as a probe compound were conducted to study the reactivity of PbO₂ in water. To distinguish the species of lead, both atomic absorbance spectrometry (measuring total lead) and anodic stripping voltammetry [measuring Pb(II)] were employed for lead measurements.

RESULTS/FINDINGS:

Lead oxide is relatively unstable in water. It is reduced to Pb(II) by water itself but only very slowly at near neutral pH values. It is also reduced by NOM quite readily. The reductive capacity of the NOM however is reduced if it is pre-oxidized by free chlorine, consistent with a destruction of reductive NOM functionalities. However, if free chlorine were present with NOM, no apparent reduction of lead oxide occurs presumably because it would oxidize any released Pb(II) back to lead oxide.

Monochloramine, generally considered an oxidant, was found to reduce lead oxide. The amount of Pb(II) formed was linearly related to the amount of monochloramine that decomposed via auto-oxidation. This suggests that the reaction mechanism involved a reaction of intermediates produced from the auto-decomposition of monochloramine, acting as potent reductants of lead oxide. Because NOM was also slowly oxidized by monochloramine, the effect of mixtures of monochloramine and NOM led to more complex behavior as each component did not act synergistically or independently. Studies also showed that lead oxide could oxidize iodide and lead to the formation of iodoform in the presence of NOM, a DBP formation mechanism that does not involve a disinfectant.

IMPACT:

Treatment processes that can alter the oxidation potential of the treated water in the distribution system need to be carefully evaluated for lead release from PbO₂ or lead bearing materials. Water systems that have adopted or are contemplating adoption of chloramination for secondary disinfection may want to conduct a more thorough lead monitoring program after switching disinfectants. Pre-oxidation of NOM or its removal should reduce the potential for lead release from lead oxide. The findings also suggest that a reservoir of lead oxide may eventually be completely reduced resulting in a possible reduction in soluble lead levels.