Kinetics of Lead Release from Brass Water Meters and Faucets

摘要

As part of AwwaRF project 3018, the release of Pb from new faucets and new and usedwater meters was measured. Data were obtained for brass compositions, scalecompositions, and water chemistry. Pb (or Bi) is present in these brasses as small'islands' of metal, whereas Cu and Zn are mixed in a solid solution. With time, the brassshows preferential loss of Zn relative to Cu. Lead releases from the faucets in 6 hourstagnation runs increased rather than decreased with time. This behavior is inconsistentwith formation of passivating scale layers, but is consistent with progressivedezincification producing a porous surface layer through which Pb can diffuse morerapidly, or from which Pb particulates can be detached more readily with time. The rateof Pb release was found to correlate inversely with the Zn/Cu ratio of the brass.For meters, metal release rates could be effectively modeled as controlled by twoprocesses. Initially, the release curves for Cu, Zn, and Pb follow t1/2 kinetics, indicatingthat the reaction is transported limited by diffusion. At longer times, the reaction ratefollows first-order kinetics consistent with consumption of oxygen as the rate limitingstep. Rate constants for the first stage correlate to the age of the meters, whereas for thesecond correlate to the Zn/Cu ratio – higher Zn brasses corrode more slowly. Thisrelationship is likely caused by autocatalyic oxidation of copper viaCu metal + Cu~(2+) → 2Cu~(1+)As long as oxygen remains in the water, the cupric ions can be regenerated via4Cu~(1+) + 4H~+ + O_2 → 4Cu~(2+) + 2H_2Oand the reaction continues. The higher Cu brasses (those with lower Zn/Cu ratios) releasemore Cu to solution in short stagnation cycles; therefore they corrode more quickly thanlower Cu brasses. In flowing water, the buildup of the copper ions in solution isprecluded, and the autocatalytic effect does not occur. In this case, the corrosion rate iscontrolled by the diffusion of Zn (and Pb) from the metal, which should increase withincreasing Zn content.