Addition of sodium silicate has been recommended as a corrosion control method for lead pipes used for drinking water supply, but its application in real tap water conditions has shown mixed effects. To determine the mechanism of corrosion inhibition, bench-scale experiments were carried out to explore the interactions between dissolved silicate and corrosion scales developed inside of lead pipes at conditions relevant to drinking water supply. The experiments used real scales removed from an aged lead pipe as well as pure hydrocerussite (Pb3(CO3)2(OH)2), the dominant lead-containing solid under these conditions, to distinguish if lead release is controlled by chemical interactions or mass transfer. The study also examined the effects of silicate doses (0.33-0.83 mM), the amounts of corrosion (0.5 g/L and 2 g/L), and pH (7.7 and 8.8). The study complements previous flow-through experiments with intact pipes by allowing a focus on chemical interactions in the absence of any mass transfer limitations. The study shows that silicate uptake was a biphasic reaction with a fast uptake in first 4 to 6 hours followed by a slower uptake phase that continued over 4 days. The silicate taken up by the scales and hydrocerussite was mainly due to adsorption of silicate on the solid surface. Despite the uptake of silicate by the powdered scales and hydrocerussite, the lead release did not decrease at either pH 7.7 or pH 8.8. This indicates that adsorption of silicate to these solid phases does not inhibit the dissolution of these solid phases, and it suggests that the benefits of silicate addition in controlling lead release in a previous study were associated with silicate uptake impacts on mass transfer and not on chemical interactions.

Silicate uptake by lead corrosion products and its effect on lead release

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