Lead silicate solubility and the control of lead contamination in drinking water

The intake of lead into the human body has become an area of major concern because high levels of lead are harmful and can cause physiological damage, especially in children. It has been suggested that adding NaSiO₃might control Pb²⁺ contamination of drinking water supplies (c.g., Thresh 1922). PbSiO₃ (am), the white, gelatinous precipitate formed by mixing Na₂SiO₃ solutions with Pb²⁺ bearing solutions, dissolves at pH<7 by the reaction:

PbSiO₃ + H₂O + 2H⁺ = Pb²⁺ + H₄SiO₄

Measurements of the solubility of PbSiO₃ show that the K_eq for this reaction is 7.41x10⁵ and the ΔG_f° (PbSiO₃ (am)) is 1061.81 kJ mol⁻¹. This high value of K means that extreme amounts of a Na₂SiO₃ additive are required in a water supply system to reduce the Pb²⁺ concentration to the EPA MCL action level of 15 ppb. Furthermore, the high pH values that result from NaSiO₃ water treatments lead to the formation of lead hydroxycarbonate (hydrocerussite) because this carbonate phase is more stable in the pH range of natural waters (pH 5-8) than lead silicate.