Understanding Of Silicate-based Corrosion Inhibitor For Controlling Lead Release And Water Quality In Drinking Water Distribution Systems

Abstract

Sodium silicates have been used in drinking water treatment for decades, both as sequestrants and as corrosion inhibitors. For the latter purpose they are poorly understood; a lack of information contributes uncertainty and risks drinking water quality. This work explored three approaches to investigate the effectiveness and mechanisms of sodium silicate on lead release control: lead release of corroded coupons, dissolution rate of lead carbonate, and lead release in a pilot-scale distribution system. Here, silicate treatment did not effectively mitigate the lead release, except for the impact of elevated pH with its use. Sodium silicate also provided minimal protection for galvanic corrosion while orthophosphate effectively mitigated lead release. Impact of silicate adsorption on lead carbonate dissolution was negligible and direct interaction between sodium silicate and lead appears unlikely. Lead carbonates are the major corrosion scales of lead pipes and lead coupons in silicate-treated systems. Although silicate formed a nanometer-thick coating on the top surface of corrosion scales, this mechanism appeared to be negligible in term of lead release. As a sequestrant, sodium silicates dispersed the colloidal metals (e.g., iron, manganese and aluminum) which in turn elevated the lead concentration. Elevated pH is the major mechanism by sodium silicate for lead corrosion mitigation.