Date of Award
10-30-2024
Degree Name
Doctor of Philosophy (PhD)
Degree Type
Dissertation
Abstract
Lead release from lead pipes into drinking water is a critical threat to public health. Orthophosphate addition is an established method of controlling lead concentrations in tap water. Many phosphate-based inhibitors currently used by utilities are blends of orthophosphate and polyphosphate. Orthophosphate can limit lead release from pipes by forming low-solubility lead phosphate solids on pipe inner surfaces. In contrast, polyphosphate can accelerate lead release by forming soluble lead-phosphate complexes. Due to the potential conversion from polyphosphate to orthophosphate in the distribution system and to complex dynamics of water chemistry in lead service lines (LSLs), the net impact of a blended phosphate chemical on lead in tap water had been challenging to assess. In addition to lead-phosphate solids, lead(IV) oxide (PbO2) solids in the scales of LSLs can play an important role in controlling lead concentrations. PbO2 has an extremely low solubility (below 0.1μg/L for most situations) that has the potential to maintain desired low lead concentrations in water. However, PbO2 is only formed and stable at a high redox potential environment, as Pb(IV) is highly oxidative and can easily undergo reductive dissolution. Free chlorine, as an important tap water infectant, is a strong oxidant that can help form and maintain PbO2 in LSLs. However, the threshold free chlorine to maintain PbO2 had been determined for layers of PbO2 on the surfaces of elemental lead materials. There were also unresolved questions regarding the impacts of stagnation time and orthophosphate dosing on the stability of PbO2 in scales of harvested lead pipes. This dissertation includes five studies aimed at providing a better understanding of the role of phosphate and residual free chlorine on the precipitation, stability and dissolution of lead(IV) oxide and lead-phosphate solids at conditions relevant to LSLs involved in residential drinking water supply. In the first study, the effects of polyphosphate presence as a component of a mixture with orthophosphate on lead release were investigated on harvested lead pipes. This study confirmed that orthophosphate will be a better corrosion control choice for LSLs than a blended phosphate for a system that has a major goal of lead corrosion control and no specific need for metal sequestration. The second study investigated the stability of PbO2 on the surface of metallic lead coupons and its potential role in corrosion control. This study found that for systems that have PbO2 already present in lead scales, promoting conditions that maintain PbO2 stability could be an effective method for limiting lead release to drinking water. A threshold concentration of residual free chlorine was identified to maintain PbO2 stability. In the third study, the effect of residual free chlorine and water stagnation on lead release associated with PbO2 dissolution was evaluated with actual LSLs harvested from a public water system. Similarly to in the study with lead coupons, this study with lead pipes identified a threshold free chlorine concentration below which PbO2 undergoes reductive dissolution. Long water stagnation times (five days) can exacerbate the lead release from pipes into water especially for particulate lead. Scale analysis reveals that plattnerite (β-PbO2) was the primary component in the pipe scales and that most PbO2 pipe scales maintained their integrity during periods of water stagnation even when stagnation did result in substantial increases in dissolved lead. The fourth study investigated the effect of orthophosphate on lead release from PbO2-rich layers on lead pipes and coupons. The study found that the use of orthophosphate in LSLs with rich PbO2 layers in scales may not provide a substantial benefit for lead corrosion control during the initial phase of its addition. The fifth study explored the variations in lead pipe scales from 43 different drinking water systems based on laboratory scale analysis together with data from previous research. It also evaluated the applications and limitations of chemical equilibrium models for predicting the solid phases present in lead pipe scales. The study found that the compositions of lead pipe scales depend on water chemistry, corrosion control methods, and disinfectant types. While equilibrium predictions are not completely predictive and have certain limitations, they remain valuable for making initial assessments and screening lead solids in pipe scales.
Language
English (en)
Chair
Daniel Giammar
Committee Members
Daniel Giammar; Fangqiong Ling; Jeffrey Catalano; Peng Bai; Young-Shi Jun