Date of Award
Summer 8-15-2022
Degree Name
Doctor of Philosophy (PhD)
Degree Type
Dissertation
Abstract
Lead (Pb) in tap water is mainly contributed by lead service lines (LSLs). The dissolved lead concentration is controlled by the solubilities of lead corrosion products that form on the inner surface of LSLs. Among these lead corrosion products, lead(IV) oxide (PbO2) has the lowest solubility and can maintain a low lead level when free chlorine is present. Current strategies to control the lead concentration include replacing the LSL, optimizing the water chemistry conditions, and using point-of-use (POU) filters. The present study investigated the effect of manganese (Mn) on the oxidation of the lead(II) carbonate solids by free chlorine, the impact of dissolved copper and zinc on PbO2 reductive dissolution, and the redox-driven recrystallization of PbO2. Opportunities to use POU filters as lead monitoring devices were explored, and a specific challenge to POU filter performance for lead removal was also investigated.The effect of manganese on lead(II) carbonate oxidation was determined in batch and flow-through experiments, solid characterization, and kinetic modeling. Lead(IV) oxide can form via the oxidation of lead(II) carbonate solids by free chlorine. Manganese is present at appreciable concentrations in many water distribution systems. The oxidation of lead(II) carbonate is an auto-catalytic process, which means the product, PbO2, acts as a catalyst for the reaction. The presence of manganese can boost oxidation by forming Mn(IV) oxide, which catalyzes lead(II) oxidation. The effect of copper (Cu) and zinc (Zn) on PbO2 reductive dissolution was investigated under drinking water conditions. Although PbO2 has a low solubility, it is only stable when free chlorine is present. When free chlorine is depleted, PbO2 could undergo reductive dissolution. Other solutes in tap water may affect this reaction. Dissolved Zn(II) and Cu(II) are commonly detected in tap water and can be released into the water by copper pipes and brass fittings. The presence of Zn(II) and Cu(II) inhibits PbO2 reductive dissolution for reaction times less than12 h and enhances its reductive dissolution after 48 h. The inhibitory and enhancement effects were delineated with experimental methods coupled with solid characterization. Molecular-scale structures of surface complexes of Cu(II) and Zn(II) on PbO2 were determined with extended X-ray absorption fine structure spectra. Interaction of dissolved Pb(II) and PbO2 was determined under free chlorine-depleted conditions. Lead(II) can be released by the dissolution of lead-containing corrosion products. The atom exchange between aqueous Pb(II) and solid Pb(IV)O2 was delineated using 207Pb(II) as an isotope tracer. After experiments, solids were collected for characterization, and no changes in the solid phase were detected using X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electron microscopy. Results from isotope exchange experiments and solid characterization establish that redox-driven of PbO2 recrystallization occurs upon its reaction with Pb(II). Challenges and opportunities were identified for POU filters. POU filters are certified to remove lead from tap water. Opportunities for POU filters include using them as lead monitoring devices and as lead exposure evaluation devices. These opportunities are built on (1) the ability of POU filters to accumulate the vast majority of lead from tap water and (2) the ability to fully recover the accumulated lead. Experiments showed that an acid flow-through method could achieve a >99% recovery from used POU filters. This method works for both dissolved and particulate lead and was tested with synthetic and real tap water. A challenge for POU filters is their underperformance for lead removal for some water compositions. When challenged in the laboratory with water containing lead phosphate particles with different ionic strengths and hardness, these filters failed to maintain a low lead concentration in the filtered water under low hardness and low ionic strength conditions.
Language
English (en)
Chair
Daniel E. Giammar
Committee Members
Jeffrey G. Catalano, Young-Shin Jun, Peng Bai, William E. Buhro,