AWWA WQTC62405

Because of the many questions surrounding the occurrence of tetravalent lead solids andtheir relationships to lead release, this research investigation was initiated in 2002 in order to:explore the water quality conditions and pathways that lead to the formation of PbO₂ in waterduring long term precipitation experiments; and, to determine the possible sensitivity of PbO₂scales to treatment changes that would affect oxidation-reduction potential (ORP) by analyzing the stability of the solids inresponse to changes in ORP. The physical properties, mineralogy, and solubility of lead solids atseveral pH values in well chlorinated water were examined, and the effect of sulfate on PbO₂formation was also briefly investigated. A series of long-term precipitation experiments were conducted to evaluate the effects of waterchemistry and time on the mineralogy and corresponding solubilities of lead solids in water.During these tests, redox conditions were chosen to represent the high end of realistic utilitypractices and were maintained with the intent to form Pb(IV) solids (plattnerite and/or scrutinyite).High redox potential of the water was maintained with free chlorine, which was replenished asneeded to maintain a goal concentration of 3 mg Cl₂/L throughout most of the studies. Late intothe test runs, chlorine residuals were permitted to dissipate to test the reversibility of mineral phasedevelopment. One run was performed in water containing sulfate to investigate the role of anadditional major anion on the development of lead solids. Lead (II) sulfate and hydroxl-carbonate-sulfatesolids have been observed in some lead drinking water pipes. All experiments wereconducted at room temperature (-23 ºC).Lead aging studies were conducted in a 3.0 liter glass beaker. Secured at the top of thebeaker were pH and two redox electrodes, dissolved oxygen (DO) and temperature probes, amechanical stirrer and an injection line for both acid and base. The computer software recordedpH values and titrant volumes. Data was collected on the pH, dissolved oxygen, redox potential,free and total chlorine, and concentrations of lead, sulfur and total inorganic carbon. In addition thecrystalline phases of lead solids were identified using X-ray diffraction (XRD). Includes 26 references, table, figures.

Product Details

Edition:

Vol. - No.

Published:

11/01/2005

Number of Pages:

14

File Size:

1 file , 360 KB