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Microelectrode investigation of the reactions between metallic pipe materials and monochloramine
Citation:
Gonzalez, B., J. Liggett, J. Pressman, D. Wahman, AND D. Lytle. Microelectrode investigation of the reactions between metallic pipe materials and monochloramine. Presented at Water Quality Technology Conference, Salt Lake City, UT, November 15 - 19, 2015.
Impact/Purpose:
The information presented will be useful for government, utilities, and university academics on the reactions between chloramines and drinking water distribution system materials.
Description:
Water quality parameters (i.e., pH, dissolved oxygen [DO], and phosphate) are known to impact metal reactivity with disinfectants and therefore corrosion and metals release into drinking water supplies. With various water utilities switching from free chlorine to chloramines for secondary disinfection or considering a changeover in the near future, additional research is needed to understand the interactions of chloramines with typical pipe materials. Typically, bulk water measurements are used to infer the reactions of disinfectant with metal surfaces, but there is evidence that the localized surface concentrations may differ substantially from bulk water measurements. Microelectrodes have contributed to a greater understanding of biological mechanisms in biofilm for decades; however, their application to provide a more detailed knowledge of the micro scale processes of pipe corrosion is just beginning. The tiny tip (5-15µm) of a microelectrode enables the measurement of water chemistry conditions within 10-30 µm of the surface, providing new information on concentrations at metal surfaces to study corrosion and metals release. Thus, the goals of this study were to use microelectrodes to investigate the microscale in-situ reaction of monochloramine with aged abiotic pipe materials (ductile-iron and copper) under various water conditions.Four different water conditions were created by varying the pH (7 or 9) and phosphate concentration (0 or 3 mg/L). Using these four water conditions, the reactions between aged ductile iron and copper coupons and monochloramine (2 mg Cl2/L) were analyzed through the use of pH, DO, and monochloramine microelectrodes during both flowing and stagnant conditions.For both metals, the monochloramine profiles showed a decreasing monochloramine concentration from the bulk water to the metal coupon’s surface; however, for the ductile iron coupons, the monochloramine loss was greater, decreasing from 2.0mg Cl2/L to 0.5mg Cl2/L. This is noteworthy as maintaining monochloramine residual in the surface layer is critical for disinfection of biological contaminants. For DO profiles, the DO concentration showed a minimal decrease from the bulk water to the copper coupon’s surface, but for ductile iron coupons, a sharp DO decrease was seen for all conditions tested as one moved from the bulk water to the metal’s surface. Considering the pH profiles, there were again minimal changes using copper coupons, but for the ductile iron coupons, the pH increased up to 1.4 pH units from the bulk water to the metal’s surface for pH 7.0 and 9.0 in the absence of phosphate. Overall, these results indicated that even after 250 days of aging, the iron coupons were more reactive than the copper coupons for all conditions tested.
URLs/Downloads:
http://www.awwa.org/Portals/0/files/education/conferences/wqtc/wqtc15/pdfs/2015WQTCTechnicalProgramWEB.pdf