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Formation of Pb(IV) Oxides in Chlorinated Water

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Formation of Pb(IV) Oxides in Chlorinated Water Recent research has shown that Pb(IV) oxides play a significant geochemical role in drinking water distribution systems. However, most of the guidance for lead control in drinking water is based on the presumption that Pb(II) solids control lead solubility. Therefore, a better understanding of the chemistry of Pb(IV) in water is needed. Long-term lead precipitation experiments were conducted in chlorinated water (1–3 mg/L Cl2) at pH 6.5, 8, and 10, with ␤ and without sulfate. Results showed that two Pb(IV) dioxide polymorphs—plattnerite ( -PbO2) ␣ and scrutinyite ( -PbO2)—formed over time, as long as a high suspension redox potential BY DARREN A. LYTLE was maintained with free chlorine. Neither mineral formed spontaneously, and the rate of AND MICHAEL R. SCHOCK formation increased with increasing pH. Hydrocerrusite and/or cerrusite initially precipitated out and over time either disappeared or coexisted with PbO2. Water pH dictated mineralogical presence. High pH favored hydrocerrusite and scrutinyite; low pH favored cerrusite and plattnerite. Along with a transformation of Pb(II) to Pb(IV) came a change in particle color from white to a dark shade of red to dark grey (differing with pH) and a decrease in lead solubility. If free chlorine was permitted to dissipate, the aging processes (i.e., mineralogy, color, and solubility) were reversible. Formation of Pb(IV) oxides in chlorinated water ontrolling plumbosolvency and lead release in drinking water distribu- tion systems from lead pipes, brass fixtures, and lead-based solders is a goal of all water utilities. The US Environmental Protection Agency (USEPA) Lead and Copper Rule (LCR) established an action level for lead C at the consumer’s tap of 0.015 mg/L in a 1-L first-draw sample (the sample must sit for at least 6 h before being measured; USEPA, 1992; 1991a; 1991b). Since passage of the LCR, the understanding of relationships between water quality and the solubility of lead-containing minerals found in drinking water distribution systems has grown extensively. The effects of such factors as pH, dissolved inorganic carbon (DIC), and orthophosphate on the solubility of Pb(II) solids are relatively well known (Schock & Clement, 1998; Schock et al, 1996; Schock & Wagner, 1985; Schock & Gardels, 1983). When adjusted appro- priately, these parameters can be used to control lead levels at the tap. Although most water treatment and water chemistry specialists are familiar with the radically different solubilities and scale formation properties of ferrous iron versus ferric iron or cuprous copper versus cupric copper, the same potential 2005 © American Water Works Association 102 NOVEMBER 2005 | JOURNAL AWWA • 97:11 | PEER-REVIEWED | LYTLE & SCHOCK This series of micrographs shows the color transformation of lead solids during experiment A (dissolved inorganic carbon = 10 mg/L C, temperature = 24oC, pH = 7.75–8.19). Numbers indicate the elapsed time in days. Over the first 82 days of the study, the color of the lead precipitate changed from white to dark orange-red in color, but by the end of the experiment (day 397), the red solid had disappeared. behavior of lead is less well known. Potential-pH dia- Pb(IV) in drinking water systems. Schock and colleagues grams for the lead system going back many years have a (Schock, 2001; Schock et al, 1996) have published the prominent stability field for the highly insoluble lead only reports of the presence of Pb(IV) in drinking water dioxide (PbO2) solid (Schock, 1999; Schock et al, 1996; distribution systems. USEPA analyses of pipe scales from Schock & Wagner, 1985; Schock, 1981; Schock, 1980; lead pigtails and service lines collected in the late 1980s Pourbaix et al, 1966; Delahay et al, 1951). Thus an anal- and early 1990s verified that one or both of the common ␤ ogy can be drawn between the Pb(IV)–Pb(II) redox cou- polymorphs of PbO2—plattnerite ( -PbO2) and scruti- ␣ ple and the Fe(III)–Fe(II) redox couple. The higher-oxi- nyite ( -PbO2)—were present in varying degrees on the dation-state forms of the iron oxide or oxyhydroxide pipes from several different water systems (Schock et al, pipe-scale phases have much lower solubility than those 1996). In subsequent years, pipe-scale analyses from many of the lower oxidation state. In the case of lead, how- additional water systems have been performed. Of more ever, the oxidation–reduction potential (ORP) required for than 85 lead pipe specimens obtained from 34 water sys- the transformation of Pb(II) to Pb(IV) is much higher tems, at least 16 specimens representing 9 systems have ␣ ␤ than for the ferrous to ferric iron transformation. -PbO2, -PbO2, or both present in clearly identifiable quantities based on x-ray diffraction (XRD) analysis. More samples may have trace amounts that are hard to positively confirm. Usually the PbO2 was found to exist in the form of patches or a thin surficial layer at the water boundary. In the case of lead-service line deposits from Cincinnati, Ohio, the pipe scales were in contact with relatively high disinfectant concentrations for long periods of time, followed by a combi- nation of granular activated carbon and free chlorine treatment (Schock, 2001). Only traces of basic cerussite (PbCO3) were found. It is not known when the PbO2 scale formed in the Cincinnati pipe, because no historical scale analyses were performed during several earlier treatment schemes. The distribution system area where the Cincinnati pipe specimens were obtained has recorded elevated pH (8.5–9.2) since the 1980s for corrosion control. In con- trast, lead service line specimens obtained from Madison, Wis., had a thin layer of continuous PbO2 at the water interface, with PbCO3 mak- ing up the bulk of the underlying scale. Madison represents a groundwater low in total organic carbon but with high alkalinity (~300 mg/L as calcium carbonate [CaCO3]) and a pH near neutral. Because of the existence of the PbCO3 Stereomicrographs collected on filters during experiment A show lead particles phase, lead concentrations were expected to be at 18 elapsed days (A), 64 elapsed days (B), 82 elapsed days (C), 145 elapsed high. Instead, the 90th-percentile lead concen- days (D), and 397 elapsed days (E). (Experimental conditions were: dissolved tration during LCR monitoring in 1992 was inorganic carbon = 10 mg/L C, temperature = 24oC, pH = 7.75–8.19; all images remarkably low (only 0.016 mg/L), which is are at 65× magnification). more than a factor of 10 below the solubility of 2005 © American Water Works Association LYTLE & SCHOCK | PEER-REVIEWED | 97:11 • JOURNAL AWWA | NOVEMBER 2005 103 Micrographs show the color difference in lead solids produced in experiment A at pH 8, day 145 (left), and experiment D at pH 10, day 30 (right). tain a high concentration of free chlorine to combat biofilm problems or overcome corrosivity toward iron and its pipe wall demand. Questions raised. The previous scale analysis observa- tions raised several important questions. How prevalent PbCO3 (Cantor, 2004). Subsequent targeted lead service is Pb(IV) in drinking water distribution systems? Can it line sampling also showed that approximately 90% of occur in some parts of a water distribution system and not all of those samples were <0.015 mg/L in total lead (Can- others? What is the significance of the presence of Pb(IV) tor, 2004). relative to lead release, and how does it affect lead treat- A significant amount of tetravalent lead scale mater- ment guidance? How quickly and by what pathways are ial was also found in pipe specimens from different parts Pb(IV) solids formed, and how stable are they in response of the Oakwood, Ohio, distribution system. The water has to a variety of water treatment changes? What is the a pH of 7 and a high alkalinity (~350 mg/L as CaCO3); chemical pathway by which PbO2 solids are broken down, greensand filtration is used for iron removal. Taken in and why does it cause periodic increases in lead concen- combination, these observations provide a reasonable trations? Furthermore, these questions draw attention to Controlling plumbosolvency and lead release in drinking water distribution systems from lead pipes, brass fixtures, and lead-based solders is a goal of all water utilities. hypothesis to explain the apparent anomaly observed in a large gap in the thermochemical data in terms of the sol- many field studies in which high-alkalinity waters did ubility and speciation of tetravalent lead for these min- not tend to produce nearly as high levels of lead release erals, which makes it difficult to precisely and accurately as would be expected based on the knowledge of lead model either characteristic (Schock & Giani, 2004; solubility chemistry and bench-scale tests (Edwards et al, Schock, 2001). 1999; Dodrill, 1995; Dodrill & Edwards, 1995; Dodrill Purpose of this research. This investigation was initiated & Edwards, 1994). in 2002 to investigate the many questions surrounding the PbO2 and ORP. The presence of PbO2 is associated with occurrence of tetravalent lead solids and their relationships water of persistently high ORP, which is only possible to lead release. The research was undertaken with two by maintaining sufficient levels of free chlorine in water principal objectives. First was the initial exploration of the in contact with lead service lines and other lead materi- water quality conditions and pathways that lead to the for- als. Certain water chemistry conditions favor higher and mation of PbO2 in water during long-term precipitation persistent ORP. These conditions
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