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Crystal and morphological phase transformation of Pb(II) to Pb(IV) in chlorinated water
Journal of Hazardous Materials 165 (2009) 1234–1238
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Short communication
Crystal and morphological phase transformation of Pb(II) to Pb(IV) in chlorinated water夽 Darren A. Lytle a,∗ , Colin White a , Mallikarjuna N. Nadagouda b , Adam Worrall c a
United States Environmental Protection Agency, ORD, NRMRL, WSWRD, TTEB, Cincinnati, OH 45268, United States Pegasus Technical Services, 46 E Hollister Street, Cincinnati, OH 45219, United States c University of Cincinnati, Department of Chemical and Materials Engineering, Cincinnati, OH 45221, United States b
a r t i c l e
i n f o
Article history: Received 22 February 2008 Received in revised form 24 September 2008 Accepted 7 October 2008 Available online 1 November 2008 Keywords: Morphology PbO2 X-ray diffraction Crystal structure Drinking water
a b s t r a c t Herein, we show an important transformation of Pb(II) to Pb(IV) in chlorinated water under laboratory conditions. The study results will give an insight toward understanding how corrosion by-products on lead materials found in drinking water distribution systems develop and breakdown with time. The experiments were conducted to elucidate the morphology of lead (IV) oxide mineral transformation from hydrocerussite and its relationship to color change over a period of time. Scanning electron microscopy and transmission electron microscopy were used to describe the surface morphology, shape and size of lead solids. X-ray diffraction (XRD) analysis was performed to determine the mineral structure of lead solids. Solids analysis results were compared over a 14-day period of time to define changes in the crystal structure and morphology of lead solids. XRD analysis results of freshly synthesized lead solids showed that hydrocerussite, [Pb3 (CO3 )2 (OH)2 ], was the only lead mineral present. After 14 days, a mixture of cerussite (PbCO3 ) and ␣-PbO2 and -PbO2 was present. Lead precipitates, i.e. hydrocerussite changed color from white to reddish brown confirming a transformation of the lead phase with time. This was correlated to a change in morphology from flower shaped crystals to hexagonal bars and submicron particles. Published by Elsevier B.V.
1. Introduction Controlling plumbosolvency and lead release in drinking water distribution systems (DWDS) from lead pipes, brass fixtures, and lead-based solders is a goal of all water utilities. In 1991, the U.S. Environmental Protection Agency’s (U.S. EPA) Lead and Copper rule established an action level for lead 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) [1–3]. Since the rule’s passage, the understanding of relationships between water quality and the solubility of lead-containing minerals found in DWDS has grown extensively. The impacts of water quality parameters such as pH, dissolved inorganic carbon (DIC), and orthophosphate on the solubility of Pb(II) solids are relatively well known [4–7]. When adjusted
夽 Any opinions expressed in this paper are those of the author(s) and do not, necessarily, reflect the official positions and policies of the U.S. EPA. Any mention of products or trade names does not constitute recommendation for use by the U.S. EPA. ∗ Corresponding author. Fax: +1 513 569 7892. E-mail addresses: [email protected] (D.A. Lytle), [email protected] (C. White), [email protected] (M.N. Nadagouda), [email protected] (A. Worrall). 0304-3894/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.jhazmat.2008.10.058
appropriately, these parameters can be used to control lead levels at the tap. Recent attention has focused on these factors since the widespread reported occurrence of elevated lead levels in Washington D.C.’s distribution system. The cause was attributed to Pb(IV) to Pb(II) transformations of lead corrosion by-products that followed disinfection practice changes [8]. Historically, lead control in drinking water distribution systems has focused on Pb(II) mineralogy and solubility, and little attention has been given to Pb(IV). As a result, a number of recent research efforts have focused on Pb(II) to Pb(IV) transformation in drinking waters [9–11]. Schock et al. [6] have identified the presence of Pb(IV) in drinking water distribution systems. Lead pig-tail and service line samples collected by U.S. EPA had common polymorphs of Pb(IV) oxide, PbO2 (plattnerite and scrutinyite), in varying degrees [6]. In subsequent years, more pipe scale analyses have been performed from many additional water systems. Thus far, of more than 85 lead pipe specimens obtained from 34 water systems, at least 16 specimens representing 9 systems have either ␣-PbO2 , -PbO2 , or both present in clearly identifiable quantities based on X-ray diffraction (XRD) analysis. Usually, the PbO2 was found to exist in the form of patches or a thin surface layer at the water boundary. These observations are important because the historical approaches to controlling lead levels in drinking water distribution systems (DWDS) are based on Pb(II) chemistry.
D.A. Lytle et al. / Journal of Hazardous Materials 165 (2009) 1234–1238
Potential-pH diagrams for the lead system going back many years have a prominent stability field for the highly insoluble lead dioxide, PbO2 , solid [12–15,5,6,16]. Unlike Pb(II), a large gap in the thermodynamic data in terms of the solubility and speciation of Pb(IV) for PbO2 minerals in water makes it difficult to precisely and accurately model either [17–18]. The objective of this work is to document the important transformation from Pb(II) to Pb(IV) in chlorinated water with the intent to represent the formation of Pb(IV) species in drinking water lead pipes over time with a focus on the morphological and crystalline features. These results will provide insight toward an understanding of how corrosion by-product scales form on lead pipes, and the dissolution of lead into DWDS. 2. Experimental 2.1. Materials Lead precipitation experiments were prepared in chemically adjusted double-deionized (DDI) water. Double-deionized water was prepared by passing building demineralized water through a cartridge deionized water system with a resistivity ≥18.2 M cm. All chemicals used in this research were analytical reagent-grade. Diluted 0.6 M hydrochloric acid and 0.5N sodium hydroxide were used to adjust the pH, and sodium bicarbonate was used to adjust DIC concentration. Chloride was added to test water as sodium chloride. Sodium hypochlorite (4–6% NaOCl, purified-grade) was added to maintain chlorine residual, and lead was added as lead chloride. 2.2. Synthesis of PbO2 The synthesis PbO2 was conducted in a 1 L glass beaker at room temperature (∼23 ◦ C). Secured to the top of the beaker were a pH electrode, two redox electrodes, a mechanical stirrer, and an injection line for both acid and base. A computer software-controlled dual titrator system was used to adjust the initial pH as well as rapidly compensate for pH changes due to chemical additions and reactions; adjustments were made with additions of small increments of acid or base. Sodium bicarbonate, sodium chloride, and sodium hypochlorite were added to 1 L of deionized water at initial concentration goals of 10 mg C/L, 300 mg Cl/L and 3 mg Cl2 /L, respectively. The titration system was programmed to maintain
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the test water at pH 8.0. Once stabilized, lead chloride was added to provide an initial lead concentration of 20 mg/L. After 30 min, “fresh” samples were drawn for transmission electron microscopy (TEM), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) analysis. A 60 mL suspension volume was filtered through a 0.2 m polypropylene (PP) syringe 25 mm disk filter (Whatman Inc., Clifton, NJ) at a rate of 100 mL/min to separate colloidal lead for metal analysis. A second volume of 120 mL was filtered through a 0.02 m sterile Anotop® 47 mm filter (glass fiber prefilter and Anopore® inorganic membrane) (Whatman Inc., Clifton, NJ) using in-house vacuum line to be used for solids analysis. Once the precipitation cake formed, the filter was used for XRD and microscopy analysis. Additional samples were collected in two 250 mL PTFE bottles with no headspace, and placed in a tumbler to age at room temperature (23 ◦ C) for a period of 14 days. Samples were taken from the aged test water for XRD, TEM, SEM and energy dispersive X-ray analysis (EDS) analysis. 2.3. Characterization A JEOL-1200EXII TEM with a side-mounted Gatan digital camera was used for the imaging of precipitated PbO2 . 15 L of lead solution was placed on a formvar-carbon coated nickel grid and allowed to air dry. Images were collected using Gatan software at an accelerating voltage of 120 kV. For SEM, a JEOL-6490LV with an Oxford X-Act EDS system was used for imaging and elemental analysis generally following ASTM E1508 procedures. Grids containing PbO2 , previously viewed on the TEM, were placed on double-sided carbon tape and adhered to an aluminum stub. Images and EDS spectra were collected using an accelerating voltage of 15 kV. Spectra were collected for 50 live seconds using a process time of 5 and a 30% dead-time. A Scintag (Scintag, Inc., Santa Clara, CA) XDS-2000 theta-theta diffractometer with a copper K␣ source was used to record XRD patterns of the lead precipitates. The tube was operated at 35 kV and 40 mA for the analyses. Scans were performed over a 2-theta range between 5 and 90◦ with a step of 0.02◦ with a 1-s count time at each step. Pattern analysis was performed generally following ASTM procedures [19] using the computer software Jade (Versions 5–7, Materials Data, Inc.) with reference to the 1995–2002 ICDD PDF-2 data files (International Center for Diffraction Data, Newtown Square, PA). XRD d-spacing results were not corrected for sample displacement by the filters.
Fig. 1. Photographic image of (a) freshly precipitated lead (hydrocerussite) and (b) aged lead (PbO2 and cerussite) samples.
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D.A. Lytle et al. / Journal of Hazardous Materials 165 (2009) 1234–1238
Fig. 2. XRD patterns of (a) hydrocerussite (Pb3 (CO3 )2 (OH)2 ) aged for 14 days, i.e. cerussite (b) as precipitated hydrocerussite (Pb3 (CO3 )2 (OH)2 ). H = hydrocerussite, pattern No. 013-0131, C = cerussite, pattern 047–1134, P = plattnerite, pattern 041–1492, and S = scrutinyite, pattern 045–1416.
3. Results and discussion PbO2 synthesis was conducted in the initial conditions of 15.1 mg Pb/L, 10 mg C/L DIC, 2.82 mg Cl2 /L free chlorine, and a pH of 8.02. White lead precipitate was visibly present and 0.2 m filtered lead concentration was 0.228 mg/L, indicating that the majority of lead was in the particulate form. Following 14 days of aging, the suspension pH decreased to 7.56 and 1.91 mg Cl2 /L of free chlorine was consumed. The chlorine consumption was attributed largely to the oxidation of Pb(II) to Pb(IV). A filtered (0.2 m) lead concentration after aging was not detectable (<0.002 mg/L), indicating a shift in lead solubility and mineralogy with time. During the 14-day aging period, the color of the precipitated lead turned from white to reddish-brown (Fig. 1). Similar observations were made by Edwards and Dudi [20] and Lytle and Schock [11]. Lytle and Schock [11] found that the color of the lead precipitate visible in the water changed gradually but dramatically from white to dark orange-red over an 82-day period of time when free chlorine was present. They found that the orange-red color disappeared with more time after the chlorine was consumed to the eventual point of depletion. At the completion of the study, only white colored lead solids were present in the water and no visible red color was
Fig. 4. SEM images of (a) lower and (b) higher magnification image of aged cerussite sample.
observed [11]. In order to understand the crystal and morphological changes along with the color and time, XRD and TEM analysis were performed. The results were in agreement with the recent work by Liu et al. [21] on reactions of lead (II) solid phases (hydrocerussite, cerussite) with chlorine. Chlorine consumption profiles for these solids exhibited a lag phase, during which little consumption of chlorine occurred, and an ensuing rapid reaction phase. The durations of these phases were affected by the pH, carbonate, and chlorine concentrations. They observed that hydrocerussite started
Fig. 3. TEM images of (a) precipitated hydrocerussite and (b) aged cerussite samples with submicron PbO2 particles.
D.A. Lytle et al. / Journal of Hazardous Materials 165 (2009) 1234–1238
to be transformed into cerussite during the lag phase and was confirmed by SEM and XRD [21]. XRD patterns (Fig. 2) collected before and after aging of PbO2 (14 days) clearly confirms the transformation of Pb(II) to Pb(IV) solids. As synthesized lead precipitate was indexed to only hydrocerussite [Pb3 (CO3 )2 (OH)2 ]. After aging, all of the peaks for hydrocerussite disappeared and new peaks were observed. These new peaks were indexed to cerussite, PbCO3 , and the Pb(IV) oxides plattnerite, PbO2 , and scrutinyite, ␣-PbO2 . The change in solid color from white to reddish brown indicates the transformation of phase, and is in agreement with the XRD findings (Pb(II) carbonates are white and Pb(IV) oxides are red to brown). TEM images of precipitated samples showed flower shaped crystals representing hydrocerussite. The crystals were electron dense, but not opaque, suggesting a thickness of less than 1500 Å. TEM images of aged samples showed elongated bars generally greater than 5 m in length with submicron particles adhered to the surface. Various crystals of the aged sample, imaged on the SEM, suggested twinning and/or paring on the plane of the crystal surface (Fig. 3). SEM images of aged samples revealed that the bars had a hexagonal longitudinal structure. Submicron particles observed by TEM adhering/nucleating to/at the surface in Fig. 3 resolved as cubic to spherical structures indiscriminately attached to the surface of the hexagonal bars (Fig. 4). EDS analysis confirmed the crystal’s elemental composition as lead (data not shown). Fig. 5 shows selected area diffraction pattern (SAED) of precipitated hydrocerussite and
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aged cerussite samples. The SAED pattern can be indexed to hydrocerussite and cerussite as shown in Fig. 5. The aged cerrusite sample also contain significant amount of PbO2 which can be indexed to major planes of PbO2 such as (1 1 0), (1 1 2) and (1 2 1). 4. Conclusions In summary, we have shown possible tendencies for the transformation of hydrocerussite, [Pb3 (CO3 )2 (OH)2 ] to cerussite, PbCO3 , and the Pb(IV) oxides plattnerite, -PbO2 , and scrutinyite, ␣-PbO2 , i.e. Pb(II) to Pb(IV) compounds. The understanding of these transformations brought about by this work calls attention to the possibility that these transformations may be occurring on a full-scale in DWDS, and further investigation into the formation of these compounds is necessary. Gaining more knowledge factors associated with transformation and surface morphology will provide useful insight into dissolution of Pb in water. Acknowledgements The authors would like to acknowledge fellow U.S. EPA staff Keith Kelty, Brittany Almassalkhi, and Bill Kayler for analytical support, and Michael R. Schock for assistance with XRD analysis. Finally, we would like to thank Melissa Steckhahn of Miami University (Ohio) and Robert Hyland with Pegasus Technical Services for editorial comments. References
Fig. 5. Selected area diffraction pattern of (a) as precipitated hydrocerussite and (b) aged cerussite samples.
[1] Federal Register, Drinking water regulations: maximum contaminant level goals and national primary drinking water regulations for lead and copper, 40 CRF Parts 141 and 142. U.S. EPA, 56 (July) (1991) 32112. [2] Federal Register, Maximum contaminant level goals and national primary drinking water regulations for lead and copper, U.S. EPA, 56 (1991) 26460. [3] Federal Register, Drinking Water Regulations: maximum contaminant level goals and national primary drinking water regulations for lead and copper, 40 CFR Parts 141 and 142, U.S. EPA, 57 (1992) 28785. [4] M.R. Schock, M.C. Gardels, Plumbosolvency reduction by high pH and low carbonate-solubility relationships, J. Am. Water Works Assoc. 75 (2) (1983) 87–91. [5] M.R. Schock, I. Wagner, The corrosion and solubility of lead in drinking water. Internal corrosion of water distribution systems, AWWA Research Foundation/DVGW Forschungsstelle, Denver, CO, 1985, pp. 213–316. [6] M.R. Schock, I. Wagner, R. Oliphant, The Corrosion and Solubility of Lead in Drinking Water. Internal Corrosion of Water Distribution Systems, second ed., AWWA Research Foundation/TZW, Denver, CO, 1996. [7] M.R. Schock, J.A. Clement, Lead and copper control with non-zinc orthophosphate, J. N. Engl. Water Works Assoc. 112 (1) (1998) 20–33. [8] R. Renner, Plumbing the depths of DC’s drinking water crisis, Environ. Sci. Technol. 38 (12) (2004) 224A–227A. [9] D.J. Dryer, G.V. Korshin, Investigation of the reduction of lead dioxide by natural organic matter, Environ. Sci. Technol. 41 (15) (2007) 5510–5514. [10] Y. Lin, R.L. Valentine, The release of lead from the reduction of lead oxide (PbO2 ) by natural organic matter, Environ. Sci. Technol. 42 (3) (2008) 760– 765. [11] D. Lytle, M.R. Schock, Formation of Pb(IV) oxides in chlorinated water, J. Am. Water Works Assoc. 97 (11) (2005) 102–114. [12] P. Delahay, M. Pourbaix, P.V. Rysselberghe, Potential-pH diagram of lead and its applications to the study of lead corrosion and to the lead storage battery, J. Electrochem. Soc. 98 (2) (1951) 57–64. [13] M. Pourbaix, N. De Zoubov, C. Vanleuenhaghe, P. Van Rysselberghe, Atlas of Electrochemical Equilibria in Aqueous Solutions, National Association of Corrosion Engineers, Houston, TX, 1966, vol. Chapter IV, Section 17.5, pp. 485– 492. [14] M.R. Schock, Response of lead solubility to dissolved carbonate in drinking water, J. Am. Water Works Assoc. 72 (12) (1980) 695–704. [15] M.R. Schock, Erratum—response of lead solubility to dissolved carbonate in drinking water, J. Am. Water Works Assoc. 73 (3) (1981) 36. [16] M.R. Schock, Internal Corrosion and Deposition Control. Water Quality and Treatment: A Handbook of Community Water Supplies, fifth ed., McGraw-Hill, Inc., New York, 1999. [17] M.R. Schock, Tetravalent lead: a hitherto unrecognized control of tap water lead contamination, water quality technology conference, American Water Works Association, Nashville, TN, 2001.
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[18] M.R. Schock, R. Giani, Oxidant/disinfectant chemistry and impacts on lead corrosion, Sunday Workshop, AWWA Water Quality Technology Conference, November 14, San Antonio, TX, 2004. [19] ASTM, Standard practices for identification of crystalline compounds in waterformed deposits by x-ray diffraction, Volume 11.02, D 934-80 American Society for Testing and Materials, Conshohocken, PA, 1996.
[20] M. Edwards, A. Dudi, Role of chlorine and chloramine in corrosion of lead-bearing plumbing materials, J. Am. Water Works Assoc. 96 (2004) 69–81. [21] H. Liu, G. Korshin, J. Ferguson, Investigation of the kinetics and mechanisms of the oxidation of cerussite and hydrocerussite by chlorine, Environ. Sci. Technol. 42 (2008) 3241–3247.
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Short communication
Crystal and morphological phase transformation of Pb(II) to Pb(IV) in chlorinated water夽 Darren A. Lytle a,∗ , Colin White a , Mallikarjuna N. Nadagouda b , Adam Worrall c a
United States Environmental Protection Agency, ORD, NRMRL, WSWRD, TTEB, Cincinnati, OH 45268, United States Pegasus Technical Services, 46 E Hollister Street, Cincinnati, OH 45219, United States c University of Cincinnati, Department of Chemical and Materials Engineering, Cincinnati, OH 45221, United States b
a r t i c l e
i n f o
Article history: Received 22 February 2008 Received in revised form 24 September 2008 Accepted 7 October 2008 Available online 1 November 2008 Keywords: Morphology PbO2 X-ray diffraction Crystal structure Drinking water
a b s t r a c t Herein, we show an important transformation of Pb(II) to Pb(IV) in chlorinated water under laboratory conditions. The study results will give an insight toward understanding how corrosion by-products on lead materials found in drinking water distribution systems develop and breakdown with time. The experiments were conducted to elucidate the morphology of lead (IV) oxide mineral transformation from hydrocerussite and its relationship to color change over a period of time. Scanning electron microscopy and transmission electron microscopy were used to describe the surface morphology, shape and size of lead solids. X-ray diffraction (XRD) analysis was performed to determine the mineral structure of lead solids. Solids analysis results were compared over a 14-day period of time to define changes in the crystal structure and morphology of lead solids. XRD analysis results of freshly synthesized lead solids showed that hydrocerussite, [Pb3 (CO3 )2 (OH)2 ], was the only lead mineral present. After 14 days, a mixture of cerussite (PbCO3 ) and ␣-PbO2 and -PbO2 was present. Lead precipitates, i.e. hydrocerussite changed color from white to reddish brown confirming a transformation of the lead phase with time. This was correlated to a change in morphology from flower shaped crystals to hexagonal bars and submicron particles. Published by Elsevier B.V.
1. Introduction Controlling plumbosolvency and lead release in drinking water distribution systems (DWDS) from lead pipes, brass fixtures, and lead-based solders is a goal of all water utilities. In 1991, the U.S. Environmental Protection Agency’s (U.S. EPA) Lead and Copper rule established an action level for lead 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) [1–3]. Since the rule’s passage, the understanding of relationships between water quality and the solubility of lead-containing minerals found in DWDS has grown extensively. The impacts of water quality parameters such as pH, dissolved inorganic carbon (DIC), and orthophosphate on the solubility of Pb(II) solids are relatively well known [4–7]. When adjusted
夽 Any opinions expressed in this paper are those of the author(s) and do not, necessarily, reflect the official positions and policies of the U.S. EPA. Any mention of products or trade names does not constitute recommendation for use by the U.S. EPA. ∗ Corresponding author. Fax: +1 513 569 7892. E-mail addresses: [email protected] (D.A. Lytle), [email protected] (C. White), [email protected] (M.N. Nadagouda), [email protected] (A. Worrall). 0304-3894/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.jhazmat.2008.10.058
appropriately, these parameters can be used to control lead levels at the tap. Recent attention has focused on these factors since the widespread reported occurrence of elevated lead levels in Washington D.C.’s distribution system. The cause was attributed to Pb(IV) to Pb(II) transformations of lead corrosion by-products that followed disinfection practice changes [8]. Historically, lead control in drinking water distribution systems has focused on Pb(II) mineralogy and solubility, and little attention has been given to Pb(IV). As a result, a number of recent research efforts have focused on Pb(II) to Pb(IV) transformation in drinking waters [9–11]. Schock et al. [6] have identified the presence of Pb(IV) in drinking water distribution systems. Lead pig-tail and service line samples collected by U.S. EPA had common polymorphs of Pb(IV) oxide, PbO2 (plattnerite and scrutinyite), in varying degrees [6]. In subsequent years, more pipe scale analyses have been performed from many additional water systems. Thus far, of more than 85 lead pipe specimens obtained from 34 water systems, at least 16 specimens representing 9 systems have either ␣-PbO2 , -PbO2 , or both present in clearly identifiable quantities based on X-ray diffraction (XRD) analysis. Usually, the PbO2 was found to exist in the form of patches or a thin surface layer at the water boundary. These observations are important because the historical approaches to controlling lead levels in drinking water distribution systems (DWDS) are based on Pb(II) chemistry.
D.A. Lytle et al. / Journal of Hazardous Materials 165 (2009) 1234–1238
Potential-pH diagrams for the lead system going back many years have a prominent stability field for the highly insoluble lead dioxide, PbO2 , solid [12–15,5,6,16]. Unlike Pb(II), a large gap in the thermodynamic data in terms of the solubility and speciation of Pb(IV) for PbO2 minerals in water makes it difficult to precisely and accurately model either [17–18]. The objective of this work is to document the important transformation from Pb(II) to Pb(IV) in chlorinated water with the intent to represent the formation of Pb(IV) species in drinking water lead pipes over time with a focus on the morphological and crystalline features. These results will provide insight toward an understanding of how corrosion by-product scales form on lead pipes, and the dissolution of lead into DWDS. 2. Experimental 2.1. Materials Lead precipitation experiments were prepared in chemically adjusted double-deionized (DDI) water. Double-deionized water was prepared by passing building demineralized water through a cartridge deionized water system with a resistivity ≥18.2 M cm. All chemicals used in this research were analytical reagent-grade. Diluted 0.6 M hydrochloric acid and 0.5N sodium hydroxide were used to adjust the pH, and sodium bicarbonate was used to adjust DIC concentration. Chloride was added to test water as sodium chloride. Sodium hypochlorite (4–6% NaOCl, purified-grade) was added to maintain chlorine residual, and lead was added as lead chloride. 2.2. Synthesis of PbO2 The synthesis PbO2 was conducted in a 1 L glass beaker at room temperature (∼23 ◦ C). Secured to the top of the beaker were a pH electrode, two redox electrodes, a mechanical stirrer, and an injection line for both acid and base. A computer software-controlled dual titrator system was used to adjust the initial pH as well as rapidly compensate for pH changes due to chemical additions and reactions; adjustments were made with additions of small increments of acid or base. Sodium bicarbonate, sodium chloride, and sodium hypochlorite were added to 1 L of deionized water at initial concentration goals of 10 mg C/L, 300 mg Cl/L and 3 mg Cl2 /L, respectively. The titration system was programmed to maintain
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the test water at pH 8.0. Once stabilized, lead chloride was added to provide an initial lead concentration of 20 mg/L. After 30 min, “fresh” samples were drawn for transmission electron microscopy (TEM), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) analysis. A 60 mL suspension volume was filtered through a 0.2 m polypropylene (PP) syringe 25 mm disk filter (Whatman Inc., Clifton, NJ) at a rate of 100 mL/min to separate colloidal lead for metal analysis. A second volume of 120 mL was filtered through a 0.02 m sterile Anotop® 47 mm filter (glass fiber prefilter and Anopore® inorganic membrane) (Whatman Inc., Clifton, NJ) using in-house vacuum line to be used for solids analysis. Once the precipitation cake formed, the filter was used for XRD and microscopy analysis. Additional samples were collected in two 250 mL PTFE bottles with no headspace, and placed in a tumbler to age at room temperature (23 ◦ C) for a period of 14 days. Samples were taken from the aged test water for XRD, TEM, SEM and energy dispersive X-ray analysis (EDS) analysis. 2.3. Characterization A JEOL-1200EXII TEM with a side-mounted Gatan digital camera was used for the imaging of precipitated PbO2 . 15 L of lead solution was placed on a formvar-carbon coated nickel grid and allowed to air dry. Images were collected using Gatan software at an accelerating voltage of 120 kV. For SEM, a JEOL-6490LV with an Oxford X-Act EDS system was used for imaging and elemental analysis generally following ASTM E1508 procedures. Grids containing PbO2 , previously viewed on the TEM, were placed on double-sided carbon tape and adhered to an aluminum stub. Images and EDS spectra were collected using an accelerating voltage of 15 kV. Spectra were collected for 50 live seconds using a process time of 5 and a 30% dead-time. A Scintag (Scintag, Inc., Santa Clara, CA) XDS-2000 theta-theta diffractometer with a copper K␣ source was used to record XRD patterns of the lead precipitates. The tube was operated at 35 kV and 40 mA for the analyses. Scans were performed over a 2-theta range between 5 and 90◦ with a step of 0.02◦ with a 1-s count time at each step. Pattern analysis was performed generally following ASTM procedures [19] using the computer software Jade (Versions 5–7, Materials Data, Inc.) with reference to the 1995–2002 ICDD PDF-2 data files (International Center for Diffraction Data, Newtown Square, PA). XRD d-spacing results were not corrected for sample displacement by the filters.
Fig. 1. Photographic image of (a) freshly precipitated lead (hydrocerussite) and (b) aged lead (PbO2 and cerussite) samples.
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D.A. Lytle et al. / Journal of Hazardous Materials 165 (2009) 1234–1238
Fig. 2. XRD patterns of (a) hydrocerussite (Pb3 (CO3 )2 (OH)2 ) aged for 14 days, i.e. cerussite (b) as precipitated hydrocerussite (Pb3 (CO3 )2 (OH)2 ). H = hydrocerussite, pattern No. 013-0131, C = cerussite, pattern 047–1134, P = plattnerite, pattern 041–1492, and S = scrutinyite, pattern 045–1416.
3. Results and discussion PbO2 synthesis was conducted in the initial conditions of 15.1 mg Pb/L, 10 mg C/L DIC, 2.82 mg Cl2 /L free chlorine, and a pH of 8.02. White lead precipitate was visibly present and 0.2 m filtered lead concentration was 0.228 mg/L, indicating that the majority of lead was in the particulate form. Following 14 days of aging, the suspension pH decreased to 7.56 and 1.91 mg Cl2 /L of free chlorine was consumed. The chlorine consumption was attributed largely to the oxidation of Pb(II) to Pb(IV). A filtered (0.2 m) lead concentration after aging was not detectable (<0.002 mg/L), indicating a shift in lead solubility and mineralogy with time. During the 14-day aging period, the color of the precipitated lead turned from white to reddish-brown (Fig. 1). Similar observations were made by Edwards and Dudi [20] and Lytle and Schock [11]. Lytle and Schock [11] found that the color of the lead precipitate visible in the water changed gradually but dramatically from white to dark orange-red over an 82-day period of time when free chlorine was present. They found that the orange-red color disappeared with more time after the chlorine was consumed to the eventual point of depletion. At the completion of the study, only white colored lead solids were present in the water and no visible red color was
Fig. 4. SEM images of (a) lower and (b) higher magnification image of aged cerussite sample.
observed [11]. In order to understand the crystal and morphological changes along with the color and time, XRD and TEM analysis were performed. The results were in agreement with the recent work by Liu et al. [21] on reactions of lead (II) solid phases (hydrocerussite, cerussite) with chlorine. Chlorine consumption profiles for these solids exhibited a lag phase, during which little consumption of chlorine occurred, and an ensuing rapid reaction phase. The durations of these phases were affected by the pH, carbonate, and chlorine concentrations. They observed that hydrocerussite started
Fig. 3. TEM images of (a) precipitated hydrocerussite and (b) aged cerussite samples with submicron PbO2 particles.
D.A. Lytle et al. / Journal of Hazardous Materials 165 (2009) 1234–1238
to be transformed into cerussite during the lag phase and was confirmed by SEM and XRD [21]. XRD patterns (Fig. 2) collected before and after aging of PbO2 (14 days) clearly confirms the transformation of Pb(II) to Pb(IV) solids. As synthesized lead precipitate was indexed to only hydrocerussite [Pb3 (CO3 )2 (OH)2 ]. After aging, all of the peaks for hydrocerussite disappeared and new peaks were observed. These new peaks were indexed to cerussite, PbCO3 , and the Pb(IV) oxides plattnerite, PbO2 , and scrutinyite, ␣-PbO2 . The change in solid color from white to reddish brown indicates the transformation of phase, and is in agreement with the XRD findings (Pb(II) carbonates are white and Pb(IV) oxides are red to brown). TEM images of precipitated samples showed flower shaped crystals representing hydrocerussite. The crystals were electron dense, but not opaque, suggesting a thickness of less than 1500 Å. TEM images of aged samples showed elongated bars generally greater than 5 m in length with submicron particles adhered to the surface. Various crystals of the aged sample, imaged on the SEM, suggested twinning and/or paring on the plane of the crystal surface (Fig. 3). SEM images of aged samples revealed that the bars had a hexagonal longitudinal structure. Submicron particles observed by TEM adhering/nucleating to/at the surface in Fig. 3 resolved as cubic to spherical structures indiscriminately attached to the surface of the hexagonal bars (Fig. 4). EDS analysis confirmed the crystal’s elemental composition as lead (data not shown). Fig. 5 shows selected area diffraction pattern (SAED) of precipitated hydrocerussite and
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aged cerussite samples. The SAED pattern can be indexed to hydrocerussite and cerussite as shown in Fig. 5. The aged cerrusite sample also contain significant amount of PbO2 which can be indexed to major planes of PbO2 such as (1 1 0), (1 1 2) and (1 2 1). 4. Conclusions In summary, we have shown possible tendencies for the transformation of hydrocerussite, [Pb3 (CO3 )2 (OH)2 ] to cerussite, PbCO3 , and the Pb(IV) oxides plattnerite, -PbO2 , and scrutinyite, ␣-PbO2 , i.e. Pb(II) to Pb(IV) compounds. The understanding of these transformations brought about by this work calls attention to the possibility that these transformations may be occurring on a full-scale in DWDS, and further investigation into the formation of these compounds is necessary. Gaining more knowledge factors associated with transformation and surface morphology will provide useful insight into dissolution of Pb in water. Acknowledgements The authors would like to acknowledge fellow U.S. EPA staff Keith Kelty, Brittany Almassalkhi, and Bill Kayler for analytical support, and Michael R. Schock for assistance with XRD analysis. Finally, we would like to thank Melissa Steckhahn of Miami University (Ohio) and Robert Hyland with Pegasus Technical Services for editorial comments. References
Fig. 5. Selected area diffraction pattern of (a) as precipitated hydrocerussite and (b) aged cerussite samples.
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