Effect of Phosphate Inhibitors on the Formation of ... - ACS Publications

Effect of Phosphate Inhibitors on the Formation of Lead Phosphate/ Carbonate Nanorods, Microrods, and Dendritic Structures Mallikarjuna N. Nadagouda,*,† Michael Schock,*,‡ Deborah H. Metz,§ Michael K. DeSantis,† Darren Lytle,‡ and Meghan Welch†

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 4 1798–1805

Pegasus Technical SerVices, 46 East Hollister Street, Cincinnati, Ohio 45219, Drinking Water Research DiVision, Risk Reduction Engineering Laboratory U.S. EnVironmental Protection Agency, 26 West Martin Luther King DriVe Cincinnati, Ohio 45268, and Greater Cincinnati Water Works, 4747 Spring GroVe AVenue, Cincinnati, Ohio 45232 ReceiVed September 1, 2008; ReVised Manuscript ReceiVed December 11, 2008

ABSTRACT: There are several factors which influence the corrosion rate of lead, which in turn morphs into different crystal shapes and sizes. Some of the important factors are alkalinity, pH, calcium, orthophosphate, and silica. Low to moderate alkalinity decreases corrosion rates, while higher alkalinities have a tendency to increase the corrosion rates of lead. This work describes the effect of orthophosphate inhibitor and pH on the formation of different structures of lead phosphate/carbonate nanorods, nanobelts, microrods, and dendritic structures. The experiments were carried out at different pHs both with and without orthophosphate inhibitor under laboratory conditions, which were intended to represent actual drinking water distribution system (DWDS) conditions. The surface morphology and crystal structure of the different crystals were obtained using scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDS), transmission electron microscopy (TEM), and selected area diffraction pattern (SAED). The phase identification was done using powder X-ray diffraction (PXRD). With the increase in pH from 6.5 to 8.5, the formation of uniform thickness coating of phosphate containing minerals was observed, which was in contrast to the different crystal growth under low pH conditions. The XRD patterns indicate that the surface solids contain a mixture of many phases. Introduction One-dimensional nanostructures, such as nanotubes, nanowires, and nanobelts, have attracted extraordinary attention for their novel physical properties and potential applications in constructing nanoscale electronic and optoelectronic devices.1-4 One-dimensional nanostructures with different morphologies and compositions have been successfully prepared using microwave,5 vitamins,6,7 wet chemistry,8 laser ablation,9,10 chemical vapor deposition,11 thermal evaporation,12,13 and soft chemistry.14 Among these one-dimensional structures, lead (Pb) and its related compounds are gaining importance due to their useful applications in ever expanding technology and industries. Historically, lead has been used for service lines to household drinking water systems, and is a component of brass alloys used in faucets and plumbing fixtures. Lead contamination of drinking water from these sources is a major issue, and is subject to regulation under the U.S. EPA Lead and Copper Rule.15 Many studies have been conducted in order to understand lead corrosion and its prevention.16-18 Phosphates play a vital role in preventing calcium scale build-up in water distribution systems, and can prevent lead corrosion in a relatively narrow pH range. Insoluble lead phosphate compounds form a film on the surface of the pipe, and subsequently reduce lead in a consumer’s tap water.19 Consequently, when the U.S. EPA Lead and Copper Rule effectively expanded the meaning of the term “corrosion control” at utilities to explicitly consider lead and copper concentrations at the consumer’s tap, it was hoped that orthophosphate dosing would provide a low cost approach in achieving multiple water quality objectives. * To whom correspondence should be addressed. E-mail: [email protected] (M.N.N.); [email protected] (M.S.). † Pegasus Technical Services. ‡ Risk Reduction Engineering Laboratory U.S. Environmental Protection Agency. § Greater Cincinnati Water Works.

This work investigates how pH and orthophosphate control the morphology and mineralogy of various shapes and sizes of lead crystals and solids formed on lead pipe surfaces under water distribution system conditions. The study aimed to achieve two possible applications (1) to contribute to the understanding of how orthophosphate impacts the lead corrosion process in water and its reduction, and (2) to illustrate the versatility available when making different lead one-dimensional structures at room temperature without employing any special equipment or procedure, which could serve many applications in science and technology. Experimental Procedures The experiments were conducted in recirculating systems comprised of 55 gallon polyethylene tanks with floating lids. Magnetic drive centrifugal pumps were used to maintain the flow through the recirculation system at approximately 3 gallons per minute, yielding a velocity through the piping of 2 feet per second. This rate and velocity reflected conditions that would be realistic in a drinking water distribution system (DWDS). The tanks and lids were constructed of high-density polyethylene. All plumbing, aside from the lead, was PVC, nylon, or Tygon. Sampling ports and chemical feed ports were positioned on each system so that sampling could be performed without exposing the water to air. Floating o-rings of Tygon tubing were used beneath the lids to ensure minimal air contact and gas transfer. Four-foot lengths of new lead pipe (1/2in.-in ID) were conditioned and placed in the recirculation systems. The lead pipe sections were conditioned by chemical cleaning with surfactant soap (Contrad 70), followed by three deionized water rinses. This testing system offered considerable flexibility in the level of control of the critical variables, and in the ability to gain considerable worst-case scenarios relative to solubility and metal release information in a limited amount of time. Two orthophosphate conditions were tested (no inhibitor and orthophosphate, 3 mg/L), making 12 water conditions total (six water qualities × two inhibitor conditions) and allowing for direct and unambiguous comparison of inhibitor effects in a given water. Water used for the study was building DI water amended with sodium bicarbonate, potassium chloride, and calcium chloride.

10.1021/cg8009699 CCC: $40.75  2009 American Chemical Society Published on Web 02/20/2009

Formation of Lead Phosphate/Carbonate Nanorods

Crystal Growth & Design, Vol. 9, No. 4, 2009 1799

Figure 1. SEM images of lead crystals grown on the surface of lead pipe (a) at pH 6.5 without inhibitor, (b) at pH 6.5 with orthophosphate inhibitor, (c) at pH 7 with inhibitor, and (d) 7 pH without orthophosphate inhibitor. Table 1 pipe loop

pH

0201 0202 0203 0204 0205 0206 0207 0208 0209 0210 0211 0212

6.5 6.5 7.0 7.0 7.5 7.5 8.0 8.0 8.5 8.5 9.0 9.0

PO4 added yes yes yes yes yes yes

Table 2 element

weight %

atomic %

CK OK Pb M totals

4.55 17.57 77.87 100.00

20.45 59.27 20.28

Table 3

Figure 2. SEM images of (a, b) dendritic structures grown on lead microrods at pH 7 with inhibitor. Dissolved inorganic carbon (DIC) was maintained at 100 mg/L as carbon. The pH was adjusted and maintained by adding HCL and/ or NaOH, as needed. Stock solutions of orthophosphate inhibitors were prepared at concentrations of 3.0 mg/L using NaH2PO4, and the experiments were done in pairs (with vs without PO4) for pH 6.5, 7.0, 7.5, 8.0, 8.5, and 9.0, respectively (Table 1). To test the effects of aging on lead corrosion byproduct release, one batch of pipes was exposed to water for one year beginning in March 2006. Only two of the study’s samples, sample 0201 and 0202, were pulled from the recirculation system when, after 3 months, a plumbing leak and poor control of DIC were detected. Each 48-in. pipe was cut into four 12-in. length pieces that were subsequently split longitudinally with a band saw using a metal-cutting blade (Sears Craftsman 12” band saw. Blade is a Morse 1/4 25 14R HB). During this process, small lead particles are scattered throughout the pipe and can become embedded in the existing scale. These particles were blown off with a laboratory air jet as much as possible. For SEM analysis one or

element

weight %

atomic %

CK OK Al K PK Cl Pb M total

4.06 12.54 0.09 3.48 3.33 76.49 100.00

19.88 46.08 0.20 6.61 5.53 21.70

more 1-cm wide sections exhibiting characteristic scale morphology for each experimental condition was subsampled from the previously cut sections using the band saw. These pipe pieces were mounted on aluminum sample holder and the images were recorded. For transmission electron microscopy (TEM) measurements a small sample was scratched from the surface and dispersed in water. A drop of dispersion was casted onto copper grid and dried at room temperature. A JEOL-1200EXII TEM with a side mounted Gatan digital camera was used for the imaging of lead pipe surfaces. Fifteen microliters of lead solution was placed on a Formvar-carbon coated nickel/copper grid and allowed to air-dry. Images were captured at an accelerating voltage of 120 kV, and collected using Gatan software. For scanning electron microscopy (SEM), a JEOL-6490LV with an Oxford X-Act EDS system was used for imaging and

1800 Crystal Growth & Design, Vol. 9, No. 4, 2009

Nadagouda et al.

Figure 3. SEM images of lead crystals grown on the surface of lead pipe (a) at pH 7.5 without inhibitor, (b) at pH 7.5 with orthophosphate inhibitor, (c) at pH 8 without inhibitor, and (d) at pH 8 with orthophosphate inhibitor.

Figure 4. EDS analysis of lead crystals grown on the surface of lead pipe (a) at pH 7.5 without inhibitor, (b) at pH 7.5 with orthophosphate inhibitor, (c) at pH 8 without inhibitor, and (d) at pH 8 with orthophosphate inhibitor (the inset figures show where the EDS pattern was done). elemental analysis. Images and EDS spectra were captured 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 KR source was used to identify crystalline phases of the lead precipitates. The tube was operated at 40 kV and 40 mA for the analyses. Scans were performed over a 2-theta ranging from 5 to 90° with a step of 0.02° and a 1-s count time at each step.



Figure 5. X-ray mapping image of lead pipe (a) at pH 6.5 with inhibitor (red-phosphorus and green-calcium), (b) at pH 6.5 (blue-lead, red-carbon and green-oxygen), (c) at pH 7 with inhibitor (red-phosphorus and green-calcium), and (d) at pH 7 with inhibitor (red-lead, green-oxygen and blue-carbon).

Figure 6. SEM images of lead crystals grown on the surface of lead pipe (a) at pH 8.5 without inhibitor, (b) at pH 8.5 with orthophosphate inhibitor, (c) at pH 9 without inhibitor, and (d) at pH 9 with orthophosphate inhibitor. Pattern analysis was performed by generally following ASTM procedures using the computer software Jade (Versions 8, Materials Data, Inc.), with reference to the 1995-2002 ICDD PDF-2 data files (International Center for Diffraction Data, Newtown Square, PA).

Results and Discussion At a lower pH (6.5) without inhibitors, the surface corrosion and Pb solubility is high, and the formation of microrods is favored (Figure 1a). The size of the microrods varies from 1 to 50 µm. Addition of orthophosphate inhibitor at a lower pH (6.5) yielded tubular structures, and no major change in surface morphology was observed (Figure 1b). The tubular structures vary from a few hundred nanometers to 2 µm. Increase in pH to 7 with orthophosphate inhibitor generated dendritic structures (Figure 1c) with a backbone. These microrod dendritic structures grew on the surface to nearly 1 mm long (Figure 2a,b). The

size of these dendritic structures is approximately 40-50 µm, and side aligned growth structures are in the nanometer range. Without addition of orthophosphate inhibitor at pH 7 the dendritic structure changed to irregular microrods, which could be the crystallization of different minerals under these conditions (Figure 1d). The energy dispersive X-ray analysis (EDS) analysis on selected crystals, and additional SEM images, are shown in Supplementary Figures 1-4 and Tables 1-4, Supporting Information. SEM images of lead crystals grown on the surface of the lead pipe at a pH of 7.5 and 8 with and without inhibitor are shown in Figure 3a-d. Without inhibitor, at pH 7.5, hexagonal structures (Figure 3a) with minor irrational structures are generated and, with inhibitor, dendritic structures are yielded on very long microrods similar to those observed at pH 6.5 without inhibitor. However, without inhibitor at pH 8 very long micron-sized nanorods (Figure


Nadagouda et al.

Figure 7. X-ray mapping of images of lead crystals grown on the surface of lead pipe (a) at pH 8.5 without inhibitor (blue-lead and red-carbon), (b) at pH 8.5 with orthophosphate inhibitor (blue-lead, red-carbon, and green-oxygen), (c) at pH 8.5 with orthophosphate inhibitor (blue-lead, red-phosphorus), and (d) at pH 8.5 with orthophosphate inhibitor (blue-lead, red-carbon, and green-oxygen).

Figure 8. TEM image (a) at pH 6.5 with orthophosphate inhibitor, (b) at pH 7.0 without orthophosphate inhibitor, (c) at pH 7.5 with orthophosphate inhibitor, (d) at pH 8.0 without orthophosphate inhibitor.

3c) are observed, and the addition of orthophosphate inhibitor yielded ultra high micron rods with a deposition of very minute structures on top of them (Figure 3d). The representative EDS analyses are shown in Figure 4a-d, along with SEM images and tabulated results in Tables 2 and 3. From EDS analysis it has been found that the major phase is PbCO3 (see Supplementary Figures 5-6, Supporting Information for additional SEM images and Supplementary Tables 5 and 6, Supporting Information). Other minor phases can not be ruled out, as was discuss in the XRD section. X-ray mapping studies indicate that phosphorus was precipitated and well dispersed on the surface of lead pipes with phosphate corrosion inhibitor (Figure 5a). Along with phosphorus, the presence of calcium phosphate scale precipitation was also observed in certain samples (Figure 5b). Similarly uniform precipitation of phosphorus was observed for all the pH tested. An increase in pH to 8.5 without

inhibitor generated very thin platelets of lead, which is probably a basic lead carbonate Pb3(CO)3(OH)2 or Pb10(CO3)6(OH)6O (Figure 6a). The addition of phosphorus inhibitor at the same pH helped phosphorus to precipitate uniformly on the surface of lead pipe (Figure 6b). With a further increase of pH to 9, with or without inhibitor, significant crystal growth on the surface of lead pipes (Figure 6c,d) was not observed. The EDS analyses on selected crystals at different zones are shown in Supplementary Figures 7-10 and Tables 7-10, Supporting Information. X-ray mapping revealed that formation of scaling was uniform with lead, carbon, and oxygen (Figure 7a,b). At higher pH the scaling was very uniform, as compared to lower pH range, without generating any major crystal growth and scaling formation and with phosphorus also uniform (Figure 7c,d).


Crystal Growth & Design, Vol. 9, No. 4, 2009 1803 Table 4 compound

phases involved

PbR0201

Pb - lead, PbCO3 - cerussite, Pb3(CO3)2(OH)2 hydrocerussite, PbO - litharge, Pb4O3Cl2 · H2O - lead oxide chloride hydrate, Pb(OH)Cl - lead chloride hydroxide pyromorphite - Pb5(PO4)3Cl, PbCO3 - cerussite, Pb3(CO2)2(OH)2 - hydrocerussite, Pb(OH)Cl - lead chloride hydroxide, Pb4O3Cl2 · H2O - lead oxide chloride hydrate PbCO3 - cerussite, Pb - lead, Pb(OH)Cl - lead chloride hydroxide, Pb4O3Cl2 · H2O - lead oxide chloride hydrate, Pb3(CO3)2(OH)2 - hydrocerussite, PbO - lithrage PbCO3 - cerussite, Pb5(PO4)3Cl - pyromorphite, Pb9(PO4)6 - lead phosphate, Pb(OH)Cl - lead chloride hydroxide, Pb3(CO2)2(OH)2 - hydrocerussite Pb - lead, PbO - lithrage, PbCO3 - cerussite, Pb3(CO2)2(OH)2 - hydrocerussite lithrage - PbO, hydrocerussite - Pb3(CO3)2(OH)2, pyromorphite - Pb5(PO4)3Cl, Pb(OH)Cl - lead chloride hydroxide, massicot - PbO Pb3(CO3)2(OH)2 - hydrocerussite, Pb3(CO2)2(OH)2 hydrocerussite, Pb - lead, PbO - lithrage, Pb4O3Cl2 · H2O - lead oxide chloride hydrate, Pb(OH)Cl - lead chloride hydroxide, PbCO3 cerussite Pb - lead, PbO - lithrage, Pb3(CO3)2(OH)2 hydrocerussite, Pb9(PO4)6 - lead phosphate, Pb9(PO4)6 - lead phosphate Pb3(CO3)2(OH)2 - hydrocerussite, Pb(OH)Cl - lead chloride hydroxide, PbO - lithrage, Pb10(CO3)6(OH)6O - plumbonacrite (lead oxide carbonate hydroxide) Pb3(CO3)2(OH)2 hydrocerussite, Pb lead, PbO lithrage, Pb10(CO3)6(OH)6O - plumbonacrite (lead oxide carbonate hydroxide), Pb9(PO4)6 - lead phosphate, Pb2(P4O12) · 2H2O - lead phosphate hydrate Pb - lead, PbO - lithrage, Pb10(CO3)6(OH)6O plumbonacrite (lead oxide carbonate hydroxide), Pb3(CO3)2(OH)2 - hydrocerussite Pb - lead, PbO - lithrage, Pb10(CO3)6(OH)6O plumbonacrite (lead oxide carbonate hydroxide), Pb3(CO3)2(OH)2 - hydrocerussite, Pb9(PO4)6 - lead phosphate, Pb2(P4O12) · 2H2O - lead phosphate hydrate

PbR0202

PbR0203

PbR0204 PbR0205 PbR0206 PbR0207

Figure 9. SAED patterns of representative lead crystals (a) at pH 6.5 with orthophosphate inhibitor, (b) at pH 7.0 without orthophosphate inhibitor (taken from thick coating of lead white patch), (c) at pH 7.5 with orthophosphate inhibitor, and (d) at pH 8.0 without orthophosphate inhibitor.

PbR0208 PbR0209

PbR0210

PbR0211 PbR0212

Figure 10. TEM image (a) at pH 7 without orthophosphate inhibitor (in different location where normal coating is observed), (b) at pH 8.5 with orthophosphate inhibitor; and SAED patterns (c) at pH 7 without orthophosphate inhibitor and (d) at pH 8.5 with orthophosphate inhibitor.

It is well-known that there are several factors which influence the mineralogy, growth rate, and morphology of lead corrosion byproduct, which in turn grow into different crystal shapes and sizes of lead. Some of the important factors are alkalinity, pH, calcium (if carbonate stabilization is used), orthophosphate, and silica. Low to moderate alkalinity decreases corrosion rates, while higher alkalinities have been shown to increase the corrosion rates of lead.20 Other constituents added to water supplies can also have an effect on alkalinity. The addition of orthophosphate, ammonia, silica, and hypochlorite will serve to increase the alkalinity of water. Bicarbonate and carbonate present in the water affects many important reactions in corrosion chemistry, including water’s ability to lay down a protective coating of calcium carbonate or complex films with other metals to form precipitates of insoluble carbonates such as Pb3(CO3)(OH)2.

These precipitates form as films in the immediate proximity to where the lead corrosion is occurring, and serve to deposit a coating over the actively corroding area. The chemical reduction/breakdown pathway of the PbO2 to divalent lead ion, and subsequent plumosolvency through complexation, can be possibly represented as follows:

PbO2(s)+ 4H+ f PbO2+ + 2H2O PbO2++ 2H+ + 2e- a PbO(s) + H2O or

PbO2(s)+ 2H+ + 2e- a PbO(s) + H2O PbO(s) + 2H+ a Pb2+ + H2O Pb(II) oxide and hydroxide are both extremely soluble at any drinking water pH, so other precipitation reactions would have to be operative to limit lead levels. Once in solution, the activity of the free lead ion will be governed by the amount of complexation, primarily by bicarbonate, carbonate, and hydroxide ions.21,22 The free lead ion can then react with


Nadagouda et al.

Figure 11. XRD Pattern for Selected Lead Samples.

carbonate or orthophosphate in the water to precipitate one of the conventional passivating solids as shown below: + 3Pb2+ + 2H2O + 2CO23 a Pb3(CO3)2(OH)2(S) + 2H

5Pb2++ 3PO34 + H2O a Pb5(PO4)3(OH)(S) 9Pb2++ 6PO34 a Pb9(PO4)6(S) The pH of the water plays a vital role in the ability to deposit protective films on the surface of the water conduit. Many of the film forming mechanisms in corrosion protection methods are directly dependent upon maintaining a proper pH range. Most of the low solubility films, such as lead carbonate, will not form if the pH is too low. It is very important to know the surface morphology of the lead in order to understand how the pH and alkalinity influences the corrosion rate. In order to understand the crystal shape and size more precisely, TEM studies were conducted, and representative images are shown in Figure 8a-d. At lower pH of 6.5 with orthophosphate inhibitor, tubes were observed with a thickness of measurable only in nanometers and several microns in length (Figure 8a). An increase in pH to 7 without inhibitor (Figure 8c) yielded dendritic structures on top of microrods, as evidenced by the SEM images mentioned earlier (Figure 1). At pH 7.0 without inhibitor, nanorods were observed with thickness ranging from 75-100 nm and several microns in length, as observed in the case of pH 6.5 with orthophosphate inhibitor.

With an increase in pH to 8.0 without orthophosphate inhibitor, the nanorods were transformed into spheres with a uniform coating on the surface, as observed with SEM images. The corresponding electron diffraction of TEM images in Figure 8a-d is shown in Figure 9a-d. Similarly TEM images at pH 7 without orthophosphate inhibitor (in a different location from where normal coating is observed), at pH 8.5 with orthophosphate inhibitor and SAED patterns, at pH 7 without orthophosphate inhibitor, and at pH 8.5 with orthophosphate inhibitor are shown in Figure 10a-d, respectively. X-ray diffraction was used to identify the phase present on the surface and the resulting phases are listed in Table 4. The representative XRD pattern for selected samples is shown in Figure 11. All of the samples tested both with and without orthophosphate inhibitor, along with the different pH, have one major phase such as PbCO3 along with other minor phases. At lower pH without orthophosphate inhibitor, cerrusite (PbCO3) was the main phase, and at higher pH hydrocerrusite or plumbonacrite was the main phase. With orthophosphate, a similar trend was observed regarding the lead carbonates, which are still present but minor compared to Pb phosphates. Under low pH these seem to be pyromorphite. At higher pH the main phase is Pb9(PO4)6, or something similar to it. Conclusions Effect of orthophosphate inhibitor and pH on the formation of different structures of lead phosphate/carbonate nanorods,


nanobelts, microrods, and dendritic structures have been described. At lower pH (6-7) the formation of different shapes, such as nanorods, microrods, and dendritic structures, were favored with and without inhibitor. However, an increase in pH to 8 and above with orthophosphate inhibitor started the formation of spheres and of a uniform layer of phosphate/PbCO3 coating. Different crystal growth at higher pH, as contrasted to a lower pH, was not observed. The X-ray diffraction patterns indicate the presence of one major phase along with minor other phases, as listed in the Table 4. Understanding these inhibitor- and pH-dependent crystal formations may help with development of better monitoring and control of corrosion processes in DWDSs. Understanding the pH and orthophosphate inhibitor-dependent crystal growth of different shaped and sized lead minerals may also be useful in various technological applications. Acknowledgment. M.N.N. is thankful to Cristina BennetStamper for the microscopic facility and to Dr. Thomas Speth for the encouragement. The U.S. Environmental Protection Agency, through its Office of Research and Development, funded and managed, or partially funded and collaborated in, the research described herein. It has been subjected to the Agency’s peer and administrative review and has been approved for external publication. Any opinions expressed are those of the author(s) and do not necessarily reflect the views of the Agency, and therefore, no official endorsement should be inferred. Any mention of trade names or commercial products does not constitute endorsement or recommendation for use. Supporting Information Available: SEM images and tables of EDS analysis. This information is available free of charge via the Internet at http://pubs.acs.org.


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Effect of Phosphate Inhibitors on the Formation of ... - ACS Publications

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