Role of Phosphate Inhibitors in Mitigating Lead and Copper Corrosion

Transcription

1 Role of Phosphate Inhibitors in Mitigating Lead and Copper Corrosion Subject Area: Distribution Systems

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3 Role of Phosphate Inhibitors in Mitigating Lead and Copper Corrosion

4 The mission of the AWWA Research Foundation (AWWARF) is to advance the science of water to improve the quality of life. Funded primarily through annual subscription payments from over 1,000 utilites, consulting firms, and manufacturers in North America and abroad, AWWARF sponsors research on all aspects of drinking water, including supply and resources, treatment, monitoring and analysis, distribution, management, and health effects. From its headquarters in Denver, Colorado, the AWWARF staff directs and supports the efforts of over 500 volunteers, who are the heart of the research program. These volunteers, serving on various boards and committees, use their expertise to select and monitor research studies to benefit the entire drinking water community. Research findings are disseminated through a number of technology transfer activities, including research reports, conferences, videotape summaries, and periodicals.

5 Role of Phosphate Inhibitors in Mitigating Lead and Copper Corrosion Prepared by: Marc Edwards and Laurie S. McNeill Department of Civil Engineering, 407 NEB Virginia Tech Blacksburg, VA Thomas R. Holm and Michael C. Lawrence Illinois State Water Survey 2204 Griffith Drive Champaign, IL Sponsored by: AWWA Research Foundation 6666 West Quincy Avenue Denver, CO Published by the AWWA Research Foundation and American Water Works Association

6 DISCLAIMER This study was funded by the AWWA Research Foundation (AWWARF). AWWARF assumes no responsibility for the content of the research study reported in this publication or for the opinions or statements of fact expressed in the report. The mention of trade names for commercial products does not represent or imply the approval or endorsement of AWWARF. This report is presented solely for informational purposes. Library of Congress Cataloging-in-Publication Data Role of phosphate inhibitors in mitigating Pb and Cu corrosion / prepared by Marc Edwards [et al.]. p. cm. Includes bibliographical references. ISBN Lead--Corrosion. 2. Copper--Corrosion. 3. Chemical inhibitors. 4. Water-pipes--Corrosion. I. Edwards, Marc. TA480.L4 R '8323--dc Copyright 2001 by AWWA Research Foundation and American Water Works Association Printed in the U.S.A. ISBN Printed on recycled paper.

7 CONTENTS LIST OF TABLES... LIST OF FIGURES... FOREWORD... ACKNOWLEDGMENTS... EXECUTIVE SUMMARY... ix xi xv xvii xix CHAPTER 1: INTRODUCTION... 1 Impacts of Phosphate Inhibitors on Corrosion By-Products... 2 Previous Research Results... 4 CHAPTER 2: MATERIALS, METHODS, AND QUALITY ASSURANCE/ QUALITY CONTROL... 7 Copper Complexation Experiments... 7 Lead Complexation Experiments Representative CU ISE Result and Interpretation Pipe Experiments Selection of Base Water Qualities for Pipe Study General Experimental Protocol Head-to-Head Comparison of Pipes at Different Ages Particulate Versus Soluble Metal Release: Pipe Experiments and Utility Survey CHAPTER 3: HEXAMETAPHOSPHATE REVERSION Materials and Methods Results and Discussion Laboratory Experiments Practical Field and Pipe Data CHAPTER 4: COMPLEXATION OF LEAD AND COPPER BY METAPHOSPHATE General Experimental Approach Lead v

8 CHAPTER 5: DISSOLUTION AND PRECIPITATION OF LEAD CARBONATES Methods and Materials Results CHAPTER 6: INFLUENCE OF PHOSPHATE INHIBITORS ON TOTAL AND SOLUBLE COPPER CORROSION BY-PRODUCT RELEASE Effects of Inhibitors and Aging on Copper Corrosion By-Product Release Copper Release From Pipes During Long-term Exposure Detailed Examination of By-Product Release Data Impact of Inhibitors on Soluble and Particulate Copper Release CHAPTER 7: INFLUENCE OF WATER QUALITY, PIPE AGE, AND PHOSPHATE INHIBITORS ON TOTAL AND SOLUBLE LEAD CORROSION BY-PRODUCT RELEASE Effects of Aging, Inhibitors, and Stagnation Time on Total Lead Release Soluble Lead Release CHAPTER 8: IMPORTANCE OF PARTICULATES IN MITIGATING LEAD AND COPPER CORROSION BY-PRODUCTS The Role of Particulates in Corrosion By-Product Release in Pipes Copper Pipe Corrosion Lead Pipe Corrosion Survey of Lead and Copper Speciation at Drinking Water Utilities Results for Copper Results for Lead Relative Effect of Zinc Orthophosphate Versus Orthophosphate CHAPTER 9: MAINTENANCE AND CONDITIONING DOSING OF INHIBITORS Results and Discussion By-Product Release Before and After Cutting Results of Conditioning and Maintenance Experiment APPENDIX A: THEORY OF DIFFERENTIAL SPECTROSCOPY APPENDIX B: EQUILIBRIUM CONSTANTS USED FOR DATA MODELING AND CHEMICAL EQUILIBRIUM CALCULATIONS APPENDIX C: SOLUBLE AND TOTAL METALS ANALYSIS QUALITY ASSURANCE/QUALITY CONTROL vi

9 APPENDIX D: ADDITIONAL FIGURES OF COPPER AND LEAD SPECIATION BASED ON UTILITY SURVEY APPENDIX E: ADDITIONAL PREEXPERIMENTAL SAMPLING RESULTS FOR COPPER PIPES BEFORE AND AFTER CUTTING REFERENCES ABBREVIATIONS vii

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11 TABLES 1.1 Synthesis of previous research on phosphate inhibitors in lead corrosion Effect of phosphate-based inhibitors on copper corrosion pcu buffers used to calibrate the Cu ISE Log [Pb 2+ ] values in equilibrium with PbCO 3 and Pb 3 (CO 3 ) 2 (OH) 2 for 1 mm alkalinity (50 mg CaCO 3 /L), Pb(OH) 2, and Pb 5 (PO 4 ) 3 OH Water qualities for pipe experiments Typical ionic constituents in base water Pipe ages, as represented in specific experiments presented in subsequent text Operational definition of size distribution Rates of sodium metaphosphate reversion Reported reversion from polyphosphate to orthophosphate calculated based on data of Goldberg (1995) Complexation parameters for models fit to Cu ISE titrations Comparison of HSuSal 2 and Cu(SuSal) 2 4 concentrations measured spectrometrically and calculated from complexation parameters fit to Cu ISE data Metaphosphate Pb-complexation parameters for two-ligand model Summary of lead carbonate and basic lead carbonate dissolution experiments Summary of lead carbonate and basic lead carbonate precipitation experiments Copper XRD samples for pipe samples after 4 years of aging Phosphorus consumption in lead pipes, 72-hour stagnation time Lead samples on XRD Grouping of utilities Summary of copper samples Summary of lead samples Copper pipes and water qualities tested in maintenance and conditioning experiments Experiment preparation and timeline Observed trends in corrosion for maintenance and conditioning experiments C.1 Average and maximum contamination during filter blank runs ix

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13 FIGURES 1.1 Schematic of factors controlling corrosion by-product release Spectra of solutions containing 20 µm 4-sulfosalicylic acid and 0 10 µm Cu, ph Spectra of solutions containing 19.1 µm NaHPAR and 0 17 µm Pb, ph A Cu ISE experiment, ph Abbreviated version of instructions sent with sampling kits Sodium metaphosphate reversion, ph 8, 25 C Metaphosphate reversion, 2 mg P/L, ph 8, 25 C Sodium metaphosphate reversion, ph 9, 25 C Metaphosphate reversion, ph 7, 25 C Perspectives on hexametaphosphate reversion to orthophosphate in copper and lead pipes after 72 hours stagnation Comparison of Cu ISE titration curves for ph 7, 8, and 9 and metaphosphate 2 mg P/L Cu ISE titration, metaphosphate 2 mg P/L, ph 8, run 1, and model fit to the data Spectra of solutions containing 20 mm 4-sulfosalicylic acid (SuSal), ph Spectra of solutions containing 20 mm 4-sulfosalicylic acid (SuSal), ph Calibration spectra (no metaphosphate), 19.1 µm PAR, mm Pb, ph Spectra of metaphosphate-containing solutions, 2 µg P/L, 19.1 µm PAR, mm Pb, ph Metaphosphate-bound Pb as a function of Log Pb 2+, 2 µg P/L, ph Comparison of metaphosphate-bound Pb versus Log Pb 2+ and two-ligand model, ph 8, 2 mg/l Lead solubility for systems in equilibrium with PbCO 3 (cerussite), ph 7.0, precipitation of Pb 5 (PO 4 ) 3 OH (hydroxypyromorphite) forbidden Lead solubility for systems in equilibrium with PbCO3 (cerussite), ph 7, precipitation of Pb 5 (PO 4 ) 3 OH allowed PbCO 3 dissolution, ph 7, NaHCO 3 1 mm, 25 C Final Pb concentrations in dissolution experiments, metaphosphate 2 mg P/L Final Pb concentrations in dissolution experiments, ph Precipitation kinetics of Pb for different metaphosphate concentrations, ph 8, 25 C, 1 mm NaHCO 3 (DIC 12 mg C/L) xi

14 5.5 Effect of Ca on Pb precipitation kinetics, ph 8, 25 C, 1 mm NaHCO 3 (DIC 12 mg/l) Copper release from pipes after 72-hours of stagnation Effect of aging on highest copper release from pipes at ph Results of triplicate sampling event for total copper Results of triplicate sampling event for total copper Relative impact of indicated inhibitor on total copper release for 8- and 72-hour sampling; results of triplicate sampling Impact of inhibitors on final concentration of soluble copper after 8 hours of stagnation Perspectives on phosphate residuals Scatter plot of polyphosphate after stagnation in pipes versus increase in soluble copper for hexametaphosphate inhibitor versus orthophosphate Simplest conceptualization of factors influencing soluble copper concentrations at equilibrium after stagnation in waters dosed with hexametaphosphate Best-fit complexation constant and resultant predictive ability for simplistic model Lead release from pipes after 72-hours of stagnation Effects of aging on lead release at constant alkalinity or at constant ph Results of triplicate 72-hour sampling event for total lead Results of triplicate 8-hour sampling event for total lead Relative impact of indicated inhibitor on total copper release for 8- and 72-hour triplicate sampling Lead release with time for old (aged 3.5 years) and brand new (aged 4.5 months) lead pipes at ph 7.8, alkalinity 45 mg/l Final ph after 72-hour stagnation for new (aged 6 months) pipes Phosphate residuals in pipes dosed with orthophosphate and hexametaphosphate Soluble lead in the 8-hour stagnation event Predicted soluble lead concentrations in equilibrium with different solid phases at constant DIC and ph Percentage increase in soluble Pb resulting from use of hexametaphosphate versus orthophosphate xii

15 7.12 Excess soluble Pb generally increased with residual Poly-P regardless of pipe age or ph Relative importance of soluble versus particulate copper in the 8-hour stagnation event In copper pipes dosed with hexametaphosphate, particulate corrosion by-products contained a high concentration of both phosphorus and copper Relative importance of soluble versus particulate lead in the 8-hour stagnation event Relationship between particulate P and particulate Pb Lead size distribution for utilities with ph , alkalinity mg/l Lead size distribution for utilities with ph , alkalinity mg/l Lead size distribution for utilities with ph , alkalinity mg/l Lead size distribution for utilities with ph , alkalinity 50 mg/l Relative comparison of zinc orthophosphate performance and speciation of copper in pipes Copper pipes dosed with zinc orthophosphate always had a lower soluble orthophosphate residual than pipes dosed with orthophosphate; the opposite trend was observed for lead pipes dosed with zinc orthophosphate Relative effects of zinc orthophosphate versus orthophosphate or no inhibitor After stagnation, there was a rough correlation between particulate Zn and particulate P in both copper and lead pipes; consistent with this observation, Mineql + predicts that a large fraction of the initial phosphorus would have precipitated in our zinc phosphate Hypothetical conceptualization of a conditioning dose and maintenance dose if inhibitor is mitigating corrosion by-product release General experimental approach for conditioning and maintenance experiments Sampling results for indicated pipe sections before cutting and for individual pipe samples after cutting Impact of various inhibitor doses on pipes never exposed to inhibitors Effects of reducing inhibitor dosage on pipes previously exposed to inhibitors maintenance experiment Effects of reducing inhibitor dosage for pipes previously exposed to inhibitors pipes aged for 4 years A.1 Concentrations of PAR species for 19.1 µm NaHPAR, 0 18 µm Pb, ph C.1 Extent of metal leaching and sorption by centrifuge tubes xiii

16 C.2 Extent of copper sorption by the filters from adjusted Boulder tap water C.3 Extent of lead sorption by the filters from adjusted Boulder tap water D.1 Total copper released as a function of treated-water ph D.2 Total copper released as a function of treated-water alkalinity D.3 Total copper released as a function of residual phosphorus D.4 Copper size distribution for utilities with ph , alkalinity mg/l D.5 Copper size distribution for utilities with ph , alkalinity mg/l D.6 Copper size distribution for utilities with ph , alkalinity mg/l D.7 Copper size distribution for utilities with ph , alkalinity 50 mg/l E.1 Sampling results for indicated pipe sections before cutting and for individual pipe samples after cutting E.2 Sampling results for indicated pipe sections before cutting and for individual pipe samples after cutting xiv

17 FOREWORD The AWWA Research Foundation is a nonprofit corporation that is dedicated to the implementation of a research effort to help utilities respond to regulatory requirements and traditional high-priority concerns of the industry. The research agenda is developed through a process of consultation with subscribers and drinking water professionals. Under the umbrella of a Strategic Research Plan, the Research Advisory Council prioritizes the suggested projects based upon current and future needs, applicability, and past work; the recommendations are forwarded to the Board of Trustees for final selection. The foundation also sponsors research projects through the unsolicited proposal process; the Collaborative Research, Research Applications, and Tailored Collaboration programs; and various joint research efforts with organizations such as the U.S. Environmental Protection Agency, the U.S. Bureau of Reclamation, and the Association of California Water Agencies. This publication is a result of one of these sponsored studies, and it is hoped that its findings will be applied in communities throughout the world. The following report serves not only as a means of communicating the results of the water industry s centralized research program but also as a tool to enlist the further support of the nonmember utilities and individuals. Projects are managed closely from their inception to the final report by the foundation s staff and large cadre of volunteers who willingly contribute their time and expertise. The foundation serves a planning and management function and awards contracts to other institutions such as water utilities, universities, and engineering firms. The funding for this research effort comes primarily from the Subscription Program, through which water utilities subscribe to the research program and make an annual payment proportionate to the volume of water they deliver and consultants and manufacturers subscribe based on their annual billings. The program offers a costeffective and fair method for funding research in the public interest. A broad spectrum of water supply issues is addressed by the foundation s research agenda: resources, treatment and operations, distribution and storage, water quality and analysis, toxicology, economics, and management. The ultimate purpose of the coordinated effort is to assist water suppliers to provide the highest possible quality of water economically and reliably. The true benefits are realized when the results are implemented at the utility level. The foundation s trustees are pleased to offer this publication as a contribution toward that end. xv

18 This project was undertaken to improve understanding of phosphate-based inhibitor use in meeting provisions of the U.S. Environmental Protection Agency Lead and Copper Rule. Previous anecdotal evidence, theory, and laboratory testing strongly suggested that polyphosphate dosing may be detrimental to lead and copper release under at least some circumstances. First and foremost, this work was aimed at conclusively examining this possibility for a range of waters in longterm tests. The performance of orthophosphate inhibitors, zinc orthophosphate, and aspects of inhibitor chemistry were examined as part of that evaluation. We hope that this research will help utilities understand the potential benefits and detriments of phosphate-based inhibitors in drinking water treatment. Julius Ciaccia, Jr. Chair, Board of Trustees AWWA Research Foundation James F. Manwaring, P.E. Executive Director AWWA Research Foundation xvi

19 ACKNOWLEDGMENTS This work was supported by a grant from the AWWA Research Foundation. We appreciate the extensive insights and constructive criticism provided by our project advisory committee members: Michael Schock (Chemist, US Environmental Protection Agency), Brad Segal (Water Quality Specialist, City of Boulder, Colorado), and Jonathan Clement (Senior Engineer, Black and Veatch). Special thanks to Dawn Gladwell and Loay Hidmi (research assistants, University of Colorado at Boulder) for helping with pipe experiments and to Jonathan Talbott of the Illinois Waste Management and Research Center, who analyzed selected water samples for lead by inductively coupled plasma mass spectrometry. Kent Smothers of the Illinois State Water Survey analyzed the lead carbonate and basic lead carbonate using X-ray diffraction. Finally, we acknowledge the invaluable assistance of participating utilities that conducted at-the-tap sampling for particulate lead and copper corrosion by-products. xvii

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21 EXECUTIVE SUMMARY Surprisingly, although phosphate-based inhibitors are used at utilities serving more than half the U.S. population, at the time this work was undertaken, many fundamental questions remained unanswered regarding their effect(s) on lead and copper corrosion. This research successfully answered some questions and provided some new insights as to important reactions occurring in pipes as well. Finally, some additional questions were raised that should be addressed in future research. At the outset, we note the potential limitations of the work. First, there is no guarantee that trends noted in this study for inhibitors will be directly transferable to other waters of similar ph and alkalinity. Although it is believed that ph and alkalinity are two of the most important factors influencing corrosion and inhibitor behavior, other factors, such as chloride, hardness, disinfectant type and dose, stagnation and flow, and temperature, are undoubtedly important. Second, with respect to lead release, it is well known that mechanisms of release differ based on the source of lead (brass, pure lead, solder, etc.), whether the material is galvanically coupled to a more noble metal such as copper, and other factors. This work exclusively examined the effects of water quality on pure lead pipes. Given the lack of information regarding corrosion of other leadcontaining plumbing materials, it is often assumed that trends noted for pure lead apply. Although this assumption provides some basis for decision making, we note that the validity of this hypothesis has not been rigorously tested, and in fact there is reason to believe that it is not valid in many circumstances. Thus, as with any research report, results described below must be interpreted and applied with caution. Is polyphosphate reversion likely to be significant in water distribution systems? The rate of reversion (hydrolysis) of sodium metaphosphate to orthophosphate generally increased as the ph decreased and as the Ca concentration increased. The reversion rate in the presence of 120 mg Ca/L was approximately 10 times the rate in the absence of Ca. Although the rate of orthophosphate production from metaphosphate was essentially constant for about 1 month, the degree of reversion was negligible within the time of the complexation, dissolution, and precipitation experiments. On the basis of these results, it is expected that only up to 35 percent polyphosphate reversion to orthophosphate could be expected within a week or so in a distribution system xix

22 under some conditions. Re-examination of previous research results, with samples collected from the distribution system before stagnation in lead and copper pipes, supports this rough estimate. On the other hand, within stagnant lead and copper pipes, much higher reversion (up to 80 percent) occurred, even after a few days of stagnation. Clearly, the pipe surfaces themselves, or possibly bacteria on the surfaces, catalyzed the reversion reaction, and the extent of catalysis increased with age of the pipe in the case of lead pipes. This discovery deserves additional research. In summary, within distribution systems, modest levels of reversion can typically be expected, whereas much higher levels of reversion can be anticipated during stagnation in pipes. What fundamental evidence was gathered that polyphosphates will increase levels of copper and lead release? Experiments were conducted to examine complexation of copper and lead, solid dissolution rates, and solid precipitation in the presence of polyphosphate. Fundamental chemistry experiments demonstrated that polyphosphates can effectively complex lead and copper in solution; by extension, these experiments established the likelihood that these inhibitors would enhance dissolution of lead and copper during stagnation within pipes. For the model polyphosphate used in this work, every 1 mg/l of phosphate inhibitor dosed (as P) had the potential to hold 2 mg/l lead in solution. This may be considered a maximum capacity for lead dissolution, as this value rarely would be achieved in practice because of the ameliorating effects of calcium, magnesium, kinetic limitations, and other factors. Considering the stringent lead action limit of 15 µg/l, however, only 1 percent of this theoretical capacity translates to 20 µg/l Pb at a dose of 1 mg/l polyphosphate P. This highlights the danger of polyphosphate complexation of lead in the context of the U.S. Environmental Protection Agency action limit. For lead, fundamental laboratory experiments established that complex formation is insensitive to ph. Lead complexation is less strong in the presence of Ca (40 mg/l) but is still relatively important. Experiments also indicated that not only does hexametaphosphate increase the maximum level of soluble lead; it also increases the rate of dissolution from representative lead scales including PbCO 3 and Pb 3 (CO 3 ) 2 (OH) 2. In beaker experiments, the amount of Pb dissolved after xx

23 just 6 hours was roughly equal to the hexametaphosphate complexation capacity, and higher hexmetaphosphate produced higher dissolved Pb concentrations. Although the dissolution rate in actual plumbing systems will depend on the accessible surface area of Pb deposits, diffusion rates, and the pipe surface area to volume, the clear acceleration of scale dissolution by hexametaphosphate cannot be beneficial in the context of Pb regulation or consumer exposure. Finally, precipitation of Pb from solutions containing NaHCO 3 was inhibited by sodium metaphosphate. The final dissolved Pb concentration was roughly equal to the metaphosphate complexing capacity. Higher metaphosphate concentrations resulted in higher dissolved Pb concentrations. This laboratory result suggests that polyphosphate can influence the kinetics of scale formation in pipes as well. What practical evidence was gathered on relative effects of polyphosphate and orthophosphate for copper corrosion? With a few exceptions, dosing of orthophosphate and hexametaphosphate inhibitors had beneficial effects on copper release. The exceptions were for very new pipes at ph 7.2, in which hexametaphosphate had very significant adverse short-term effects and for well-aged pipes at ph 7.2 and alkalinity 300 mg/l as CaCO 3. In the latter case, although the orthophosphate had dramatic short-term benefits, a few years of aging caused marked decreases in release when inhibitors were absent. A more detailed look at hexametaphosphate versus orthophosphate was instructive and it provides some insights into mechanisms of inhibition. In every instance, systems dosed with hexametaphosphate exhibited higher concentrations of soluble copper after stagnation than in the same system dosed with orthophosphate. To describe this trend in greater detail, a term was defined to quantify the magnitude of this adverse effect for each equivalent condition tested for both orthophosphate and hexametaphosphate: Excess soluble Cu = soluble Cu poly-p soluble Cu ortho-p xxi

24 A simplistic model then was formulated to attempt to explain trends observed in these experiments. The first step was to assume that the scales formed in the presence of hexametaphosphate (with some reverted orthophosphate) and orthophosphate are essentially identical, and that Cu +2 ion concentrations are fixed, for example, by a solid such as Cu 3 (PO 4 ) 2 solid. Moreover, it is assumed that the concentration of polyphosphate complexes can be predicted by a simplistic complexation reaction of the form: Cu +2 + Poly-P CuPoly-P (complex) K pp in which copper and poly-p concentrations are expressed in terms of moles Cu +2 and moles Poly-P as P. If polyphosphate complexes are exclusively responsible for the excess concentration of soluble copper observed in pipes dosed with hexametaphosphate versus orthophosphate, and assuming equilibrium with a copper phosphate or similar scale, at ph above about 7.5, it can be shown that: Excess soluble Cu = K [Total Poly-P] [ PO 4 ] 0.66 where K is a constant and the orthophosphate and polyphosphate concentrations are in brackets. The basic form of this admittedly simplistic equation suggests that the relative impact of hexametaphosphate is a balance between two factors: the beneficial presence of reverted orthophosphate versus the adverse effects of polyphosphate. At higher ph, even if the extent of reversion is relatively small, a relatively large fraction of the total orthophosphate present will be in the form of [PO 3 4 ] because of acid base reactions; consequently, the extra hexametaphosphate had only a small adverse effect at higher ph. At lower ph, the net effect is a balance between the benefits of orthophosphate (driving down free copper concentrations) and polyphosphate concentrations that can cause complexation and increased soluble copper. Note, however, that the model predicts that the net effect of hexametaphosphate will be negative relative to orthophosphate regardless of how these factors balance. xxii

25 What practical evidence was gathered on relative effects of polyphosphate and orthophosphate for lead corrosion? The relative benefits of inhibitors can be assessed by the following equation: ( Pb release with inhibitor Pb release without inhibitor) % change = % Pb release without inhibitor In this calculation, benefits from inhibitor dosing result in negative numbers, whereas detrimental effects result in positive numbers. Orthophosphate dosing often produced significant benefits. This was true for every stagnation time and water quality tested at 6 months pipe age. In pipes aged 3 years, orthophosphate significantly (greater than 95 percent confidence) reduced lead release for all conditions other than ph 7.2 and alkalinity 15 mg/l. The only detrimental impacts from dosing orthophosphate were in new pipes (aged 2 weeks) with a 72-hour stagnation time in waters at ph 7.2 and alkalinity 300 mg/l and in waters at ph 7.8 and alkalinity 45 mg/l. In contrast, effects of hexametaphosphate dosing were more frequently detrimental than beneficial. In fact, for an 8-hour stagnation event, the only benefits were at ph 7.2 and 15 mg/l alkalinity at 6 months pipe age. In very new (2 weeks) and the oldest pipes (3 years), hexametaphosphate was highly detrimental to lead release at ph 7.2 and alkalinity 300 mg/l for both an 8- and a 72-hour stagnation time. On the other hand, ph 7.2 and alkalinity 300 mg/l as CaCO 3 was the only condition for which significant adverse effects from hexametaphosphate were still noteworthy after 3 years. The work also closely examined the role of phosphate inhibitors in controlling soluble lead release, as opposed to total lead, as discussed in the preceding section. In every instance, soluble lead concentrations were lower in the presence of orthophosphate than in an equivalent system without inhibitor. Conversely, with few exceptions, soluble lead concentrations were higher in systems dosed with hexametaphosphate than without inhibitor. Therefore, in terms of relative performance of the two inhibitors in controlling soluble lead release, orthophosphate has an enormous advantage. To highlight this effect, the percentage increase in soluble lead resulting from hexametaphosphate dosing compared to orthophosphate can be calculated as: ( Pb release with hexametaphosphate Pb release with orthophosphate) % change = % Pb release with orthophosphate xxiii

26 Consistent with the fundamental studies discussed earlier, hexametaphosphate was demonstrated to increase soluble lead release in every instance when compared to an equivalent dose of orthophosphate. Moreover, the minimum increase was 100 percent, or a doubling of the soluble lead concentration. The most pronounced effect was at ph 7.2 and alkalinity 45 mg/l in pipes aged 3 years, for which hexametaphosphate increased soluble lead by 3,500 percent compared to the same system but with orthophosphate. There no longer can be any doubt that hexametaphosphate substantially increases problems with soluble lead. Moreover, the magnitude of the observed effects are disturbing, to say the least. Extensive attempts were made to model the increase in soluble lead using the simplistic approach outlined for copper. Although there was a good correlation between the excess soluble Pb and residual polyphosphate, strongly suggesting the formation of lead polyphosphate complexes, no improvement in correlation could be obtained using the model based on lead phosphate equilibrium. This is probably because the lead phosphate complex is very strong and is affected only slightly by phs as high as 9.5. In the relatively soft water tested, each milligram per litre of residual polyphosphate led to an increase of 1.6 mg/l of soluble lead. This is 80 percent of the maximum complexation capacity determined for soluble lead in the fundamental experiments described previously. How important are particulate lead and copper corrosion by-products? Significant fractions of particulate and colloidal lead and copper were found in tap water samples from various utilities and in pipe experiments. Copper was mostly soluble, however, especially when total copper levels were high. In contrast, most of the lead found in the tap water samples and the pipe experiments was particulate. Particulate lead was very high at ph 7.2 and alkalinity 15 mg/l as CaCO 3, and in fact, the problems with lead release for this condition are attributable almost exclusively to particulate lead. Orthophosphate and polyphosphate tended to decrease particulate lead release in this water, adding to the reductions in soluble lead release noted earlier that are attributable to orthophosphate. One of the most interesting findings was that, consistent with previous research, lead release was highest in pipes where hydrocerrusite was present in the scale. Thus, from this perspective, hydrocerrusite caused problems with lead plumbosolvency. Clearly, however, in this xxiv

27 work the problem arose from increased particulate lead release and not soluble lead as expected. It is possible that hydrocerrusite is not very adherent to pipe compared to other scales that can form on lead. This issue deserves additional research, especially given that most existing lead-control strategies are based on controlling lead solubility and the solubilities of the cerrusite and hydrocerrusite solids are so high compared to the action limits (except at conditions of relatively high ph and low alkalinity). Are any extra benefits attributable to zinc orthophosphate compared to orthophosphate alone? In these experiments, the addition of zinc did not enhance the performance of orthophosphate. To the contrary, in all cases, zinc tended to detract from the benefits of orthophosphate, and this adverse effect was significant at 95 percent confidence. In fact, for copper corrosion, the only condition for which zinc orthophosphate had significant benefits relative to the condition without any inhibitor at all was ph 7.2 and alkalinity 45 mg/l as CaCO 3. In summary, unless direct evidence to the contrary exists for a particular water of interest, the work indicated that zinc orthophosphate cannot be recommended for copper or lead corrosion control when compared to orthophosphate alone. What effects can be expected when dosing lower levels of inhibitors? Can a clear optimum be identified? Can utilities reduce doses to save money? In concept, the ideas of conditioning dose and maintenance dosing remain attractive. Nevertheless, although exhaustive testing of these concepts was undertaken in this work and some preliminary results are presented, unambiguous answers to these questions could not be obtained. Their resolution will have to await future research. In general, even for pipes exposed to constant water quality, variation in corrosion by-product release was high. When future experiments are conducted, researchers should anticipate that the variation will be even greater in responses to changes in inhibitor dosing. xxv

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29 CHAPTER 1 INTRODUCTION Holm and Schock (1991a, 1991b) do a disservice to the water industry by creating unwarranted concern about the increase in lead solubility from polyphosphates in potable water. Boffardi, 1991 Applying inhibitors whose effects are not well understood may be viewed in the extreme sense as an uncontrolled toxicological experiment on the general population. Holm and Schock, 1991a, 1991b The article wrongly assumes the operating conditions under which polyphosphates are used. Polyphosphates are normally applied to potable water systems at neutral ph, not at ph 8. Boffardi, 1991 at ph above 8.4 when inhibitors increased lead release by 91 percent, all 13 utilities were using polyphosphate inhibitors had highly variable effects on corrosion byproduct release. Dodrill and Edward, Jour. AWWA 1995 The preceding quotes appropriately frame the debate over use of phosphate-based corrosion inhibitor at water utilities and the need for additional research on the subject. Even though inhibitors can reduce lead and copper corrosion by-product release under some circumstances, anecdotal evidence has been gathered regarding serious adverse effects in at least some circumstances. The lack of understanding of how inhibitors actually work should be troubling to utilities, vendors concerned with the safety of their products, and consumers. This project is aimed at providing answers to some of the most important questions regarding inhibitor use. 1

30 IMPACTS OF PHOSPHATE INHIBITORS ON CORROSION BY-PRODUCTS Lead and copper plumbing materials are thermodynamically unstable under aqueous oxygenated conditions and, thus, are continually being converted to more oxidized species (corroding). The oxidized metals may be released to and contaminate the drinking water as corrosion byproducts, or they may react with anions in the water to form solids on the metal surface (scale) that hinder corrosion reactions. In general, the satisfactory performance of lead and copper materials in drinking water is critically dependent on the identity and type of scale that is formed. Phosphate-based inhibitors can alter characteristics of water quality and scale that control corrosion by-product release. These changes, detailed below, affect both soluble and particulate metal release mechanisms, conceptualized pictorially in Figure Scale solubility. Scale solubility quantitatively determines the maximum free concentration of soluble metal ion (i.e., Cu +2, Pb +2 ) present after a long stagnation time in a pipe. The presence of orthophosphates may change the type of scale that forms in pipe and the solubility of a given scale. Scales containing orthophosphates are hypothesized to have lower solubility than scales that form in the absence of orthophosphates in certain ph ranges (Schock, 1980a, 1980b; Schock, Wagner, and Oliphant, 1996). Although polyphosphate could react to form insoluble scale directly, it is more commonly believed that scales form through reversion to orthophosphates, consistent with X-ray diffraction (XRD) data for lead pipes (Schock, Wagner, and Oliphant, 1996). 2. Metal complexation/chelation. The maximum concentration of soluble lead and copper (free + complexed metal) after long stagnation times in pipes depends on the type of scale and the complexation ability of the water. That is, water contains organic and inorganic species that can bond with the lead and copper, holding more of these corrosion by-products in a soluble form and increasing soluble metal concentrations. Polyphosphates are widely applied in industry as complexation agents; thus, their use in drinking water is predicted to markedly increase soluble metal concentrations under some circumstances (Holm et al., 1989). 3. Scale durability. Poorly adherent or weak scales will produce higher corrosion byproduct release during first-draw sampling. Orthophosphates, which alter the type of solids formed, could make a scale more or less durable. Likewise, the conventional 2

31 Water Velocity Total Copper Corrosion and By-Product Release = Particulate Cu + Soluble Cu Kinetic Limitations May Control Soluble or Particulate Cu Release -Diffusion Limitations -Ligand-Promoted Dissolution - Inhibitors Can Shear of Cu Particles and Colloid Detachment and Reattachment Increase or Free Cu (i.e., Cu +2 ) Decrease Dissolution Rates Maximum Soluble Copper = Free Cu + Complexed Cu Complexed Cu Phosphates Can Complex More Copper Equilibrium With Scale: Phosphate Scales May Be Relatively Insoluble Solid Scale ( Rust ) Layer Formed on Corroded Copper Pipe Uncorroded Copper Pipe Wall Figure 1.1 Schematic of factors controlling corrosion by-product release wisdom is that polyphosphates make certain types of scale softer and more likely to slough during flow, although this phenomenon is mentioned most commonly in the context of iron corrosion. 4. Scale surface charge. Because of development of electrostatic repulsion among particles on the scale surface at high negative or positive surface charge, it might be expected that surfaces with high charge would tend to cause release of more particulates. If the scale had a strong positive surface charge before the addition of inhibitor, low doses of negatively charged polyphosphates would tend to make the charge closer to neutral and decrease release rates. The converse is also true, with adverse effects expected from inhibitor addition (i.e., higher rates of particulate metal release) if the scale already had a slightly negative surface charge. Though different mechanisms will dominate depending on the specific circumstances, the relative effects of phosphate inhibitors will always depend on water quality (i.e., ph, natural organic matter concentration, ionic constituents, temperature), the type of phosphate inhibitor added (i.e., orthophosphates that form insoluble scale versus polyphosphates that can complex soluble metal), the inhibitor dose, and kinetic considerations. Even though only a small amount of 3

32 research has been completed to date on phosphate-inhibitor action, nearly all of that work has presumed that inhibitors work by altering scale solubility and metal complexation. This work also will explore the notion that inhibitors could affect release of particulate metals. PREVIOUS RESEARCH RESULTS Given the complexity of phosphate reactions and the scarcity of previous research, our poor current understanding of inhibitor effects is not surprising. Although a comprehensive analysis of the literature for lead and copper is beyond the scope of this work, a summary of previous peer-reviewed research results is instructive. For lead corrosion (Table 1.1), the literature is replete with benefits resulting from orthophosphate (O-P) or Zn orthophosphate dosing. Whether any additional benefit is realized from Zn orthophosphate versus orthophosphate alone, however, has not been established. In one study by Karalekas and co-workers (1983), zinc orthophosphate was believed to have caused slightly adverse impacts on at-the-tap lead release. Similarly, effects of polyphosphate inhibitors are inconsistent. Dodrill and Edwards (1995) and Holm and Schock (1991a, 1991b) demonstrated adverse impacts from polyphosphates under some circumstances, whereas the Dodrill study and others have clearly illustrated beneficial effects in others. The qualitative effects appear to be dependent on water quality. Dodrill found that the most significant benefits for lead were obtained from dosing inhibitors in low-alkalinity (< 30 mg/l as CaCO 3 ) water and that impacts became variable at higher alkalinities. The extent to which the beneficial effects of polyphosphates can be attributed to orthophosphates formed via reversion has never been addressed. The situation for copper corrosion (Table 1.2) is much the same as for lead. That is, orthophosphates are clearly beneficial sometimes, but whether any additional benefits can be attributed directly to polyphosphates or the presence of zinc is speculative. Werner et al. (1994) demonstrated adverse impacts of orthophosphates under some unusual conditions of dissolved oxygen and pipe geometry. In general, benefits of orthophosphates are most significant for copper at ph < 8.0 and effects were highly variable at higher ph (Dodrill and Edwards, 1995; Schock, Lytle, and Clement 1995). 4

33 Table 1.1 Synthesis of previous research on phosphate inhibitors in lead corrosion Reference Type of study Water quality Remarks Karalekas et al. (1983) First-draw tap sampling Soft water with low alk. (8 12 mg/l as CaCO 3 ), ph 6.7 Zn O-P had adverse or insignificant effects in reducing lead release Colling et al. (1987) High alk. ( mg/l as CaCO 3 ), ph 7 8 (water of high and low plumbosolvency) O-P reduced lead release Cousino et al. (1987) Soft water, ph 7.2 Poly-P reduced Pb release Schock (1989) Solubility models Wide range Addition of O-P at ph < 7.6 predicted to reduce Pb release Lee et al. (1989) First-draw, 1-L samples at-the-tap Unknown Zn O-P reduced Pb release compared to simply raising the ph >8.0; Zn HMP and Na HMP had similar Pb as raising ph > 8.0 Holm et al. (1989) Solubility calculations Hard water, ph 8.0, alk mg/l as CaCO 3 Na Poly-P increases Pb solubility Gregory (1990) ph , alk. low to high; low to high SO 4 2 and Cl conc. Zn O-P effectively reduces Pb release Holm and Schock (1991a; 1991b) Modeling and complexation tests ph 8.0, Ca 40 mg/l, I 0.1M Lead complexation constants determined experimentally Poly-P addition increases lead solubility Boffardi et al. (1991) ph 7.3, 50 mg/l PO 4 (O-P and Poly-P), 60 C O-P and Poly-P reduced Pb release Colling et al. (1992) High and low plumbosolvency waters O-P reduced Pb release Dodrill and Edwards (1995) Survey of large utilities Monitoring experiences of over 365 large utilities Phosphate inhibitors reduced Pb release in low-alkalinity waters; highly variable impacts at higher alkalinity Richards and Moore (1984) Tap water sampling Low ph, 6.3 and very low alk. (negligible) Scotland water Adding O-P in addition to ph adj. (ph 8 9) reduced Pb release to an even greater extent than ph adj. Schock et al. (1996) Scale analysis and comprehensive review Wide range of water qualities Hydroxypyromorphite and tertiary lead orthophosphate scales noted on pipes Schock et al. (1985, 1996) ph 8.0, alk. 20 mg/l, mg/l O-P Fraction of particulate lead was high in presence of orthophosphate Johnson et al. (1993) Pipe loop tests Low ph ( ), low alk.(6 12 mg/l as CaCO 3 ) water; ph was adj. to and Zn O-P was added Pb release was reduced by 85 percent in lead pipe and 53 percent in Cu pipe with Pb/Tn solder 5

34 Table 1.2 Effect of phosphate-based inhibitors on copper corrosion Reference Type of study Water quality Remarks Bancroft (1988) Reiber (1989) Corrosion rate measurements Water of low ph, alkalinity, and hardness ph 7.7 Werner et al. (1994) ph ; med. hardness; med. to low Cl, med. SO 2 4 Schock et al. (1995) Theoretical study Cupric hydroxide model for Cu solubility Dodrill and Edwards (1995) Statistical analysis for data from large utilities Wide range of ph and alkalinities Johnson et al. (1993) Pipe-loop testing Low ph (<7.3), low alk. water (<12 mg/l as CaCO 3 ) Zn O-P reduced Cu by-product release O-P dosed at 1 and 5 mg/l as P resulted in three-fold to five-fold decreases in cum. wt. loss over noninhibited ph 7.7 water Inhibitors reduced the formation of cuprous oxide leading to higher Cu concentration Predicts that O-P reduces Cu levels in water at ph values > 6.5 Phosphate addition had beneficial effects when ph < 7.8 Zn-PO 4 (w/ ph adj. to ) reduced Cu release by 82 percent vs. control and by 71 percent in Cu pipe; Zn PO 4 addition was no more effective than increasing ph to >8.3 In sum, the previous practical research has demonstrated that phosphate-based inhibitors profoundly impact lead and copper corrosion by-product release. In research done to date, researchers rarely have been able to draw conclusions demonstrating cause-and-effect relationships, much less identify how inhibitors actually work. Moreover, the existing literature (Tables 1.1 and 1.2) cannot be revisited to compare and contrast inhibitor effects because key data such as temperature, ph, and alkalinity usually were not reported. Because inhibitors can act through a variety of water quality-dependent mechanisms, an important first step toward improved understanding would be to investigate inhibitor impacts in detail for a few important water qualities. When coupled with additional laboratory work designed to examine fundamentals of polyphosphate reversion, complexation, dissolution, and precipitation, a foundation for improved understanding may be provided. 6

35 CHAPTER 2 MATERIALS, METHODS, AND QUALITY ASSURANCE/QUALITY CONTROL The experimental methods and materials include those necessary for well-defined laboratory experiments (led by Illinois State Water Survey [ISWS] and pipe experiments by the Virginia Tech research group, which moved from the University of Colorado at Boulder to Virginia Tech in the middle of the project). The ISWS experiments focused on hexametaphosphate (HMP) reversion, copper and lead complexation by hexametaphosphate, and dissolution/precipitation of lead solids in the presence of phosphate-based inhibitors. The Virginia Tech experiments focused on pipe testing. In this chapter, we describe general experimental approaches used. Specifics are described as appropriate in subsequent chapters. COPPER COMPLEXATION EXPERIMENTS A copper-specific electrode was the indicator electrode (Ruzicka, Lamm, and Tjell, 1972) and was used in conjunction with a double-junction reference electrode. A combination ph electrode was used to determine ph. The electrode was calibrated with National Institute of Standards and Testing traceable buffers. The ph was buffered at ph 7.0 and 8.0 using HEPES (4-(2-hydroxyethyl)-1-piperazineethane sulfonic acid, pk a 7.5) and at ph 9.0 using borate. HEPES was chosen for ph buffering because it has a negligible affinity for metal ions (Good et al., 1966). The titration vessel was a jacketed 250-mL beaker. The outer vessel was connected to a constant-temperature circulator that maintained the temperature at 25 C. The titration vessel had a cover with holes for the electrodes and a gas-dispersion tube. For each experiment, the ph of the solution containing Cu, sodium metaphosphate, and NaNO 3 was adjusted to < 5 by adding HNO 3 and the solution was sparged to remove CO 2. After sparging, the HNO 3 was neutralized with CO 2 -free NaOH; the buffer (HEPES or borate), which had been prepared from freshly deionized water and carbonate-free NaOH, was added to adjust the ph. The surface of the solution was blanketed with N 2 to exclude atmospheric CO 2. The Cu ion-selective electrode (Cu ISE) was calibrated for every titration, using pcu buffers (Hansen, Lamm, and Ruzicka 1972). (The pcu of a solution is the negative logarithm of the free Cu 2+ activity. It is analogous to the ph.) Preliminary titrations showed that at low total Cu 7