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Effect of corrosion inhibitors on in-situ leak repair by precipitation of calcium carbonate in potable water pipelines Fei Wang, Christina L. Devine, and Marc A. Edwards Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b01380 • Publication Date (Web): 29 Jun 2017 Downloaded from http://pubs.acs.org on June 29, 2017
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Effect of corrosion inhibitors on in-situ leak repair by precipitation of calcium
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carbonate in potable water pipelines
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Fei Wang, Christina L. Devine and Marc A. Edwards *
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The Charles E. Via, Jr. Department of Civil & Environmental Engineering
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Virginia Tech
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Blacksburg, VA 24061
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*Corresponding author:
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Durham Hall 407
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Blacksburg, VA 24061
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Phone: (540) 231-7236
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Fax: (540) 231-7916
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Email:
[email protected] 15 16
To submit to Environmental Science & Technology
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June 28, 2017
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Abstract
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Corrosion inhibitors can affect calcium carbonate precipitation and associated in-situ and
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in-service water distribution pipeline leak repair via clogging. Clogging of 150 µm
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diameter leak holes represented by glass capillary tubes, in recirculating solutions that are
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supersaturated with calcite (Ωcalcite=13), demonstrated that Zn, orthophosphate,
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tripolyphosphate and hexametaphosphate corrosion/scaling inhibitors hinder clogging but
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natural organic matter (NOM) has relatively little impact. Critical concentrations of
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phosphates that could inhibit leak repair over the short term in one water tested were:
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tripolyphophate (0.05 mg/L as P) < hexametaphosphate (0.1 mg/L) < orthophosphate (0.3
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mg/L). Inhibitor blends (Zn+orthophosphate and Zn+NOM+orthophosphate) had
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stronger inhibitory effects compared to each inhibitor (Zn, orthophosphate or NOM)
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alone, whereas Zn+NOM showed a lesser inhibitory effect than its individual component
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(NOM) alone presumably due to formation of smaller CaCO3 particles with a much more
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negative Zeta-potential. Overall, increased dosing of corrosion inhibitors is probably
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reducing the likelihood of scaling and in-situ and in-service leak repair via clogging with
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calcium carbonate solids in potable water systems.
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Introduction
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Calcium carbonate (CaCO3) precipitation has undoubtedly been a critical operational
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concern since potable water distribution systems were first invented. Waters highly
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supersaturated with respect to CaCO3 can cause severe scaling problems including pipe
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blockage and energy loss in heat transfer elements of hot water systems.1 Nearly a
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century ago, waters that were highly undersaturated with respect to CaCO3 were also
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assumed to be corrosive to metallic and concrete pipe infrastructure,2 leading to studies
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attempting to reduce corrosivity by increasing the likelihood of CaCO3 precipitation as
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monitored with Langelier 3 and similar indices.
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Although use of Langelier index to guide corrosion control fell out of favor,3-7
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engineering practice and water quality are changing in a manner that will potentially
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thrust the problems related to CaCO3 back into the spotlight. These changes include (1)
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potential CaCO3 precipitation inhibitors are now more commonly present naturally or
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added to distribution systems;8 (2) an increased target temperature (> 51 °C) is suggested
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for hot water systems in buildings to reduce the growth of pathogens such as Legionella
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pneumophila;9 (3) rising atmospheric CO2 levels might increase calcium levels in natural
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waters;10, 11 and (4) increasing use of deicing salt including CaCl2,12, 13 can contribute to
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higher Ca entering both ground and surface waters.14 Researchers have found that CaCO3
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particles can clog small leaks in potable water pipes, and may extend the lifetime of
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distributions systems through both in-situ and in-service leak repair mechanism,15-23
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highlighting heretofore under-appreciated benefits from CaCO3 scaling in preventing
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pipeline degradation.
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In-situ and in-service repair of leaks by CaCO3 precipitation is a potentially
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transformative approach that attempts to use flowing waterborne or waterformed CaCO3
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solids to clog leaks while a pipeline is still in service. Although there is no record of
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intentional use of this approach in the context of modern potable water distribution
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systems, the concept dates back at least to Roman engineers (≈15 BC), who dosed
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alkaline wood-ash to new terracotta pipelines to permanently seal pressure pipe leaks.24
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In the 1960s, leaks in concrete pipelines for irrigation waters were sealed by adding
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anhydrous ammonia to raise pH and induce CaCO3 precipitation.25,
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CaCO3 lining was attempted to coat the inner surface of potable water pipes to combat
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corrosion, by targeting supersaturation of CaCO3, although the concept of small leak
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clogging was never mentioned.27,
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feasibility of leak crack clogging with CaCO3 precipitation, as a function of
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supersaturation level (14-30 mg/L of Ca), surface materials (glass, hydraulic cement), and
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crack widths (10 - 25 µm),22 and other recent studies also demonstrated the capability of
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water-formed deposits, including CaCO3, to clog leaks and restore some strength of large
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cracks in reactive new concrete.21 Changes in water supplies that might hinder formation
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of CaCO3, could potentially leave a water distribution system more vulnerable to damage
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by disabling processes that once allowed pipes to naturally heal themselves -- an
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important problem given that upgrading damaged water infrastructure is expected to cost
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about $1 trillion dollars over the next 25 years.29
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In the 1980s,
Snyder and Letterman explicitly considered the
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Two mechanisms can be operative during in-situ and in-service repair of leaks by CaCO3
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precipitation.23 Physical clogging can occur when waterborne particles, such as soil,
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bacteria, sand, clay, Al(OH)3, Fe(OH)3 or CaCO3, seal leaks by blockage. Precipitation on
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the inner surfaces of leak holes can also seal leaks. Physical clogging and precipitation
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could occur in pipes of all materials, although the importance of autogenous in-situ and
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in-service leak repair is believed most important for concrete pipe leaks that have reactive
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lime present in the matrix.23 Precipitation of CaCO3 is less likely in aged concrete in
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which lime has been converted to CaCO3. Snyder’s use of inert glass cracks with width of
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10-25 µm simulated a typical worst-case condition of tiny holes in the aged concrete of
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the Delaware aqueduct after over 60 years (as of 2007) of exposure to a low alkalinity
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water prone to leaching of lime.22
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Corrosion inhibitors are now widely added at water utilities,8 and it is logical to suspect
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they might have inadvertently prevented in-situ and in-service repair by CaCO3
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precipitation in the last few decades. Specifically, to control lead and copper corrosion in
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the USA, water treatment plants have been increasingly dosing phosphates and
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polyphosphates since the advent of the Lead and Copper Rule (LCR) in 1991 by US
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Environmental Protection Agency (EPA), to meet the action limit for lead of 15 µg/L in
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tap water.8 Similarly, the Water Supply (Water Quality) Regulations of the United
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Kingdom, the Drinking Water Directive of the European Union, the Drinking Water
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Guidelines of Canada, the Drinking Water Standards in Japan, and the Standards for
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Drinking Water Quality in China, have recently decreased the drinking water standard for
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lead to 10 ug/L which will increase the impetus for phosphate dosing.30-34 However,
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phosphates can also prevent CaCO3 precipitation.35,
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Lin and Singer systematically
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studied the inhibitory effects of orthophosphate (PO43-), pyrophosphate (P2O74-),
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tripolyphosphate (P3O105-), hexametaphosphate (P6O186-) and binary-polyphosphate
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blends at P concentrations of 10-8 M and above, and suggested that phosphates could
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adsorb onto calcite crystal growth sites (or kinks) to inhibit calcite precipitation.35,
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They did not identify threshold values (or the smallest values needed) for phosphates to
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inhibit CaCO3 precipitation. In addition, NOM (e.g. fulvic acid and humic acid) that are
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naturally present in aquatic systems, can also inhibit calcite precipitation through a
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similar mechanism as phosphates.37-39
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Zinc cations (or Zn2+) are sometimes present in potable water systems as a byproduct of
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brass or galvanized iron corrosion, or are sometimes dosed along with phosphates as
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corrosion inhibitors by water utilities,8 and these practices might also influence the
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likelihood of leak repair. Addition of Zn2+ by itself at a concentration of 0.1-10 ppm can
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significantly inhibit calcite precipitation,40, 41 and the formation of aragonite and vaterite
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in addition to calcite has been observed.40, 42 Ghizellaoui and Euvrard
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the adsorption of Zn2+ onto calcite crystal growth sites and subsequent blocking (or kink
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blocking) or the co-precipitation of Zn2+ with CaCO3 might be operative. Wada et al.
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also found an inhibitory effect by Zn2+, as it favored formation of aragonite instead of
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calcite. It is possible that this change in mineralogy could also affect the likelihood of
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scaling problems.
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suggested that
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Changes under consideration for dosing corrosion inhibitors (e.g. phosphates and Zn)
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may also be important for CaCO3 precipitation in potable water systems. Although water
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utilities were increasingly dosing phosphate corrosion inhibitors from 1991 to present to
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control lead and copper corrosion, some water utilities are now considering whether they
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should decrease the use of phosphates due to initial costs of adding the chemical and later
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removing it at the sewage treatment plant, and a desire to conserve phosphate
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resources.43-46 The dosage of Zn may also have been reduced, due to recent studies
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inability to find it contributes to corrosion control of materials including lead, copper and
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cement.47, 48 On the other hand, other water utilities are considering the need to increase
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the levels of phosphates and Zn to better control lead corrosion. A key point is that there
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is presently pressure on water utilities to consider both increasing or decreasing corrosion
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inhibitor dose, and an improved understanding of phosphates holistic impacts on water
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system performance including scaling is much needed.
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The objective of this study was to: (1) Examine leak repair using controlled CaCO3
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precipitation and determine the effects of corrosion inhibitors on leak repair kinetics and
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morphology. We hypothesize that small leaks can be blocked via CaCO3 precipitation,
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and corrosion inhibitors may impair this process, because they also inhibit CaCO3
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precipitation. (2) Investigate the effect of single inhibitors (Zn, NOM, orthophosphate,
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tripolyphosphate and hexametaphosphate) on leak repair over a range of conditions
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relevant to portable water systems. Each corrosion inhibitor can exhibit different degrees
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of impact on leak repair, because some are better inhibitors of CaCO3 precipitation (e.g.,
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tripolyphosphate and hexametaphosphate are stronger than orthophosphate). (3)
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Investigate the effect of mixed inhibitors on leak repair via CaCO3 precipitation, as
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combinations of inhibitors will sometimes have stronger effects compared to single
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inhibitors. (4) Shed light on why some existing pipelines may develop leaks that grow to
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failure, while other leaks are self-limiting, which may be critical factors to improving the
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longevity of these critical infrastructure assets.
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Materials and methods
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Materials
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Solutions of CaCl2 and NaHCO3 were prepared by dissolving CaCl2·2H2O and NaHCO3
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(Fisher Scientific) into deionized water. Similarly, solutions of Zn, NOM, orthophosphate
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(or P1), tripolyphosphate (or P3) and hexametaphosphate (or P6) were prepared using
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ZnCl2 (Sigma Aldrich), Na3PO4•12H2O (Fisher Scientific), Na5P3O10 and Na8P6O19
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(Sigma Aldrich).
162 ൛మశ ൟ∙{ைయమష }
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All test solutions had the same degree of supersaturation (Ω௧ =
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calcite precipitation, which can be described in terms of the calcite solubility product
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(Ksp,calcite=10-8.48) and the activities of dissolved Ca2+ and CO32-.
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Ωcalcite=13 were prepared by adding CaCl2 and NaHCO3 in a 1:1 molar concentration
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ratio (Table S1). Although Ωcalcite=13 is on the higher end of what is encountered in
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potable water systems, it was selected to be a “worst case” test of inhibitor effectiveness
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in preventing scaling.
49, 50
ೞ,ೌ
) to drive
The solutions with
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Methods
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CaCO3 precipitation and leak repair tests were tracked in recirculation systems with
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simulated leaks (Figure S1). Initially, glass bottles (or reservoirs) were filled with 2 liter 8 ACS Paragon Plus Environment
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of Ωcalcite=13 solutions. Within 10 minutes of making the solutions, a peristaltic pump
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(model 7553-70, Cole Parmer) delivered solutions at a flow rate of 1100 mL/min through
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hard-wall rigid clear PVC tubing (1/4” ID, McMaster Carr). To simulate concrete leaks, a
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method similar to that of Snyder
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capillary tubing with 150 µm ID (VitroCom) were mounted onto the side of rigid clear
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PVC tubing using Epoxy.
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was used, in which borosilicate heavy-wall glass
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Leak rates through the glass capillaries were measured at certain times (0 h – 24 h) to
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monitor the kinetics of clogging. Reservoir solution pH was measured every time the leak
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rates were measured. The pH at t=10 min was measured right after a peristaltic pump
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initially delivered solutions through the recirculating system, in addition to the initial pH
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at t=0 min. After the experiments, liquid samples were collected from reservoirs, and
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solids remaining in the reservoirs were collected using vacuum filtration and dried at
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room temperature. Liquid samples were analyzed by inductively coupled plasma mass
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spectroscopy (ICP-MS) for concentrations of dissolved Ca, by laser scattering particle
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size distribution analyzer (Horiba LA-300) for CaCO3 particle sizes, and by Zeta-Meter
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for zeta-potential of particles (Model 3.0+, Zeta-Meter, Inc.). Solid samples were
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characterized by XRD and SEM. Glass capillaries were removed from the recirculation
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systems and observed with an Amscope microscope (model IN300TC) with digital
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camera to characterize the nature of the leak repair. Reproducibility was tested by
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studying the onset of CaCO3 precipitation in the reservoir using beaker tests in triplicates,
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and the system error was confirmed as
