Lead enters drinking water by a process of corrosion, dissolution or particle detachment from lead bearing plumbing materials. Preventing contamination of water from lead-tin solder corrosion and achieving effective removal of particulate lead by point-of-use (POU) filters are important public health goals. These topics are especially timely given forthcoming revisions to the Lead and Copper Rule and ongoing efforts to reduce lead levels at the tap.
Recently a switch from non-corrosive groundwater to a surface water source at a utility in Illinois caused unusual drinking water contamination from the release of large lead solder chunks from plumbing to water. Point-of-use (POU) filters distributed to remove the lead at this utility and elsewhere were not always completely effective. Here, we elucidate the mechanism of lead solder release in two chapters, followed by two more chapters examining lead removal by POU filters.
The lead solder contamination arose after the water utility switched sources from high sulfate and low nitrate groundwater to a surface water with lower sulfate and high nitrate during runoff events. Such problems were unexpected because the surface water with high nitrate was not considered corrosive according to current theory. A chapter entitled A Novel Mechanism of Lead-Tin Solder Spallation in the Presence of Nitrate describes how 1) nitrate is extremely corrosive to lead:tin solder galvanically connected to copper, 2) nitrate corrosion can sometimes cause detachment of solder chunks to water, and 3) nitrate corroded the metal by reduction to ammonia and other reaction products.
Another Chapter reports a follow up study, that reproduced the essence of nitrate induced spallation corrosion as observed in homes, using copper pipe with beads of lead-tin solder attached. During a 4-month experiment, the non-corrosive groundwater with high sulfate caused no solder beads to detach and only about 1% of the total lead was released to water. But in the surface water with high nitrate believed to cause the lead problem, 100% of the solder beads detached after just two months, and 80% of the total lead in the solder was released to water after 4 months. In the same surface water that had lower nitrate, with or without zinc orthophosphate or polyphosphate inhibitors, only 8 to 17% of the solder beads detached. Electrochemical studies also found that equimolar concentrations of chloride did not cause the disintegration of tin solder or as much weight loss as nitrate. Moreover, sulfate concentrations as low as 0.75 mM could effectively inhibit tin corrosion caused by 10 mg/L NO3-N.
Studies focused on efficacy of POU filters have indicated that soluble lead in water is reliably removed, but sometimes particulate lead can escape capture and contaminate the treated water. To better understand this issue and practical limitations of filter use, field studies were performed in occupied and unoccupied homes in Enterprise, LA and New Orleans under both normal and extreme conditions of water lead contamination. For severe lead contamination present after lead pipes were disturbed or when a very long lead service line was present, and filters were tested to 200% of their rated capacity, the treated water occasionally had more than 15 ppb lead even when a very high percentage of the lead was removed. In Enterprise and New Orleans water with more typical levels of influent lead, the treated water was always below 1 ppb lead. But in Enterprise water with high iron and manganese the filters clogged quickly, causing higher costs for filtered water and consumer dissatisfaction.
The occasional problems in removing particulate lead observed in this and prior research gave impetus to a series of bench-scale experiments elucidating particulate lead removal mechanisms by conventional ion-exchange media in sodium (Na+), strong acid (H+), chloride (Cl-) or strong base (OH-) form. Suspensions of lead phosphate particles of varying sizes and age revealed marked differences in dissolution rates under acidic, circa neutral and basic pHs that are caused by treatment with H+, Na +, OH -, Cl- form resin. Fresh nanoparticle lead phosphate particles were very labile, and immediately dissolved at pH 4 to form soluble Pb+2 ions which were quickly removed by strong acid media. High pHs > 10 and phosphate removal by OH– form resin could also dissolve the particles, and then remove the anionic soluble lead formed at high pHs. Na+ and Cl- resin caused little or no dissolution at the circa neutral pHs associated with their use and had lower rates of lead removal from water as a result. Older lead phosphate particles acquired from a New York City harvested lead pipe loop rig or purposefully synthesized in the laboratory, did not dissolve as readily as fresh nanoparticles which profoundly affected their relative removal efficiency by the different media. Overall, dissolution of lead phosphate particles in the ion-exchange media can sometimes have a range of important effects that can enhance or hinder lead removal dependent on circumstance.
This thesis enhances our understanding of water lead contamination mechanisms by spallation of lead-tin solder and factors affecting lead removal by some POU filters. These novel insights can be helpful in preventing and mitigating future water lead contamination events. / Master of Science / Lead enters drinking water by a process of corrosion, dissolution or particle detachment from lead bearing plumbing materials. Preventing contamination of water from lead-tin solder corrosion and achieving effective removal of particulate lead by point-of-use (POU) filters are important public health goals. These topics are especially timely given forthcoming revisions to the Lead and Copper Rule and ongoing efforts to reduce lead levels at the tap.
Recent studies have revealed that high nitrate sometimes causes severe lead contamination of water in homes with lead soldered copper pipe. This thesis elucidates a novel mechanism of lead solder corrosion from nitrate attack in two chapters, followed by two more chapters examining problems associated with lead removal from water by point-of-use (POU) filters.
In a recent water lead contamination event, nitrate somehow caused large chunks of metallic lead solder to fall off pipes into the drinking water, a novel process that we term "spallation" corrosion. This observation inspired experiments to recreate this problem in the laboratory which found 1) nitrate and its reduced reaction products create a very low pH at the lead or tin anode during the nitrate-accelerated corrosion 2) the corrosion eats at the bond between the lead-tin solder and the copper pipe, cracking the lead:tin solder, causing chunks of metal to completely detach into water, and 3) corrosion of metal via nitrate reduction to ammonia at the tin anode.
Follow-up electrochemical studies reproduced the essence of field nitrate induced spallation corrosion as seen in homes using copper pipe with beads of lead-tin solder attached. These beads detached to water during a 4-month experiment in some water chemistries and not others. No solder beads detached, and only about 1% of the total lead in the solder was released to water, during exposure to a non-corrosive groundwater with high sulfate. But all the solder beads detached in just two months, and 80% of the total lead was released to the water in 4 months, in a surface water with high nitrate. Electrochemical studies found that sulfate concentrations as low as 0.75 mM effectively inhibited the extreme tin corrosion caused by 10 mg/L NO3-N.
Testing of lead certified POU filtration performance under varying conditions offers insight into challenges facing consumers. Field filtration studies were conducted in occupied homes for typical water lead challenges, or in unoccupied homes for testing of potentially dangerous water lead hazards, in Enterprise and New Orleans, LA. Results illustrate the difficulty of always achieving effective lead removal in cases where 1) the lead service line is very long, or 2) there is high erratic particulate lead after a lead service line is disturbed. Although effective lead removal occurred in other situations, the presence of very high levels of iron caused premature filter clogging and associated consumer frustration.
Problems observed in removing particulate lead informed a series of bench-scale studies evaluating the role of particle age and size on filtration effectiveness by cation and anion form exchange resins (H+, Na +, OH -, Cl-). Batch tests demonstrated that fresh lead phosphate particles less than 1 micron in size are quickly dissolved at pH less than 4 caused by H+ form ion-exchange resin and were dissolved moderately fast at pH higher than 10 caused by OH- form ion-exchange resin. But the particles hardly dissolved at all at the moderate pHs present when Na+ and Cl- form resins are used. Dissolved lead was readily removed by H+, OH - and Na+ form resins at the pH range they created during treatment, but not by Cl- form resins. Lead phosphate particles from New York City did not dissolve as quickly as fresh nanoparticles, which sometimes enhanced or hindered their relative removal efficiency in the range of media tested. Overall, dissolution of lead phosphate particles within the media had important effects on the overall lead removal and could even cause previously removed lead to be released in some cases.
This thesis enhances our understanding of water lead contamination mechanisms by spallation of lead-tin solder and lead removal by some POU filters. These novel insights can be helpful in preventing and mitigating future water lead contamination events.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/119507 |
| Date | 25 June 2024 |
| Creators | Villalona, Chantaly |
| Contributors | Civil and Environmental Engineering, Edwards, Marc A., Krometis, Leigh Anne Henry, Cohen, Alasdair Gordon, Sarver, Emily Allyn |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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