Understanding Mechanisms of Water Lead Contamination by Nitrate Spallation Corrosion and Lead Removal by Point-of-Use (POU) Filters

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.