Effect of Competing Metals and Humic Substances on Uranium Mobilization from Noncrystalline U(IV) Induced by Anthropogenic and Biogenic Ligands.

Anthropogenic and biogenic ligands may mobilize uranium (U) from tetravalent U (U(IV)) phases in the subsurface, especially from labile noncrystalline U(IV). The rate and extent of U(IV) mobilization are affected by geochemical processes. Competing metals and humic substances may play a decisive role in U mobilization by anthropogenic and biogenic ligands. A structurally diverse set of anthropogenic and biogenic ligands was selected for assessing the effect of the aforementioned processes on U mobilization from noncrystalline U(IV), including 2,6-pyridinedicarboxylic acid (DPA), citrate, N,N'-di(2-hydroxybenzyl)ethylene-diamine-N,N'-diacetic acid (HBED), and desferrioxamine B (DFOB). All experiments were performed under anoxic conditions at pH 7.0. The effect of competing metals (Ca, Fe(III), and Zn) on ligand-induced U mobilization depended on the particular metal-ligand combination ranging from nearly complete U mobilization inhibition (e.g., Ca-citrate) to no apparent inhibitory effects or acceleration of U mobilization (e.g., Fe(III)-citrate). Humic substances (Suwannee River humic acid and fulvic acid) were tested across a range of concentrations either separately or combined with the aforementioned ligands. Humic substances alone mobilized appreciable U and also enhanced U mobilization in the presence of anthropogenic or biogenic ligands. These findings illustrate the complex influence of competing metals and humic substances on U mobilization by anthropogenic and biogenic ligands in the environment.

Exchange of Adsorbed Pb(II) at the Rutile Surface: Rates and Mechanisms.

The dynamics of Pb(II) at mineral surfaces affect its mobility in the environment. Pb(II) forms inner- and outer-sphere complexes on mineral surfaces, and this adsorbed pool often represents a large portion of the bioaccessible Pb in contaminated soils. To assess the lability of this potentially reactive adsorbed Pb(II) pool at metal oxide surfaces, we performed Pb(II) isotope exchange measurements between dissolved Pb(II) enriched in 207Pb and natural isotopic abundance Pb(II) adsorbed to rutile at pH 5, 6, and 7. We find that ∼95% of the adsorbed lead is exchangeable. An initially fast exchange (<1 h) is followed by a slower exchange that occurs on a time scale of hours to days. Pb LIII-edge extended X-ray absorption fine structure spectra indicate that similar binding mechanisms are present at all pH values and Pb(II) loadings, implying that differences in exchange rates across the pH range examined are not attributable to changes in the coordination environment. The slower exchange at pH 5 may be associated with interparticle and intraparticle diffusion resulting from particle aggregation. These findings demonstrate that the dissolved Pb(II) pool can be rapidly replenished by adsorbed Pb(II) if this pool is drawn down incrementally by biological uptake or a shift in chemical conditions.

Redox-Driven Recrystallization of PbO2.

Lead(IV) oxide (PbO2) is one of the lead corrosion products that forms on the inner surface of lead pipes used for drinking water supply. It can maintain low dissolved Pb(II) concentrations when free chlorine is present. When free chlorine is depleted, PbO2 and soluble Pb(II) will co-occur in these systems. This study used a stable lead isotope (207Pb) as a tracer to examine the interaction between aqueous Pb(II) and solid PbO2 at conditions with no net change in dissolved Pb concentration. While the dissolved Pb(II) concentration remained unchanged, significant isotope exchange occurred that indicated that substantial amounts (24.3-35.0% based on the homogeneous recrystallization model) of the Pb atoms in the PbO2 solids had been exchanged with those in solution over 264 h. Neither α-PbO2 nor ß-PbO2 displayed a change in mineralogy, particle size, or oxidation state after reaction with aqueous Pb(II). The combined isotope exchange and solid characterization results indicate that redox-driven recrystallization of PbO2 had occurred. Such redox-driven recrystallization is likely to occur in water that stagnates in lead pipes that contain PbO2, and this recrystallization may alter the reactivity of PbO2 with respect to its stability and susceptibility to reductive dissolution.

Ligand-Induced U Mobilization from Chemogenic Uraninite and Biogenic Noncrystalline U(IV) under Anoxic Conditions.

Microbial reduction of soluble hexavalent uranium (U(VI)) to sparingly soluble tetravalent uranium (U(IV)) has been explored as an in situ strategy to immobilize U. Organic ligands might pose a potential hindrance to the success of such remediation efforts. In the current study, a set of structurally diverse organic ligands were shown to enhance the dissolution of crystalline uraninite (UO2) for a wide range of ligand concentrations under anoxic conditions at pH 7.0. Comparisons were made to ligand-induced U mobilization from noncrystalline U(IV). For both U phases, aqueous U concentrations remained low in the absence of organic ligands (<25 nM for UO2; 300 nM for noncrystalline U(IV)). The tested organic ligands (2,6-pyridinedicarboxylic acid (DPA), desferrioxamine B (DFOB), N,N'-di(2-hydroxybenzyl)ethylene-diamine-N,N'-diacetic acid (HBED), and citrate) enhanced U mobilization to varying extents. Over 45 days, the ligands mobilized only up to 0.3% of the 370 µM UO2, while a much larger extent of the 300 µM of biomass-bound noncrystalline U(IV) was mobilized (up to 57%) within only 2 days (>500 times more U mobilization). This work shows the potential of numerous organic ligands present in the environment to mobilize both recalcitrant and labile U forms under anoxic conditions to hazardous levels and, in doing so, undermine the stability of immobilized U(IV) sources.

Lead phosphate deposition in porous media and implications for lead remediation.

Phosphate addition is commonly applied as an effective method to remediate lead contaminated sites via formation of low solubility lead phosphate solids. However, subsequent transport of the lead phosphate particles may impact the effectiveness of this remediation strategy. Hence, this study investigates the mechanisms involved in the aggregation of lead phosphate particles and their deposition in sand columns as a function of typical water chemistry parameters. Clean bed filtration theory was evaluated to predict the particle deposition behavior, using Derjaguin-Landau-Verwey-Overbeek (DLVO) theory to estimate particle-substrate interactions. The observed particle deposition was not predictable from the primary energy barrier in clean bed filtration models, even in simple monovalent background electrolyte (NaNO3), because weak deposition in a secondary energy minimum prevailed even at low ionic strength, and ripening occurred at ionic strengths of 12.5 mM or higher. For aged (aggregated) suspensions, straining also occurred at 12.5 mM or higher. Aggregation and deposition were further enhanced at low total P/Pb ratios (i.e., P/Pb = 1) and in the presence of divalent cations, such as Ca2+ (≥ 0.2 mM), which resulted in less negative particle surface potentials and weaker electrostatic repulsion forces. However, the presence of 5 mg C/L of humic acid induced strong steric or electrosteric repulsion, which hindered particle aggregation and deposition even in the presence of Ca2+. This study demonstrates the importance of myriad mechanisms in lead phosphate deposition and provides useful information for controlling water chemistry in phosphate applications for lead remediation.

Reduction of U(VI) on Chemically Reduced Montmorillonite and Surface Complexation Modeling of Adsorbed U(IV).

Adsorption and subsequent reduction of U(VI) on Fe(II)-bearing clay minerals can control the mobility of uranium in subsurface environments. Clays such as montmorillonite provide substantial amounts of the reactive surface area in many subsurface environments, and montmorillonite-containing materials are used in the storage of spent nuclear fuel. We investigated the extent of reduction of U(VI) by Fe(II)-bearing montmorillonite at different pH values and sodium concentrations using X-ray absorption spectroscopy and chemical extractions. Nearly complete reduction of U(VI) to U(IV) occurred at a low sodium concentration at both pH 3 and 6. At pH 6 and a high sodium concentration, which inhibits U(VI) binding at cation-exchange sites, the extent of U(VI) reduction was only 70%. Surface-bound U(VI) on unreduced montmorillonite was more easily extracted into solution with bicarbonate than surface-bound U(IV) generated by reduction of U(VI) on Fe(II)-bearing montmorillonite. We developed a nonelectrostatic surface complexation model to interpret the equilibrium adsorption of U(IV) on Fe(II)-bearing montmorillonite as a function of pH and sodium concentration. These findings establish the potential importance of structural Fe(II) in low iron content smectites in controlling uranium mobility in subsurface environments.

Metal-Catalyzed Hydrolysis of RNA in Aqueous Environments.

The stability of RNA in aqueous systems is critical for multiple environmental applications including evaluating the environmental fate of RNA interference pesticides and interpreting viral genetic marker abundance for wastewater-based epidemiology. In addition to biological processes, abiotic reactions may also contribute to RNA loss. In particular, some metals are known to dramatically accelerate rates of RNA hydrolysis under certain conditions (i.e., 37 °C or higher temperatures, 0.15-100 mM metal concentrations). In this study, we investigated the extent to which metals catalyze RNA hydrolysis under environmentally relevant conditions. At ambient temperature, neutral pH, and ∼10 µM metal concentrations, we determined that metals that are stronger Lewis acids (i.e., lead, copper) catalyzed single-stranded (ss)RNA, whereas metals that are weaker Lewis acids (i.e., zinc, nickel) did not. In contrast, double-stranded (ds)RNA resisted hydrolysis by all metals. While lead and copper catalyzed ssRNA hydrolysis at ambient temperature and neutral pH values, other factors such as lowering the solution pH and including inorganic and organic ligands reduced the rates of these reactions. Considering these factors along with sub-micromolar metal concentrations typical of environmental systems, we determined that both ssRNA and dsRNA are unlikely to undergo significant metal-catalyzed hydrolysis in most environmental aqueous systems.

Impact of dissolved oxygen and pH on the removal of selenium from water by iron electrocoagulation.

Removing dissolved selenium (i.e., selenate and selenite) from wastewater is a challenging issue for a range of industries. Iron electrocoagulation can produce Fe(II)-containing solids that can adsorb and chemically reduce dissolved Se. In a series of bench-scale experiments we investigated the effects of dissolved oxygen (fully oxic, partially oxic, and strictly anoxic) and pH (6 and 8) on the rate and extent of dissolved selenate and selenite removal by iron electrocoagulation. These studies combined measurements of the aqueous phase with the direct characterization of the resulting solids. Among the conditions studied the rate and extent of dissolved selenium (Se) removal were highest at pH 8 and strictly anoxic conditions. X-ray absorption spectroscopy demonstrated that in the absence of oxygen, Se was primarily transformed to elemental selenium (Se0) and selenide. Green rust that formed in the suspension during electrocoagulation played a key role as a reductant and sorbent of Se. At pH 6 dissolved oxygen did not affect the rates and extents of dissolved Se removal. Under all the conditions studied, dissolved Se removal was more effective with iron electrocoagulation than with the direct addition of pre-synthesized green rust or ferrous hydroxide. The most rapid and substantial dissolved Se removal was achieved by freshly-formed green rust and ferrous hydroxide, which are both Fe(II)-bearing solids. With an improved understanding of the products and mechanisms of the process, iron electrocoagulation can be optimized for removal of Se from wastewater.

Influence of point-of-use filters and stagnation on water quality at a preschool and under laboratory conditions.

A local preschool installed NSF/ANSI 42 and 53 certified point-of-use (POU) filters in its classroom sinks and drinking fountains to protect children from the possibility of elevated lead (Pb) levels in drinking water. We examined the effects of these filters during flowing water and immediately following stagnation periods on Pb, chlorine, and bacterial concentrations in the field and the laboratory. Before and after typical school stagnation periods, we collected samples from filtered classroom sinks, a filtered drinking fountain and nearby unfiltered sinks for a year. No unfiltered samples exceeded Illinois State limits of 5 µg/L for Pb in pre-K through 5th grade schools. However, following stagnation periods as short as overnight (14.5 h), over half of post-stagnation filtered samples from classroom sinks exceeded 5 µg/L while post-stagnation unfiltered samples remained below 5 µg/L. Laboratory testing showed no significant increases in Pb with stagnation, suggesting that the preschool classrooms may have had Pb-bearing plumbing downstream of the filters which released Pb into the filtered drinking water. The filters effectively removed free chlorine (99% decrease) in both the preschool and laboratory. Installing the filters had the unintended consequence of significantly increasing the bacterial concentrations (as measured by qPCR) in the preschool's drinking water and in laboratory filter effluent. Legionella pneumophila, Pseudomonas aeruginosa, and Mycobacterium spp. were not detected in pre-stagnation unfiltered and post-stagnation filtered samples. These results suggest that the installation of POU filters be considered as one component of an overall strategy to decrease Pb concentrations in school drinking water that holistically considers the premise plumbing system. A 5-minute flush significantly decreased concentrations of Pb and bacteria in filtered sinks. Replacing Pb-bearing plumbing components downstream of a POU filter may also be needed to combat Pb levels in drinking water.

Effects of Cu(II) and Zn(II) on PbO2 Reductive Dissolution under Drinking Water Conditions: Short-term Inhibition and Long-term Enhancement.

Lead oxide (PbO2) has the lowest solubility with free chlorine among Pb corrosion products, but depletion of free chlorine or a switch from free chlorine to monochloramine can cause its reductive dissolution. We previously reported that Cu(II) and Zn(II) inhibited PbO2 reductive dissolution within 12 h. Here, we expanded on this work by performing longer duration experiments and further exploring the underlying mechanisms. Between 12 and 48 h, Cu(II) and Zn(II) had no discernible effect on PbO2 reductive dissolution. From 48 to 192 h, Cu(II) and Zn(II) enhanced PbO2 reductive dissolution. Dissolved oxygen (DO) concentrations followed the same trends as PbO2 reductive dissolution, indicating that the DO was produced by PbO2 reductive dissolution. On the basis of extended X-ray absorption fine structure spectra, we hypothesize that the inhibitory effect of Cu(II) and Zn(II) on PbO2 reductive dissolution (<12 h) is caused by decreasing abundance of protonated sites on the PbO2 surface. The enhanced dissolution (>48 h) may be caused by competitive adsorption of Cu(II) and Zn(II) with Pb(II), which could limit the adsorption of Pb(II) onto PbO2 that could otherwise inhibit reductive dissolution. This study indicates that stagnation time plays a vital role in determining cations' effects on the stability of Pb corrosion products.

Intercomparison and Refinement of Surface Complexation Models for U(VI) Adsorption onto Goethite Based on a Metadata Analysis.

Adsorption of uranium onto goethite is an important partitioning process that controls uranium mobility in subsurface environments, for which many different surface complexation models (SCMs) have been developed. While individual models can fit the data for which they are parameterized, many perform poorly when compared with experimental data covering a broader range of conditions. There is an imperative need to quantitatively evaluate the variations in the models and to develop a more robust model that can be used with more confidence across the wide range of conditions. We conducted an intercomparison and refinement of the SCMs based on a metadata analysis. By seeking the globally best fit to a composite dataset with wide ranges of pH, solid/sorbate ratios, and carbonate concentrations, we developed a series of models with different levels of complexity following a systematic roadmap. The goethite-uranyl-carbonate ternary surface complexes were required in every model. For the spectroscopically informed models, a triple-plane model was found to provide the best fit, but the performance of the double-layer model with bidentate goethite-uranyl and goethite-uranyl-carbonate complexes was also comparable. Nevertheless, the models that ignore the bidentate feature of uranyl surface complexation consistently performed poorly. The goodness of fitting for the models that ignore adsorption of carbonate and the charge distributions was not significantly compromised compared with that of their counterparts that considered those. This approach of model development for a large and varied dataset improved our understanding of U(VI)-goethite surface reactions and can lead to a path for generating a single set of reactions and equilibrium constants for including U(VI) adsorption onto goethite in reactive transport models.

Pilot-scale comparison of sodium silicates, orthophosphate and pH adjustment to reduce lead release from lead service lines.

Sodium silicate is thought to mitigate lead release via two mechanisms: by increasing pH and by forming a protective silica film. A pilot-scale study using an excavated lead service line (LSL) fed with water from a Great Lakes source was undertaken to: (1) clearly distinguish the pH effect and the silica effect; (2) compare sodium silicate to orthophosphate and pH adjustment; (3) determine the nature of silica accumulation in the pipe scale. The LSL was cut into segments and acclimated with water at pH 7.1. Median dissolved lead was 197 µg/L in the last 8 weeks of acclimation and dropped to 16 µg/L, 54 µg/L, and 85 µg/L following treatment with orthophosphate (dose: 2.6 mg-PO4/L, pH: 7.9), pH adjustment (pH: 7.9) and sodium silicate (dose: 20 mg-SiO2/L, pH: 7.9), respectively. When silica dose was increased from 20 mg-SiO2/L to 25 mg-SiO2/L (pH: 8.1), lead release destabilized and increased (median dissolved lead: 141 µg/L) due to formation of colloidal dispersions composed mainly of lead- and aluminum-rich phases as detected by field flow fractionation used with inductively coupled plasma mass spectrometry. Si was present in the scale at a maximum of 2.2 atomic % after 17 weeks of silica dosing at 20 mg- SiO2/L. Under the conditions tested, sodium silicate did not offer any benefits for reducing lead release from this LSL other than increasing pH. However, sodium silicate resulted in lower levels of biofilm accumulation on pipe walls, as measured by heterotrophic plate counts, when compared to orthophosphate.

Effect of sodium silicate on lead release from lead service lines.

The effect of sodium silicate addition on lead release from lead service lines (LSLs) was investigated using laboratory-based pipe loop experiments with LSLs harvested from a water utility that has one of the Great Lakes as its source water. The LSLs were first conditioned with a synthetic water similar to that of Buffalo Water that matched the major water chemistry that the pipes had experienced in the field; the one exception was the absence of dissolved organic carbon in the synthetic water. After conditioning, the LSLs were used in tests with the same synthetic water and with sodium silicate added to the water for half of the LSLs. In one test sodium silicate addition was performed with adjustment of the pH to maintain it at the same value (pH 7.7) as before addition. In this test sodium silicate effectively reduced the dissolved and particulate lead concentrations in the water within six weeks of treatment. Periodic assessments of the corrosion scales in the pipes found that sodium silicate accumulated throughout the scale thickness and gradually decreased the lead release. In the other test the pH was allowed to increase from 7.7 to 8.8 upon addition of 20 mg/L as SiO2 sodium silicates, and parallel control experiments were performed with the same pH increase made using sodium hydroxide addition. In these tests the lead concentrations decreased in both the silicate-treated and control pipes, and the decreases were not significantly different between the silicate-treated and control pipes.

Impact of orthophosphate on lead release from pipe scale in high pH, low alkalinity water.

This study explored the ability of orthophosphate addition to limit lead release from lead service lines delivering high pH, low alkalinity water. We built pipe loop reactors with lead pipes harvested from Providence, RI, and we operated them with high pH and low alkalinity water of a composition similar to that in Providence. Orthophosphate addition decreased the release of both dissolved and particulate lead to the water. The most substantial decreases in total lead concentrations occurred after 15 weeks of orthophosphate addition, which was associated with the formation of calcium-lead-phosphorus (Ca-Pb-P) solids as part of the pipe scale. Pre-existing hydrocerussite (Pb3(CO3)2(OH)2(s)) in the scale of the lead pipe appeared to promote the formation of a Ca-Pb-P solid similar to phosphohedyphane (Ca2Pb3(PO4)3(Cl,F,OH)(s)). Continuous orthophosphate addition was also associated with the formation of a calcium phosphate solid with features like those of fluorapatite (Ca5(PO4)3F(s)) on the outermost layer of the scale. Through promoting the formation of these new solids within and on top of the scales, orthophosphate addition limited release of dissolved and particulate lead. These results demonstrate the ability of orthophosphate to control lead release at higher pH conditions than those for which it has typically been used. In addition to the formation of phosphate solids, PbO2(s), which was not present on the as-received pipes, was formed due to the constant supply of free chlorine in the laboratory-scale experiment.

Effect of Aluminum on Lead Release to Drinking Water from Scales of Corrosion Products.

The occurrence of aluminum in scales on lead pipes is common. This study aimed to identify factors that influence Al accumulation on oxidized lead surfaces and to determine whether the presence of Al impacts Pb release from corrosion products to water. Al accumulation and Pb release were monitored both with and without the addition of phosphate as a corrosion inhibitor. Pb coupons with corrosion scales were exposed to chlorinated water for up to 198 days to investigate Al accumulation and Pb release. Al accumulation was facilitated by Pb corrosion products, but its accumulation was inhibited by phosphate addition. During the study period, the formation of Al deposits did not affect Pb release when phosphate was absent. In an Al-free system, the addition of 1.0 mg/L phosphate (as P) lowered the dissolved Pb concentration below 1.0 µg/L. In a system containing 200 µg/L Al, the emergence of phosphate's effect on Pb control was delayed, and the dissolved Pb concentration decreased but stabilized at a higher value (10-12 µg/L) than in the Al-free system. Phosphohedyphane (Ca2Pb3(PO4)3Cl) was formed in all phosphate-containing systems, and PbO2 was formed independent of phosphate addition. The effect of Al on Pb release was probably related to its influence on the composition and morphology of Pb-containing minerals on coupon surfaces. The laboratory study has unavoidable limitations in its ability to simulate all conditions in real lead service lines, but this study still highlights the importance of considering the influence of Al when designing Pb corrosion control strategies.

Formation and Transport of Cr(III)-NOM-Fe Colloids upon Reaction of Cr(VI) with NOM-Fe(II) Colloids at Anoxic-Oxic Interfaces.

Natural organic matter-iron (NOM-Fe) colloids are ubiquitous at anoxic-oxic interfaces of subsurface environments. Fe(II) or NOM can chemically reduce Cr(VI) to Cr(III), and the formation of Cr(III)-NOM-Fe colloids can control the fate and transport of Cr. We explored the formation and transport of Cr(III)-humic acid (HA)-Fe colloids upon reaction of Cr(VI) with HA-Fe(II) colloids over a range of environmentally relevant conditions. Cr(VI) was completely reduced by HA-Fe(II) complexes under anoxic conditions, and the formation of Cr(III)-HA-Fe colloids depended on HA concentration (or molar C/Fe ratio) and redox conditions. No colloids formed at HA concentrations below 3.5 mg C/L (C/Fe ratio below 1.6), but Cr(III)-HA-Fe colloids formed at higher HA concentrations. In column experiments, Cr(III)-HA-Fe(III) colloids formed under oxic conditions were readily transported through sand-packed porous media. Colloidal stability measurements further suggest that Cr(III)-HA-Fe colloids are highly stable and persist for at least 20 days without substantial change in particle size. This stability is attributed to the enrichment of free HA adsorbed on the Cr(III)-HA-Fe colloid surfaces, intensifying the electrostatic and/or steric repulsion interactions between particles. The new insights provided here are important for evaluating the long-term fate and transport of Cr in organic-rich redox transition zones.

Effect of Cu(II) on Mn(II) Oxidation by Free Chlorine To Form Mn Oxides at Drinking Water Conditions.

The chemical oxidation of dissolved Mn(II) to Mn(III/IV) oxides (MnOx) can lead to the accumulation of Mn deposits in drinking water distribution systems. However, Mn(II) oxidation by free chlorine is quite slow under mild conditions (e.g., pH 7.7 and 1.0 mg/L Cl2). This study found a significant role for Cu(II) in Mn(II) oxidation under conditions relevant to the supply of chlorinated drinking water. At pH 7.7, dissolved Cu(II) accelerated Mn(II) oxidation more than 10 times with a dose of 20 µg/L. Solid characterization revealed that during Mn(II) oxidation, Cu(II) adsorbed to freshly formed MnOx and produced Mn-Cu mixtures (denoted as MnOx-Cu(II)). An autocatalytic model for the reaction kinetics suggested that the freshly formed MnOx-Cu(II) had a much higher catalytic activity than that of pure MnOx. Solid CuO also catalyzed Mn(II) oxidation, and kinetic modeling indicated that after an initial oxidation of Mn(II) facilitated by the CuO surface, the freshly formed MnOx-Cu(II) on CuO surface played the dominant role in accelerating further Mn(II) oxidation. This study indicates a high potential for the formation of Mn oxides at locations in a drinking water distribution system or in premise plumbing where both Mn(II) and Cu(II) are available. It provides insights into the co-occurrence of other metals with Mn deposits that is frequently observed in distribution systems.

The Ability of Phosphate To Prevent Lead Release from Pipe Scale When Switching from Free Chlorine to Monochloramine.

For lead pipes that contain PbO2(s) as a major component of their scales, a change in the residual disinfectant from free chlorine to monochloramine can destabilize the PbO2(s) and result in dramatic increases in aqueous lead concentrations. Such a scenario occurred in Washington, D.C., in late 2000. That problem was ultimately addressed by the addition of phosphate as a corrosion inhibitor, but it took several months for lead levels to drop below regulatory values. This study sought to determine whether adding phosphate prior to switching the disinfectant could mitigate lead release. Using synthetic tap water and new lead pipes, we developed a set of lead pipes with scales rich in PbO2(s) and then studied their response to a change from free chlorine to monochloramine. Total lead concentrations remained below 10 µg/L for pipes that received phosphate prior to and during the switch. In contrast, total lead concentrations increased from less than 5 µg/L to more than 150 µg/L as a result of the disinfectant switch when phosphate was not present. Characterization of the pipe scales demonstrated that plattnerite (ß-PbO2(s)) was the dominant component of the scale prior to the switch, and that the scale gradually transformed into one containing a lead phosphate solid chemically similar to phosphohedyphane (Ca2Pb3(PO4)3(Cl,F,OH)(s)) when phosphate was present.