Lead( iv ) oxide PbO 2 is one dominant solid phase in lead corrosion scales of drinking water distribution systems. Understanding the colloidal dispersion of PbO 2 is important for lead control in drinking water, especially under scenarios of switching the residual disinfectant from chlorine to chloramine. This study investigated the changes in lead release and colloidal dispersion from PbO 2(s) associated with the presence of natural organic matter (NOM), the introduction of chloramine, and the addition of a phosphate corrosion inhibitor in drinking water distribution systems. Experimental data showed that when NOM was present, the surface charges of PbO 2 exhibited a prominent negative shift, leading to colloidal dispersion of Pb( iv ) particles. The presence of chloramine did not significantly change the detrimental effects of NOM on the colloidal behavior of PbO 2 . In contrast, the addition of phosphate greatly reduced colloidal lead release in the size range between 0.1 and 0.45 μm, and limited lead release with colloidal sizes less than 0.1 μm to below 15 μg L −1 , i.e. , the U.S. EPA regulatory standard. The beneficial effects of phosphate addition are mainly attributed to the suppression in colloidal dispersion of Pb( iv ) particles. Meanwhile, the presence of phosphate also limits the reductive dissolution of PbO 2 via the formation of hydroxypyromorphite Pb 5 (PO 4 ) 3 OH particles. Results from this study suggest that phosphate limits the dispersion of PbO 2(s) by NOM and prevented the release of Pb( iv ) colloids into drinking water.
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Effects of residual disinfectants on the redox speciation of lead( ii )/( iv ) minerals in drinking water distribution systems
This study investigated the reaction kinetics on the oxidative transformation of lead( ii ) minerals by free chlorine (HOCl) and free bromine (HOBr) in drinking water distribution systems. According to chemical equilibrium predictions, lead( ii ) carbonate minerals, cerussite PbCO 3(s) and hydrocerussite Pb 3 (CO 3 ) 2 (OH) 2(s) , and lead( ii ) phosphate mineral, chloropyromorphite Pb 5 (PO 4 ) 3 Cl (s) are formed in drinking water distribution systems in the absence and presence of phosphate, respectively. X-ray absorption near edge spectroscopy (XANES) data showed that at pH 7 and a 10 mM alkalinity, the majority of cerussite and hydrocerussite was oxidized to lead( iv ) mineral PbO 2(s) within 120 minutes of reaction with chlorine (3 : 1 Cl 2 : Pb( ii ) molar ratio). In contrast, very little oxidation of chloropyromorphite occurred. Under similar conditions, oxidation of lead( ii ) carbonate and phosphate minerals by HOBr exhibited a reaction kinetics that was orders of magnitude faster than by HOCl. Their end oxidation products were identified as mainly plattnerite β-PbO 2(s) and trace amounts of scrutinyite α-PbO 2(s) based on X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) spectroscopic analysis. A kinetic model was established based on the solid-phase experimental data. The model predicted that in real drinking water distribution systems, it takes 0.6–1.2 years to completely oxidize Pb( ii ) minerals in the surface layer of corrosion scales to PbO 2(s) by HOCl without phosphate, but only 0.1–0.2 years in the presence of bromide (Br − ) due the catalytic effects of HOBr generation. The model also predicts that the addition of phosphate will significantly inhibit Pb( ii ) mineral oxidation by HOCl, but only be modestly effective in the presence of Br − . This study provides insightful understanding on the effect of residual disinfectant on the oxidation of lead corrosion scales and strategies to prevent lead release from drinking water distribution systems.
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- Award ID(s):
- 1653931
- NSF-PAR ID:
- 10219614
- Date Published:
- Journal Name:
- Environmental Science: Water Research & Technology
- Volume:
- 7
- Issue:
- 2
- ISSN:
- 2053-1400
- Page Range / eLocation ID:
- 357 to 366
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Chlorinated and brominated forms of salicylic acid (SA) have recently been identified as a new class of disinfection byproducts (DBPs) in drinking water. Herein, we report the inherent reactivity of several aqueous halogenating agents toward hydrogen salicylate, the predominant species of salicylic acid under environmental conditions. Using synthetic waters, halogenation rates associated with the formation of 3-chloro, 5-chloro, 3-bromo, and 5-bromosalicylate were measured as a function of pH, [Cl − ], [Br − ], free chlorine dose, and the initial concentration of SA. Halogenating-agent specific second-order rate constants were determined and decrease in the order: BrCl > BrOCl > Br 2 > Br 2 O > Cl 2 > Cl 2 O > HOBr > HOCl. Chloride is capable of enhancing rates of bromination and chlorination, ostensibly by promoting the formation of BrCl and Cl 2 , both of which are several orders of magnitude more inherently reactive than HOBr and HOCl, respectively. Kinetic data also support the participation of salicyloyl hypochlorite as a chlorination intermediate capable of influencing chlorination rates at pH >8. Experiments in which buffer concentrations were varied indicate that phosphate buffers can enhance rates of SA bromination but not chlorination; carbonate and borate buffers did not appreciably influence rates of bromination or chlorination. Under conditions representative of chlorinated drinking water, rates of SA bromination will generally exceed rates of SA chlorination. The results discussed herein demonstrate the importance of considering halogenating agents beyond HOBr and HOCl when developing kinetic models to describe and predict halogenation rates and selectivity in waters containing free chlorine.more » « less
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Free chlorine and free bromine ( e.g. , HOCl and HOBr) are employed as disinfectants in a variety of aqueous systems, including drinking water, wastewater, ballast water, recreational waters, and cleaning products. Yet, the most widely used methods for quantifying free halogens, including those employing N , N -diethyl- p -phenylenediamine (DPD), cannot distinguish between HOCl and HOBr. Herein, we report methods for selectively quantifying free halogens in a variety of aqueous systems using 1,3,5-trimethoxybenzene (TMB). At near-neutral pH, TMB reacted on the order of seconds with HOCl, HOBr, and inorganic bromamines to yield halogenated products that were readily quantified by liquid chromatography or, following liquid–liquid extraction, gas chromatography-mass spectrometry (GC-MS). The chlorinated and brominated products of TMB were stable, and their molar concentrations were used to calculate the original concentrations of HOCl (method quantitation limit (MQL) by GC-MS = 15 nmol L −1 = 1.1 μg L −1 as Cl 2 ) and HOBr (MQL by GC-MS = 30 nmol L −1 = 2 μg L −1 as Cl 2 ), respectively. Moreover, TMB derivatization was efficacious for quantifying active halogenating agents in drinking water, pool water, chlorinated surface waters, and simulated spa waters treated with 1-bromo-3-chloro-5,5-dimethylhydantoin. TMB was also used to quantify bromide as a trace impurity in 20 nominally bromide-free reagents (following oxidation of bromide by HOCl to HOBr). Several possible interferents were tested, and iodide was identified as impeding accurate quantitation of HOCl and HOBr. Overall, compared to the DPD method, TMB can provide lower MQLs, larger linear ranges, and selectivity between HOCl and HOBr.more » « less
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Abstract Total iron and total lead concentrations were correlated in water that had stagnated in laboratory‐scale experiments with sections of 10 harvested lead service lines (LSLs) from Providence, Rhode Island. One of these sections had much greater lead release and pH decrease during stagnation, and the inner surface of this service line had a thick coating of iron oxide scale. The iron‐rich scale was composed of coarse‐grained iron oxides (lepidocrocite and magnetite) covered with a thin lead‐bearing layer (hydrocerussite). Complementary batch experiments with pure iron oxides found that their surfaces accelerated the oxidation of Pb(II) from hydrocerussite, which produced PbO2(s)and decreased the pH. While the findings presented are for LSLs from Providence, the co‐occurrence of iron oxides and lead corrosion products is widespread. The results highlight the importance of considering iron corrosion when evaluating processes that control lead concentrations in tap water.
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/d0ew00706d
