Environ. Sci. Technol. 2009, 43, 4412–4418
Identification and Distribution of Vanadinite (Pb5(V5+O4)3Cl) in Lead Pipe Corrosion By-Products TAMMIE L. GERKE,† KIRK G. SCHECKEL,‡ A N D M I C H A E L R . S C H O C K * ,‡ Department of Geology, University of Cincinnati, Cincinnati, Ohio 45221-0013, and U.S. Environmental Protection Agency, National Risk Management Research Laboratory, 26 West Martin Luther King Drive, Cincinnati, Ohio 45268
Received February 16, 2009. Revised manuscript received April 20, 2009. Accepted April 28, 2009.
This study presents the first detailed look at vanadium (V) speciation in drinking water pipe corrosion scales. A pool of 34 scale layers from 15 lead or lead-lined pipes representing eight different municipal drinking water distribution systems in the Northeastern and Midwestern portions of the United States were examined. Diverse synchrotron-based techniques, including bulk XANES (X-ray absorption near edge spectroscopy), µ-XANES, µ-XRD (X-ray diffraction), and µ-XRF (X-ray fluorescence) mapping were employed along with traditional powder XRD, SEMEDXA (scanning electron microscopy-energy dispersive X-ray analysis), and ICP-OES (inductively coupled plasma-optical emission spectrometry) to evaluate vanadium speciation and distribution in these deposits. Vanadinite (Pb5(VO4)3Cl) was positively identified, and occurred most frequently in the surface layers. Low Vtot in these waters is likely the limiting factor in the abundance of vanadinite in the pipe scales, along with the existence of divalent lead. The occurrence of V in these samples as a discrete mineral is important because it is formed in the presence of very low concentrations of V in the finished water, it provides a mechanism to concentrate µg · L-1 amounts of V from the water to near-percent levels in the pipe scales, and the robustness of V accumulation and release in response to water chemistry changes is likely different than it would be with a sorption accumulation mechanism. Extrapolation from limited existing water chemistry data in this study provides an estimate of ∆Gf° for vanadinite as approximately -3443 kJ · mol-1, or less, leading to a log Ks0 value of approximately -86 for the reaction 3 Pb5(VO4)3CI(s) a {Pb2+}5 + {VO34 } + {Cl }, in which {}denotes activity.
Introduction Vanadium can be either beneficial or toxic to humans, depending on its oxidation state and concentration and its toxicity increases with increasing oxidation state (1–4). In natural and drinking waters, vanadium dominantly exists in either the +4 or +5 oxidation state as the aquatic species vanadyl (VO32-) or vanadate (VO43-), respectively (5–8). VIV * Corresponding author phone: 513-487-7412; fax: 513-569-7172; e-mail:
[email protected]. † University of Cincinnati. ‡ National Risk Management Research Laboratory. 4412
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 12, 2009
would be the most stable in reducing environments and VV (vanadate) is expected to be prevalent in systems consistently exposed to atmospheric oxygen (5), and hence, would be expected in disinfected or preoxidized drinking waters. Vanadate behaves similarly to orthophosphate, and will form ligand exchange surfaces with hydrous phases such as iron oxyhydroxides (8, 9). VIII, however, would rarely be found in potable waters because of being readily oxidized by air or dissolved oxygen. Researchers have modeled the V3+ metal ion, forming several hydroxide complexes including VOH2+, V(OH)2+, and V(OH)3°. Modeling of aqueous vanadyl speciation shows greater variation. Wehrli and Stumm (8) assumed the aqueous vanadyl species included the oxocations VO2+ and VO(OH)+; the neutral species VO(OH)2°; the oxoanion VO(OH)3-; and the polymeric oxocation (VOOH)22+. However, Langmuir (10) only included VO2+ and VO(OH)+ as vanadyl species (10, 11). Wanty and Goldhaber (12) have presented perhaps the most extensive critically evaluated compilation of thermochemical data for aqueous V species, including several vanadyl complexes with carbonate, oxalate, acetate, sulfate, fluoride, and chloride. In spite of the differences in the details of the aqueous model for VIV, the general predominance trends predicted by Wehrli and Stumm (8), Wanty and Goldhaber (12), and Langmuir (10) are similar (Supporting Information Figure SI1). Ingestion of vanadium from drinking water has recently been considered a potentially serious threat to human health because of the growing body of evidence regarding the human toxicity of vanadium (1–4). The concentrations of vanadium in United States drinking waters have not been comprehensively surveyed, because without a regulatory driving force, vanadium is rarely monitored by public or private water utilities. Regardless, in 2007 the California Office of Environmental Health Hazard Assessment set a notification level of 15 µg/L for vanadium in drinking water (13), and it has been listed by the U.S. Environmental Protection Agency in the draft Drinking Water Contaminant Candidate List 3 (14). Recently, the accumulation of vanadium up to as much as 2% by weight, has been reported for corrosion deposits in lead drinking water pipes from numerous United States public drinking water systems (15). Thus, a potential reservoir for human exposure already exists if vanadium were to be mobilized by changes in drinking water characteristics resulting from source or treatment changes. This study presents the first detailed look at vanadium speciation and distribution in drinking water pipe corrosion scales. Diverse synchrotron-based techniques, including bulk XANES (X-ray absorption near edge spectroscopy), µ-XANES, µ-XRD (X-ray diffraction), and µ-XRF (X-ray fluorescence) mapping along with traditional powder XRD, SEM-EDXA (scanning electron microscopy-energy dispersive X-ray analysis) and ICP-OES (inductively coupled plasma-optical emission spectrometry) were used in this study to evaluate vanadium speciation and distribution in these deposits. Fifteen lead pipe samples from eight different municipal drinking water distribution systems in the Northeastern and Midwestern regions of the United States representing a wide range of water chemistries and treatments were selected for this study, and were chosen on the expectation of giving the highest potential for differences in V speciation (Table 1). Six pipe samples received treated water from surface sources and two from groundwater sources. Corrosion control treatment strategies listed in Table 1 reflects current utility operations, not historical treatment strategies. Therefore, 10.1021/es900501t CCC: $40.75
2009 American Chemical Society
Published on Web 05/12/2009
TABLE 1. Summary of Samples Analyzed in This Study, with Type of Source Water, Current Applied Corrosion Treatment, V and Pb Concentrations, Types of Analyses Conducted on Each XANES sample ID
source water
corrosion treatment
A1
S
OP
A2
S
OP
A3
S
OP
A4
S
OP
A5
S
OP
A6
S
OP
A7
S
OP
B1 C1
S S
CP CP
D1 E1 F1 G1 H1 I1
GW S S S S GW
CP BP BP CP+PP CP CP
layer
V ppm
Pb %
macro XRD
L1 L2 L1 L2 L1 L2 L3 L1 L2 L3 L4 L1 L2 L3 L1 L2 L3 L1 L2 L3 whole L1 L2 L3 L1 L1 L2 L3 L1 L1 L1 L1 L2 L3
2760 1900 1200 6540 1380 4310 2730 2710 1560 6330 3600 8500 2750 5480 2650 716
