Environmental Impact of Metal and Metalloid Leaching from Highway

(1) Used on highways since the 1930s, glass beads are increasingly being imported; recently elevated concentrations have been observed.(1-4) The prese...
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Environmental Impact of Metal and Metalloid Leaching from Highway Marking Glass Beads Nimrat K. Sandhu,† Lisa Axe,†,* Kauser Jahan,‡ Kandalam V. Ramanujachary,§ and Kelsey Coolahan§ †

Department of Civil & Environmental Engineering, New Jersey Institute of Technology, Newark, New Jersey 07102, United States Department of Civil & Environmental Engineering, Rowan University, Glassboro, New Jersey 08028, United States § Department of Chemistry & Biochemistry, Rowan University, Glassboro, New Jersey 08028, United States ‡

S Supporting Information *

ABSTRACT: Recently, metals and metalloids have been observed at elevated concentrations in glass beads imported to the US. Average total concentrations in imported batches ranged from 103 to 683 mg kg−1 for As, 62 to 187 mg kg−1 for Sb, and 23 to 179 mg kg−1 for Pb. The labile fraction associated with the glass beads resulted in leached concentrations as great as 538 μg L−1 for As, 1092 μg L−1 for Pb, and 160 μg L−1 for Sb. Sequential extraction was conducted as well to better understand the form of metals and metalloids associated with the glass beads. Only 0.23% of As, 3.40% of Pb, 2.37% of Ba, and 1.92% of Mn were extracted in the exchangeable (As, Mn, and Ba) and the oxidizable forms (Pb), whereas greater than 97% of metals and metalloids present were associated with the glass matrix. Nonparametric statistics were applied to test total concentrations that resulted in exceedances in the groundwater quality criteria. Results demonstrated that the As, Pb, and Sb limits were exceeded for 98%, 58%, and 15% of the samples tested respectively suggesting a potential environmental impact to groundwater used as a drinking water source.

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INTRODUCTION Glass beads are embedded on pavement markings to provide retroreflectivity, an optical phenomenon that plays a crucial role in maintaining the guiding function of highway striping to ensure safe driving.1 Used on highways since the 1930s, glass beads are increasingly being imported; recently elevated concentrations have been observed.1−4 The presence of metals and metalloids in the glass beads is of environmental concern due to their potential for leaching under environmentally relevant conditions.1 Sandhu1 evaluated a total of 18 batches of glass beads (6 domestic and 12 imported) for total concentrations of As, Pb, and Sb. Whereas samples from the batches were observed to be normally distributed, concentrations in imported batches were 1 to 2 orders of magnitude greater than those in the domestic batches. As a result, further study of the potential for leaching from the glass beads was conducted.1 In addition to the toxicity characteristic leaching procedure (TCLP)5 and the synthetic precipitation leaching procedure (SPLP),6 a fractional factorial experiment was carried out on one imported batch based on © 2013 American Chemical Society

environmentally relevant factors (pH, deicing salt application, particle size, time, and ionic strength). The factors most significant in affecting leaching were pH and time. For As and Sb, leached concentrations tended to increase with increasing pH, whereas for Pb leached concentrations increased as pH decreased. The TCLP and SPLP tests proved to be insufficient as compared to the factorial studies where leached concentrations were 1 to 2 orders of magnitude greater. TCLP results revealed that the glass beads studied would not be classified as hazardous waste.1 However, elevated leached concentrations from the factorial study indicated a potential impact when disposed as a bulk solid waste. For storage on a surface or subsurface and in considering a worst case scenario, leached concentrations can diffuse through the surrounding soils and impact groundwater (used as a drinking water source). Received: Revised: Accepted: Published: 4383

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Estimating pore water concentrations can be challenging.7 Several states including New Jersey, Florida, and Wisconsin require an assessment of risk through application of the SPLP test.6,8−10 In theory, SPLP simulates leaching of elements from acid rain using HNO3/H2SO4 solution at an initial pH of 4.20 and a liquid to solid ratio (L/S) of 20:1. The extract represents the leachate and is gauged against criteria for assessing potential risk to groundwater. These criteria are derived from state-based groundwater quality limits (GQL) that, in some cases, are equivalent to the U.S. EPA maximum contaminant levels (MCL) for drinking water and for others are multiplied by a dilution attenuation factor (DAF) to account for dilution in an aquifer. Florida and Wisconsin use a DAF of 1.7,8,10,11 Other states assume the leachate represents pore water and specify site-specific DAF values. For example, the New Jersey Department of Environmental Protection (NJDEP) developed remediation standards on a site-by-site basis and applied a default DAF of 13 based on their hydrogeological studies.9 The U.S. EPA12 specifies a range for DAF values 1 (for no dilution) to 20. However, the SPLP approach may under-predict leaching from glass beads as concentrations were 2 orders of magnitude less than those observed in other batch studies using environmentally relevant conditions.1 In our earlier studies,1 over the course of 160 days, less than 2% of the total metal or metalloid concentration leached from the glass beads. This result is similar to that found by Dijkstra et al.13 for example, where leached concentrations made up a small fraction (approximately 2 to 10%) of the total metal present in the soils. Nonetheless, this fraction (2%) resulted in leached concentrations as great as 6213 μg L−1 for As, 520 μg L−1 for Pb, and 110 μg L−1 for Sb,1 where the New Jersey Default Leachate Criteria for Groundwater of 3 μg L−1 for As, 65 μg L−1 for Pb, and 78 μg L−1 for Sb were exceeded, thereby suggesting one potential impact would be that to groundwater used as a drinking water source. On the basis of these initial studies, environmentally relevant conditions under which leaching was most significant were applied to better understand the relationship between the total and leached concentrations. Moreover, the mobility of metal and metalloids from the soda-lime glass beads may be affected by the oxides present. The soda lime silica glasses are Si−O networks in which the SiO4 tetrahedral are joined together by the oxygen atoms located at the vertices. These tetrahedral connections become modified by the incorporation of the Na, Ca, Mg, Fe, K, and Al ions.14 The basic unit is a trigonal antiprism in which Na is coordinated to three O atoms at a distance of 2.3 Å with another three O atoms at nonbonding distances of approximately 3 Å.15 The degree of a chemical attack on glasses increases with increasing concentration of alkali oxides due to formation of a more open glass network.16−18 EDX microanalysis on soda lime glass exposed to chemical attack with simulated marine water showed loss of Na2O and SiO2 with respect to the original glass.16 The type of leaching solution affects the degree of leaching of glass.17,19,20 El-Batal et al.19 observed that the maximum leaching rates of soda lime silica glass occurred under acidic conditions (0.1 N HCl) and was followed by alkaline (0.1 N NaOH); the lowest rates were reported for distilled water. To assess the metal mobility, sequential extraction procedure was carried out on a selected batch of the glass beads.

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METHODS AND MATERIALS

Laboratory quality assurance and quality control procedures were based on Standard Methods for the Examination of Water and Wastewater.21 The plastic and glass containers were soaked in 10% nitric acid for 24 and 48 h respectively, rinsed with Millipore-Q water, dried, and stored in particle free environment before use. All reagents were of trace metal or analytical grade. Batches of glass beads (six domestic and twelve imported) were provided by New Jersey Department of Transportation (NJDOT) and its vendors. Ninety eight percent of the glass beads as received were observed to be greater than 100 μm using Beckman Coulter Particle Size Analyzer (Table S1 of the Supporting Information). Total metal concentrations were measured using the Niton XL3t 600 Series field portable X-ray fluorescence analyzer (FP-XRF) (Thermo Scientific, ̀ Massachusetts), consistent with methodology used in Billerica, earlier studies.1 Three types of studies were conducted: batch leaching experiments based on environmentally relevant factors (i.e., pH, salt, and ionic strength), sequential extraction, and field emission scanning electron microscopy (FE-SEM). Each of these methods is discussed in detail in the following sections. Batch Leaching Experiments and Potential Environmental Impact. These experiments consisted of batch leaching assessments conducted on eight imported batches. The objective was to address leaching under environmentally relevant conditions (pH, salt, and ionic strength) and as a function of the initial metal and metalloid concentration in the glass beads imported. Domestic batches were not studied as metal and metalloid concentrations were 1 to 2 orders of magnitude lower than those in imported batches.1 Moreover, for a number of batches concentrations were observed to be less than the detection limit of FP-XRF (Table S2 of the Supporting Information). Samples studied in the leaching experiments were first analyzed with FP-XRF for total concentrations. Leaching from these glass bead samples was then examined under environmentally relevant conditions where leaching was most significant for As, Pb, and Sb based on results from our previous studies1 (Table S3 of the Supporting Information). In a fractional factorial study involving batch experiments, the affect of pH, salt, ionic strength, and time was investigated.1 For As and Sb, maximum leaching was observed at pH values of 7 and 10 with 10−2 M CaCl2·2H2O. However, the most significant leaching for Pb was found at pH 4 with 10−2 M NaCl.1 Three samples from each of the eight imported batches were studied and run in triplicate resulting in 72 samples. Of these samples, 48 were analyzed for leaching of As and Sb, whereas 24 were examined under the conditions for the leaching of Pb. This study was conducted for a period of 30 days (under completely mixed conditions using a shaker) as this represented approximately 80% of the maximum leaching observed over the 160 days studied.1 After 30 days, filtered samples were acidified with 1% HNO3 to a pH less than 2 and analyzed for metals and metalloids using inductively coupled plasma mass spectroscopy (ICP-MS).22 Sequential Extraction. Sequential extraction was carried out on six samples collected from one of the imported batches. Initial total concentrations of As, Pb, and Sb in these samples were determined using FP-XRF. The extractions were conducted with the modified three-step sequential extraction procedure proposed by the Commission of the European Communities Bureau of Reference (BCR) of the Standards, Measurements and Testing (SM&T) Programme for soils and 4384

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sediments.23 A major component of soils and sediments is silica (>50%),13,23 this procedure is applicable for glass beads (silica content >60%). Furthermore, whereas the BCR procedure was used to study Cd, Cr, Cu, Ni, Pb, and Zn,23 this extraction technique has also since been used by a number of researchers to study leaching of As, Pb, and Sb.24−27 Extractions were performed in 40 mL borosilicate glass centrifuge bottles with an end-overend shaker rotated at 30 rpm. Between successive extractions, the residue was separated through centrifugation at 3000 g for 20 min. The supernatant was decanted into high density polyethylene (HDPE) Nalgene bottles, acidified to pH less than 2, and refrigerated until analysis with ICP-MS.22 The residue was washed with 20 mL of deionized water, centrifuged for 20 min, and the supernatant discarded. In the first step, metals associated with the exchangeable and carbonate fraction were extracted with 0.11 M acetic acid solution (CH3COOH) at a pH of 3. The fraction of metals and metalloids observed to leach are hypothesized to be associated with alkali and alkaline earth metal oxides, which form nonbridging bonds with the silicate structure16 and are considered soluble; this fraction would be observed in the exchangeable fraction. In the second step, sorption to iron and manganese fractions are extracted with 0.5 M hydroxylammonium chloride (NH2OH·HCl) and 0.05 M HNO3 solution; this contribution is not expected to be significant with the soda-lime glass. In the third step, remaining residue is treated at 85 ± 2 °C with two sequential 10-mL portions of 30% H2O2 adjusted to pH 2 with HNO3, and then extracted at pH 2.0 ± 0.1 with 1.0 M ammonium acetate (CH3COONH4) solution for the organics and sulfide fractions. Step 4, which employs digestion to measure residual concentrations, was not carried out because, whereas aqua regia (HCl and HNO3) cannot achieve complete dissolution of glass,28,29 dissolution with hydrofluoric acid (HF) is hazardous. FP-XRF was used to determine the initial total metal concentrations, and thus based on a mass balance; the difference between initial and extracted concentrations from the first three steps of sequential extraction provides an estimate of the metals associated with labile fraction, whereas the residue remaining for the fourth step provides the fraction associated with the silicate structure, considered sequestered throughout the study. FE-SEM Analysis. The LEO 1530 FE-SEM equipped with an energy dispersive X-ray micro analyzer (EDX) Inca series 200 was utilized to study the surface morphology of domestic and imported beads. During sequential extraction, the residual from each step is applied in further extractions. The fraction remaining at the conclusion of the procedure was then analyzed by FE-SEM/EDX. For the FE-SEM, samples were coated under high vacuum with a layer of carbon using an Edward’s 12E6/ 1266 coating unit.

Figure 1. Total average concentrations of As, Pb, and Sb measured using FP-XRF based on triplicate samples from batches 9, 10, 11, 12, 13, 14, 16, and 18. Variability of concentrations was observed to range from 10 to 42% for As, 30−89% for Pb, and 9−24% for Sb, respectively.

reached 1092 μg L−1, this degree of leaching is plausible given the significant variability observed (30−89%). In this study, although a relationship between the leached and initial total concentrations was not discernible, the potential impact associated with glass bead application, use, and management is considered. A better understanding of contaminant speciation and behavior may involve several types of studies, such as extractable concentrations, SPLP, and leaching as a function of pH, time, salt, and ionic strength. In this study, the NJ Default Leachate Criteria with a DAF of 13 was applied to consider the potential impact using a conservative condition of leaching from bulk glass beads (i.e., a storage or solid waste condition) into groundwater (used as a drinking water source) (Figure 2). For As, 96% of the leached concentrations observed exceeded the NJ Default Leachate Criterion of 3 μg L−1 where the initial As concentration in the beads ranged from 92 to 823 mg kg−1 (Figure 2). For Pb, 75% of leached concentrations exceeded the criterion of 65 μg L−1 with initial Pb concentrations ranging between 19 mg kg−1 and 204 mg kg−1 (Figure 2). Leached concentrations of Sb exceeded the threshold limit of 78 μg L−1 for 27% of the samples where the initial concentration varied between 54 mg kg−1 and 192 mg kg−1 (Figure 2). Leached concentrations of As, Pb, and Sb also exceeded the Florida and Wisconsin groundwater quality criteria. With regard to other potential impacts, surface water quality limits (340 μg L−1 for As and 65 μg L−1 for Pb in freshwater, 69 μg L−1 for As and 210 μg L−1 for Pb in saltwater) set by the U.S. EPA12 as well as NJDEP30 for aquatic life were also exceeded for As and Pb. The distribution of leached concentrations was not normal (Figure 3). Specifically, for As 2.1% of leached concentrations lied outside the (μ ± 3σ) limit, where broadening of the tails suggest the need for additional moments to describe the distribution. For Pb, 4.2% of the leached concentrations fell outside the μ ± 3σ limits. However, samples from measuring total concentrations revealed Gaussian distributions for As (99.7%) and Pb (99%). For Sb, both the total and leached concentrations measured were normally distributed. On the basis of these distributions, nonparametric statistics were applied to further evaluate leached concentrations. These

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RESULTS AND DISCUSSION Batch Leaching Experiments and Potential Environmental Impact. Results from using FP-XRF revealed that average total concentrations for the imported batches of glass beads ranged from 103 to 683 mg kg−1 for As, 62 to 187 mg kg−1 for Sb, and 23 to 179 mg kg−1 for Pb (Figure 1). Variability was significant with the average error for each batch ranging from 10 to 42% for As, 30 to 89% for Pb, and 9 to 24% for Sb.1 Leached concentrations were as great as 538 μg L−1 for As, 1,092 μg L−1 for Pb, and 160 μg L−1 for Sb (Figure 2). Similar concentrations of As and Sb have been observed in the previous studies.1 Whereas the peak concentration for Pb 4385

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Figure 2. Leached concentrations versus the total concentrations for As, Pb, and Sb. NJ Default Leachate Criteria is shown by dashed lines. Florida and Wisconsin both use a Default Leachate Criteria of 10 μg L−1 and 15 μg L−1 for As and Pb, respectively. For Sb, FL limit is 14 μg L−1and WI uses 6 μg L−1.

A survey conducted across the U.S. indicates that the present allowable total metal concentrations in glass beads set by various states (NJ, GA, LA, CA, TX, CO, AZ, WA, OR, and KS) range between 50 to 200 mg kg−1 for As, Pb, and Sb.36 The hypothesis was tested down to the lowest total metal concentration observed. In the case of As, the analysis was applied at 100, 150, and 200 mg kg−1. For Pb and Sb, the lowest metal concentrations were approximately 20 and 50 mg kg−1, respectively. The analysis for Pb was applied at X = 20, 50, 75, and 100 mg kg−1 and for Sb at X = 50, 75, 100, 150, and 175 mg kg−1 as total concentrations of Sb did not exceed 200 mg kg−1. The minimum total concentration resulting in a leached concentration exceeding the NJ Default Leachate Criterion and for which the results of the nonparametric test applied was observed to be statistically significant is identified as the concentration that may potentially pose a risk to groundwater used as a drinking water source. This analysis (illustrated for Pb with initial concentrations exceeding 100 mg kg−1 in Table S4 of the Supporting Information) implies that as the total concentrations of As, Pb, and Sb in the glass beads exceed these thresholds there is a potential risk based on this approach.9,12 For As, when the total concentration was greater than or equal to 100 mg kg−1, the leached concentrations exceeded the NJ Default Leachate Criterion of 3 μg L−1 at a 99% confidence level (Table 1). Similarly, in the case of Sb, total concentrations greater than or equal to 175 mg kg−1 resulted in leachates

methods use relative ranks of the sample observations rather than their actual numerical values. The Wilcoxon Signed Rank test was applied31 and is stronger than the simple Sign Test.32 This test has been frequently used in environmental studies to compare water quality characteristics as a function of seasonal changes,33 metal concentrations in contaminated sediments,34 and lead concentrations in workplace air samples.35 The sample differences from a test median (M) were ranked and statistically evaluated. The test median (M) in this study was assumed to be the NJ Default Leachate Criterion for each metal and metalloid (3 μg L−1 for As, 65 μg L−1 for Pb, and 78 μg L−1 for Sb). The following hypothesis was tested: Ho: When the initial metal concentration is less than X mg kg−1, the median of the leached concentrations (MY) is less than the NJ Default Leachate Criteria (3 μg L−1 for As, 65 μg L−1 for Pb, and 78 μg L−1 Sb), that is MY < 3 μg L−1 (for As), MY < 65 μg L−1 (for Pb), and MY < 78 μg L−1 (for Sb). H1: When the initial metal concentration is greater than or equal to X mg kg−1, the median of the leached concentrations (MY) is greater than or equal to the NJ Default Leachate Criteria (3 μg L−1 for As, 65 μg L−1 for Pb, and 78 μg L−1 for Sb), that is MY ≥ 3 μg L−1 (for As), MY ≥ 65 μg L−1 (for Pb), and MY ≥ 78 μg L−1 (for Sb). 4386

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Figure 3. Gaussian plots of (a) total vs (b) leached concentrations for As, Pb, and Sb. For As and Sb, 48 data points were used and for Pb, 24 points were used.

exceeding the criterion at the 90% confidence level. The NJ Default Leachate Criterion of 65 μg L−1 for Pb was exceeded at total concentrations greater than 20 mg kg−1 with the Wilcoxon Signed Rank test. The maximum concentration for lead allowable in food (glass) containers is regulated at 100 ppm.37 This limit was established by testing the hydrolytic/ corrosion resistance of glass food containers with hydrochloric acid. Food containers, as a virtue of their function, come in contact with a variety of acidic chemicals (e.g., vinegar). To address corrosion resistance, a standard hydrolytic resistance test is carried out on all food containers made from glass. In processing glass beads, more than 90% of the melt is from recycled glass. If source concentrations of 100 mg kg−1 are assumed to be representative in manufacturing, then leachates may exceed the criterion at the 99% confidence level. Sequential Extraction. The metal and metalloid species associated with the alkali oxide, alkaline earth oxides, and the silica surface offer the most plausible explanation for the

leached concentrations observed. The hypothesis in this study is that the labile fraction of metals are either adsorbed on the surface through electrostatic interactions38,39 or are released in oxide dissolution (e.g., Na2O, CaO, MgO, and PbO).17,40 To test this hypothesis, the modified BCR sequential extraction procedure was conducted. This approach makes use of several reagents consecutively to extract operationally defined segments.41−43 The recoveries of fractions from sequential extraction were computed based on the total metal concentrations initially present as determined by FP-XRF. The first step in sequential extraction (pH 3) revealed 0.04 to 0.23% (5 to 35 μg L−1) of As, 0.46 to 0.76% (16 to 30 μg L−1) of Pb, 0.76 to 1.66% (82 to 221 μg L−1) of Ba, and 0.46 to 0.60% (9 to 13 μg L−1) of Mn (Figure 4). This fraction represents the physically adsorbed elements.38 Alkali and alkaline earth oxides are also extracted.17 During the leaching experiments, this form is expected to be released in the aqueous phase. In the manufacturing of glasses, As is introduced in the 4387

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Table 1. Results of Wilcoxon Signed Rank Testa

a sparingly soluble form (Ca2As2O7), to a lesser extent.47−50 These structures may also be found in glass.50 Past studies have reported that arsenic vapor is captured by lime and calcium silicates.51,52 As the glass beads are subjected to an aqueous solution of 0.11 M acetic acid at a pH of 3 (pKa = 4.76 at 20 °C53), sodium and calcium ions are preferentially extracted from the glass bead surface due to dissolution (e.g., CaO, pKso = 3.34 at 20 °C54); thereby releasing associated metals and metalloids. Pb, Ba, and Mn are also present in the structure of the glass and are associated with alkali oxides, silica, or nonbridging oxygens. Using X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), PbO, Pb2SiO4, BaO, and MgMnSi2O6 were observed in bottom ash samples;55 of these, the two oxides are ones most likely to undergo dissolution in the extraction and leaching experiments.46,55 The second step of the extraction (pH 1.5 and 0.5 M) resulted in the leaching of 0.23 to 0.26% (9 to 34 μg L−1) of Pb, 0.05 to 0.20% (7 to 21 μg L−1) of Ba, and 0.57 to 0.63% (11 to 14 μg L−1) of Mn (Figure 4). Because hydroxylamine hydrochloride is applied to dissolve Mn/Fe oxides, strong scavengers for trace metals,56 this extraction did not result in significant concentrations. The third extractant, hydrogen peroxide followed by ammonium acetate (pH 2) resulted in 1.2 to 2.43% (41 to 94 μg L−1) of Pb, 0.17 to 0.51% (22 to 53 μg L−1) of Ba, and 0.46 to 0.50% (9 to 11 μg L−1) of Mn (Figure 4). In this study, the dominant part for Pb was observed to be associated with the oxidizable fraction similar to results observed by others.42,57,58 The third stage is used to extract organic and oxidizable sulfides leading to the release of associated metals. As there is no sulfur present in the glass beads and hence no oxidizable sulfides, the observed increase in Pb is most likely associated with the organic oleophobic coating on the glass beads. Glass beads are coated with nontoxic organo-titanium derivatives to support their ability to float in the paint marking process. Through this application the surface is exposed providing retroreflectivity.59,60 During successive extractions, redistribution of metals is likely,61 which may explain the association of Pb with this surface. The residual fraction consists of metals embedded in the lattice structure and is considered the immobilized fraction. Using a mass balance, approximately 99% of As, 98% of Ba, 98% of Mn, and 97% of Pb were associated with the residual fraction. The degree of mobilization observed in this extraction followed the trend of Pb > Ba ≈ Mn > As. Results of the sequential extraction suggest that a significant fraction of As, Pb, and Sb in the glass bead samples are likely adsorbed through ionic bonds on the alkali oxides,38,39 which in turn are bound in the lattice structure through nonbridging bonds.16,17,40 Nevertheless, the extracted fraction resulted in leached concentrations that have potential toxicological relevance as they exceed the New Jersey Default Leachate Criteria. FE-SEM Analysis. The morphology of the domestic glass beads revealed the surface was smooth and spherical with no distinguishable features (Figure 5). However, imported glass bead surfaces appeared to be rougher with visible surface irregularities. FE-SEM analysis of the extracted glass bead showed corrosion on the surfaces and the formation of porous layers due to leaching of alkali ions. Whereas EDX provides various types of qualitative and quantitative information such as elemental mapping, morphology, crystal habits, and mineral composition,62,63 silicon was the main element present at 69 ± 4% for the domestic as well as imported beads. Fe was detected in the domestic glass beads; however, it was absent in the

significance

element As

Pb

Sb

total conc (X) (mg kg−1)

calcd zvalue

P-value

90% (α = 0.1)

95% (α = 0.05)

99% (α = 0.01)

100 150 200 20 50 75 100 150 50 75 100 150 175

5.710 5.300 4.920 3.450 2.896 3.006 2.480 1.753 −3.892 −3.744 −3.945 −2.017 1.355

10−5 10−5 10−5 10−5 0.002 0.001 0.007 0.040 1.000 1.000 1.000 0.978 0.088

yes yes yes yes yes yes yes yes no no no no yes

yes yes yes yes yes yes yes no no no no no no

yes yes yes yes yes yes yes no no no no no no

a A p-value less than the selected level of confidence, results in the rejection of Ho and indicates a statistically significant result.

Figure 4. Mass balances of selective sequential extraction fractions for As, Ba, Pb, and Mn.

form of As2O3 in the glass melt. As a fining agent it is converted to As2O5, then back again to As2O3 at higher melt temperatures, and may be present as As (III) or As (V) in the final glass product.44−46 However, studies of matrices similar to glass (soils, sediments, and fly ash) have revealed that the dominant form is As(V) present was calcium arsenate (pharmacolite, Ca3(AsO4)2, pKso = 14.40 at 20 °C) with calcium pyroarsenate, 4388

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Figure 5. FE-SEM micrographs of (a) domestic, (b) imported, and (c) leached glass bead surfaces. EDX analysis normalized to 100% is shown in the insets.

imported beads. The concentrations of the remaining nontrace (>0.5%) elements (i.e., Na, Ca, and Mg) were consistent in both domestic and imported glass beads (Figure 5). Trace metals and metalloids were observed at concentration less than the detection limit of the Inca series 200 EDX (0.5% by wt). Results indicated the presence of trace elements on the surface at low concentrations (As at 0.38% by wt), which can be extracted over time during precipitation and salt application. As the alkali ions undergo leaching from the surface, dissolution of

alkali oxides takes place, and the trace metals associated with the oxides and silica are released.

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ASSOCIATED CONTENT

S Supporting Information *

Particle size distribution of glass beads using the Beckman Coulter particle size analyzer in the gas phase, detection limits of FP-XRF, summary of leaching studies, and Wilcoxon signed 4389

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rank test applied on Pb. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Tel: +1 973-596-2477. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank NJDOT for providing funding for this research under the grant FHWA-NJ-2010-014. The contents of this report reflect the views of the authors who are responsible for the facts and the accuracy of the data presented herein. The contents do not necessarily reflect the official views or policies of the NJDOT or the Federal Highway Administration. This article does not constitute a standard, specification, or regulation.

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REFERENCES

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