Selenium Speciation in Coal Ash Spilled at the Tennessee Valley

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Selenium Speciation in Coal Ash Spilled at the Tennessee Valley Authority Kingston Site Yu-Ting Liu, Tsan-Yao Chen, William Greer Mackebee, Laura Ruhl, Avner Vengosh, and Heileen Hsu-Kim Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 22 Nov 2013 Downloaded from http://pubs.acs.org on November 25, 2013

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Environmental Science & Technology

Selenium Speciation in Coal Ash Spilled at the Tennessee Valley Authority Kingston Site

Yu-Ting Liu§†, Tsan-Yao Chen※, William Greer Mackebee§, Laura Ruhl#, Avner Vengosh#, ※

Heileen Hsu-Kim§*

§

Department of Civil and Environmental Engineering, 121 Hudson Hall, Box 90287, Duke University, Durham, North Carolina 27708, United States

†

Department of Environmental Science and Engineering, Tunghai University, Taichung, Taiwan

※

Department of Engineering and System Science, National Tsing Hua University, Hsinchu, Taiwan

#

Department of Earth and Ocean Sciences, Nicholas School of the Environment, Duke University, Durham, North Carolina 27708, United States

*Corresponding Author: Phone (919) 660-5109; Fax: (919) 660-5219; Email: [email protected] #

Current Affiliation: Department of Earth Sciences, University of Arkansas at Little Rock, Little Rock, AR 72204

1

ACS Paragon Plus Environment


1 2

Abstract Selenium (Se) in coal ash spills poses a threat to adjacent ecosystems because of its

3

potential to mobilize and bioaccumulate in aquatic organisms. Given that the mobility and

4

bioavailability of Se is controlled by its valence states, we aimed to define Se speciation in coal

5

ash solids and examine the relationships between Se speciation and the magnitude of its

6

mobilization from coal ash. We used coal ash samples from the Tennessee Valley Authority

7

(TVA)-Kingston fossil plant and a site of coal ash spill that occurred in 2008 in Tennessee, USA.

8

Results of X-ray absorption spectroscopic analyses showed that Se in coal ash samples was a

9

mixture of elemental Se0 and Se oxyanions. The amount of leachable Se increased with an

10

increase of pH from 3 to 13. At the natural pH of coal ash samples (from pH 7.6 to 9.5), the

11

leachable Se was comprised of Se oxyanions, mainly selenite. This was observed by both direct

12

quantification of Se oxyanions in the leachate and the corresponding loss of Se oxyanions in the

13

solid phase. At pH 12, however, the Se release appeared to derive from both desorption of Se

14

oxyanions and oxidative dissolution of elemental Se0. Our results indicate that Se oxyanions are

15

the most labile species; however, the magnitude of Se mobilization will increase if the waste

16

material is subjected to alkaline conditions.

17

2


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Introduction A dike failure at Tennessee Valley Authority (TVA) Kingston Fossil Plant on December 22,

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2008 released over 3.7 million cubic meters of coal combustion products into the adjacent Emory

21

River. The release of the coal ash posed a great risk to the local ecosystem health, mainly due to

22

higher levels of trace elements in the spilled coal ash compared with background soils. Aquatic

23

organisms are particularly vulnerable to trace elements such as selenium (Se) if they are leached

24

from the coal ash material1-6. Selenium can biomagnify in the food web and impart deleterious

25

effects for ecosystem functions7. In the process of bioaccumulation in aquatic food webs, Se

26

content in primary producers varied by 2 to 6 orders of magnitude when exposed to selenite6, 8.

27

Selenium, in particular, is of environmental interest because of a narrow range between

28

nutritionally required and toxic-effect concentrations in many organisms9. The mobility and

29

bioavailability of Se in the environment mainly depends on the partitioning among different

30

valence states of Se. Oxidized forms of Se like selenite (SeIV) and selenate (SeVI) are highly

31

soluble, hence more mobile when compare to less soluble forms such as elemental Se0. The

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oxidized forms of Se exist primarily as oxyanions dissolved in solution or adsorbed to mineral

33

surfaces10. In terms of bioaccumulation, selenite poses an approximately 10-fold higher

34

bioconcentration factor than selenate for the phytoplankton Se uptake11. Although organo-Se

35

compounds with oxidation state of -2 or 0 are less soluble, they are relatively easily absorbed by

36

organisms12. Therefore the speciation of Se is an important consideration in undertaking

37

environmental risk assessments of Se-bearing wastes, predicting Se uptake and bioaccumulation,

38

and understanding Se distribution and transfer into the food chain in ecosystems.

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The negative impacts of Se have been well-documented12, 13; however, mechanisms

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pertaining to the sustained release of Se from coal ash spills into the aquatic environment remain

41

undefined. Several studies have indicated that total Se contents in ash materials are poor 3



42

predictors for Se leachability14, 15. Thus, Se speciation could be used as a complementary and

43

comparative manner for developing a better informed risk assessment, management and

44

remediation strategies for spilled coal combustion products.

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The objective of this research was to determine the leaching potential of Se in relation to the

46

speciation of Se in the coal ash materials. A reliable speciation analysis is needed to elucidate the

47

mechanisms for Se migration in this ecosystem. We performed pH- and redox-dependent

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leaching experiments with a variety of coal ash materials obtained from the TVA Kingston Fossil

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Plant and from the nearby site of the 2008 coal ash spill in Tennessee. The speciation of Se was

50

quantified in both leachates and solid phases (before and after leaching). The combination of the

51

leaching and speciation results was used to identify factors that can influence the magnitude of

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mobilization of leachable Se from coal ash.

53 54

MATERIALS AND METHODS

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Characteristics of Coal Ash Samples. The coal ash materials tested in this study included

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coal combustion products generated at the TVA Kingston Fossil Plant in Tennessee, USA. These

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samples comprised of fly ash collected in 2009 and 2010 from the plant (labeled KIF FA and KIF

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TVA C) and bottom ash (KIF BA) collected in 2009. We also studied three spilled coal ash

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samples collected from the Emory River (VB3, VB4) and the cove site (TN10S - referred as

60

"spilled TVA ash" in Ruhl et al.4). These samples were collected in 2009 within 6 months after

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the spill event. Sampling sites and details are provided in Figure S1 and Table S1 in the

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supporting information (SI) section.

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Concentrations of Se and other elements were quantified first by acid digestion of

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approximately 0.5 g of coal ash sample in concentrated HNO3 heated at 80 °C for 6 h. Elemental

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analysis of the digestate was performed by inductively coupled plasma mass spectrometry 4


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(ICP-MS). Mineral phases of coal ash samples were determined using X-ray diffraction (XRD).

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Subsamples for XRD analysis were mounted in 1-mm-thick glass sample holders. Diffraction

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patterns were recorded in transmission mode using a Mar3450 image plate with a wavelength of

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0.6199 Ǻ at the beamline BL01C2 in the National Synchrotron Radiation Research Center

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(NSRRC), Hsinchu, Taiwan.

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Leaching Experiments. Batch leaching experiments were first performed at the materials'

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‘natural’ pH, as defined by the aqueous phase pH when the coal ash sample was mixed with

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Nanopure-grade water (>18M-cm) for 48 h (Table S2). The leaching experiments were

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performed with liquid to solid (L/S) ratios ranging from 1 to 20 mL g-1, where the solids mass

75

refers to the sample dry weight. The rate of trace element leaching was also tested over the

76

course of 217 h for mixtures with L/S ratio of 10 mL g-1. The pH-dependent leaching

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experiments of coal ash samples were tested at a L/S ratio of 10 mL g-1-dry across from pH 3 to

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13 (adjusted using 1 N HNO3 or KOH) and mixed for 48 h.

79

All coal ash suspensions were continuously mixed end-over-end for 48 h at room

80

temperature (22-25ºC) to examine the Se leachability. The pH was measured before mixing and

81

after incubation prior to the filtration step. Separation of supernatant from the residual solids was

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conducted using a 0.2 µm nylon membrane syringe filter (VWR). Aqueous samples were

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preserved with the acid mixture of 2% HNO3 and 0.5% HCl (v/v) and held at room temperature

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for at least overnight prior to analysis for Se and other trace elements. Separate aliquots of

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filtered aqueous samples were adjusted to pH 5.2 for analysis of Se speciation.

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A subset of leaching experiments was performed in oxygen-free conditions. These

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experiments involved the KIF FA sample suspended in water with a liquid-to-solid ratio of 10

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mL g-1. Sodium borate (50 mmol L-1) was used to control the pH in the range from 8 to 13.

89

Oxygen-free conditions were created by boiling buffer solutions dispensed in the serum bottles 5



90

for 5 min and then purging the solution with 99.999% N2(g) for 2 h to achieve a redox potential

91

below about -110 mV as indicated by a resazurin redox indicator. The coal ash sample was added

92

to the oxygen-free buffer solution in an anaerobic chamber (Coy Labs) containing an ambient

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atmosphere of 90% N2(g), 5% H2(g), and 5% CO2(g). Similar ash suspensions with the borate

94

buffer were prepared in oxic (i.e. air saturated) conditions. The suspensions sealed in serum

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bottles were tumbled end-over-end for 48 h. The end-point pH was measured prior to the

96

filtration. Separation of the supernatant from the residual solid for the anaerobic samples was

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conducted in the glovebox using a 0.2 µm nylon syringe filter. Similar leaching tests were

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performed with elemental (gray) Se0 powder (99.99%, Sigma-Aldrich) suspended in air-saturated

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and oxygen-free buffer water to 1g Se L-1 (0.1%)16.

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Instrumentation. The concentrations of Se and other elements in acid digestates and

101

leachates were determined using an Agilent 7700x ICP-MS equipped with an Octopole Reaction

102

System (ORS). Se analysis was performed with a hydrogen reaction gas at a H2(g) flow rate of 4

103

mL min-1. Other trace elements were analyzed in no gas mode or helium reaction gas mode.

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Dissolved Se speciation was performed by high-performance liquid chromatography

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(Agilent 1100) interfaced with the ICP-MS. Chromatographic separation was performed with a

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Hamilton PRPX-100 HPLC column (10 µm particle size, 25 cm length × 4.1 mm internal

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diameter) and a mobile phase comprised of 5 mmol L-1 ammonium citrate and 2% v/v methanol

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with pH adjusted 5.2 and flow rate at 1 mL min-1. The HPLC-ICP-MS method was calibrated

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with separate stock solutions of selenocystine, selenomethionine, sodium selenite, and sodium

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selenate.

111

Selenium K-Edge XANES Data Collection and Analysis. Selenium speciation in the solid

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phase was determined by Se K-edge X-ray absorption near edge structure (XANES). Three sets

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of samples were prepared for Se-XANES analysis: the original coal ash samples with no further 6


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modification, residual solids after 48-h leaching experiments at the natural pH of the coal ash

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samples, and residual solids after 48-h leaching of the sample at pH 12 (Table S2 in SI). The

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leached solids were separated from the ash-water mixtures by centrifugation for 15 min at

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approximately 6000 × g and analyzed as moist pastes.

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Coal ash samples for Se-XANES analysis were prepared by mounting a portion

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(approximately 0.2 g) of the sample in acrylic sample holders, sealing with Kapton tape to avoid

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desiccation, and storing at 4 °C until analysis17. Spectra were collected at room temperature at

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the Beamline BL17C1 at the NSRRC. The speciation of Se in the samples was determined using

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the linear combination fitting (LCF) across from 30 eV below to 40 eV above the Se absorption

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edge. Commercially-purchased powders of iron selenide (Alfa Aesar), elemental (gray) Se0

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(99.99%, Sigma-Aldrich), selenomethionine (Sigma-Aldrich), and selenocystine (Sigma-Aldrich)

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were used as reference materials in LCF. Reference materials for selenite and selenate were

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prepared by sorbing these species to a concentration of 500 mmol kg-1on poorly crystalline

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aluminum hydroxide at pH 7. Spectral artifacts caused by radiation-induced reduction or

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oxidation were not observed in the study coal ash samples (Figure S2). Additional details for

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data collection and processing are provided in the SI.

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RESULTS AND DISCUSSION

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Coal Ash Composition. The fly ash and spilled coal ash samples were enriched in major and

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trace elements such as iron, cadmium, arsenic, and selenium, relative to the bottom ash sample

134

(Table S3). Total Se concentrations in these samples range from 4.5 to 6.9 mg kg-1, which are

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similar to the values reported by Bednar et al.3 (5.4 to 7.0 mg kg-1) but considerably greater than

136

that reported by Ruhl et al.1 (0.2 mg kg-1). No significant difference in Se content was observed

137

between the fly ash from the coal plants and the ash samples from the nearby spill site. However, 7



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total Se concentration in the bottom ash sample (0.3 mg kg-1) was only about one-tenth of the

139

other samples (Table S3). Results of XRD revealed the presence of approximately 70% mullite

140

or quartz in all of the tested ashes (Figure S3). These crystalline phases are commonly observed

141

in coal fly ash samples15. No other crystalline minerals were recognized in XRD spectra of the

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bulk powder samples.

143

Coal Ash Leaching Experiments. The leaching experiments were first conducted at the natural

144

pH of each sample to test the trends of Se dissolution as functions of L/S ratio and time.

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Dissolved Se in the coal ash samples generally increased with the decreasing L/S ratio (Figure

146

S4) and increasing reaction time (Figure S5). Se dissolution appeared to reach equilibration

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within 48 h at the L/S ratio of 10 mL g-1 for all samples except for the fly ash of KIF TVA C.

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Therefore, we selected a L/S ratio of 10 mL g-1 and time period of 48 h to perform subsequent

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pH-dependent leaching experiments. Endpoint pH values at completion of the 48-h leaching

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experiments at the natural pH of the coal ash samples were 7.6 to 9.5 (Table S2). At the natural

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pH values, the concentration of dissolved Se was greater than 30 µg L-1 for all samples except

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the bottom ash (Figure 1). At the L/S ratio of 10 mL g-1, this concentration exceeded the EPA

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freshwater chronic criterion concentration of 5 µg L-1 18. Although the absolute concentration of

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dissolved Se may decrease with increasing L/S ratio, our leaching results implied a potential

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release of Se from the spilled coal ash, which may be harmful to the adjacent aquatic ecosystems.

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For the pH-dependent leaching experiments, the content of dissolved Se increased as the

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leachate pH was artificially increased from 3 to 13 (Figure 1). The release of Se was relatively

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small (< 20 µg L-1) for all coal ash samples at low pH values between 3 and 5, but enhanced with

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increasing pH values. At pH near 13, the dissolved Se concentration was 13 to 81 times greater

160

than leachate concentrations at pH 5. This observed trend in increasing Se dissolution as a

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function of pH could be attributed to multiple liquid-to-solid partitioning phenomena, including 8


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dissolution, anion exchange at the solid-water interface, and aqueous complexation14. Selenium

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association with solid phases of coal ash have been described as: (1) adsorption of Se oxyanions,

164

i.e. selenite and selenate, on mineral surfaces such as Fe-, Al-, and Mn-(hydr)oxides19, 20, (2)

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formation of solid solutions with minerals such as gypsum (CaSO4) and hydrocalumite

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(Ca4[Al(OH)6]2(OH)2‧6H2O)20, 21, and (3) co-precipitation with multivalent cations such as Ca2+,

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Pb2+, and Ba2+.20, 22 At alkaline conditions, the desorption of Se oxyanions can result from fewer

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protonated surface sites that serve as binding sites for Se oxyanions5, 19 . However, alkaline pH

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can also decrease the solubility of Se precipitates21, 22. In our leaching experiments, the enhanced

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Se dissolution with increasing pH indicated that desorption of Se oxyanions controlled the

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magnitude of Se release and the formation of Se-containing precipitates, if they existed, played

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rather a minor role.

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For each coal ash sample, the amount of dissolved Se at both the sample’s natural pH and

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pH 12 corresponded with a decrease of total Se content in the solid phase after leaching

175

experiments (Figure 2a). The relationship between the dissolved Se concentration and the change

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in solid phase Se content was nearly 1:1 (regression slope = -1.10 ± 0.18 and -0.88 ± 0.18 for

177

natural pH and pH 12, respectively). This mass balance indicated a conservation of Se leaving

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from the solid phase and entering into the liquid phase at both pH conditions.

179

While our collection of ash samples did not include a very wide range of Se concentrations,

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we observed a relatively weak correlation between the total Se content in the solid phase of coal

181

ash samples and the amounts released to the liquid phase (Figure 2b). Approximately 12% of

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total Se (as indicated by the slope) was dissolved after 48 h of leaching at the natural pH, and this

183

proportion increased to around 35% when the leaching experiment was performed at pH 12

184

(Figure 2b). The proportion of leachable Se varied with pH values, indicating that total Se

9



185

content in the solid phase was not a good indicator for the magnitude of leachable Se. Several

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studies have provided a similar conclusion and recommended that environmental assessment

187

should not be based solely on the total Se contents of coal ash samples14, 15.

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Selenium Speciation in Leachates. Selenium speciation in the leachates indicated that

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selenite was the major dissolved species, consist of 75% to 94% of the total Se (Figure 3). A

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small amount of selenate was also observed in a few samples (up to 25%). These results agreed

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with the thermodynamic stability of Se: selenite is the most stable species under aerobic

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condition at the natural pH of coal ash samples ranging from 7.6 to 9.5 3, 23. Bednar et al.3

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indicated the predominance of selenite in leachates of spilled coal ash and in water samples

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collected from the sites with submerged fly ash. However, they also found an enrichment of

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selenate (44% of total dissolved Se) in the stilling pond effluent. Although the kinetics of the

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oxidation from selenite to selenate is rather slow and is a function of the redox potential16, 24, the

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distribution of Se species is sensitive to subtle alterations such as redox potential and the

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presence of redox catalysts.

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Selenium Speciation in Solid Phases. Major forms of Se in the solid phase of pre- and

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post-leached coal ash samples were quantified using Se K-edge XANES. XANES data in Figure

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4a showed minor spectral variations across the original coal ash samples with the exception of

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the bottom ash KIF BA. For this sample, the position of the white line peak was 12660.3 eV,

203

approximately 5 eV lower than other samples. The 48 h leaching experiment for the fly ash and

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spilled ash samples caused the position of the white line peaks to shift to a lower energy (from

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0.8 to 2.8 eV) relative to spectra of the original samples. Leaching of the bottom ash sample did

206

not result in a change in the white-line energy (Figure 4a-c).

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The presence and proportion of the Se species in solid phases was determined using LCF

208

with spectra obtained for reference materials including selenide, elemental Se0, selenomethionine, 10


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selenocystine, and selenite and selenate sorbed on Al-hydroxides. Results of LCF shown in

210

Figure 4d-f and Table S4 in SI indicate that only inorganic Se species were found in all of the

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pre- and post-leached samples. The Se inventory in pre-leached sample was dominated mainly

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by elemental Se0 (> 57%) with lesser amounts of selenite (11-42%) and selenate (< 5%). This

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composition generally agrees with the abundance of elemental Se0 shown for sediments in

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freshwater systems10, 25, 26 and for soils in reclaimed mine sites27, 28, but differs from that in other

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reported fly ash samples29 and flue gas desulphurization residues30, wherein selenite was

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reported to be dominant. In addition to selenite, however, Huggins et al.29 did find the presence

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of reduced form of Se in one of the fly ash samples collected during injection of activated carbon

218

upstream for Hg control. The apparent discrepancy in Se species may be caused by the various

219

coal ash treatments.

220

The post-leached samples at natural pH had a greater proportion of elemental Se0 and

221

smaller fraction of selenite when compared to the original Se composition in the pre-leached

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samples (Figure 4d and f; Table S4). The bottom ash sample was an exception to this trend. The

223

changes in solid-phase speciation suggested that adsorbed selenite was the main component that

224

was lost from the solid phases during leaching experiments. This result is consistent with

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speciation measurements of the leachates by HPLC-ICP-MS analysis: selenite accounted for

226

more than 75% of the dissolved Se in the leachates at natural pH of coal ash samples (Figure 3).

227

In leaching experiments performed at pH 12, greater proportions of selenite were lost from the

228

solid phase relative to leaching experiments at the samples’ natural pH values.

229

Selenium Speciation and Dissolution. We performed a mass balance of individual Se

230

species in the leaching experiments by calculating the concentrations of each Se species in the

231

solid phase from the LCF results and total Se content (Table S4 and S5) and comparing the

232

changes in the amount of Se species in the solids to the dissolved Se data in the leachate 11



233

solutions. For example, the change in solid phase concentrations of Se oxyanions

234

∆[selenite+selenate] was calculated by the selenite and selenate contents in the post-leached ash

235

samples minus that in the pre-leached ash samples. These ∆[selenite+selenate] values strongly

236

correlated to the concentration of dissolved Se at natural pH (regression slope = -0.95 ± 0.06; R2

237

= 0.98, Figure 5a). The nearly 1:1 relationship demonstrates that the release of Se oxyanions,

238

mainly selenite (Table S4), controlled the composition of Se in the leachates. However, at more

239

alkaline conditions, the ∆[selenite+selenate] values showed a relatively weaker correlation with

240

dissolved Se . The loss of selenite and selenate oxyanions from the solid phase could only

241

account for a portion of the bulk Se released into solution, as indicated by the regression slope of

242

-0.60 ± 0.15 (R2 = 0.83, Figure 5b). While we observed an increase in dissolved Se with

243

increasing pH (Figure 1), the weak correlation between loss of Se oxyanions from the solids and

244

dissolved Se at pH 12 (Figure 5b) indicate that other mechanisms and/or Se source may

245

influence the partitioning behavior of Se at high pH leaching conditions.

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246

Over the course of leaching processes, the changes in concentrations of elemental Se0

247

showed a weak correlation with dissolved Se, and were highly variable if data are separated by

248

the pH of the leachates (Figure 5c). While the proportion of elemental Se0 tended to increase

249

with elevated pH upon leaching (Table S4 in SI), the absolute concentration of elemental Se0

250

either stayed constant or decreased in the post-leached solids; the magnitude of this change

251

seems to be dependent on the pH of the leachates (Figure 5d). At the natural pH of the ash

252

samples, a measureable change of elemental Se0 in the solid phase was observed in three samples:

253

KIF TVA C, KIF BA, and TN10S. The largest drop in solid phase Se0 content ∆[Elemental Se0]

254

was observed at -0.29 mg kg-1 in KIF TVA C, which corresponds to 5% of total Se in the original

255

ash. No significant changes were found in the other three samples. For the leaching experiments

256

performed at pH 12 however, a much greater change in elemental Se0 content was observed for 12


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the two fly ash samples and the three spilled ash samples (Figure 5d). ∆[Elemental Se0] values

258

for the pH-12 experiments ranged from -0.46 to -0.62 mg kg-1, corresponding to approximately 7

259

to 11% of total Se in original coal ash samples (N/A for KIF BA). These results suggest that the

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Se release at higher pH could be derived from not only desorption of Se oxyanions but also

261

oxidative dissolution of elemental Se0 from the original coal ash materials.

262

Oxidative Dissolution of Elemental Se0. The desorption or dissolution of Se oxyanions

263

into solution appeared to be the primary process that changed Se content in the solid phase. This

264

process appeared to be accompanied by a loss of elemental Se0 in some ash samples at pH 12.

265

Under alkaline conditions, the oxidation of elemental Se0 could be facilitated by a lower proton

266

concentration if the oxidative dissolution reaction were to occur according to the following:

267

Se0 + O2 + H2O → SeO32- + 2 H+

[1]

268

In order to confirm the importance of oxidative dissolution of elemental Se0 and the role of pH,

269

we performed a 48-h leaching experiment under aerobic and anaerobic conditions for the fly ash

270

material (KIF FA) and commercially-purchased elemental Se0. For the fly ash sample, we

271

observed only a small increase of dissolved Se in the anaerobic experiment relative to the aerobic

272

experiment (11% to 35%, Figure 6a). In contrast, for pure elemental Se0 the release of leachable

273

Se in the aerobic experiment increased appreciably with increasing pH, a trend that was

274

insignificant under the anaerobic condition (Figure 6b).

275

Overall, the aerobic and anaerobic leaching experiments with the KIF FA sample

276

demonstrated a discernible, yet small difference in leachable Se concentrations that suggests a

277

possible role for other dissolution processes that do not involve oxygen as the electron acceptor

278

for redox reactions. For example, the reductive dissolution of iron-(hydr)oxides could enhance

279

the release of Se oxyanions that was associated with these minerals31, 32. Moreover, ferrous iron

280

could further promote the reduction of elemental Se0 to iron selenide and the subsequent release 13



281

of Se oxyanions33: 3 Se0 + 2 Fe2++ 3 H2O = 2 FeSe + SeO32- + 6 H+

282 283

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[2]

The process described by equation 2 would be more significant at alkaline pH as a result of

284

proton consumption, and thereby, could partially account for the increasing Se dissolution with

285

increasing pH under anaerobic conditions in the fly ash leach test (Figure 6a). Inspection of

286

dissolved Fe concentration data did not reveal discernible differences between the aerobic and

287

anaerobic leaching experiments (Figure S6). Thus, if reductive dissolution of Fe-oxides were

288

occurring, then another process, such as one described by equation 2, was facilitating the

289

re-precipitation of Fe-containing mineral phase in the anaerobic experiment.

290

Taken together, the oxidative dissolution of elemental Se0, in particular at alkaline condition,

291

is supported by the combination of solid phase Se speciation and aqueous chemistry data.

292

However, the role of the electron acceptor and the specific mechanism of this process are not

293

clear.

294

Others have also described oxidative dissolution of Se in environmental settings relevant to

295

coal ash holding ponds and ash spill events34, 35. For example, Sarathchandra and Watkinson35

296

indicated that 0.008% of pure gray elemental Se0 was abiotically oxidized to Se oxyanions after a

297

42-d incubation at pH 7, which is one order of magnitude lower than our result: 0.05% of added

298

pure elemental Se0 was oxidatively dissolved after a 48-h reaction at pH 8 (Figure 6b). Oxidation

299

of elemental Se0 was also demonstrated in incubation experiments with Se-contaminated

300

sediments16. In this case, the amount of Se-II,0 seemed to decrease as the incubation pH increased

301

from 6.5 to 9 and for both redox potentials of 0 and -200 mV. In our coal ash leach system, the

302

oxidant acting on elemental Se0 is still unclear and could include a number of possibilities (e.g.

303

O2, Fe(III), photochemical oxidation). Moreover, given that we did not autoclave coal ash

304

samples, the leachability of Se may be influenced by the indigenous microbes by directly 14


Page 15 of 29


305

affecting the Se redox cycling and/or indirectly enhancing the reduction of metals like Fe35, 36.

306

We note, however, that the leaching experiments were not amended with additional substrates to

307

promote microbial metabolism (e.g., carbon source), and we believe that microbial activity was

308

minimal. Nevertheless, the large repository of elemental Se0 in coal ash materials might play a

309

crucial step in the oxidative processes related to Se biogeochemical cycling.

310

Implications for Coal Ash Disposal and Release to the Environment. One of the major

311

environmental impacts of the coal ash spill was the release of Se into aquatic ecosystems, which

312

is of significance as Se accumulation in organisms is directly related to the content of dissolved

313

Se37. For the site of the TVA Kingston coal ash spill, Se is one of the drivers of long term

314

ecological risks in the area38. This study improves our understanding of the abiotic processes

315

contributing to the leaching of Se from the waste materials.

316

Our XANES results of the ash samples indicated that the Se is comprised mainly of

317

elemental Se0 with smaller amounts of selenite and selenate, irrespective of total Se

318

concentrations and types of coal ash materials. However, the release of Se oxyanions, with the

319

predominance of selenite, controlled Se mobilization from coal ash materials at their natural pH

320

ranging from 7.6 to 9.5. This is of environmental concern as selenite bioaccumulates in

321

organisms and incorporates into food webs more readily than selenate6, 11.

322

Our results also suggest the occurrence of oxidative Se0 dissolution from the ash materials

323

at high pH. At alkaline pH values, Se dissolution from mineral phases has been generally

324

interpreted to occur via desorption of Se oxyanions. However, our results indicated an additional

325

mechanism, oxidative dissolution of elemental Se0, which should be considered in modeling the

326

enhanced Se release at high pH conditions. Alkaline leaching conditions could occur if coal ash

327

materials are subjected to treatments such as limestone amendments that are widely employed as

328

sorbents for sulfur capture in coal power generation processes39. Recognition of this oxidative 15



329

step could lead to a better quantification of the dynamics and mass balances of Se in

330

contaminated ecosystems and result in improved strategies for their management and

331

remediation.

332 333

SUPPORTING INFORMATION

334

Additional information includes sampling details, XANES data collection analysis, examination

335

for the radiation-induced reduction or oxidation of Se during XANES data collection, metal

336

concentrations, XRD patterns, leaching pH and preliminary results of coal ash samples, and LCF

337

fitting results. This information is available free of charge via the Internet at http://pubs.acs.org/.

338 339

ACKNOWLEDGEMENTS

340

We thank Prof. Dean Hesterberg (Department of Soil Science, North Carolina State University,

341

USA) for the advice in Se XANES data collection and analysis, Dr. Jyh-Fu Lee and Dr.

342

Hwo-Shuenn Sheu for providing the beamtime of BL17C1 and BL01C2 at NSRRC. We also

343

thank Dr. Clement Levard (CEREGE, Aix-en-Provence, France) for his assistance with XRD

344

analyses. This work was supported by grants from the Oak Ridge Associated Universities and the

345

National Science Foundation (CBET-1235661).

346

16


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347 348 349

REFERENCES (1) Ruhl, L.; Vengosh, A.; Dwyer, G. S.; Hsu-Kim, H.; Deonarine, A.; Bergin, M.; Kravchenko, J. Survey of the potential environmental and health impacts in the immediate aftermath of the

350

coal ash spill in Kingston, Tennessee. Environ. Sci. Technol. 2009, 43, (16), 6326-6333.

351

(2) Kim, A. G. Proceedings of the Coal Combustion By-products and Western Coal Mines: A

352

Technical Interactive Forum; Golden, CO, 2002; pp 25-42.

353 354

(3) Bednar, A. J.; Chappell, M. A.; Seiter, J. M.; Stanley, J. K.; Averett, D. E.; Jones, W. T.; Pettway, B. A.; Kennedy, A. J.; Hendrix, S. H.; Steevens, J. A. Geochemical investigations of

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metals release from submerged coal fly ash using extended elutriate tests. Chemosphere 2010, 81, (11), 1393-1400.

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(4) Ruhl, L.; Vengosh, A.; Dwyer, G. S.; Hsu-Kim, H.; Deonarine, A. Environmental impacts of

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the coal ash spill in Kingston, Tennessee: an 18-month survey. Environ. Sci. Technol. 2010, 44, (24), 9272-9278.

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(5) Wang, T.; Wang, J. M.; Tang, Y. L.; Shi, H. L.; Ladwig, K. Leaching characteristics of

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arsenic and selenium from coal fly ash: role of calcium. Energy Fuels 2009, 23, 2959-2966.

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(6) Conley, J. M.; Funk, D. H.; Hesterberg, D. H.; Hsu, L.-C.; Kan, J.; Liu, Y.-T.; Buchwalter, D. B. Bioconcentration and Biotransformation of Selenite versus Selenate Exposed Periphyton and

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Subsequent Toxicity to the Mayfly Centroptilum triangulifer. Environ. Sci. Technol. 2013, 47, (14), 7965-7973.

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(7) Janz, D. M.; DeForest, D. K.; Brooks, M. L.; Chapman, P. M.; Gilron, G.; Hoff, D.; Hopkins, W. A.; McIntyre, D. O.; Mebane, C. A.; Palace, V. P.; Skorupa, J. P.; Wayland, M., Selenium toxicity to aquatic organisms. In Ecological Assessment of Selenium in the Aquatic Environment, Chapman, P. M., Ed. CRC Press: Boca Raton, FL, 2010 pp 141-232.

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(8) Baines, S. B.; Fisher, N. S. Interspecific differences in the bioconcentration of selenite by

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phytoplankton and their ecological implications. Mar. Ecol.-Prog. Ser. 2001, 213, 1-12.

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(9) Ralston, N. V. C.; Unrine, J.; Wallschläger, D. Biogeochemistry and Analysis of Selenium and its Species. Prepared for North American Metals Council.

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(10) Wiramanaden, C. I. E.; Liber, K.; Pickering, I. J. Selenium speciation in whole sediment using x-ray absorption spectroscopy and micro x-ray fluorescence imaging. Environ. Sci. Technol.

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(11) Besser, J. M.; Canfield, T. J.; Lapoint, T. W. Bioaccumulation of organic and inorganic

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selenium in a laboratory food-chain. Environ. Toxicol. Chem. 1993, 12, (1), 57-72.

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(12) Luoma, S. N.; Presser, T. S. Emerging opportunities in management of selenium

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contamination. Environ. Sci. Technol. 2009, 43, (22), 8483-8487.

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(13) Lindberg, T. T.; Bernhardt, E. S.; Bier, R.; Helton, A. M.; Merola, R. B.; Vengosh, A.; Di Giulio, R. T. Cumulative impacts of mountaintop mining on an Appalachian watershed. Proc.

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fate of metals in air pollution control residues from coal-fired power plants. Environ. Sci. Technol.

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by sediment oxidation-reduction potential and pH. Environ. Sci. Technol. 1990, 24, (1), 91-96.

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Coprecipitates. Environ. Sci. Technol. 2011, 45, (15), 6283-6289.

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(19) Balistrieri, L. S.; Chao, T. T. Adsorption of selenium by amorphous iron oxyhydroxide and

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manganese dioxide. Geochim. Cosmochim. Acta 1990, 54, (3), 739-751.

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(20) Cornelis, G.; Johnson, C. A.; Van Gerven, T.; Vandecasteele, C. Leaching mechanisms of

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oxyanionic metalloid and metal species in alkaline solid wastes: A review. Appl. Geochem. 2008, 23, (5), 955-976.

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(21) Baur, I.; Johnson, C. A. The solubility of selenate-AFt (3CaO·Al2O3·3CaSeO4·37.5H2O)

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and selenate-AFm (3CaO·Al2O3·CaSeO4·xH2O). Cem. Concr. Res. 2003, 33, (11), 1741-1748.

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I. soil composition and pH effects. Soil Sci. Soc. Am. J. 1987, 51, (5), 1161-1165.

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(24) White, A. F.; Dubrovsky, N. M., Chemical oxidation-reduction controls on selenium

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mobility in groundwater systems. In Selenum in the Environment, Frankenberger, W. T.; Benson, S., Eds. Marcel Dekker, Inc.: New York, 1994; pp 185-221.

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(25) Martin, A. J.; Simpson, S.; Fawcett, S.; Wiramanaden, C. I. E.; Pickering, I. J.; Belzile, N.; Chen, Y. W.; London, J.; Wallschlager, D. Biogeochemical mechanisms of selenium exchange

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between water and sediments in two contrasting lentic environments. Environ. Sci. Technol. 2011, 45, (7), 2605-2612.

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(26) Tokunaga, T. K.; Pickering, I. J.; Brown, G. E. Selenium transformations in ponded

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sediments. Soil Sci. Soc. Am. J. 1996, 60, (3), 781-790.

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(27) Ryser, A. L.; Strawn, D. G.; Marcus, M. A.; Fakra, S.; Johnson-Maynard, J. L.; Moller, G.

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Microscopically focused synchrotron X-ray investigation of selenium speciation in soils

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developing on reclaimed mine lands. Environ. Sci. Technol. 2006, 40, (2), 462-467.

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(28) Oram, L. L.; Strawn, D. G.; Moller, G. Chemical speciation and bioavailability of selenium in the rhizosphere of symphyotrichum eatonii from reclaimed mine soils. Environ. Sci. Technol.

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2011, 45, (3), 870-875.

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(29) Huggins, F. E.; Senior, C. L.; Chu, P.; Ladwig, K.; Huffman, G. P. Selenium and arsenic

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speciation in fly ash from full-scale coal-burning utility plants. Environ. Sci. Technol. 2007, 41, (9), 3284-3289.

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(30) Al-Abed, S. R.; Jegadeesan, G.; Scheckel, K. G.; Tolaymat, T. Speciation, characterization,

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and mobility of As, Se, and Hg in flue gas desulphurization residues. Environ. Sci. Technol. 2008, 42, (5), 1693-1698.

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(31) van der Hoek, E. E.; Comans, R. N. J. Modeling arsenic and selenium leaching from acidic

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fly ash by sorption on iron (hydr)oxide in the fly ash matrix. Environ. Sci. Technol. 1996, 30, (2), 517-523.

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(32) Mattigod, S. V.; Rai, D.; Eary, L. E.; Ainsworth, C. C. Geochemical factors controlling the mobilization of inorganic constituents from fossil-fuel combustion residues. 1. Review of the 19



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major elements. J. Environ. Qual. 1990, 19, (2), 188-201.

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(33) Kang, M. L.; Ma, B.; Bardelli, F.; Chen, F. R.; Liu, C. L.; Zheng, Z.; Wu, S. J.; Charlet, L. Interaction of aqueous Se(IV)/Se(VI) with FeSe/FeSe2: Implication to Se redox process. J.

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Hazard. Mater. 2013, 248, 20-28.

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(34) Dowdle, P. R.; Oremland, R. S. Microbial oxidation of elemental selenium in soil slurries

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and bacterial cultures. Environ. Sci. Technol. 1998, 32, (23), 3749-3755.

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(35) Sarathchandra, S. U.; Watkinson, J. H. Oxidation of elemental selenium to selenite by

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Bacillus megaterium. Science 1981, 211, (4482), 600-601.

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(36) Lovley, D. R. Dissimilatory metal reduction. Annu. Rev. Microbiol. 1993, 47, 263-290.

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(37) Wiramanaden, C. I. E.; Forster, E. K.; Liber, K. Selenium distribution in a lake system receiving effluent from a metal mining and milling operation in northern Saskatchewan, Canada

445

Environ. Toxicol. Chem. 2010, 29, (3), 606-616.

446 447 448

(38) Tennessee Valley Authority. Kingston Ash Recovery Project Non-Time Critical Removal Action River System Engineering Evaluation/Cost Analysis.Document No. EPA-AO-051. In 2012.

449 450

(39) Diaz-Somoano, M.; Lopez-Anton, M. A.; Huggins, F. E.; Martinez-Tarazona, M. R. The stability of arsenic and selenium compounds that were retained in limestone in a coal gasification

451 452 453

atmosphere. J. Hazard. Mater. 2010, 173, (1-3), 450-454.

20


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Page 21 of 29


Figure Captions Figure 1. The concentration of dissolved selenium that leached from coal ash samples after 48-h in coal ash-water mixtures adjusted to a range of pH values. The fly ash (KIF FA, KIF TVA C), bottom ash (KIF BA), and spilled coal ash (VB3, VB4, TN10S) slurries comprised of a liquid-to-solid ratio of 10 ml g-1. Leaching results at the natural pH of the sample (without amendments of acid or base) are indicated by circled data points. Figure 2. The relationship between (a) dissolved Se after 48-h leaching and the difference in solid phase Se content before and after leaching (∆ solid phase Se), and (b) solid phase and dissolved Se contents measured after 48-h leaching experiments. The experiments were performed at two pH values (natural pH of the samples and pH 12) with liquid-to-solid ratio of 10 ml g-1 for fly ash, bottom ash, and spilled coal ash samples. Dissolved Se data are reported as mg of dissolved Se per kg of ash in the slurry. Each data point shown is the mean of three replicate leaching experiments. Error bars indicate one standard deviation. The significance levels for the correlations are indicated by * for p < 0.05; ** for p < 0.01. Figure 3. Total and speciation of dissolved Se after 48-h leaching experiments at natural pH with liquid to solid ratio of 10 ml g-1-dry for fly ash (KIF FA, KIF TVA C), bottom ash (KIF BA), and spilled coal ash (VB3, VB4, TN10S) samples. The end-point pH is 9.1, 9.5, 8.8, 8.7, 9.1,and 7.6, respectively. The recoveries of Se species (determined by HPLC-ICP-MS) relative to total dissolved Se were 106, 91, 115, 101, 114, and 113%, respectively. Total amount of dissolved Se was directly determined using ICP-MS. Figure 4. XANES spectra (solid lines) and results of linear combination fitting (LCF - dashed lines) for original coal ash samples (fly ash - KIF FA, KIF TVA C; bottom ash - KIF BA; spilled coal ash - VB3, VB4, TN10S), and the residual coal ash solids collected after 48-h leaching experiments at natural pH and ~pH 12 are shown in (a)-(c), respectively. Figures (d)-(f) represent the corresponding proportion of elemental selenium [Se(0)], selenite [Se(IV)], and selenate [Se(VI)] determined from LCF. Figure 5. The change in solid phase selenite and selenate content (∆ [selenite + selenate]) for all ash materials as a function of the dissolved Se measured after 48-h leaching experiments at (a) the natural pH of the ash and (b) pH 12. Dissolved Se was normalized to the mass of ash, based on leaching experiments performed with liquid to solid ratio of 10 ml g-1. The change in solid phase Se0 content (∆ elemental Se0) after 48-h leaching experiments at natural pH and pH 12 are shown (c) as the function of dissolved Se and (d) for individual coal ash samples (fly ash - KIF 21



FA, KIF TVA C; bottom ash - KIF BA; spilled coal ash - VB3, VB4, TN10S). The ∆ values were calculated from the Se species concentration in post-leached samples minus the concentration in pre-leached samples. Error bars indicate one standard deviation of triplicate samples. The significance levels for the correlations are indicated by * for p < 0.05; ** for p < 0.01. Figure 6. Selenium dissolution as a function of pH from (a) the fly ash material (KIF FA) and (b) commercially-purchased elemental Se0 after a 48-h incubation at aerobic and anaerobic conditions. The L/S ratio of 10 mL g-1-dry was used in the ash material. The initial concentration for the pure elemental Se0 is 1g Se L-1 (0.1%). Each data point shown is the mean of three replicates. Error bars indicate one standard deviation.

22


Page 22 of 29

Page 23 of 29


350

KIF FA KIF TVA C KIF BA VB3 VB4 TN10S

300

-1

Dissolved Se (g L )

250 200 150 100 50 0 2

4

6

8

10

12

14

pH Figure 1. The concentration of dissolved selenium that leached from coal ash samples after 48-h in coal ash-water mixtures adjusted to a range of pH values. The fly ash (KIF FA, KIF TVA C), bottom ash (KIF BA), and spilled coal ash (VB3, VB4, TN10S) slurries comprised of a liquid-to-solid ratio of 10 ml g-1. Leaching results at the natural pH of the sample (without amendments of acid or base) are indicated by circled data points.

1 ACS Paragon Plus Environment

Environmental Science & Technology -3.0

Page 24 of 29

(a)

-1

 Solid phase Se (mg kg )

-2.5

-2.0

-1.5

-1.0

-0.5

Natural pH; y = -1.10 (0.18) x + 0.10 (0.13); R2 = 0.90 ** pH 12; y = -0.88 (0.18) x - 0.37 (0.29); R2 = 0.87 1:1

0.0 0.0 3.5

-1

1.0

1.5

2.0

2.5

3.0

3.5

-1

(b)

Dissolved Se (mg kg ) ;

pH 12; y = 0.35 (0.09) x - 0.24 (0.50); R2 = 0.72* Natural pH; y = 0.12 (0.05) x - 0.02 (0.30); R2 = 0.48

3.0

Dissolved Se (mg kg )

0.5

**

2.5 2.0 1.5 1.0 0.5 0.0 0

1

2

3

4

5

6

7

-1

Total Se (mg kg )

Figure 2. The relationship between (a) dissolved Se after 48-h leaching and the difference in solid phase Se content before and after leaching (Δ solid phase Se), and (b) solid phase and dissolved Se contents measured after 48-h leaching experiments. The experiments were performed at two pH values (natural pH of the samples and pH 12) with liquid-to-solid ratio of 10 ml g-1 for fly ash, bottom ash, and spilled coal ash samples. Dissolved Se data are reported as mg of dissolved Se per kg of ash in the slurry. Each data point shown is the mean of three replicate leaching experiments. Error bars indicate one standard deviation. The significance levels for the correlations are indicated by * for p < 0.05; ** for p < 0.01. 2 ACS Paragon Plus Environment

Page 25 of 29


Total Se Selenite Selenate

KIF FA

KIF TVA C

KIF BA

VB3

VB4

TN10S 0

20

40

60

80

100

-1

Dissolved Se (g L ) Figure 3. Total and speciation of dissolved Se after 48-h leaching experiments at natural pH with liquid to solid ratio of 10 ml g-1-dry for fly ash (KIF FA, KIF TVA C), bottom ash (KIF BA), and spilled coal ash (VB3, VB4, TN10S) samples. The end-point pH is 9.1, 9.5, 8.8, 8.7, 9.1,and 7.6, respectively. The recoveries of Se species (determined by HPLC-ICP-MS) relative to total dissolved Se were 106, 91, 115, 101, 114, and 113%, respectively. Total amount of dissolved Se was directly determined using ICP-MS.



Page 26 of 29

(d) Original coal ash

(a) Original coal ash

KIF FA

KIF FA

KIF TVA C

KIF TVA C KIF BA

KIF BA

VB3

VB3

VB4

VB4

TN10S

TN10S 0

40

60

80

100

80

100

80

100

(e) Leached at natural pH

(b) Leached at natural pH

KIF FA

KIF FA Normalized XANES

20

KIF TVA C

KIF TVA C

KIF BA

KIF BA

VB3

VB3

VB4

VB4

TN10S

TN10S 4

0

20

40

60

(f) Leached at pH 12

(c) Leached at pH12

KIF FA

KIF FA

3

KIF TVA C

KIF TVA C

VB3

VB3

A

2

VB4

VB4

TN10S

1

TN10S 0

0 12640

12660

12680

40

60

Percent composition

12700

Energy (eV)

20

Gray Se

Adsorbed Se(IV)

Adsorbed Se (VI)

Figure 4. XANES spectra (solid lines) and results of linear combination fitting (LCF - dashed lines) for original coal ash samples (fly ash - KIF FA, KIF TVA C; bottom ash - KIF BA; spilled coal ash - VB3, VB4, TN10S), and the residual coal ash solids collected after 48-h leaching experiments at natural pH and ~pH 12 are shown in (a)-(c), respectively. Figures (d)-(f) represent the corresponding proportion of elemental selenium [Se(0)], selenite [Se(IV)], and selenate [Se(VI)] determined from LCF. 4 ACS Paragon Plus Environment


-1.0

(a) Natural pH

1:1

-0.6 -1

-0.8

Elemental Se (mg kg )

-1

Selenite + Selenate) (mg kg )

Page 27 of 29

-0.6 -0.4 -0.2

y = -0.95 (0.06) + 0.04 (0.04) R2 = 0.98**

0.0 0.0

0.2

0.4

0.6

0.8

(c)

-0.4

-0.2

0.0

Natural pH pH12

0.2

1.0

0.0

-1

-1

1.5

2.0

2.5

3.0


(c)

1:1

(b) pH 12

-0.8

-2.5

 Elemental Se (mg kg )

-2.0

Natural pH pH12

(d)

-0.6

-1

Selenite + Selenate) (mg kg )

1.0

-1

Dissolved Se (mg kg ) -3.0

0.5

-1.5 -1.0 -0.5

y = -0.60 (0.15) - 0.41 (0.28) R2 = 0.83*

0.0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

-0.4

-0.2

0.0

0.2

-1


KIF

A FA A C IF B TV K F KI

3 VB

4 VB

S 10

TN

Figure 5. The change in solid phase selenite and selenate content (Δ [selenite + selenate]) for all ash materials as a function of the dissolved Se measured after 48-h leaching experiments at (a) the natural pH of the ash and (b) pH 12. Dissolved Se was normalized to the mass of ash, based on leaching experiments performed with liquid to solid ratio of 10 ml g-1. The change in solid phase Se0 content (Δ elemental Se0) after 48-h leaching experiments at natural pH and pH 12 are shown (c) as the function of dissolved Se and (d) for individual coal ash samples (fly ash - KIF FA, KIF TVA C; bottom ash - KIF BA; spilled coal ash - VB3, VB4, TN10S). The Δ values were calculated from the Se species concentration in post-leached samples minus the concentration in pre-leached samples. Error bars indicate one standard deviation of triplicate samples. The significance levels for the correlations are indicated by * for p < 0.05; ** for p < 0.01.



250

Page 28 of 29

(a) Fly ash (KIF FA) aerobic anaerobic

-1


200

150

100

50

0 8 12000

-1


10000

9

10

11

12

13

pH (b) Pure elemental Se aerobic anaerobic

8000 6000 4000 2000 0 8

9

10

11

12

13

pH

Figure 6. Selenium dissolution as a function of pH from (a) the fly ash material (KIF FA) and (b) commercially-purchased elemental Se0 after a 48-h incubation at aerobic and anaerobic conditions. The L/S ratio of 10 mL g-1-dry was used in the ash material. The initial concentration for the pure elemental Se0 is 1g Se L-1 (0.1%). Each data point shown is the mean of three replicates. Error bars indicate one standard deviation.


Page 29 of 29


~pH12

350

Coal Ash

-1


300

Desorption

Se oxyanions

Elemental Se

Oxidative dissolution

250 200

Se oxyanions

150 100 50 0 4

6

8

10

12

pH Selenium dissolution from coal ash samples as a function of pH in relation to the ACS Paragon Plus Environment processes of desorption of Se oxyanions and oxidative dissolution of elemental Se.

Selenium Speciation in Coal Ash Spilled at the Tennessee Valley

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