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In situ quantification of [Re(CO)3]+ by fluorescence spectroscopy in simulated Hanford tank waste Shirmir D. Branch, Amanda D. French, Amanda M. Lines, Brian M Rapko, William R. Heineman, and Samuel A. Bryan Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b04222 • Publication Date (Web): 14 Dec 2017 Downloaded from http://pubs.acs.org on December 21, 2017

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Environmental Science & Technology

1

In situ quantification of [Re(CO)3]+ by fluorescence

2

spectroscopy in simulated Hanford tank waste

3

Shirmir D. Branch,a,b Amanda D. French,b Amanda M. Lines,b Brian M. Rapko,b William R.

4

Heineman,a* Samuel A. Bryanb*

5

a

Department of Chemistry, University of Cincinnati, Cincinnati, OH, 45221-0172, USA

6

b

Pacific Northwest National Laboratory, Richland, WA, 99352, USA

7

ABSTRACT

8

A pretreatment protocol is presented that allows for the quantitative conversion and subsequent

9

in situ spectroscopic analysis of [Re(CO)3]+ species in simulated Hanford tank waste. In this test

10

case, the non-radioactive metal rhenium, is substituted for technetium (Tc-99), a weak beta

11

emitter, to demonstrate proof of concept for a method to measure a non-pertechnetate form of

12

technetium in Hanford tank waste. The protocol encompasses adding a simulated waste sample

13

containing the non-emissive [Re(CO)3]+ species to a developer solution that enables the rapid,

14

quantitative conversion of the non-emissive species to a luminescent species which can then be

15

detected spectroscopically. The [Re(CO)3]+ species concentration in an alkaline, simulated

16

Hanford tank waste supernatant can be quantified by the standard addition method. In a test case,

17

the [Re(CO)3]+ species was measured to be at a concentration of 38.9 µM, which was a

18

difference of 2.01% from the actual concentration of 39.7 µM.

ACS Paragon Plus Environment

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INTRODUCTION

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The Hanford Site in southeastern Washington State is one of the largest and most costly

21

remediation efforts in the United States. The Site is home to 9 decommissioned nuclear reactors

22

and numerous associated processing facilities, including 177 underground nuclear waste storage

23

tanks containing approximately 55 million gallons (2.1 × 105 m3) of waste generated as a result

24

of decades of plutonium production.1 Consequently, it has been declared a major EPA Superfund

25

site that is undergoing cleanup. One aspect of the remediation efforts includes the separation of

26

low- and high-level activity waste.2 The need to characterize and monitor various constituents of

27

interest in these underground storage tanks presents a major scientific challenge, particularly

28

concerning moving high activity constituents, such as water-soluble radionuclides, prior to

29

immobilization. Current methods of analysis are hazardous, expensive, and time consuming.3, 4

30

These methods typically require extensive sample collection and preparation, putting workers at

31

risk of exposure, as well as lengthy analysis and data interpretation. A more efficient approach

32

might be to use sensors to perform rapid, sensitive, and economic in situ analyses for key

33

constituents. A major issue for the environmental remediation of the Hanford site is the ability to

34

measure for specific contaminants of interest given the chemical complexity, harsh radiological

35

environment, and limited tank access complicating well-established laboratory-based analytical

36

techniques employed to analyze the wastes. This paper continues our interest in developing

37

sensors based on optical spectroscopy, electrochemistry and selective partitioning for the direct

38

measure of specific target analytes within the waste.5

39

Technetium (Tc) is one such constituent where an analytical method exists, but a rapid, direct

40

sensor does not.6 Technetium is not found in substantial quantities in nature. However, the

41

isotope

99

Tc is a byproduct of the thermal nuclear fission of

235

U,

233

U, and

239

Pu (at yields of


2

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6.1%, 5.0%, and 6.2%, respectively),7, 8 and therefore is generated in large quantities at nuclear

43

sites. 99Tc accounts for ~100% of all Tc isotopes sources. The total 99Tc content at the Hanford

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Site approximates to 2000 kg (~36,000 Ci), of which ~4% has been lost to the environment.9

45

Although 99Tc exhibits only a weak β- decay (0.292 keV), it is of environmental concern for two

46

reasons: (1) it has a half-life of 2.13 × 105 years,7 and (2) its most abundant environmental

47

species, pertechnetate, TcO4-, migrates quickly with groundwater.10 Pertechnetate has a very low

48

soil retention and a high solubility in water. These properties allow technetium, after leaking

49

from the underground storage tanks, to pass quickly through soil into subsurface waters.11, 12

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One of the basic necessities in Tc remediation is to meet regulatory requirements for disposal

51

of Tc found in US Department of Energy (USDOE) waste streams and released into the

52

environment. Current techniques to analyze technetium include ion exchange chromatography

53

(IC),13 mass spectrometry (MS),3 capillary electrophoresis (CE),14 coulometric titration,15 surface

54

enhanced Raman scattering (SERS),16 and extended X-ray absorption fine structure (EXAFS).17

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These techniques are often limited for routine analytical use by arduous sample treatment and

56

long processing times. Many of these techniques also lack the sufficient selectivity to identify the

57

Tc(CO)3+ species that this work is interested in, which presently inhibits the remediation of total

58

technetium from tank waste.

59

Several significant uncertainties remain regarding the understanding and modeling of the fate 99

99

60

and speciation of

61

Hanford tank wastes using ion-exchange or ion-pair extraction processes specific to

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pertechnetate were only partially successful due to the presence of a significant fraction of other

63

valence and complexed forms of

64

reported evidence of a Tc(I)-tricarbonyl-type compound present in the waste.17 Under tank

Tc in Hanford tank waste. Previous attempts to remove

99

Tc from selected

Tc in the waste supernatants. A study into this problem


3


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65

conditions, the tricarbonyl species is expected to react with constituents within the waste,

66

forming a [Tc(CO)3(L)3]n- complex, where (L)3 represents a multidentate ligand binding to the

67

metal center, such as citrate, oxalate, and gluconate. The presence of the non-pertechnetate

68

species (which has been found in select Hanford tank supernatants at concentrations ranging

69

from 2% – 60% of total Tc) has hampered the selective movement of Tc high level waste

70

streams prior to immobilization.9 It is also believed that the [Tc(CO)3(L)3]n- species have the

71

potential to leak from the tanks into the underlying vadose zone and groundwater. Once it

72

reaches this point, the species can change into the more prevalent TcO4- form, which has a high

73

mobility rate in the arid soil and groundwater, further impacting the environment around the

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Hanford Site. 11, 12 The objective of this work is to develop a sensor for the direct detection of the

75

[TcI(CO)3]+ species that will help reduce the uncertainties in the fate and speciation of Tc in

76

Hanford waste storage tanks prior to immobilization.

77

Optical spectroscopy is a quick non-destructive method that would be useful for detection of

78

the [TcI(CO)3]+ species. Previously, the quantitative measurement of various Tc species using

79

both absorption2 and fluorescence spectroscopy,18 as well as the luminescence-based

80

spectroelectrochemical detection of Tc(II) species, both in solution19 and in film,20 have been

81

demonstrated. One challenge with detecting the [TcI(CO)3(L)3]n- species is that it does not have a

82

unique signature using routine optical spectroscopic techniques and is indistinguishable from

83

other constituents stored in the waste tanks. To overcome this challenge, a sensing method that

84

converts the non-emissive [TcI(CO)3(L)3]n- species into a luminescent species that would be

85

detectable at the concentrations found in tank waste, which are estimated to be in the range of 60

86

µM, will be developed. For optical detection at such low concentrations, fluorescence would be a

87

more suitable choice than other methods, such as UV-vis absorption spectroscopy. Absorption


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88

spectroscopy is limited by several factors, including a high sample concentration and a long path

89

length required for signal collection, due to the relatively low molar extinction coefficient of the

90

analyte. Absorption spectroscopy is also limited by selectivity, in that many other species present

91

in Hanford tank supernatants also absorb light in the UV-vis region. Fluorescence spectroscopy

92

has the benefits of higher sensitivity and selectivity than absorption. Detection using

93

fluorescence can achieve detection limits at least 1,000-fold lower than absorption spectroscopy.

94

Fluorescence is also more selective in that while many of the constituents in tank waste absorb

95

light, far fewer also emit light. Selectivity is further increased in that very few constituents, if

96

any, both excite and emit light at the same wavelengths as the target species.

97

The direct reaction of [TcI(CO)(L)3]n- with bidentate and tridentate ligands has been

98

established under conditions (neutral and basic pH, at room temperature, saline) suitable for

99

radiotherapeutic applications.21-23 By complexing the Tc(I)-tricarbonyl species with sensitizing

100

ligands such as 2,2′-bipyridine (bpy), 1,10′-phenanthroline (phen), or functionalized bpy and

101

phen ligands, we propose to convert the [TcI(CO)3(L)3]n- species into optically emissive

102

complexes with the formula [Tc(CO)3(LᴖL)(L)]n- as shown in Figure 1, where LᴖL is a bidentate

103

sensitizing ligand.

104 105

Figure 1. Schematic of conversion of [TcI(CO)(L)3]n- to [Tc(CO)3(LᴖL)(L)]n-; LᴖL = bpy.

106

For initial laboratory testing, rhenium was used as the surrogate of technetium to study in

107

simulated tank waste. The conversion of [Re(CO)3(H2O)3]+ into a luminescent species has been

108

well characterized in the interest of radiopharmaceutical imaging.24,

25

A range of Re(I)


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109

complexes, based on the Re(I)-tricarbonyl core, have previously been shown to be emissive,26

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and are used in this work as a non-radioactive surrogate for the Tc(I)-tricarbonyl system.

111

Previous work has shown that the luminescence from technetium and rhenium, second- and

112

third-row congeners, containing the d2 metal-oxo core,

113

(where M = Re or Tc; and dmpe is bis-1,2-(dimethylphosphino)ethane)), 19, 29-31 are comparable

114

with respect to the lifetimes and electronic states of emission. In this study, a number of Re(I)-

115

carbonyl

116

[Re(CO)3(bpy)(H2O)]+, to serve as the standard species for comparison to the product formed in

117

the proposed conversion method. This conversion, or pretreatment method, starts by dissolving

118

the non-emissive [Re(CO)3]+ species in a solution matrix, either with or without simulated tank

119

waste. The pretreatment method performs the following elements: 1) dissolves the waste matrix

120

containing the non-emissive [Re(CO)3]+ species into a developer solution containing a high

121

concentration of sensitizing ligand, 2) introduces a high dilution factor to minimize matrix

122

effects from waste sample, and 3) is designed to optimize the conversion of the non-emissive

123

[Re(CO)3]+ species into the target luminescent species. The reaction is allowed to occur at a

124

chosen temperature for a short amount of time. Finally, the sample is excited and the

125

fluorescence is measured. We have purposely designed our methodology based on the

126

preparation and measurement of fluorescent complexes in order to decrease the LOD as well and

127

enhance the selectivity of target analyte detection. Fe and Cr are two competing metals known

128

to be present in actual tank waste samples, and whose complexation with the sensitizing ligand,

129

2,2′-bipyridyl (bpy) is known. For this reason, we have chosen to use a pretreatment method

130

containing a sensitizing ligand in large excess concentration sufficient to bind to the target Re (or

131

Tc) metal as well as any other competing metals. Using fluorescence detection instead of

complexes

have

been

synthesized,

27, 28

or the d5 [M(dmpe)3]2+ complexes

including

[Re(CO)3(H2O)3]+

and


6

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132

absorption spectroscopy is extremely useful in this situation, as fluorescence is more selective

133

and sensitive for conditions such as these Cr-bpy and Fe-bpy species do not have the same

134

spectroscopic signature as the target Re-bpy, or the Tc-bpy species.

135

The quantity of [Re(CO)3]+ in the simulated waste samples is determined using the accepted

136

method of standard addition.32 The application of the standard addition method in conjunction

137

with the proposed pretreatment method greatly reduces the actions of interfering species – by

138

way of target species conversion and spectroscopic signature – in both simulated and real waste

139

samples. Preliminary studies have shown that this conversion can be successfully applied to

140

[Re(CO)3]+ and [Tc(CO)3]+ in simulated waste samples (Figure 2A), as well as [Tc(CO)3]+ in an

141

actual Hanford waste sample (Figure 2B). All three samples were treated as described above,

142

using a developer solution containing a high concentration of 2,2′-bipyridyl in acetonitrile. This

143

information indicates that the proposed method presents a convenient method for the detection of

144

[Tc(CO)3]+ species both in simulated waste and actual tank waste samples containing .

(A)

(B)

145 146

Figure 2. Measurement of [Re(CO)3]+ and [Tc(CO)3]+ in simulated and actual Hanford waste

147

samples using the Pretreatment Protocol Solution consisting of 0.125 M bpy in CH3CN. (A)

148

Excitation (red, dashed line) and emission (red, solid line) of [Re(CO)3]+ in simulated waste;


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excitation (blue, dashed line) and emission (blue, solid line) of [Tc(CO)3]+ in simulated waste;

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(B) emission of [Tc(CO)3]+ in simulated waste (blue, solid line) and an actual tank waste sample

151

(green, solid line). Actual tank waste was taken from tank 241-AN-102 at the Hanford Site.

152

EXPERIMENTAL SECTION

153

Chemicals and materials

154

The following chemicals and solvents were obtained from Sigma-Aldrich (unless otherwise

155

indicated) and used without further purification: dirhenium decacarbonyl (98%); 2,2′-bipyridyl

156

(≥99%); bromine (≥95%); dichloromethane (anhydrous, ≥99.8%); acetonitrile (anhydrous,

157

99.8%); methanol (≥99.8%; Fisher); diethyl ether (anhydrous, ≥99.0%). Aqueous solutions were

158

prepared with deionized water (D2798 Nanopure system; Barnstead, Boston, MA).

159

Instrumentation

160

Emission spectra were collected using a Horiba Jobon Yvon Fluorolog III fluorimeter

161

equipped with a 450-W xenon lamp, double-emission monochromator blazed at 500 nm, and a

162

single-excitation monochromator blazed at 300 nm, and an InSpectrum 150 spectrometer-CCD,

163

using SpectraSense data-acquisition software. A 405 nm laser source was used for excitation

164

with the InSpectrum 150 spectrometer. Signal integration times were 999 msec using a 2 mm slit

165

width using a 600-gr/mm grating blazed at 500 nm.

166

Preparation of Re-carbonyl complexes

167

The preparation of the target [Re(CO)3]+ species begins with Re2(CO)10, which is reacted with

168

elemental bromine to form the complex Re(CO)5Br (see Figure 3).33 The pentacarbonyl complex

169

is allowed to react with water under reflux conditions to yield the aquo complex,

170

[Re(CO)3(H2O)3]+ (1). This complex is then reacted with a sensitizing ligand, such as bpy, to

171

form the luminescent [Re(CO)3(bpy)(X)]+ (2) species (X = H2O or CH3OH). Complex 1 can also


8

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172

be added to a waste simulant to prepare [Re(CO)3(L)3]n-, where R is some organic complexant,

173

such as oxalate. This serves as an analogue for the non-pertechnetate technetium species believed

174

to be within the tank supernatant.

175

[Re(CO)3(H2O)3]Br (1).34 Approximately 1.2 mmol Re(CO)5Br was added to a round-

176

bottomed flask, to which 20 mL of DI water was added. The starting material did not dissolve in

177

room-temperature water. The solution was allowed to reflux for at least 24 hours at 100°C.

178

Periodically, the condenser was rinsed as the starting material deposited at the bottom of the

179

condenser. Upon completion of the reflux, the solvent was removed using rotary evaporation.

180

The product remaining was a white solid and stored for later use.

181

[Re(CO)3(bpy)(X)]Br (2).35 Approximately 1 mmol [Re(CO)3(H2O)3]Br was added to a

182

round-bottomed flask, to which 40 mL of methanol was added. The solution was heated to 65°C,

183

while stirring. More solvent was added, as necessary, so that all of the [Re(CO)3(H2O)3]Br was

184

dissolved before the ligand addition. Approximately 1.1 mmol of the sensitizing ligand was

185

added to the flask. A color change was observed immediately upon ligand addition. The solution

186

was allowed to reflux for approximately 5 hours at 65°C. The solution was then allowed to cool

187

back to room temperature. If no precipitate was observed, the solvent was removed using rotary

188

evaporation. If a precipitate was observed, the sample was filtered and rinsed with diethyl ether.

189

The product [Re(CO)3(bpy)(X)]Br was dried and stored for use (where X = CH3OH). Other

190

solvates (where X = H2O or CH3CN) were prepared by substituting water or acetonitrile in place

191

of methanol.


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192 193

Figure 3. Synthesis schematic for [Re(CO)3(bpy)(X)]+.

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Preparation of simulated tank waste supernatant

195

The simulated waste supernatant is based on a generic Hanford tank waste supernatant

196

simulant composition used for flowsheet development testing.36 This simulant was chosen for its

197

high alkalinity, high ionic strength characteristics, the presence of interfering metal species, and

198

the presence of organic complexants. Table 1 lists the components of the simulated waste.

199

Table 1. Composition of Hanford Waste Supernatant simulant. Component

Chemical Formula

Concentration (g/L)

Sodium oxalate

Na2C2O4

1.9

Aluminum nitrate

Al(NO3)3•9H2O

78 (60% solution of Al(NO3)3•9H2O)

Sodium phosphate

Na3PO4•12H2O

25

Sodium sulfate (anhydrous)

Na2SO4

25

Sodium nitrate

NaNO3

104

Sodium hydroxide (50% solution)

NaOH

104 (50% solution of NaOH)

Sodium nitrite

NaNO2

35

Sodium carbonate (anhydrous)

Na2CO3

58.57


10

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200 201

RESULTS AND DISCUSSION

202

Spectroscopic signature of [Re(CO)3(bpy)(X)]+

203

To establish the spectroscopic signature of the converted Re-tricarbonyl complexes by

204

pretreatment, the excitation and emission spectra were acquired for the luminescent Re species

205

synthesized by the standard route. The solvation of the [Re(CO)3(bpy)(X)]+ complex is verified

206

based on the known emission maximum for the aquated and acetonitrile complex: X = H2O, λmax

207

= 575 nm; X = CH3CN, λmax = 595 nm. Complex 2 (where X = CH3CN) excites at 396 nm and

208

fluoresces at 595 nm (Figure S1). The emission of complex 2 was measured at concentrations

209

ranging from 0 – 1 mM (Figure 4A). The linear range of the emission at 595 nm is plotted

210

against the concentration of 2 (Figure 4B). The limit of detection for [Re(CO)3(bpy)(CH3CN)]+

211

was 16.7 nM. The detection limit for rhenium was calculated using the equation:

212

=

∗

,

(1)

213

where s is the standard deviation for the noise measurements, and m is the slope of the linear

214

region of the plot. The full concentration range for the concentration-emission profile of complex

215

2 is shown in Figure S2.


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emission intensity

(A)

max emission

(B)

216 217

Figure 4. Concentration-emission profile of [Re(CO)3(bpy)(CH3CN)]+ in acetonitrile. (A) Emission

218

spectra of [Re(CO)3(bpy)(CH3CN)]+ in acetonitrile; excitation at 405 nm. (B) Emission at 595 nm vs.

219

concentration of [Re(CO)3(bpy)(CH3CN)]+ (linear range). The equation for the line is y = 6.307e+09 x +

220

1168 (R2 = 0.9998).

221

Conditions of Pretreatment Protocol: [Re(CO)3]+ complex conversion

222

The purpose of the pretreatment protocol is to provide an alternative method of detection that

223

is quicker and less hazardous than other techniques currently being used to analyze tank waste

224

samples. The protocol comprises adding a tank sample (or simulated waste), to a developer

225

solution that enables the conversion of the target analyte into a rhenium (or technetium, in the


12

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case of actual waste) complex that can be measure spectroscopically. The pretreatment protocol

227

is designed to accomplish several things simultaneously:

228 229 230 231 232 233

1. The developer solution contains a diamine sensitizing ligand in high enough concentration that allows for binding to the target [Re(CO)3]+ species. 2. The sensitizing ligand can be chosen based on its affinity for the target [Re(CO)3]+ species over other competing ligands in a waste sample. 3. The developer solution can also adjust the pH and/or ionic strength of the waste sample to diminish the interference of competing metals.

234

4. The developer solution is used at a volume to allow for high dilution of the waste

235

sample in order to diminish the competition from other organic ligands within the

236

waste matrix.

237 238 239

5. The waste/developer matrix can be heated to encourage quick formation of the target [Re(CO)3(LᴖL)(L)]+ species. 6. The solvent

in

the

developer solution

is

able to

dissolve the formed

240

[Re(CO)3(LᴖL)(L)]+ complex allowing for ease of measurement and simultaneously

241

enabling the precipitation of some interfering salt species.

242

A variety of adjustments were tested to ensure the final developer solution met the acceptable

243

criteria described above. Some parameters included: adjusting the pH of the simulated waste

244

sample before addition to the developer solution; testing a variety of aqueous and nonaqueous

245

solvents for the developer solution; adjusting the choice and concentration of the sensitizing

246

ligand in the developer solution, including mixed solvent solutions; optimizing the volume

247

dilution ratio of the simulated waste sample in the developer solution; and adjusting the heating

248

temperature and time to allow for species conversion. The decision to use 2,2′-bipyridyl in


13


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249

acetonitrile was based on reducing the time/number of steps to pretreat a waste sample for

250

spectroscopic analysis. The simulated waste sample is added in a 1:200 volume dilution to the

251

developer solution. The chosen solvent for the developer solution is sufficient to keep the

252

sensitizing ligand, the target [Re(CO)3]+ species, and the converted [Re(CO)3(LᴖL)(L)]+ complex

253

in soluiton at high enough concentrations.

254

Pretreatment standard calibration

255

A solution of [Re(CO)3]+ in water was prepared, of which aliquots were added in increasing

256

amounts from 0 – 50 µL to 2 mL aliquots of the developer solution containing 0.125 M bpy in

257

CH3CN. The solutions were heated at 30°C for 10 min. The solutions were then allowed to cool

258

to room temperature, and the fluorescence was measured. The maximum fluorescence occurred

259

at 563 nm. This protocol was repeated with heating of the solution at either 40 or 50°C. Figure 5

260

shows the emission spectra at various concentrations of [Re(CO)3]+ added and a calibration plot

261

of the maximum emission at all three temperatures vs. the concentration of [Re(CO)3]+. Heating

262

at 40°C is shown to have the highest sensitivity, with a detection limit of 38.4 nM (Figure 5A).

263

Heating at 30°C and 50°C yielded detection limits of 42.3 nM and 42.0 nM, respectively (Figure

264

5B). Though it has been shown that the complete conversion of the non-emissive [Re(CO)3]+

265

species to the luminescent complex can occur within 30 min at 40°C,37 there was no significant

266

difference in the detection limit between heating at 10 min as opposed to 30 min. The in situ

267

detection limit of 38.4 nM is 56.5% higher than that of the complex synthesized by standard

268

route (16.7 nM). However, the fluorescence of the standard synthesized complex was measured

269

using a more selective detector in order to fully determine the excitation and emission

270

characteristics that set the basis for our pretreatment protocol. In either case, the detection limit


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271

calculated using this protocol falls well under the expected concentration of [Tc(CO)3]+ species

272

in the Hanford tank waste estimated to be ~60 µM.

(A)

(B) 273 274

Figure 5. Pretreatment of [Re(CO)3]+ in water using 0.125 M bpy in CH3CN. (A) Emission spectra after

275

heating at 40°C (405 nm excitation). (B) Emission at 563 nm vs. [Re(CO)3]+ concentration; 30°C

276

regression line: y = 7.960 × 108 x + 186.2 (R2 = 0.9962); 40°C regression line: y = 8.789 × 108 x + 490.2

277

(R2 = 0.9973); 50°C regression line: y = 8.028 × 108 x + 249.0 (R2 = 0.9957).

278

The maximum emission intensity of the pretreated samples was shifted about 30 nm to higher

279

energy compared to the standard synthesis method, which could be due to the water environment

280

that the [Re(CO)3]+ species was in upon being introduced to the developer solution. The key

281

differences in the reaction conditions between this pretreatment method and the standard


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282

synthesis method is the concentration of the sensitizing ligand and the time allowed for complex

283

formation. In the standard method, the ligand and metal are added in equal mole fractions and

284

any impurities are removed from the final product before characterization studies take place. The

285

standard synthesis process also involves more purification steps than would be required when

286

handling a real waste sample. In this protocol, the signal of the free ligand does not interfere with

287

the signal from the target [Re(CO)3(bpy)(H2O)]+ species.38

288

Pretreatment in simulated waste supernatant

289

A solution of [Re(CO)3]+ in water was added to a simulated waste supernatant to form a matrix

290

emulating a real waste sample. Aliquots of the matrix were added in increasing amounts from 0 –

291

50 µL to 2 mL aliquots of the developer solution containing 0.125 M bpy in CH3CN. The

292

solutions were then heated at 40°C for 10 min. The solutions were cooled to room temperature,

293

then the fluorescence was measured. Figure 6A shows the fluorescence of each solution. The

294

emission at 635 nm is plotted against the concentration of [Re(CO)3]+ in solution (Figure 6B),

295

showing a linear concentration-emission response, with a detection limit of 5.17 µM, well below

296

the concentration of [Tc(CO)3]+ reported to date in actual waste.9 Compared to the pretreatment

297

of Re(CO)3+ in water (Figure 5), the maximum emission of the [Re(CO)3]+ complex formed in

298

the simulated waste solution is shifted, as well as the intensity being reduced by a factor of ~50

299

(Figure 6). The components in the simulated waste were tested for quenching of the emission

300

(Figure S3) , results shown in Figure S3. The waste species including sodium salts of nitrate,

301

nitrite, sulfate, oxalate, and aluminum nitrate did not quench the emission signal of the Re-ligand

302

complex, even though these potential quenchers were added at significant levels relative to their

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respective concentrations in actual waste solutions (Figure S3A). Sodium phosphate and sodium

304

carbonate did diminish the emission (Figure S3B) to approximately the same magnitude seen in


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305

the simulated waste measurements, presumably due to chemical interaction with the Re center,

306

since the wavelength maximum emission was also shifted in measurements containing these

307

constituents. The linear concentration-emission response for the [Re(CO)3]+ in waste simulant

308

(Figure 6) with zero intercept, indicates the emissive complex observed is due to the formation of

309

the Re-carbonyl-bpy complex. Furthermore, the proportional response of emission intensity to

310

changes in Re concentration indicate this is a viable and successful route for sensing and

311

quantifying the target Re complex.

(A)

(B) 312 313

Figure 6. Measurement of [Re(CO)3]+ in waste simulant using the Pretreatment Protocol Solution

314

consisting of 0.125 M bpy in CH3CN. (A) Emission spectra at 405 nm excitation. (B) Emission at 635 nm

315

vs. [Re(CO)3]+ concentration. The equation of the line is y = 1.642 × 107 x + 1.527 (R2 = 0.9902).


17


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Quantification of [Re(CO)3]+ in simulated waste through the use of standard addition

317

For the determination of [Re(CO)3]+ in simulated waste, the accepted method of standard

318

addition was used. A solution of [Re(CO)3]+ in water was added to a simulated waste supernatant

319

to form the matrix emulating a real waste sample. This served as the “unknown” matrix. 10 µL

320

aliquots of the “unknown” were added to 2 mL aliquots of the developer solution containing

321

0.250 M bpy in CH3CN. The simulated waste/developer matrix was spiked with a standard,

322

consisting of a known concentration of [Re(CO)3]+ in water, in increasing amounts over a range

323

of 0 – 50 µL. The spiked samples were heated at 50°C for 30 min. The solutions were cooled to

324

room temperature, then the fluorescence was measured. Figure 7A shows the fluorescence of

325

each solution. The emission at 635 nm is plotted against the concentration of [Re(CO)3]+ in

326

solution (Figure 7B). The concentration of [Re(CO)3]+ determined by the standard addition

327

method was 38.9 µM, which compares favorably to the actual value (39.7 µM) and results in a

328

2.01% difference from the “unknown” concentration.


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(A)

(B)

329 330

Figure 7. Standard addition measurement of [Re(CO)3]+ in waste simulant using the Pretreatment

331

Protocol Solution consisting of 0.250 M bpy in CH3CN. (A) Emission spectra at 405 nm excitation. (B)

332

Standard addition plot of [Re(CO)3]+ in waste simulant; emission at 635 nm vs. [Re(CO)3]+ standard

333

concentration. The equation of the line is y = 5.503 × 106 x + 214.3 (R2 = 0.9954).

334

Based on these results using [Re(CO)3]+ in simulated waste, a proposed method for the

335

convient, direct detection of [Tc(CO)3]+ within tank waste is successfully demonstrated. Further

336

steps will involve optimizing this protocol as necessary for the detection of [Tc(CO)3]+ species

337

both within and out of a simulated waste environment, then applying this protocol to the

338

treatment of real waste supernatants. For example, the selectivity and sensitivity of this system

339

could be further enhanced by moving the conversion platform from solution into a chemically


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340

selective polymer film environment. A polymer film works by charge selectivity, which would

341

isolate the target [Tc(CO)3]+ species from negatively-charged interferents, such as anionic

342

components within solution. It also may have the added benefit of preconcentrating the target

343

[Tc(CO)3]+ species within the film, further enhancing the sensitivity and limit of detection.39

344

ASSICOATED CONTENT

345

Supporting Information

346

The Supporting Information is available free of charge on the ASC Publications website.

347

Synthesis of Re(CO)5Br; excitation and emission of [Re(CO)3(bpy)(H2O)]+; and the full

348

concentration range for the emission profile of [Re(CO)3(bpy)(H2O)]+ in acetonitrile.

349

AUTHOR INFORMATION

350

Corresponding Authors

351

*E-mail: [email protected] Phone: 509/375-5648;

352

*E-mail: [email protected] Phone: 513/556-9210

353

Notes

354

The authors declare no competing financial interest.

355

ACKNOWLEDGMENTS

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This research was supported by the U.S. Department of Energy’s Office of Environmental

357

Management and performed as part of the Technetium Management Hanford Site project at the

358

Pacific Northwest National Laboratory (PNNL) operated by Battelle for the U.S. Department of


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359

Energy under Contract No. DE-AC05-76RL01830. The authors would like to especially

360

acknowledge Dr. N. P. Machara for the stewardship of this research. We would like to

361

acknowledge Zheming Wang and Jesus Romero for the use of instrumentation and of the

362

simulated Hanford waste supernatant.

363

REFERENCES

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