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Remediation and Control Technologies

Efficient Photocatalytic Reduction of Aqueous Perrhenate and Pertechnetate Hao Deng, Zijie Li, Xu-cong Wang, Lin Wang, Kang Liu, Li-Yong Yuan, Zhiyuan Chang, John K. Gibson, Lirong Zheng, Zhifang Chai, and Wei-Qun Shi Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b03199 • Publication Date (Web): 21 Aug 2019 Downloaded from pubs.acs.org on August 22, 2019

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Environmental Science & Technology

1 2

Efficient Photocatalytic Reduction of Aqueous

3

Perrhenate and Pertechnetate

4

Hao Deng1,2, Zi-jie Li1, Xu-cong Wang1, Lin Wang1, Kang Liu1, Li-yong Yuan1, Zhi-

5

yuan Chang2, John K. Gibson3, Li-rong Zheng4, Zhi-fang Chai1,5 and Wei-qun Shi*,1

6 1 Laboratory

7

of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China

8 2 Department

9

of Radiochemistry, China Institute of Atomic Energy, Beijing 102413, China

10 3 Chemical

11

Sciences Division, Lawrence Berkeley National Laboratory

(LBNL), Berkeley, California 94720, United States

12 4 Beijing

13

Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China.

14 15

5

16

Technology, Chinese Academy of Sciences, Ningbo, Zhejiang 315201, China

17

*Corresponding author at: Laboratory of Nuclear Energy Chemistry, Institute of High

18

Energy Physics, Chinese Academy of Sciences, Beijing 100049, China, Tel: +86-10-

19

88233968

Engineering Laboratory of Advanced Energy Materials, Ningbo Institute of Industrial

20 21

E-mail address: [email protected]

22 23

ACS Paragon Plus Environment


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ABSTRACT: Pertechnetate anion (99TcO4-) is a long-lived radioactive species that is

25

soluble in aqueous solution, in contrast to sparingly soluble 99TcO2. Results are reported

26

for photocatalytic reduction and removal of perrhenate (ReO4-), a nonradioactive

27

surrogate for

28

UV-visible irradiation. Re(VII) removal up to 98% was achieved at pH = 3 under air or

29

N2. The proposed mechanism is Re(VII)/Re(IV) reduction mediated by reducing radicals

30

(·CO2-) from oxidation of formic acid, not direct reduction by photogenerated electrons

31

of TiO2. Recycling results indicate photocatalytic reduction of ReO4- exhibits excellent

32

regeneration and high activity with > 95% removal even after five cycles.

33

more easily reduced than Re(VII) in the presence of NO3- with very slow redissolution of

34

reduced 99Tc. This study presents a novel method for the removal of ReO4-/99TcO4- from

35

aqueous solution, with potential application for deep geological disposal.

99TcO -, 4

using a TiO2 (P25) nanoparticle suspension in formic acid under

99Tc(VII)

is

36 37

 INTRODUCTION

38

Technetium-99 (99Tc), a β-emitting isotope (β-max = 293.7 keV), is generated from

39

thermal neutron induced fission of uranium-235 (235U), and spontaneous fission of 238U in

40

the earth’s crust.1, 2 99Tc is also formed from decay of the medical radioisotope 99mTc with

41

a half-life of only 6.0 h.3 The most common chemical form is pertechnetate,

42

which is of particular environmental concern due to the long half-life of 99Tc (2.13 × 105

43

years),1 and the resistance to adsorption on mineral surfaces and sediments that results in

44

migration with potential ecosystem risks.4-7

99TcO -, 4

45

Because, all technetium isotopes species are radioactive, research progress is

46

challenging. As a result, rhenium (Re) is often used as a non-radioactive chemical

47

analogue of 99Tc.8-11 Among the various methods used for removal of 99TcO4-/ReO4- from

48

aqueous solution is conventional solvent extraction.12,

49

shortcomings, such as utilization of large amounts of toxic and volatile organic reagents,

50

and resulting production of secondary wastes. Alternative ion exchange methods14-16

51

require high quality of raw liquid to avoid column blockage. Despite a recent

52

breakthrough toward TcO4- elimination via molecular recognition,17 long-term storage

53

stability of Tc-containing materials requires further attention and large-scale practical

54

applications have not been demonstrated.18 Solid waste forms for


13

Nevertheless, there remain

99Tc

immobilization


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55

include metals such as Tc-Zr alloys,19 and borosilicate glasses.20 A disadvantage of the

56

latter is oxidation and release of volatile Tc molecules during high-temperature

57

vitrification.1 An appealing method to immobilize

58

99Tc

is reduction of soluble Tc (VII) to

59

sparingly soluble Tc (IV) with removal from aqueous solution as 99TcO2·nH2O species,8,

60

21

61

waste forms for long-term disposal.19, 20

62

which can be separated by physical filtration and then converted to metal or other Common reducing agents such as SO32-, Sn2+ ， Fe2+

9, 22, 23

and biomass,24,

25

are

63

exhausted in one cycle and not readily reused. Using Fe(0)/Fe(II) as the reductant couple,

64

99Tc/Re

65

Electrochemical methods29-31 involve toxic chemicals and furthermore the presence of

66

SO42- suppressed Re(VII) reduction in aqueous solution. Although γ radiation-induced

67

reduction32 via hydrated electrons might efficiently reduce and seperate Re(VII), the

68

conditions are impractical. Photochemical induced reduction31,

69

broadband UV or laser irradiation over 6 h afforded 94.7% recovery of Re; unfortunately,

70

the high molar absorptivity of Re(VII) limits the practical concentration of Re(VII).

were sequestrated using a simultaneous adsorption-reduction strategy.21,

32

26-28

of Re(VII) using

71

Heterogeneous semiconductor-based photocatalytic reduction of heavy metal ions

72

such as Cu2+, Hg2+, Ag+, U(VI) and Cr(VI) 33-37 has been proposed. Many photocatalysts

73

are regarded as environmental friendly materials because of chemical inertness and

74

biological compatibility in natural systems. For example, titanium dioxide (TiO2) is a

75

good prospect for photocatalytic reduction and removal of metal ions, due to high

76

resistance to photocorrosion, non-toxicity,38 low environmental pollution, regeneration

77

ability, low cost, and convenient operations.38, 39 Evans et al.40 reported selective removal

78

(98%) of uranium from waste liquid containing strong complexing agents using TiO2 as a

79

photocatalyst. Wang et al.41 prepared a TiO2/g-C3N4 heterojunction composite that

80

facilitated rapid separation and transfer of photogenerated electrons, thus achieving

81

efficient reduction and fixation of uranium. Although TiO2 has been demonstrated for

82

photocatalytic reduction of radioactive uranium, there is evidently no report addressing

83

utilization of this approach for technetium reduction and removal.

84

The objective of this study was to provide fundamental understanding of

85

photocatalytic 99Tc/Re reduction and removal using TiO2 nanoparticles in the presence of



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as a

86

HCOOH. Most of this work was still conducted using non-radioactive ReO4

87

surrogate for 99TcO4-.8, 42 Anyway, the reported 99Tc(VII/IV) redox potential (E0 = +0.74

88

V) is somewhat more positive than that for Re(VII/IV) (E0 = +0.51 V), which means

89

photocatalytic reduction of Tc(VII) should be more energetically favorable. In addition,

90

the reduction/removal mechanism was elucidated by photoelectrochemical measurements,

91

electron paramagnetic resonance spectroscopy, X-ray photoelectron spectroscopy and X-

92

ray absorption spectroscopy. These results suggest an environmental friendly

93

photocatalytic approach for

94

solution.

99TcO − 4

−

/ReO4 − removal and sequestration from aqueous

95 96

 EXPERIMENTAL METHODS

97

Materials and Chemicals. Ammonium perrhenate (NH4ReO4, AR grade),

98

HCOOH (99%, AR) were from Aladdin Co., Shanghai, China. These and other

99

commercial (reagent grade) chemicals were used without further purification. TiO2 (P25)

100

was from Degussa. Solutions were prepared using water from a Milli-Q water

101

purification system (18.2 MΩ cm, Millipore Co.).

102

Photocatalytic Experiments. Photocatalytic experiments with TiO2 (P-25, 0.1 ~

103

0.6 g L-1) suspensions were carried out in a circulating water-cooled 100 mL jacketed

104

quartz beaker. To only evaluate the change of Re(VII) concentration under light

105

irradiation, TiO2 suspensions were stirred for 60 minutes before turning on the lamp to

106

ensure adsorption equilibrium ReO4- onto the surface of TiO2 in the solution of 50 mL

107

Re(VII) (0.5 ~ 50 mg L-1) and HCOOH (or CH3OH or CH3COOH, 0.1 ~ 2% by volume).

108

pH values of the suspensions were adjusted in the range 1 - 6 by adding a small amounts

109

of NaOH or H2SO4 (∼ 2.5 mol L-1). Bubbling N2 was used to provide an anaerobic

110

atmosphere. The mixture was illuminated for 150 min using a 300 W Xenon lamp (> 320

111

nm) to simulate UV-visible sunlight. Aliquots of 0.5 mL were periodically collected

112

during irradiation, and filtered through 0.2 μm millipore membranes before analysis of

113

Re remaining in solution. The Re concentration was obtained using an inductively

114

coupled plasma optical emission spectrometer (Horiba JY2000-2, ICP-OES). Detection

115

limit (0.06 μg mL-1) of the method and the dilutions (4% HNO3 aqueous solution) are


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116

provided in the determination of Re concentration by ICP-OES. Dissolved Re is

117

expressed as Ct/C0, where C0 and Ct are the Re concentrations (mg L-1) at the end of

118

adsorption without irradiation, and after irradiation for time t (min), respectively. In the

119

recycling experiments, the suspension was exposed to air and oxidized to Re(VII) before

120

the next experiment. After completion of the photoreaction, suspended materials were

121

separated by vacuum filtration, washed immediately with absolute ethanol, and stored

122

under vacuum until analysis. All experiments were repeated at least two times, and these

123

experimental data were average values with 95% confidence bounds.

124

Solid Characterization. TiO2 samples were characterized by X-ray diffraction

125

(XRD; Bruker D8 Advance diffractometer) with Cu Kα radiation (λ = 1.5406 Å). Raman

126

spectra were obtained using a HORIBA Scientific spectrometer with 473 nm Laser (Laser

127

quantum Ltd.). Near-surface compositions of TiO2 after photocatalytic reaction were

128

analyzed using Kratos AXIS UltraDLD X-ray photoelectron spectroscopy (XPS).

129

Radicals from photocatalytic reactions were identified using a Bruker E-500 electron

130

paramagnetic resonance (EPR) spectrometer. 5,5-dimethyl-1-pyrroline-N-oxide (DMPO)

131

is used as a spin marker of EPR to characterize the free radical. X-ray absorption fine

132

structure (XAFS) of Re samples after photocatalytic reaction, as well as reference

133

samples NH4ReO4 and ReO2, were obtained in fluorescence mode at beamline 1W1B of

134

Beijing Synchrotron Radiation Facility.

135

Tc Removal:

99Tc

was obtained as a 2% HNO3 stock solution of potassium 99Tc

136

pertechnetate (KTcO4) from China Institute of Atomic Energy. The

137

were performed in a special radiological laboratory. In accord with the above

138

experimental protocol for Re, the corresponding 99TcO4- solution was illuminated for 150

139

min under the identified optimal Re(VII) reduction/removal conditions. Residual

140

concentration of

141

PerkinElmer). Aliquots of 0.5 mL were periodically collected during light irradiation, and

142

filtered through 0.2 μm millipore membranes before analysis. 0.2 ml of the filtrate was

143

then mixed with 5 ml of liquid scintillation cocktail (ULTIMA Gold, PerkinElmer) and

144

held in a 6 ml of plastic scintillation vial for measurements. The reacted suspension was

145

stirred in air to observe the re-oxidation and release of reduced Tc.

99Tc

experiments

was analyzed by a liquid scintillation counter (Tri-Carb,



146

 RESULTS AND DISCUSSION

147

Photocatalytic Reduction of ReO4-. Figure 1A shows the time evolution of the

148

Re(VII) residual ratio in suspensions of 0.4 g L-1 TiO2 (P-25) with 1% HCOOH at pH = 3

149

under UV-visible irradiation. After adsorption onto TiO2 without irradiation for a fixed

150

time of 60 minutes, the concentration of Re(VII) at zero time (the moment of light-on)

151

remains at the initial concentration of 5 mg L-1, which indicates that perrhenate, like

152

pretechnetate, cannot be effectively adsorbed onto mineral surfaces and sediments.4

153

Furthermore, in the absence of TiO2 there was no detectable reduction of Re(VII) under

154

UV-visible irradiation for 150 min. However, reduction of Re(VII) proceeded rapidly in

155

the presence of TiO2 and irradiation, indicating an important role for TiO2 nano-particles

156

in photocatalytic reduction. After irradiation for 150 min, photocatalytic ReO4- removal

157

achieved a maximum of 98.7 %, with a reaction rate constant ( k1' ) of 0.638 min-1,

158

obtained using the retarded first-order model43 as expressed by equation (1).

159



' dCt k'  1 Ct or Ct  C0 (1   t )  k1 / dt 1   t

(1)

160

In (1), C0 and Ct are concentrations (mg L-1) of the soluble Re at the end of light-off

161

and after irradiation, respectively, for a reaction time t (min); k1' is the apparent rate

162

constant (min-1), analogous to classical pseudo first-order rate constant; and α (min-1) is a

163

retarding factor indicating the deviation from pseudo-first order behavior.43, 44 α is only a

164

fitted coefficient, as shown in Table S1. When the effect of adsorption of Re onto the

165

TiO2 surface is ignored, nearly pseudo first-order kinetics for Re(VII) reduction are

166

obtained using a simplified first-order rate equation, rather than the traditional Langmuir-

167

Hinshelwood rate equation.45 Figure 1A exhibits the differences between the pseudo

168

first-order and retarded first-order fittings for anaerobic atmosphere. Although both

169

models fit the data for short times (< 10 min), the retarded first-order model gives a

170

somewhat better fit (R2 = 0.99 versus 0.97) for longer times. Compared with N2 bubbling,

171

reduction of Re(VII) is not drastically diminished by the oxidizing effect of dissolved air

172

(Figure 1A), with only a slight decrease in net reduction for irradiation times longer than

173

20 min. The results suggest that if the reaction rate is adequately fast at short times

174

(Figure 1A), dissolved air will be a relatively unimportant factor.


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175

If photo-generated holes can be quickly removed from the TiO2 surface, there is less

176

possibility for recombination with electrons, which enhances use of the latter for

177

reduction. It has been reported46 that organic additives such as HCOOH can enhance

178

photocatalytic reduction of metal ions, including U(VI). Figures 1B and C show effects

179

of 0.1 ~ 2% by volume of HCOOH for light-off and irradiation of 0.4 g L-1 aqueous TiO2

180

in flowing N2 atmosphere at pH 2. It should be noted that the Re(VII) removal rate

181

(Figure 1C and Table S1) is negligible in the absence of HCOOH. There is considerable

182

enhancement in photocatalytic reduction due to HCOOH, with an increase in removal of

183

Re(VII) as the HCOOH concentration is increased from 0% to 1%; a decrease in Re(VII)

184

removal is apparent upon further increasing HCOOH to 2%. This may be because the

185

HCOOH induced adsorption of Re(VII) onto the TiO2 surface is almost impossible in our

186

case (Figure 1B), and thus the increase of HCOOH concentration may not lead to the

187

favorable photocatalytic reduction.47, 48 Besides, excessive HCOOH adsorbed on the TiO2

188

surface (at high HCOOH levels) could vary light absorption of photocatalytic materials,

189

which was well documented by Chenthamarakshan et al.49 It is apparent that the

190

photocatalytic reduction is controlled by some intermediates generated by HCOOH

191

photooxidation. Thus Re(VII) removal in the presence of HCOOH reflect a trend of

192

indirect reduction route based on radical intermediates, in agreement with the reaction

193

mechanism reported by Rajeshwar et al.50

194

It has been reported that photoxidation of HCOOH produces radical intermediates

195

that can reduce metal ions50, 51 such as Ni2+, Zn2+, and Cr3+. Although Re(VII) reduction

196

seems to exhibit a similar effect, there are important differences. In Cr(VI) reduction,

197

addition of formate accelerates the Cr(VI) adsorption onto TiO2 surface for

198

photoreduction.49 As the results in Figures 1A-C reveal, Re(VII) has little tendency to

199

adsorb on the surface of TiO2, with light-off adsorption apparently unimportant for

200

photocatalytic reduction of Re(VII). The present results indicate that TiO2, light

201

irradiation, and HCOOH are all essential for Re(VII) reduction and removal.



202

203 204 205 206 207 208

Figure 1. (A) Removal of Re(VII), for no TiO2 and 0.4 g L-1 TiO2 with different conditions; pH = 3, [HCOOH] = 1%, [Re(VII)] = 5 mg L-1. Removal of Re(VII) for different concentrations of HCOOH, for (B) No-light and (C) UV-visble irradiation; pH = 2, [Re(VII)] = 10 mg L-1. (D) Removal of Re(VII) with different organic additives under light irradiation; pH = 3, [organic additive] = 1%, [Re(VII)] = 5 mg L-1. V = 50 mL, T = 298 K throughout.

209

Photocatalytic Reduction Mechanism. When TiO2 is excited by light irradiation to

210

generate e-/h+ pairs that overcome recombination and transfer to the oxide/water

211

interface,52 the pairs can combine with electron acceptors (e.g. H+, O2) or donors

212

(HCOOH, OH-, H2O). Photo-oxidation of formic acid is believed to form the strongly

213

reducing radical ·CO2-.50 The reduction potential of -1.9 V for E0 (CO2/·CO2-),53 could

214

enable ·CO2- to reduce Re(VII) (E0 (Re(VII)/Re(IV) = +0.51 V). To assess the role of

215

formic acid, experiments were also performed using CH3OH or CH3COOH. The results

216

in Figure 1D show only minor photo-induced reduction of Re(VII) for TiO2/CH3COOH

217

and none for CH3OH. Minor reduction of Re(VII) with CH3COOH as an electron donor

218

might reflect that photo-generated h+ or HO· preferentially react with HCOOH due to its

219

better adsorption onto TiO2, which results in faster oxidation to generate


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220

reducing ·CO2-.46 Based on results with no electron donor (Figure 1C), we conclude that

221

photo-generated e- of TiO2 under light irradiation does not induce direct reduction of

222

Re(VII).

223

Reaction pathways were elucidated from EPR of radical intermediates. As shown in

224

Figure 2A, there are no obvious EPR signal peaks for the systems TiO2, HCOOH or

225

TiO2/HCOOH/Re(VII) without UV-vis irradiation, or for HCOOH with UV-vis

226

irradiation. However, when TiO2 alone is exposed to UV-vis irradiation for 10 min,

227

trapping of HO· radicals generates a stable paramagnetic adduct species DMPO-

228

HO· (Figure 2A, brown trace). For the system light/TiO2/HCOOH (Figure 2A, red

229

trace), the DMPO-HO· adduct is negligible while a new signal appears that is assigned to

230

a spin adduct of ·CO2-. Perissinotti et al.54 and Villamena et al.55 have shown that when

231

DMPO is used as an EPR spin marker in such study, an EPR signal peak appears that is

232

attributed

233

light/TiO2/HCOOH/Re(VII) (Figure 2A, green trace) is similar to that of

234

light/TiO2/HCOOH, though the DMPO-·CO2- adduct signal is diminished, presumably

235

due to reaction between ·CO2- and ReO4-. The indirect reaction of radical intermediates

236

can be expressed by the following reaction scheme in correspondence to previous

237

studies.50, 56

to

adduct

DMPO-·CO2-.

The

EPR

spectra

of

system

238

TiO 2 hv eCB   hVB  (2)

239

H2O + hVB+ → HO· + H+ (3)

240

hVB+/HO· + HCOOH/HCOO- → HCOO·/·CO2- + H2O (4)

241

Re(VII) + ·CO2- → Re(VI) + CO2 (5)

242

Equation (5) is based on one-electron reduction of ReO4-; Re(VI) could be

243

transformed to lower oxidation states Re(V) and Re(IV) by disproportionation.32 When

244

discussing the possible fate of ·CO2-, it is necessary to consider the reaction of ·CO2- with

245

dissolved O2 (equation 6)56. However, reaction (6) is negligible when the solution is

246

bubbled with N2 to provide an anaerobic atmosphere (Figure 1A). It should be noted



247

that ·CO2- could be oxidized to CO2 by electron injection into the conduction band of

248

TiO257 (equation 7).

249

·CO2- + O2 → CO2 + O2·- (6)

250

·CO2- → CO2 + eCB- (7)

251

Indirect reaction between the intermediate species (·CO2-) from HCOOH photo-

252

oxidation and Re(VII) in aqueous solution was further assessed from the TiO2 electrode

253

photocurrent using Bard’s approach.58 Because strongly reducing radicals (·CO2-) can

254

inject e- into the conduction band of TiO2, it is expected that effects of the “current-

255

doubling” will be observed. In Figure 2B, for 0.1 mol L-1 Na2SO4, the photocurrent (~15

256

μA) is controlled by oxidation of H2O or OH-.58 Introducing 0.1% HCOOH as a current-

257

doubling agent (0.1 mol L-1 Na2SO4+0.1% HCOOH), increases the current by up to a

258

factor of ten (i.e. ~150 μA). When Re(VII) is introduced, the reduction reaction of Re(VII)

259

competes with injection of e- into TiO2, with a resulting decrease in the photocurrent (to

260

~90 μA) (Figure 2B). The results suggest that current-doubling signals obtained in

261

photo-oxidation process of HCOOH are due to the ·CO2- intermediate, with electrons

262

simultaneously released into the conduction band.

263 264 265 266 267 268

Figure 2. (A) First-derivative EPR spectra of DMPO spin adducts. Under dark: TiO2, HCOOH and TiO2/HCOOH/Re(VII); Under light: TiO2, HCOOH, TiO2/HCOOH and TiO2/HCOOH/Re(VII). (B) TiO2 current-potential measurements: (■) Idark with 0.1 mol L-1 Na2SO4+0.1% HCOOH+5 mg L-1 Re(VII); (●) Iphoto with 0.1 mol L-1 Na2SO4; (▲) Iphoto with 0.1 mol L-1 Na2SO4+0.1% HCOOH; (▼) Idark with 0.1 mol L1 Na SO +0.1% HCOOH+5 mg L-1 Re(VII). 2 4


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269 270

Effects of Other Factors on Photocatalytic Redution.

271

Concentrations of photocatalyst and Re(VII). Figure 3A shows good fit for

272

retarded pseudo first-order kinetics at different TiO2 dosages (R2 = 0.970 - 0.998).

273

Increasing the amount of TiO2 from 0.1 to 0.2 g L-1, increased the rate constant k1' from

274

0.048 to 0.181 min-1 (~ 3.8 times). The oddly large increase might reflect enhancement of

275

light absorption by the larger amount of photocatalyst.36 Further increasing the TiO2 from

276

0.2 to 0.4 (or 0.4 to 0.6) g L-1, increased the k1' (min-1) from 0.181 to 0.344, a factor of

277

1.9, (or 0.344 to 0.565, a factor of 1.6). At the higher dosages (0.4 and 0.6 g L-1), light

278

scattering from TiO2 particles36 and other influencing factors such as mass transfer,

279

Re(VII) and NO3-concentration see below should become more important.59 To further

280

elucidate effects of the amount of TiO2 on reaction rate, surface-area normalized rate

281

constants (kBET = k1' /SBET/ mTiO2 ) were considered using the specific surface area, SBET =

282

57.96 m2 g-1, determined by Brunauer-Emmett-Teller (BET) method based on

283

N2 adsorption isotherms (Figure S1). Using the retarded k1' (Table S1), kBET is 0.008,

284

0.015, 0.015 and 0.016 [L (min·m2)-1] at 0.1, 0.2, 0.4 and 0.6 g L-1 TiO2, respectively. As

285

expected, for higher amounts of TiO2, there are more available reaction sites.

286

As displayed in Figure 3B and Table S1, upon increasing C0(Re) from 0.5 to 50 mg

287

L-1, the photoreaction rate constant k1' for Re(VII) decreases from 0.654 to 0.030 min-1. A

288

fixed dose of TiO2 (0.2 g L-1) is more effective for reduction and removal of low

289

concentration of Re(VII). Although more effective Re(VII) removal could be found at

290

higher TiO2 dose like 0.4 or 0.6 g/L, kBET stably remains to be 0.015- 0.016 [L (min·m2)-1]

291

at TiO2 dose of 0.2- 0.6 g L-1. Therefore, the reaction sites may not be fully utilized in the

292

presence of higher TiO2 dose.

293

Competition by NO3-. In legacy nuclear waste, there is often substantial nitric acid in

294

storage tanks to control corrosion and other chemistry, with a result that reduction of

295

nitrate to nitrite can compete with reduction of 99TcO4-. Results in Figure 3C for Re(VII)

296

removal for NO3- concentrations in the range of 1 to 100 mmol L-1 confirm that nitrate



297

adversely affects Re(VII) reduction, in accord with initial one-electron reduction of NO3-

298

to form the short-lived NO32- in the presence of formic acid.54

299

300 301 302 303 304 305

Figure 3. Time profiles of Re(VII) reduction during the irradiation of TiO2 suspensions with N2 bubbling, V = 50 mL, T = 298 K. (A) Various dosages of TiO2, [HCOOH] = 1%, [Re(VII)] = 10 mg L-1, pH = 2; (B) Effects of initial Re(VII) concentration, [HCOOH] = 1%, 0.2 g L-1 TiO2, pH = 2; (C) Influence of NO3concentration, [HCOOH] = 1%, [Re(VII)] = 10 mg L-1, 0.2 g L-1 TiO2, pH = 2; (D) Solution pH values, [HCOOH] = 0.2% , [Re(VII)] = 10 mg L-1, 0.4 g L-1 TiO2.

306 307

Effect of pH. pH values were adjusted in the range ~1-6 by addition of 2.5 mol L-1

308

H2SO4 or 2.5 mol L-1 NaOH. As shown in Figure S2A, without irradiation there is

309

insignificant bulk adsorption of Re(VII) onto TiO2 in this pH range. These results also

310

indicate little effect of SO42- on bulk adsorption of Re(VII) and little competition with

311

reduction of Re(VII), reflecting E0 (SO42-/SO2 = 0.158 V). Figure 3D shows the influence

312

of pH on photocatalytic reduction of Re(VII). Reduction and removal percentage of

313

Re(VII) is extremely low at lower pH (~1), which is correlated with the fact that the


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314

soluble Re(III) species may be produced under this condition, and cannot be separated by

315

simple physical filtration.2 Increasing pH from 1 to 4 enhanced Re(VII) removal from 0%

316

to 80.1%; further increasing from 4 to 6 decreased removal back down to 0%. Increasing

317

pH can decrease the valence band potential of TiO2 (EVB),60 which could inhibit oxidation

318

of formic acid. According to the Eh-pH diagram of Re,2 reduction of Re(VII) (pH = 2~5)

319

could result in precipitation of ReO2(s)·xH2O, which is the key for Re(VII) removal. The

320

dip in the initial stage of curves at pH 5 and pH 6, respectively, is due to the reduction

321

and removal of Re(VII). Nevertheless, the overall reduction of ReO4- may be represented

322

by equation (8), which would result in increasing pH (Figure S2B). At high pH

323

conditions, the soluble [Re(IV)O(OH)3]- species result in the decease of Re(VII)

324

removal.2 From the observed pH dependence, we conclude that the optimal pH range for

325

photocatalytic reduction is 3~4.

326

ReO4- + 4H+ + 3e- → ReO2 + 2H2O (8)

327

Reusability of TiO2. Along with photocatalytic activity for reduction of Re(VII),

328

TiO2 (P-25) presents excellent stability, as shown in Figure 4. After each reaction, the

329

photocatalyst was left in aqueous suspension by continuously stirring in dark with air

330

atmosphere for 24h, which resulted in a complete re-dissolution of Re. The re-oxidized

331

suspension was then re-irradiated for 150 min. Re(VII) removal during five cycles was

332

96.7%, 96.4%, 96.1%, 96.0%, and 95.7%, with no significant decrease in photocatalytic

333

activity. Unchanged characteristics of TiO2 before and after photocatalytic reactions were

334

confirmed by XRD and Raman spectroscopy (Figure. S3). XPS spectra (Re 4f region)

335

(Figure. S4A-B), X-ray absorption near-edge structure (XANES), and extended X-ray

336

absorption fine structure (EXAFS) (Figure. S4C-D) were used to analyze the surface

337

components of the solid residues (TiO2-Re). The results indicate that Re(VII)/Re(IV)

338

reduction is induced by TiO2 particles in the presence of HCOOH under UV-vis

339

irradiation. For solid residues dried in vacuum after photocatalytic reaction, the

340

percentage of Re(IV) was roughly 82 to 87%. In contrast, for wet residues in air, much

341

lower percentage of Re(IV) of 42 to 51% was observed, as Re(IV) can be oxidized to

342

Re(VII) when exposed to air under wet conditions (Figure 4B and Equation S1). The

343

ReO2·nH2O sample dried in vacuum is, as expected, more stable than that in wet solid



Page 14 of 20

344

form, which gives a hint that dried Tc(IV) form is more appropriate for deep geological

345

disposal of Tc. The results overall suggest photocatalytic reduction/removal of ReO4- as

346

an efficient, robust and environmentally friendly approach.

347 348 349 350 351

Figure 4. (A) Cycling runs of TiO2 for photocatalytic reduction of Re(VII). Time profiles of Re(VII) reduction during the irradiation of 0.4 g L-1 TiO2 suspensions at the pH = 3, with N2 bubbling, [HCOOH] = 1%, [Re(VII)] = 5 mg L-1, V = 50 mL, T = 298 K. (B) The color change of both the solid and solution before and after photocatalysis.

352 353 99Tc

354

Reduction and Removal. The results for ReO4- suggest photocatalytic

355

reduction and removal of 99TcO4- as an alternative to other separations approaches,8, 21, 42,

356

61

357

shows

358

reported above, with 20 mmol L-1 NO3-. Notably, 99Tc(VII) removal of 68% and rate k1'

359

of 0.274 min-1 are significantly higher than for Re(VII) (37% and 0.081 min-1,

360

respectively). The reported

361

positive than for Re(VII/IV) (E0 = +0.51 V), which may explain more facile reduction of

362

Tc(VII), and less interference from nitrate. The “light-off” results in Figure 5 reveal that,

363

in contrast to Re(IV), re-oxidation of Tc(IV) is not significant under these conditions,

364

which is consistent with the generally higher stability of

365

compounds.2 This may be because the photocatalytic reduction and removal of Re(VII)

366

or 99Tc(VII) is studied in an anaerobic atmosphere via N2 bubbling. After the completion

but a particular concern is competition of NO3-, as was observed with ReO4−. Figure 5 99Tc(VII)

and Re(VII) photocatalytic reduction under the optimal conditions

99Tc(VII/IV)

potential (E0 = +0.74 V)2 is somewhat more


99Tc(IV)

versus Re(IV)

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367

of photoreaction, the suspension was exposed to air and oxidized to soluble Re/Tc in

368

reoxidation experiment. Thus, the oxygen partial pressure of solution is very low in

369

reoxidation experiment within short time (~ 1440 min). The solubility of TcO2·nH2O is

370

strongly dependent upon the presence of oxygen, but ReO2·nH2O is not.62 These results

371

should stimulate further studies of photocatalytic reduction and removal of Tc(VII).

372 373 374 375

Figure 5. Time profiles of 99Tc(VII) and Re(VII) reduction during the irradiation of 0.4 g L-1 aqueous TiO2 suspensions at the pH = 3, with N2 bubbling, [HCOOH] = 1%, [99Tc(VII) or Re(VII)] = 0.05 mmol L-1, [NO3-] = 20 mmol L-1, V = 50 mL, T = 298 K.

376 377

 ASSOCIATED CONTENT

378

Supporting Information

379 380

The Supporting Information is available free of charge on the ACS Publications website at DOI:

381

Figures showing N2 sorption-desorption isotherm of TiO2, effect of pH on

382

photocatalytic reaction, XPS, XAFS, XRD and Raman characterizations of reacted

383

photocatalysts (PDF).

384 385

 AUTHOR INFORMATION

386

Corresponding Author

387

*E-mail: [email protected]. Tel: +86-010-88233968, Fax: +86-010-88235294.



388

Notes

389

The authors declare no competing financial interest.

390

 ACKNOWLEDGEMENTS

391 392 393 394 395

This work was supported by the National Natural Science Foundation of China (Grants no. 21577144, 11675192, 21790373 and 21790370) and the Science Challenge Project (TZ2016004). JKG was supported by the Center for Actinide Science and Technology, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award Number DE-SC0016568.

396 397

 REFERENCES

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