In Situ Photochemical Activation of Sulfate for ... - ACS Publications

Subscriber access provided by UB + Fachbibliothek Chemie | (FU-Bibliothekssystem)

Article

In-situ Photochemical Activation of Sulfate for Enhanced Degradation of Organic Pollutants in Water Guoshuai Liu, Shijie You, Yang Tan, and Nanqi Ren Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b05090 • Publication Date (Web): 24 Jan 2017 Downloaded from http://pubs.acs.org on January 24, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 28

Environmental Science & Technology Article

1 2

Revised Manuscript for: Environmental Science & Technology Submission date: 2017-01-20

3 4 5 6

In-situ Photochemical Activation of Sulfate for Enhanced Degradation of Organic Pollutants in Water

7 8

Guoshuai Liu, Shijie You *, Yang Tan, Nanqi Ren

9 10 11

State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, P. R. China

12 13 14 15 16 17 18

Corresponding author: * Shijie You P. O. Box 2603#, No. 73, Huanghe Road, Nangang District, Harbin, 150090, China. Tel.: +86–451–86282008; Fax: +86–451–86282110 E–mail: [email protected]

19 20 21 22 23 24 25 26 27 28 29 30 31 32 1

ACS Paragon Plus Environment

Environmental Science & Technology

Page 2 of 28 Article

33

ABSTRACT

34

The advanced oxidation process (AOP) based on SO4•− radicals has been receiving growing attention

35

in water and wastewater treatment. Producing SO4•− radicals by activation of peroxymonosulfate or

36

persulfate faces the challenges of high operational cost and potential secondary pollution. In this

37

study, we report the in-situ photochemical activation of sulfate (i-PCAS) to produce SO4•− radicals

38

with bismuth phosphate (BPO) serving as photocatalyst. The prepared BPO rod-like material could

39

achieve remarkably enhanced degradation of 2,4-dichlorophenol (2,4-DCP) in the presence of sulfate,

40

indicated by the first-order kinetic constant (k=0.0402 min–1) being approximately 2.1 times of that

41

in the absence (k=0.019 min–1) at pH-neutral condition. This presented a marked contrast with

42

commercial TiO2 (P25) whose performance was always inhibited by sulfate. The impact of radical

43

scavenger and electrolyte, combined with electron spin resonance (ESR) measurement, verified the

44

formation of •OH and SO4•− radicals during i-PCAS process. According to theoretical calculations,

45

BPO has a sufficiently high valence band potential making it thermodynamically favorable for

46

sulfate oxidation, and weaker interaction with SO4•− radicals resulting in higher reactivity toward

47

target organic pollutant. The concept of i-PCAS appears to be attractive for creating new

48

photochemical systems where in-situ production of SO4•− radicals can be realized by using sulfate

49

originally existing in aqueous environment. This eliminates the need for extrinsic chemicals and pH

50

adjustment, which makes water treatment much easier, more economical and more sustainable.

51 52 53 54 55 56 2


Page 3 of 28


57

■ INTRODUCTION

58

The advanced oxidation processes (AOPs) have been widely developed as an efficient method to

59

degrade recalcitrant organic pollutants in water environment. In many cases, the AOPs are known as

60

the process involving generation of hydroxyl radicals (•OH), an intermediate species having great

61

redox potential (E0•OH=2.80 V vs standard hydrogen electrode, SHE) to oxidize or even mineralize

62

the organic substances non-selectively.

63

hampered by the short half-life time of •OH (99 wt%) and measured using electron spin resonance

177

(ESR, Bruker, Germany).

33

The final

178 179

■ RESULTS AND DISCUSSION

180

Characterization of BPO. The XRD profiles (Figure S1) were indexed to the characteristic peaks

181

of BPO powder samples, which were consistent well with the hexagonal-phase structure (JCPDS No. 7




28

182

15-0767).

The sharp and intense diffraction peaks indicate the high crystallinity during synthesis

183

process. As shown in Figure S2, the length of the BPO gel fiber was on the centimeter scale and the

184

diameter was relatively uniform (300−400 nm) following electrospinning synthesis. The fibers were

185

seen to shrink and broken to the rod-like shape with the length of 2 µm and diameter of about 200

186

nm after calcination, which should result from the thermal decomposition of PVP, (NH4)3PO4, and

187

Bi(NO3)3. The HRTEM image and selected area electron diffraction illustrated the highest exposure

188

plane in [120] direction for BPO rods. 31 The TG-DSC curves (Figure S3) show the total weight loss

189

amounting to 92%, including decomposition of citric acid and nitrate (exothermic peak at 340 oC),

190

PVP (side chain at 400 oC and molecule at 500 oC). 34 The high-resolution XPS spectra (Figure S4) of

191

the Bi4f, P2p, O1s show two peaks at binding energy of 159.3 eV and 164.6 eV for Bi4f7/2 and

192

Bi4f5/2 of Bi(III) in BiPO4. The P2p peak could be found at binding energy of 134.6 eV for P(VI) in

193

BiPO4. 35 The O1s was also detected at binding energy of 530.0 eV.

194

Photocatalytic Degradation of 2,4-DCP in the Presence of Sulfate. As an initial tests, the removal

195

of 2,4-DCP by photocatalytic oxidation of BPO was investigated and recorded in the presence and

196

absence of sulfate at pH-neutral condition (pH 6.8±0.2) over a period of 90 min. The commercially

197

viable TiO2 (Degussa, P25) was also included for comparison under the same experimental

198

conditions. The adsorption of 2,4-DCP and sulfate on BPO was small (Table S1), and the 2,4-DCP

199

degradation in the absence of photocatalyst was negligible whether the sulfate was added or not

200

(Figure 1A), and this excluded the possibility of self degradation induced by photolysis or dissolved

201

oxygen in water. Thereafter, the time course of 2,4-DCP concentration was recorded during

202

photocatalytic reaction by using BPO and P25.

203

For the 90-min reaction lacking sulfate, the P25 could remove 2,4-DCP from initial 30 mg L−1 to

204

9.54 mg L−1 (by 68.2 %), whereas the addition of 3.5 mmol L−1 Na2SO4 was observed to decrease the

205

overall removing efficiency to 57.6%. Meanwhile, the sulfate also slowed down the rate of 2,4-DCP

206

degradation, indicated by 29.3% decline in first-order kinetic constant (k, min−1; Figure S5 in 8


Page 9 of 28


207

Supporting Information). If the adsorbed sulfate could not be converted timely, the adsorption of

208

sulfate would prevent the active sites from interacting with target pollutants, which led to a declined

209

photochemical activity. 29,36 Besides, limited evidence can support the occurrence of sulfate oxidation

210

on P25 due to thermodynamic barrier associated with low EVBM (valence band maxima), and hence

211

we attribute the •OH radicals produced from water oxidation at valence band to be responsible for

212

the degradation of 2,4-DCP. 31

213

Figure 1

214 215 216

On the contrary, the BPO behaved totally different from P25, indicated by a drastically enhanced

217

2,4-DCP degradation when pre-determined 3.5 mmol L−1 Na2SO4 (Figure S6) was incorporated into

218

the solution. As shown in Figure 1, the BPO was able to degrade 2,4-DCP with an efficiency of

219

95.2% (total organic carbon removed by 64.2%, Figure S7) and rate constant of k=0.0402 min–1 in

220

the presence of Na2SO4, a value much higher than that of 81.3% and k=0.019 min–1 in the absence

221

(Figure S5). The stability of i-PACS was also evaluated by the experiments of catalyst cycling. The

222

2,4-DCP degradation decreased slightly, i. e. 5.7% loss of pollutant removal after five-cycle tests

223

(Figure S8), possibly due to the mass loss of catalyst during operation. The substantially improved

224

organic abatement clearly suggested the significant role of sulfate during the photocatalytic oxidation

225

of 2,4-DCP by BPO. Since SO4•− radicals are well known as efficient oxidative intermediate species,

226

10,11

227

analogy to the behaviors of sulfate documented in electrochemical oxidation on BDD film anode.

228

22,23

we assume the formation of SO4•− radicals during photochemical oxidation by making an

229

Identification of Sulfate Radicals. During i-PCAS process, three pathways may be responsible for

230

the degradation of 2,4-DCP, which includes (i) direct oxidation by holes at valence band, (ii) indirect

231

oxidation by photochemically produced •OH or O2•− radicals from water splitting, and (iii) indirect 9




232

oxidation mediated by SO4•− radicals generated from sulfate oxidation.

233

Figure 2 (A and B)

234 235 236

Typically, the SO4•− and •OH radicals oxidize the 2,4-DCP through different pathways. The •OH

237

radicals mainly attack 2,4-DCP via addition to unsaturated bonds and H-abstraction, while SO4•−

238

radicals are more likely to react via electron-transfer mechanism. 38,39 To verify the existence of SO4•−

239

radicals on a qualitative basis, we carried out additional experiments by choosing different types of

240

specific probe scavengers, i. e. Methanol, tert-butyl alcohol (t-BuOH) and KI. The methanol has

241

similar reactivity towards both •OH and SO4•− radicals, with reaction rate constants of 9.7×108 M−1

242

s−1 and 1.0×107 M−1 s−1, respectively.

243

selective targeting •OH radicals, as the rate constant for •OH radicals (3.8−7.6×108 M−1 s−1) is three

244

orders of magnitude greater than that for SO4•− radicals (4−9.1×105 M−1 s−1).

245

general hole scavenger. 27

40,41

The alcohol lacking α-hydrogen, e. g. t-BuOH, is more

40,41

KI is used as

246

As illustrated in Figure 2A, the 2,4-DCP degradation by BPO was suppressed by the addition of

247

all examined scavengers, following the order of t-BuOH (k=0.0151 min–1)>methanol (k=0.0131

248

min–1)>KI (k=0.0054 min–1; Figure S9A). The performance degradation induced by t-BuOH

249

suggested that the •OH radicals produced from water oxidation played a crucial role, while the

250

discrimination between t-BuOH and methanol offered a preliminary verification for the existence of

251

SO4•− radicals because t-BuOH was less selective for trapping SO4•− radicals than methanol. 40,41 The

252

SO4•− radicals may be produced via single-electron transfer from either the direct oxidation by hole

253

or indirect oxidation by •OH radicals. For P25 as a comparison, no noticeable difference in 2,4-DCP

254

was observed for t-BuOH (k=0.0058 min–1) and methanol (k=0.0072 min–1; Figure 2B and Figure

255

S9B). This allowed to further confirm the predominant contribution of •OH radicals rather than SO4•−

256

radicals, i. e. the production of SO4•− radicals was considerably limited on P25. When KI was used as 10


Page 11 of 28


257

scavenger for the two photocatalysts, the holes produced at valence band were precedently consumed

258

by I− to form I2 such that the formation of both •OH and SO4•− radicals were inhibited substantially.

259

27

260

conduction band, which was proved by dramatic decrease in degradation of 2,4-DCP to as low as

261

6.3% when both KI (hole scavenger) and parabenzoquinone (superoxide radicals scavenger) were

262

added simultaneously. This result was also consistent with the results reported in prior studies.

263

42

In such case, organic degradation appeared more likely to proceed via O2•− radicals produced at

31, 39,

264

The ESR measurements were performed using DMPO as trapping agent. As shown in Figure 3,

265

there is no observation of definable ESR peaks for blank experiments where BPO was not added.

266

When BPO was used, the quartet lines with peak strength of 1:2:2:1 and hyperfine coupling constant

267

of αN=1.49 mT and αH=1.49 mT (g-factor of 2.0055) could be detected, which accounted for the

268

typical signals of DMPO-•OH adduct in the absence of sulfate. The addition of sulfate resulted in the

269

observation of characteristic peaks for not only DMPO-•OH, but also DMPO-SO4•− adduct in

270

accordance with the hyperfine splitting constants of αN=13.2 G, αH=9.6 G, αH=1.48 G and αH=0.78 G.

271

43–45

272

radicals should be responsible for the degradation of 2,4-DCP in i-PCAS system.

This provides a direct evidence for the formation of SO4•− radicals, and both •OH and SO4•−

273

Effect of Electrolyte. To further confirm the contribution of SO4•− radicals, the effect of electrolyte

274

was also investigated by introducing other electrolytes such as nitrate and perchlorate (3.5 mmol L–1

275

NaNO3 and NaClO4; pH 6.8). As shown in Figure 4A, for BPO, instead of performance enhancement,

276

the 2,4-DCP degradation was diminished slightly by nitrate and perchlorate. On the other hand, as

277

expected, the 2,4-DCP degradation was increased noticeably, indicated by the kinetic constant for

278

Na2SO4 (k=0.0402 min–1) being one order of magnitude higher than that for both NaNO3 (k=0.0183

279

min–1) and NaClO4 (k=0.0165 min–1) (Figure S10A). It was worth noting that there was no evidence

280

for the formation of active components by NaNO3 and NaClO4 thus far,

281

inferred that the enhanced degradation of 2,4-DCP should originate from the action of SO4•− radicals 11


29,36

and thus it could be



282

generated from i-PCAS process, which well accompanied the above results. That is, only the

283

production of SO4•− radicals could be proved on BPO; however, no performance enhancement was

284

attained for P25 and all the tested anions exhibited inhibition to different extents (Figure S10B). In

285

other words, the SO4•− radicals is unlikely to be produced at a definable amount on P25. This result

286

was in line with what was reported previously for P25 in literatures,

287

explained by the adsorption of negatively charged ions onto the surface of P25. When these anions

288

were mixed at the same molar concentration (3.5 mmol L–1), we could also obtain the enhanced

289

degradation of 2,4-DCP, i. e. the kinetic constant (k=0.0322 min–1) with sulfate was 1.69 times of

290

that without (k=0.019 min–1). This preliminarily suggests the capability of sulfate for enhanced

291

organic degradation in electrolyte competitive conditions.

46–48

which could be also

292

Figure 3

293 294 295

Mechanisms for i-PCAS. First, the band parameters of BPO were calculated in order to understand

296

the macroscopic mechanism for sulfate oxidation in view of thermodynamics. The UV-vis diffuse

297

reflectance spectrum (DRS) indicates the capability of BPO to absorb photons with the adsorption

298

edge of ca. 300 nm. Thus, the band gap (Eg, eV) of BPO used here can be estimated according to1,31

299

α hv = A(hv − Eg ) n /2

300

(2)

301 302

where α is the optical absorption coefficient, A the proportionality constant and hν the photonic

303

energy. n is determined by the type of optical transition of semiconductor (n=1.0 for direct transition

304

and n=4.0 for indirect transition).

305

substituting n=4.0 to Eq. (2) gives the Eg value of 4.2 eV (Figure S11). Furthermore, the theoretical

306

potential of valence band maxima (EVBM) and conduction band minimum (ECBM) at pH=pHpzc can be

31

Taking into account of indirect transition property for BPO,

12


31

Page 13 of 28


307

calculated according to the empirical equations 49

308 309

ECBM = (χ Bi χ P χ O )1/ 6 − 0.5 Eg + E0

4

(3)

EVBM = ECBM + E g

(4)

310 311 312 313

where E0 (−4.5 V vs SHE) is the scale factor associated with the reference electrode redox level to

314

the absolute vacuum scale, and χ the absolute electronegativities of Bi (4.69 eV), P (5.62 eV), and O

315

atom (7.54 eV). 49 By substituting these parameters to Eq. (3) and (4), we obtained the ECBM of −0.53

316

V and EVBM of 3.67 V at pH-neutral condition. Clearly visible is much higher EVBM of BPO than that

317

of E0SO4•−/SO42- and most of photocatalysts, which is in good agreement with the results in prior

318

studies.

319

sulfate to form SO4•− radicals in aqueous solution once it arrives at the BPO surface. This process

320

appears quite similar as that observed in electrochemical oxidation of sulfate on BDD film anode. 22

321

Most of previous studies reported the use of TiO2-based photocatalysts with upper limit of valence

322

band potential commonly below 2.8 V vs SHE.

323

to produce •OH radials at pH

In Situ Photochemical Activation of Sulfate for ... - ACS Publications

Recommend Documents