In Situ Photochemical Activation of Sulfate for Enhanced Degradation

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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

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Environmental Science & Technology Article

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

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

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

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

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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]

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ABSTRACT

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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■ INTRODUCTION

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

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

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

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

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the organic substances non-selectively.

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

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(ESR, Bruker, Germany).

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The final

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■ RESULTS AND DISCUSSION

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

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




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15-0767).

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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photocatalytic reaction by using BPO and P25.

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

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

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

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


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

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

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

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

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

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the degradation of 2,4-DCP. 31

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Figure 1

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

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

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

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

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

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

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

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

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

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

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10,11

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

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

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

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

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




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

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Figure 2 (A and B)

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

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

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

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

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

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

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

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

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

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general hole scavenger. 27

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

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KI is used as

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

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

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

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

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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43–45

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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•−

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

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

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

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

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

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

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

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

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


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and thus it could be



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

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

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

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

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

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

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

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

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

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organic degradation in electrolyte competitive conditions.

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which could be also

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Figure 3

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

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

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

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

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α hv = A(hv − Eg ) n /2

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(2)

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

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

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and n=4.0 for indirect transition).

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

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

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Taking into account of indirect transition property for BPO,

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calculated according to the empirical equations 49

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

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(3)

EVBM = ECBM + E g

(4)

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

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

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

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

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

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studies.

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

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

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

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band potential commonly below 2.8 V vs SHE.

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to produce •OH radials at pH

In Situ Photochemical Activation of Sulfate for Enhanced Degradation

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