Visible-light-driven photocatalytic degradation of organic water

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Visible-light-driven photocatalytic degradation of organic water pollutants promoted by sulfite addition Wei Deng, Huilei Zhao, Fuping Pan, Xuhui Feng, Bahngmi Jung, Ahmed Abdel-Wahab, Bill Batchelor, and Ying Li Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b04206 • Publication Date (Web): 30 Oct 2017 Downloaded from http://pubs.acs.org on November 4, 2017

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Environmental Science & Technology

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Visible-light-driven photocatalytic degradation of organic water pollutants

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promoted by sulfite addition

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Wei Deng1, Huilei Zhao1, Fuping Pan1, Xuhui Feng1, Bahngmi Jung2, Ahmed Abdel-Wahab2,

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Bill Batchelor3, and Ying Li1*

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1

Department of Mechanical Engineering, Texas A&M University, College Station, Texas, USA

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2

Chemical Engineering Program, Texas A&M University at Qatar, Doha, Qatar

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3

Department of Civil Engineering, Texas A&M University, College Station, Texas, USA

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*Corresponding Author: Ying Li, Tel.: +1-979-862-4465, E-mail: [email protected]

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Abstract

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Solar-driven heterogeneous photocatalysis has been widely studied as a promising technique for

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degradation of organic pollutants in wastewater. Herein, we have developed a sulfite-enhanced

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visible-light-driven photodegradation process using BiOBr/methyl orange (MO) as the model

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photocatalyst/pollutant system. We found that the degradation rate of MO was greatly enhanced

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by sulfite, and the enhancement increased with the concentration of sulfite. The degradation rate

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constant was improved by twenty-nine times in the presence of 20 mM sulfite. Studies using hole

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scavengers suggest that sulfite radicals generated by the reactions of sulfite (sulfite anions or

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bisulfite anions) with holes or hydroxyl radicals are the active species for MO photodegradation

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using BiOBr under visible light. In addition to the BiOBr/MO system, the sulfite-assisted

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photocatalysis approach has been successfully demonstrated in BiOBr/rhodamine B (RhB),

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BiOBr/phenol, BiOI/MO, and Bi2O3/MO systems under visible light irradiation, as well as in

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TiO2/MO system under simulated sunlight irradiation. The developed method implies the

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potential of introducing external active species to improve photodegradation of organic 1 ACS Paragon Plus Environment


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pollutants and the beneficial use of air pollutants for the removal of water pollutants since sulfite

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is a waste from flue gas desulfurization process.

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Introduction

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As an essential element for life from the perspectives of ecology and biology, water is

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circulated through a natural hydrologic cycle with self-cleaning capability. However, tremendous

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amounts of industrial and municipal wastewaters and landfill leachates make advanced water

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treatment techniques indispensable to maintain not only considerable available water supply but

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also a healthy large-scale hydrologic cycle. Removal of organic pollutants is one critical step in

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water treatment because many organic pollutants cause severe threats to health. For example,

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azo-dyes such as methyl orange (MO) are highly mutagenic and carcinogenic.1, 2 Currently well-

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developed techniques for organic pollutants removal include adsorption,3 biodegradation,4

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chlorination,5 ozonation,6 combined coagulation/flocculation using water treatment plant sludge,7

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electrochemical oxidation,8 and reverse osmosis,9 etc. Apart from those techniques,

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heterogeneous photocatalysis is a process for water treatment with the primary feature that it

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utilizes free and inexhaustible solar energy, implying its great potential as a low-cost,

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environmental friendly and sustainable treatment technology. As the model photocatalysis

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system, TiO2/UV has been widely demonstrated to be capable of removing various organic

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compounds in water.10 Nevertheless, UV light makes up only about 4% of the solar spectrum

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while approximately 40% of solar energy is in the visible region.11 Therefore, application of

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TiO2 in water treatment is hindered, because pristine TiO2 responds to only UV light due to its

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wide band gap (3.2 eV for anatase). To address this problem, various modifications have been

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applied to engineer TiO2, including doping, sensitization, forming heterostructures and coupling

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with π-conjugated architectures. At the same time, research efforts have been made to explore

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novel visible-light-active photocatalysts.12-18

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Within the exploration of novel visible-light-active photocatalysts, bismuth-based

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materials with low toxicity are appropriate candidates owing to the fact that Bi 6s in Bi(III) could

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hybridize O 2p levels to push up the position of valence band (VB), thus narrowing the band gap

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to harvest visible light.19 As layered ternary oxide semiconductors, bismuth oxyhalides (BiOXs,

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X=Cl, Br, I) are one category of bismuth-based materials and are crystallized in a tetragonal

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matlockite structure where [Bi2O2] slabs are interleaved with double halogen atom slabs along

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[001] direction.20 Recent studies of BiOXs focus on designing alloyed BiOX compounds with

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more than one type of halogen, like BiOCl/BiOBr and BiOBr/BiOI, and the ratio of each halogen

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could be modified to optimize the performance.21-24 BiOXs with specifically exposed facets, like

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(001) and (010) facets25, 26 and BiOX-based heterostructures, such as BN/BiOBr, BiOCl/Bi2O3,

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Ag/AgBr/BiOBr and BiOBr/Bi2MoO6,27-30 have also been studied extensively. These modified

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photocatalysts have shown exceptional photoactivity under visible light as measured by

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degradation of organic pollutants; therefore, it is promising to further study BiOXs for improved

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visible-light-driven photocatalysis.

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The fundamental mechanism of photocatalysis is based on production of photo-induced

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electron-hole pairs that react with oxygen and water to produce superoxide radicals and hydroxyl

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radicals, which are the typical active species that cause photodegradation of organic pollutants.

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Meanwhile, electrons and holes may react with other compounds in water to produce other active

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species, so providing such compounds provides a way to enhance contaminant degradation.

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Therefore, manipulating active species on carefully designed photocatalysts is an important

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aspect of study. The sulfite radical is an example of an active species that possess great reducing 3 ACS Paragon Plus Environment


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and oxidizing capabilities which can be applied to effectively degrade organics such as phenol,

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chlorpromazine, olefin, and polyunsaturated fatty acid.31-35 Sulfite radicals are usually created

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upon the photolysis of sulfite anions under middle UV light or via sulfite anions reacting with

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transition metal ions and other radicals like hydroxyl radicals.36-39 Since sulfur dioxide is a major

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air pollutant and sulfite is a product from flue-gas desulfurization process, it is an attractive idea

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to convert sulfite waste into useful sulfite radicals through a photocatalytic process and to

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enhance photodegradation of organic water pollutants.

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In this paper, we have developed a novel and generalized approach of introducing sulfite

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into photocatalyst/organic water pollutant systems and generating sulfite radicals to promote

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degradation of organics under visible light. This new concept of sulfite-assisted photocatalysis

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has been validated in BiOBr/MO, BiOBr/RhB, BiOBr/phenol, BiOI/MO, and Bi2O3/MO systems

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under visible light, and TiO2/MO under simulated sunlight. A possible mechanism is that sulfite

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anions react with photo-induced holes or hydroxyl radicals to produce sulfite radicals that attack

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organic molecules. Considering that sulfur dioxide is a major air pollutant and sulfite is a waste

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from flue-gas desulfurization process, the sulfite-enhanced photodegradation method may be

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developed into a new and cost-effective technology of beneficial use of air pollutants for the

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removal of water pollutants.

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Materials and Methods

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Materials

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Bismuth nitrate pentahydrate (Bi(NO3)3•5H2O, ≥98.0%) and sodium sulfide nonahydrate

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(Na2S•9H2O, ≥98.0%) were obtained from Aldrich-Sigma and potassium bromide (KBr) was

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obtained from Beantown Chemical Inc (New Hampshire, US). Ethylenediaminetetraacetic acid


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(EDTA, ≥99.4%), isopropyl alcohol (C3H8O, ≥99.5%), acetic acid (C2H4O2, ≥99.7%), potassium

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iodide (KI, ≥99.0%), and hydrochloric acid (HCl, 37.5%) were purchased from BDH Middle

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East LLC (Dubai, UAE). Sodium sulfite anhydrous (Na2SO3, ≥98.0%) was purchased from

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Amresco (Ohio, US). Commercial TiO2 nanopowders, P25, were obtained from Evonik Corp

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(Essen, Germany). All reagents were used directly for the experiments without any further

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purification.

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Preparation of photocatalyst materials

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BiOBr microspheres were synthesized through a facile method at room temperature.40

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Typically, bismuth nitrate (3 mmol, 1.47 g) was dissolved in a mixed solution of deionized water

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(8.5 mL) and acetic acid (4.5 mL), followed by 15 min magnetic stirring to obtain a clear and

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transparent solution. Then KBr solution (3 mmol, 0.357 g in 10 mL deionized water) was added

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into the above solution under rigorous stirring and the mixture was stirred for another 30 min to

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ensure complete precipitation. The precipitate was collected by centrifuging and washed

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thoroughly with ethanol and deionized water for six times. The final product was dried at 80 °C

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in a vacuum furnace overnight.

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BiOI was synthesized using the same manner described above with KI as the iodine

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source. Bi2O3 was obtained by annealing the synthesized BiOBr sample in air at 550 °C for 2 hr.

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Materials characterization

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X-ray diffraction (XRD) was performed on a D8 Advance diffractometer (Bruker-AXS,

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Karlsruhe, Germany) using Cu Kα irradiation at 40 kV and 40 mA. XRD patterns from 10° to 80°

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2θ were recorded at room temperature. The step increment was set as 0.05° 2θ and the counting



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time per step was 1s. The surface morphology was obtained by an ultra-high resolution field-

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emission scanning electron microscope (FE-SEM, JEOL JSM7500F, Japan) equipped with a

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cold cathode UHV field emission conical anode gun. UV–vis diffuse reflectance spectra were

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measured on a Hitachi U4100 UV–vis-NIR Spectrophotometer (Japan) with Praying Mantis

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accessory. Micromeritics ASAP 2420 physisorption analyzer was used to determine the

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Brunauer–Emmett–Teller (BET) surface area at liquid nitrogen temperature (77.3 K). Prior to

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measurement, the BiOBr powder was degassed in vacuum at 100 °C for 12 hr.

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Photocatalytic degradation experiments

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Sulfite-enhanced visible-light-driven photocatalysis was evaluated by studying various

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photocatalyst/organics systems with or without sulfite under visible light irradiation. The light

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source was a 150 W solar simulator (Oriel® Sol1A, Newport) equipped with an UV cutoff filter

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to provide visible light (λ ≥ 400 nm). The distance between the liquid surface and the lamp

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housing exit was 18 cm. When BiOBr/MO was studied as the model system, 20 mg BiOBr was

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added to 40 mL 10 ppm MO solution. A certain amount of Na2SO3 was then added to the

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BiOBr/MO system along with HCl to keep the initial pH at 7.5. The same parameters were used

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to evaluate BiOI and Bi2O3 for MO degradation. For the experiments of RhB and phenol

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degradation, 0.5 g/L BiOBr was dispersed in a 20 ppm RhB solution and 1 g/L BiOBr in a 5 ppm

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phenol solution, respectively. In a separate experiment, 0.2 g/L P25 was loaded in a 40 ppm MO

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solution to probe the sulfite-enhanced photocatalysis under simulated sunlight. Prior to

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irradiation, the photocatalyst/organics suspension systems with or without sulfite were sealed

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with a parafilm and stirred at 400 rpm in the dark for 1 hr to ensure adsorption/desorption

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equilibrium. At given time intervals, a certain volume of the suspension sample was taken and


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centrifuged for subsequent analysis. The concentrations of MO and RhB were analyzed on a

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UV–Vis spectrophotometer (UV-2600, Shimadzu) at their characteristic wavelengths of 464 nm

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and 554 nm, respectively. The concentration of phenol was measured by a high-pressure liquid

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chromatography (HPLC-2030C, Shimadzu) equipped with a reversed phase C18 column from

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Kinetex (00F-4601-E0). A mixture of acetonitrile and deionized water, flowing at a rate of 1.0

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mL/min, was used as the mobile phase. The total organic carbon (TOC) level was measured by a

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TOC analyzer (TOC-5000A, Shimadzu).

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For degradation experiments under anaerobic condition, a 17 mL transparent cylindrical

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cell (35-PX-10 from Starna Cells Inc.) was used. The BiOBr/MO suspension with or without

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sulfite was firstly fed into the cell and degassed for 10 min. The suspension was then purged with

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Argon (15 mL/min) for 20 min, sealed and stirred in dark for 30 min. After 30 min visible light

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irradiation, the suspension was taken out for analysis.

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Detection of active species

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Relevant active species produced in the BiOBr/MO/sulfite system under visible light

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were identified by adding different types of scavengers. EDTA and isopropyl alcohol (IPA) were

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used as scavengers for holes or hydroxyl radicals, and sodium nitrate was used as an aqueous

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electron scavenger.41 The degradation of MO was monitored to study the inhibition effect.

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Results and Discussion

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Structure, morphology, and optical properties

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The XRD pattern of the prepared BiOBr sample is shown in Figure 1a, which indicates

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high crystallinity of the sample with diffraction peaks well indexed to the tetragonal structure of 7 ACS Paragon Plus Environment


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BiOBr (JCPDS File No. 73-2061). The morphology of the prepared BiOBr sample is shown in

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both low (Figure 1c) and high-magnification (Figure 1d) SEM images. The BiOBr sample has a

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3D hierarchical microspherical structure with an average diameter of 6 µm and it is constructed

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by densely stacked thin microplates originating from the layered structure of the bismuth

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oxyhalide. The microspheres are interconnected to form larger aggregates, which make them

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easy to collect after water treatment due to their large sizes. The space between the microplates

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within the microspheres can promote light trapping and thus enhance the photocatalytic

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performance.42 In addition, the BET surface area of the BiOBr sample was measured to be 4.71

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m2/g through N2 adsorption-desorption analysis (Figure S1).

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Since photoabsorption and photocatalytic performance are closely related to the band

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structure of photocatalysts, the optical properties of BiOBr microspheres were examined by UV-

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vis diffuse reflectance spectra and results are shown in Figure 1 (a) XRD patterns, (b) UV-vis

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diffuse reflectance (the inset gives the band gap that is 2.82eV) and (c, d) SEM images of the

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synthesized BiOBr microspheres. . The absorption edge lies at 440 nm and covers part of the

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visible light region. The optical absorption of a crystalline semiconductor near the band edge

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follows the formula α hv = A(hv − Eg ) n /2 , where α , v , Eg and A are the absorption coefficient,

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light frequency, band gap energy, and proportionality constant, respectively.43 The value of n is 4

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for BiOBr because of its indirect transition characteristics. The band gap energy ( Eg ) of BiOBr

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can be thus extracted from a plot of (αhv) versus hv and it is estimated to be 2.82 eV from the

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inset in Figure 1 (a) XRD patterns, (b) UV-vis diffuse reflectance (the inset gives the band gap

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that is 2.82eV) and (c, d) SEM images of the synthesized BiOBr microspheres. b. The flat-band

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potential energy of BiOBr was calculated based on Mulliken electronegativity theory:

1/2


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χ and Ee

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EVB = χ − E e + 0.5Eg , where EVB ,

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BiOBr and energy of free electrons on the hydrogen scale, respectively.40 The calcuated

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conduction band and valence band positions are 0.27 V and 3.09 V vs NHE, respectively.

are the valence band enenrgy, electronegativity of

179 180

Figure 1 (a) XRD patterns, (b) UV-vis diffuse reflectance (the inset gives the band gap that is

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2.82eV) and (c, d) SEM images of the synthesized BiOBr microspheres.

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Photocatalytic degradation of organic pollutants assisted by sulfite

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The photodegradation of MO under visible light was used as a probe to evaluate the

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performance of BiOBr microspheres and the enhancement brought by sulfite (Figure 2). In the

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absence of sulfite, only 25% of MO was degraded by BiOBr under visible light irradiation for 30

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min, while 70% removal was achieved with the addition of 5 mM sulfite (Figure 2a). The



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enhancement was more evident at high sulfite concentrations, where 90% and 96% removals

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were observed with 10 mM and 20 mM sulfite, respectively. Since 10 mM sulfite alone without

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photocatalyst yielded negligible degradation of MO (Figure 2a), it is confirmed that sulfite

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participated in the photocatalytic degradation process. The apparent pseudo-first-order rate

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constants were calculated by regression using a linearized, first-order decay model

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( − ln(C / C0 ) = kt , Figure 2b), where C0 is the initial concentration of MO, C is the

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concentration after irradiation for a certain time t, and k is the rate constant. As displayed in

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Figure 2c, the rate constant increased by about 10, 20, and 29 times upon the addition of 5, 10,

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and 20 mM sulfite, respectively. The improvement greatly depended on the concentration of

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sulfite, implying that the increase in the rate constant should be ascribed to sulfite. It was noted

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in Figure 2c that the rate constant increased linearly with the sulfite concentration when sulfite

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was less than 10 mM but the rate of improvement was less prominent at a higher sulfite

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concentration, implying that the rate of production of photo-induced electrons and holes is the

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rate limiting factor at high sulfite concentrations.

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As shown in Figure 2d, rapid decrease at the wavelength of 464 nm was achieved in the

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UV-vis absorption band of MO solution under visible light in the presence of BiOBr and 10 mM

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sulfite. Neither a shift of the absorption band nor an emergence of new absorption band was

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observed, indicating that MO was degraded and no other chromophoric molecules were

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produced. Besides high photoactivity, long term durability of the photocatalyst is also required

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for practical water treatment applications. Figure S2 indicates that there was no significant

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change in degradation efficiency after four cycles, demonstrating that BiOBr has high stability in

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the developed sulfite-assisted photocatalytic process.


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Figure 2 (a) Photocatalytic activities of BiOBr for the degradation of MO under visible light

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with/without Na2SO3, and (b) kinetic fitting. (c) The dependence of MO degradation rate

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constant on sulfite concentration. (d) Time resolved UV-vis absorption of MO solution under

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visible light with BiOBr and 10 mM Na2SO3. The initial concentration of MO was 10 ppm and

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the loading of BiOBr was 0.5 g/L.

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In addition to MO, RhB and phenol were also tested as target compounds to generalize

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the effect of sulfite on photocatalysis by BiOBr. In the absence of sulfite, the degradation of RhB

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was relatively slow (Figure 3a) and the gradual blue shifts of the absorption band were attributed



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to the removal of ethyl groups.44 After 30 min irradiation, RhB molecules were fully de-ethylated

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to the core structure of rhodamine and a new absorption band at 506 nm appeared.44 Interestingly,

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in the presence of 10 mM sulfite (Figure 3b), the absorption band of RhB at 554 nm decreased

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without a blue shift, implying a direct destruction of the core rhodamine structure for faster

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decolorization. However, the addition of sulfite did not improve the mineralization of RhB;

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rather, the mineralization was slightly decreased in the presence of sulfite according to the TOC

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removal result (Figure 3c). It is probably because sulfite scavenges holes and hydroxyl radicals

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that are believed to be essential of mineralization.44 This implies that the developed approach of

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sulfite-enhanced photocatalysis is as an efficient method for decolorization of wastewaters, while

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further studies on this approach are necessary to offer improvement in terms of mineralization.

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The photodegradation of phenol, a persistent water pollutant, was also examined to confirm the

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function of sulfite. Figure S3 shows that without sulfite there was only 5% photodegradation of

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phenol, while the addition of 20 mM sulfite resulted in 78% photodegradation after visible light

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irradiation for 30 min. All these results on MO, RhB, and phenol degradation show that adding

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sulfite is a powerful approach that can significantly enhance visible-light-driven photocatalysis

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using BiOBr as the model catalyst.

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Figure 3. Time resolved UV-vis absorption of RhB solution with BiOBr under visible light

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irradiation (a) in the absence and (b) presence of 10 mM Na2SO3; (c) TOC removal of 10 ppm

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RhB after 2 h visible light irradiation with or without Na2SO3. The initial concentration of RhB

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was 20 ppm and the loading of BiOBr was 0.5 g/L.

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We have further demonstrated that sulfite-enhanced photocatalysis could be achieved

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with several other photocatalysts besides BiOBr. The kinetics of MO degradation illustrated in

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Figure S4 show that the photocatalytic activities of Bi2O3, BiOI and P25 in the presence of 10

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mM sulfite far exceeded those in the absence of sulfite. Specifically, MO photodegradation by

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Bi2O3 under 30 min visible light irradiation was increased from 16% without sulfite to 92% with

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sulfite; the degradation by BiOI under 30 min visible light irradiation was increased from 40%

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without sulfite to 86% with sulfite; and the degradation by P25 under 15 min simulated sunlight

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irradiation was increased from 39% without sulfite to 92% with sulfite.

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Moreover, we conducted experiments using other oxidants such as H2O2 and K2S2O8 to

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evaluate if they would make similar contributions to enhanced photocatalysis as sulfite did. The

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results are shown in Figure 4. H2O2 alone without BiOBr did not oxidize MO, while the addition



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of H2O2 together with BiOBr slightly increased the photodegradation of MO compared with that

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without H2O2. This is because that H2O2 may react with photo-induced e-h pairs to produce more

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reactive radicals (e.g. superoxide or hydroxyl radicals) 45 to promote photocatalysis. In contrast,

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K2S2O8 as a strong oxidant46 directly oxidized MO, and the effect was the same with or without

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BiOBr, indicating that K2S2O8 did not participate in any photo-induced reactions. More

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significant enhancement was seen due to sulfite-enhanced photocatalysis when both NaSO3 and

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BiOBr were present, resulting in more than 90% MO degradation, compared to 80% and 45%

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degradation when K2S2O8 and H2O2 were added, respectively. This result clearly demonstrates

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the unique advantage of using sulfite-enhance photocatalysis for removal of organic pollutants in

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water.

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Based on all the above experimental results of using various photocatalysts (BiOBr,

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Bi2O3, BiOI, and P25) to degrade various organic pollutants (MO, RhB, and phenol) under

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different illumination conditions (simulated sunlight and visible light), we believe that sulfite-

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enhanced photocatalysis is applicable as a general approach to promote degradation of organic

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water pollutants.

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Figure 4. Photodegradation of MO under visible light for comparison of 10 mM Na2SO3, H2O2

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and K2S2O8. The initial concentration of MO was 10 ppm and the loading of BiOBr was 0.5 g/L.

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Mechanism of sulfite-enhanced photocatalysis

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Understanding the mechanism of sulfite-enhanced photocatalysis, including the roles of

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sulfite, types of reactive species, and possible formation pathways of the reactive species will

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help extend this novel approach to broader photocatalytic applications. The mechanism was

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investigated using the BiOBr/MO/sulfite system by addressing the following three questions.

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Does the addition of sulfite promote oxidization or reduction of MO? What are the reactive

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species and how are they generated from sulfite and the photocatalyst? Are other active species

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like oxygen important to sulfite-assisted photodegradation?

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Na2SO3 and Na2S have been widely used as sacrificial agents for photoelectrochemical

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hydrogen generation due to their hole-scavenging capability.47 Thus, if sulfite-enhanced

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photocatalysis was mainly due to the effect of hole scavenging and promoting the availability of

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electrons for reduction reactions, other scavengers like EDTA and Na2S should play the same



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role. However, as indicated in Figure 5, both EDTA and Na2S almost completely inhibited the

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photodegradation of MO by BiOBr. Therefore, this suggests that holes played an important role

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in MO degradation and that the main function of sulfite is not to simply scavenge holes but

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possibly to produce more active species.

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We believe that the active species responsible for increased MO degradation is the sulfite

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•radical ( SO3 ), and its capability of reaction with organics has been reported in the literature.35

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To exclude the contributions of other possible active species involved in the photodegradation

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••process, scavenging experiments of aqueous electron, SO 4 / SO5 , and oxygen were also

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conducted. The possibility of aqueous electrons reacting with MO was studied using nitrate as an

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electron scavenger. Figure 5 shows that the addition of 20 mM nitrate to 10 mM sulfite solution

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did not have significant effect on MO photodegradation compared to that without nitrate. This

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result indicates that aqueous electrons did not make a substantial contribution to the sulfite-

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assisted enhancement in photocatalysis. To probe other sulfite related active species, such as

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SO•-4 and SO•-5 , that may be produced by reactions of SO•-3 with oxygen via a free radical chain

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mechanism,48 experiments under anaerobic conditions in a sealed cell were conducted, and the

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results in Figure 5 show that sulfite greatly improved MO degradation at about the same level in

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the absence of oxygen as in the presence of oxygen. Therefore, sulfite radical itself, rather than

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its reaction products with oxygen, was responsible for MO degradation.

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•There are two possible pathways for the formation of SO3 . In the first pathway, sulfite

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anions are oxidized by photo-induced holes directly. This is feasible, since the redox potential of


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SO•-3 / SO32- couple is 0.75 V (vs NHE, at pH=7),49 which is much lower than the VB position of

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BiOBr (3.09 V):

307

+ hVB + SO32− → SO3•−

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The second pathway to produce sulfite radicals is through reaction of sulfite anions with

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hydroxyl radicals ( OH )50. This reaction is also feasible, because the redox potential of

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(1)

•

•

OH/OH− (2.18 V vs NHE, at pH=7)51 is higher than that of SO•-3 / SO32- (0.75 V):

311 312

Figure 5. MO concentration percentages left after 30 min visible light irradiation in the absence

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or presence of various scavengers under aerobic or anaerobic conditions. The initial

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concentration of MO was 10 ppm and the loading of BiOBr was 0.5 g/L.

315 316

317

•

+ hVB +H2O → •OH+H+

(2)

OH + SO32− → SO3•− + OH−

(3)



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318

To confirm that photo-induced holes contributed to the formation of sulfite radicals via

319

the two pathways as illustrated in Eqs (1-3), active species trapping experiments were conducted

320

in the presence of 10 mM sulfite with isopropyl alcohol (IPA) or EDTA used as a scavenger for

321

holes and hydroxyl radicals (Figure 5). In the BiOBr/MO/sulfite suspension, the addition of IPA

322

significantly inhibited the degradation of MO, especially when the concentration of IPA was 330

323

mM. Likewise, the addition of 2 mM EDTA to sulfite almost completely inhibited MO

324

degradation. This supports the hypothesis that the sulfite radical is the active agent and is

325

produced by reaction with holes or hydroxyl radicals. Nevertheless, it is possible that the effect

326

•of IPA is not as a hydroxyl radical scavenger, but that it reacts with SO3 or with sulfite/bisulfite

327

and thus impeded the degradation of MO. To investigate this, the UV-vis absorption band of

328

SO32- (200 to 260 nm) was monitored in the BiOBr/MO system under visible light irradiation to

329

indicate the dependence of sulfite consumption rate upon IPA addition, and the results are shown

330

in Figure S5. In the absence of sulfite there was no absorbance band from 200 to 260 nm (Figure

331

S5a), while a clear absorption band appeared upon the addition of sulfite and the absorbance

332

decreased with time (Figure S5b), indicating the consumption of sulfite upon illumination. When

333

IPA was added at a concentration of 15 mM (Figure S5c), the rate of sulfite loss slowed down,

334

and this slowdown becomes very obvious at 330 mM IPA (Figure S5d). This result suggests that

335

IPA did not react with sulfite/bisulfite or sulfite radicals but rather competed with sulfite for

336

holes or hydroxyl radicals. It further supports the hypothesis that sulfite radicals were generated

337

by sulfite/bisulfite anions reacting with holes or hydroxyl radicals following Eqs. (1-3).

338

All of these results support the hypothesis that the sulfite radical is the main active

339

species in degrading MO in the sulfite-promoted photodegradation process. These sulfite radicals

340

are derived from sulfite/bisulfite anions reacting with holes or hydroxyl radicals. The overall 18 ACS Paragon Plus Environment

Page 19 of 25


341

process is illustrated in Figure 6. It should be noted that although holes and hydroxyl radicals can

342

attack MO molecules directly,10 they make little contribution to the degradation of MO compared

343

to sulfite radicals when sulfite is present, since the degradation rate is slow without sulfite

344

whereas significant enhancement of degradation is obtained upon the addition of sulfite.

345

Furthermore, the concentration of sulfite (10 mM) was much higher than that of MO (0.03 mM),

346

so it is more likely for holes and hydroxyl radicals to react with sulfite anions than with MO.

347

Although sulfite-enhanced photocatalysis has been successfully demonstrated in this work, the

348

pathways how sulfite radicals react with organic molecules are not clear. Further investigations

349

on sulfite radical generation using time-resolved spectroscopy are recommended, as well as the

350

identification of reaction intermediates and products in sulfite-promoted photodegradation of

351

organics by using chromatography and mass spectroscopy techniques such as GC-MS and LC-

352

MS.46, 52, 53

353 354

Figure 6. The proposed pathways of sulfite radicals formation and MO photodegradation under

355

visible light using BiOBr.

356

Supporting Information 19 ACS Paragon Plus Environment


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357

Nitrogen adsorption–desorption isotherms of BiOBr; results of cycling tests for MO degradation

358

by BiOBr; photocatalytic activity of BiOBr for phenol degradation; photocatalytic activity of

359

Bi2O3 and BiOI for MO degradation MO; and UV-vis absorption band changes of sulfite and

360

bisulfite.

361

Acknowledgement

362

This study was made possible by a grant from the Qatar National Research Fund under its

363

National Priorities Research Program award number NPRP 8-1406-2-605. The paper’s contents

364

are solely the responsibility of the authors and do not necessarily represent the official views of

365

the Qatar National Research Fund. The use of the Texas A&M University Materials

366

Characterization Facility is also acknowledged.

367

368

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Visible-light-driven photocatalytic degradation of organic water

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