Letter Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
pubs.acs.org/journal/ascecg
Solar-Driven Synchronous Photoelectrochemical Sulfur Recovery and Pollutant Degradation Jie Li,†,‡,§ Chang-Bin Chen,†,‡ Dan-Dan Wang,‡ Chen-Xuan Li,†,‡ Feng Zhang,†,‡ Dao-Bo Li,† Di Min,†,‡ Wen-Wei Li,*,†,‡ Paul K. S. Lam,*,‡,§ and Han-Qing Yu†,‡
ACS Sustainable Chem. Eng. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 07/26/18. For personal use only.
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CAS Key Laboratory of Urban Pollutant Conversion, Department of Chemistry, University of Science and Technology of China, Hefei 230000, China ‡ University of Science and Technology of China−City University of Hong Kong Joint Advanced Research Centre, Suzhou 215123, Jiangsu, China § State Key Laboratory in Marine Pollution, Department of Biology and Chemistry, City University of Hong Kong, Hong Kong, SAR, China S Supporting Information *
ABSTRACT: Hydrogen sulfide (H2S) is a hazardous contaminant in many industrial gases and wastewaters and a potential source of sulfur to be recovered, but effective and sustainable recovery technologies are still lacking. Here, we report a novel photoelectrochemical process for synchronous sulfur recovery and removal of organic pollutant, which typically coexist in waste streams, by using solar-simulating light as the sole driving force. In this system, sulfide was selectively converted into high-purity elemental sulfur (S0) particles at the photoanode, whereas efficient electrochemical oxidation of carbamazepine occurred at the cathode through Fe2+/Fe3+-mediated peroxymonosulfate activation. The formed sulfur particles with initial sizes of below one micrometer gradually grew into larger particles. Iodine ions were used as anodic redox mediator to favor a selective S0 production in the solution over the formation of sulfite/sulfate at the electrode surface. The practical feasibility of this system was demonstrated by using carbamazepine-spiked lake water samples. Our work suggests a great opportunity for sustainable recovery of sulfur resource with concomitant benefits of pollutant control by using the inexhaustible solar energy. KEYWORDS: Photoelectrochemical system, Hydrogen sulfide, Sulfur recovery, Selective oxidation, Carbamazepine, Solar light
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INTRODUCTION Hydrogen sulfide (H2S) is a toxic gas widely present in many industrial gases (especially the oil refinery), wastewaters and even in natural water bodies.1,2 In particular, the H2S content in natural gas can reach up to 10% in many gas fields.3,4 The uncontrolled production and release of H2S frequently results in catalyst deactivation, equipment corrosion, air pollution and health problems.5−8 H2S removal is thus essential from an economic, environmental and safety point of view. A more attractive route is to convert H2S into elemental sulfur (S0),9 which can be recovered as a valuable raw material for chemical engineering, lithium−sulfur batteries fertilizers, pharmaceuticals, and pesticides industries.10−13 Several technologies for recovering S0 from H2S have been established so far. The conventional Claus process14 and various electrochemical oxidation technologies enable an efficient S 0 recovery. 15−17 However, the high energy consumption, extreme reaction conditions, and the poor stability of added organic redox mediators which are used to mitigate electrode passivation by the deposited sulfur,18−22 © XXXX American Chemical Society
restrict their widespread application. Photoelectro-catalysis offers a more sustainable alternative by utilizing solar energy as the extra driving force for H2S splitting. Inorganic salts (e.g., Fe2+/Fe3+, I−/I3−, or fluorescein/fluorescein+ redox couples) are typically added as mediators to boost the electrode reactions.23−25 However, the low quality (due to mixing with H2S) and poor usability (due to storage and transport difficulty) of the resulting hydrogen gas constrains the practical utility of this approach. More effective and sustainable sulfur recovery technologies are still needed. Instead of splitting H2S for simultaneous production of sulfur and hydrogen, here we propose a novel photoelectrochemical sulfur recovery process devoid of H 2 production. In this system, the photoelectrochemical production of S0 is coupled with an electrochemical oxidation of organic pollutants, which usually coexist with H2S in the waste Received: June 7, 2018 Revised: July 16, 2018 Published: July 23, 2018 A
DOI: 10.1021/acssuschemeng.8b02678 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Letter
ACS Sustainable Chemistry & Engineering
μmol Fe2+ were generated during this period. In contrast, the unilluminated controls generated much less I2 and Fe2+ (Figure 1A), which were formed spontaneously through the following reaction (eq 1):
streams from many industries, under solar-simulating light. The organic pollutant oxidation was realized through electrochemical activation of peroxymonosulfate (PMS) mediated by Fe2+/Fe3+ loop at the cathode.26,27 In this proof-of-concept study, carbamazepine (CBZ) was used as a model of recalcitrant organic pollutant, whereas H2S or Na2S was used as the sulfide source.
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2Fe3 + + 3I− = 2Fe2 + + I−3
(1)
MATERIALS AND METHODS
Reactor Configuration. A dual-chamber glass photoelectrochemical cell (working volume of 120 mL for each chamber) was used. Titanium dioxide (TiO2)-loaded carbon paper (CP) (2 cm × 2.5 cm), prepared by directly brushing TiO2 powder on a CP,28 was used as the photoanode. A bare CP of the same size was used as the cathode. The electrodes were connected through a lead wire external circuit. A proton exchange membrane (PEM, Nafion 117, DuPont Inc., USA) was installed between the two chambers. The anodic chamber had a quartz window (3 cm × 4 cm) to allow light transmission. A xenon lamp (BL-GHX-Xe-300, Xi’an BILON Biological Technology Co.) with a full wavelength range simulating solar light was used as the light source. Evaluating the Photoelectrochemical Activities of the Redox Couples. To evaluate the electrochemical activities of the redox couples in the photoelectrochemical system, KI (0.1 M) was used as anolyte and a mixture of NaCl (0.1 M) and FeCl3 (1 mM) was used as catholyte (Supporting Information). The accumulation of I3− and Fe2+ in individual chambers was monitored. The photocurrent response of the anode was measured by an electrochemical workstation (CHI 760D, Chenhua Instrument Co., China) connected to a three-electrode cell, which contained an Ag/ AgCl reference electrode, a TiO2/CP working electrode and a CP counter electrodes. The potential of TiO2/CP electrode was set at −0.197 V. Repeated light on−off cycles (5 min−5 min) were applied, and the responsive photocurrent and Fe2+ concentration were recorded. Evaluating the Sulfur Recovery and Pollutant Removal Performance of the System. The sulfur recovery and CBZ degradation in the photoelectrochemical system was evaluated under similar operating conditions as described above with small modifications (Supporting Information). All the above experiments were conducted at room temperature (∼25 °C). Chemical Analysis. The morphology and elemental composition of the formed particles were characterized by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The valence states were analyzed by X-ray photoelectron spectroscopy (XPS). The Fe2+ concentration was determined by the ophenanthroline spectrophotometric method.29 Molecular iodine (I2) was quantified by the starch chromogenic method described in the Supporting Information. The sulfite and sulfate concentrations were measured with an ion chromatography system (DONEX ICS-1000), The sulfide concentrations was quantified by iodometric method.30 To test the practical utility of the system, filtrated lake water collected from Du Shu Lake (Suzhou, China) was used. The filtered lake water was characterized with a fluorescence spectrometer (HORIBA Aqualog). The CBZ quantification method is described in the Supporting Information.31
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(ΔG = − 45.2 kJ/mol)
Figure 1. Redox mediators regeneration and photocurrent generation in the photoelectrochemical system without sulfide and CBZ addition. (A) Influence of continuous irradiation on Fe2+ and I2 accumulation. (B) Photocurrent intensity and Fe2+ accumulation during periodic irradiation.
The photoenhanced electrochemical process was also directly shown by the high photocurrent (0.25 mA) relative to the current generatd under dark condition (−0.05 mA) during the repeated light on−off cycles (Figure 1B). There was obvious Fe2+ accumulation during each light cycle, with the cathodic columbic efficiencies reaching 91.4%−96.2%. The continuously irradiated system produced over 2-fold more Fe2+ and I3− than the dark control after 2 h of reaction (Figure 1B). These results confirm the solar-driven generation of I3− and Fe2+, which are important intermediates for the subsequent H2S oxidation and PMS activation reactions, respectively. Photoinduced Elemental Sulfur Formation in the Anodic Chamber. When H2S was bubbled into the anodic chamber, yellow precipitate gradually appeared in the bulk solution (Figure 2A), but the electrode surface remained clean (Figure S2C). In contrast, sulfur particles rapidly accumulated on the anodic surface in the KI-free control shortly after the reaction (Figure S2A,B), indicating a severe electrode fouling which is common for electrochemical S recovery process. The SEM and EDS analyses of formed precipitate in the I−mediated system confirmed the formation of plate-shaped, Srich particles (Figure 2A,B). S0 was identified as the dominant
RESULTS AND DISCUSSION
Redox Activities of the I−/I3− and Fe2+/Fe3+ Couples. In the photoelectrochemical system, the anodic H2S oxidation was mediated by the I−/I3− redox couple, whereas the cathodic PMS activation was mediated by the Fe2+/Fe3+ couple. An efficient interplay between the I−/I3− and Fe2+/Fe3+ couples is needed. We observed obvious accumulation of I2 and Fe2+ in the individual chambers during 2 h of illumination. The analyte color changed from transparent to light yellow resulting from I2 formation (Figure S1). Approximately 3.9 μmol I2 and 8.7 B
DOI: 10.1021/acssuschemeng.8b02678 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Letter
ACS Sustainable Chemistry & Engineering
Figure 2. Characteristics of the recovered S particles: (A) SEM image; (B) EDS spectrum; (C) particle size distribution; (D) XPS spectrum.
content of the particles according to the XPS analysis (Figure 2D). The average particle size grew gradually from below one micrometer (Figure 2C) to tens of micrometer due to chemical instability of the sulfur particles (zeta potential −16.9 mV) (Figure 2A). A weak XPS signal of sulfite/sulfate was detected in the precipitate (Figure 2D), indicating that a small amount of sulfite/sulfate was also formed from sulfide overoxidation. Such SO32−/ SO42− generation should be resulted from a direct oxidation of S2− or the deposited S0 at the anode surface.32 No SO32−/ SO42− was detected in the I−-mediated system after 2-h reaction, but 4.8% of the S2− was oxidized to SO42− in the I−free control during the same period (Figure S3A). Consistently, the I−-free control had more residual S2− (28.8%) than the I−-mediated system (10.0%) (Figure S3B). These results suggest that an otherwise overoxidation of S2− to SO32−/ SO42− at the electrode surface could be effectively suppressed by I−. In the I−-mediated system, the sulfur recovery ratio reached 95.9% and the selectivity was almost 100% when 1 mM Na2S was used as the sulfide source. Therefore, I− played triple roles in favoring S recovery: more efficient removal of S2−; producing higher-purity S0; mitigating electrode fouling. CBZ Degradation in the Cathodic Chamber. The CBZ was efficiently removed (up to 91.6% within 1 h) at the cathode in the photoelectrochemical system, obviously outperforming the open-circuit control (64.8%). This result highlights the coupling between photocatalytic and electrochemical processes.33 The CBZ removal was significantly impaired in the Fe3+-free and unilluminated controls (Figure 3, Figure S4A), indicating that the electrochemically regenerated Fe2+ was critically involved in CBZ degradation. The severely declined CBZ removal after addition of ethanol (as a quencher
Figure 3. Profiles of CBZ decomposition in the photoelectrochemical system under various operating conditions: no PMS in the cathodic electrolyte (PMS-free); no Fe3+ ions in the cathodic electrolyte (Fe3+free); without the external circuit connected (open circuit); with the external circuit connected (closed circuit).
of hydroxyl and sulfate radicals) (Figure S4B) confirmed the involvemet of radicals in the CBZ degradation process. Therefore, Fe2+ served as an efficient catalyst for PMS activation and CBZ degradation. There generation of Fe2+ was stimulated by light irradiation in our system. To test the system performance for practical environmental application, real lake water was used as the cathodic electrolyte. One concern is that the natural organic matters may complex with iron ions and compete with target pollutant for the oxidizing power. The 3D fluorescence spectrum confirmed the abundant presence of humic acid-like substance and proteinlike substance in the lake water34 (Figure S5). The CBZ degradation was still enhanced under illuminated condition (Figure S6), implying that our system can also be effectively used in real water environment. C
DOI: 10.1021/acssuschemeng.8b02678 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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ACS Sustainable Chemistry & Engineering
(5) Chen, X.; Jiang, J.; Yan, F.; Li, K.; Tian, S.; Gao, Y.; Zhou, H. Dry Reforming of Model Biogas on a Ni/SiO2 Catalyst: Overall Performance and Mechanisms of Sulfur Poisoning and Regeneration. ACS Sustainable Chem. Eng. 2017, 5 (11), 10248−10257. (6) Henshaw, P. F.; Zhu, W. Biological conversion of hydrogen sulphide to elemental sulphur in a fixed-film continuous flow photoreactor. Water Res. 2001, 35 (15), 3605−3610. (7) Andriamanohiarisoamanana, F. J.; Sakamoto, Y.; Yamashiro, T.; Yasui, S.; Iwasaki, M.; Ihara, I.; Tsuji, O.; Umetsu, K. Effects of handling parameters on hydrogen sulfide emission from stored dairy manure. J. Environ. Manage. 2015, 154, 110−116. (8) Tippayawong, N.; Thanompongchart, P. Biogas quality upgrade by simultaneous removal of CO2 and H2S in a packed column reactor. Energy 2010, 35 (12), 4531−4535. (9) Cai, J.; Zheng, P.; Qaisar, M.; Zhang, J. Elemental sulfur recovery of biological sulfide removal process from wastewater: A review. Crit. Rev. Environ. Sci. Technol. 2017, 47 (21), 2079−2099. (10) Chung, W. J.; Griebel, J. J.; Kim, E. T.; Yoon, H.; Simmonds, A. G.; Ji, H. J.; Dirlam, P. T.; Glass, R. S.; Wie, J. J.; Nguyen, N. A.; Guralnick, B. W.; Park, J.; Somogyi, A.; Theato, P.; Mackay, M. E.; Sung, Y. E.; Char, K.; Pyun, J. The use of elemental sulfur as an alternative feedstock for polymeric materials. Nat. Chem. 2013, 5 (6), 518−524. (11) Parcell, S. Sulfur in human nutrition and applications in medicine. Altern. Med. Rev. 2002, 7 (1), 22−44. (12) Zhou, L.; Ding, N.; Yang, J.; Yang, L.; Zong, Y.; Liu, Z.; Yu, A. Sulfur Encapsulated in Mo4O11-Anchored Ultralight Graphene for High-Energy Lithium Sulfur Batteries. ACS Sustainable Chem. Eng. 2016, 4 (7), 3679−3687. (13) Zhu, L.; You, L.; Zhu, P.; Shen, X.; Yang, L.; Xiao, K. High Performance Lithium−Sulfur Batteries with a Sustainable and Environmentally Friendly Carbon Aerogel Modified Separator. ACS Sustainable Chem. Eng. 2018, 6 (1), 248−257. (14) Nabgan, W.; Abdullah, T. A. T.; Nabgan, B.; Ripin, A.; Kidam, K. B.; Saeh, I.; Moghadamian, K. A Simulation of Claus Process Via Aspen Hysys for Sulfur Recovery. Chem. Prod. Process Model. 2016, 11 (4), 273−278. (15) Dutta, P. K.; Rabaey, K.; Yuan, Z.; Rozendal, R. A.; Keller, J. Electrochemical sulfide removal and recovery from paper mill anaerobic treatment effluent. Water Res. 2010, 44 (8), 2563−2571. (16) Selvaraj, H.; Chandrasekaran, K.; Gopalkrishnan, R. Recovery of solid sulfur from hydrogen sulfide gas by an electrochemical membrane cell. RSC Adv. 2016, 6 (5), 3735−3741. (17) Pujare, N. U.; Tsai, K. J.; Sammells, A. F. An Electrochemical Claus Process for Sulfur Recovery. J. Electrochem. Soc. 1989, 136 (12), 3662−3678. (18) Huang, H.; Yu, Y.; Chung, K. H. Recovery of Hydrogen and Sulfur by Indirect Electrolysis of Hydrogen Sulfide. Energy Fuels 2009, 23 (9), 4420−4425. (19) Kim, K.; Song, D.; Han, J.-I. A liquid redox sulfur recovery process based on heteropoly molybdophosphate (HPMo) with electricity generation. Chem. Eng. J. 2014, 241, 60−65. (20) Li, K.-T.; Yen, C.-S.; Shyu, N.-S. Mixed-metal oxide catalysts containing iron for selective oxidation of hydrogen sulfide to sulfur. Appl. Catal., A 1997, 156 (1), 117−130. (21) Zhai, L.-F.; Song, W.; Tong, Z.-H.; Sun, M. A fuel-cell-assisted iron redox process for simultaneous sulfur recovery and electricity production from synthetic sulfide wastewater. J. Hazard. Mater. 2012, 243 (Supplement C), 350−356. (22) Sun, M.; Song, W.; Zhai, L.-F.; Cui, Y.-Z. Effective sulfur and energy recovery from hydrogen sulfide through incorporating an aircathode fuel cell into chelated-iron process. J. Hazard. Mater. 2013, 263 (Part 2), 643−649. (23) Zong, X.; Han, J.; Seger, B.; Chen, H.; Lu, G. M.; Li, C.; Wang, L. An integrated photoelectrochemical-chemical loop for solar-driven overall splitting of hydrogen sulfide. Angew. Chem., Int. Ed. 2014, 53 (17), 4399−4403. (24) Jing, X.; Yang, Y.; He, C.; Chang, Z.; Reek, J. N. H.; Duan, C. Control of Redox Events by Dye Encapsulation Applied to Light-
This study demonstrated the feasibility of the solar-driven recovery of S0 particles from H2S coupled with pollutant degradation. The overall process is self-sustainable and is not plagued by the problem of high ferrous consumption common for the existing Fenton processes, providing a promising new platform for the recovery of sulfide from waste streams with concomitant pollutant removal. Nevertheless, to bring it into practical application, the recovery efficiency will need further improvement, and the system stability in long-term operation should be evaluated. Although I− is a common pollutant in water environment, more efficient and environmentally friendly redox mediators are still to be explored. Our system may preferably be used for treatment of various sulfidecontaining waste gases, including natural gas, oil refinery waste gas, and anaerobic biogases.35,36 It may also find application in sulfide-rich aquatic environments (e.g., marine sediment) for in situ remediation, which warrants future investigations.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b02678.
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Additional methods (PDF)
AUTHOR INFORMATION
Corresponding Authors
*Prof. Wen-Wei Li, Fax: +86 551 63601592, E-mail: wwli@ ustc.edu.cn. *Prof. Paul K. S. Lam, Fax: +852 3442-0522, E-mail: bhpksl@ cityu.edu.hk. ORCID
Wen-Wei Li: 0000-0001-9280-0045 Paul K. S. Lam: 0000-0002-2134-3710 Han-Qing Yu: 0000-0001-5247-6244 Author Contributions
Jie Li and Chang-Bin Chen contributed equally to this work. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China (21590812 and 51778597), the Collaborative Innovation Center of Suzhou Nano Science and Technology of the Ministry of Education of China for supporting this work.
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REFERENCES
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DOI: 10.1021/acssuschemeng.8b02678 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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ACS Sustainable Chemistry & Engineering Driven Splitting of Hydrogen Sulfide. Angew. Chem., Int. Ed. 2017, 56 (39), 11759−11763. (25) Luo, T.; Bai, J.; Li, J.; Zeng, Q.; Ji, Y.; Qiao, L.; Li, X.; Zhou, B. Self-Driven Photoelectrochemical Splitting of H2S for S and H2 Recovery and Simultaneous Electricity Generation. Environ. Sci. Technol. 2017, 51 (21), 12965−12971. (26) Ghanbari, F.; Moradi, M. Application of peroxymonosulfate and its activation methods for degradation of environmental organic pollutants: Review. Chem. Eng. J. 2017, 310, 41−62. (27) Lin, H.; Wu, J.; Zhang, H. Degradation of clofibric acid in aqueous solution by an EC/Fe3+/PMS process. Chem. Eng. J. 2014, 244, 514−521. (28) Yuan, S.-J.; Sheng, G.-P.; Li, W.-W.; Lin, Z.-Q.; Zeng, R. J.; Tong, Z.-H.; Yu, H.-Q. Degradation of Organic Pollutants in a Photoelectrocatalytic System Enhanced by a Microbial Fuel Cell. Environ. Sci. Technol. 2010, 44 (14), 5575−5580. (29) Harvey, A. E.; Smart, J. A.; Amis, E. S. Simultaneous Spectrophotometric Determination of Iron(ll) and Total Iron with 1,10-Phenanthroline. Anal. Chem. 1955, 27 (1), 26−29. (30) Wen-chao, Z.; Yong, S.; Li-chao, A.; Guang-dong, L. Improvement on determination of sulfide in waste water by Iodometry. PTCA Part B: Chem. Anal. 2017, 53 (1), 68−72. (31) Mowafy, H. A.; Alanazi, F. K.; El Maghraby, G. M. Development and validation of an HPLC−UV method for the quantification of carbamazepine in rabbit plasma. Saudi Pharm. J. 2012, 20 (1), 29−34. (32) Li, W. H.; Lei, L.; Yang, Y. N.; Yan, W. Combined sulphur cycle based system of hydrogen production and biological treatment of wastewater. Environ. Technol. 2009, 30 (12), 1297−1304. (33) Zou, J.; Ma, J.; Chen, L.; Li, X.; Guan, Y.; Xie, P.; Pan, C. Rapid acceleration of ferrous iron/peroxymonosulfate oxidation of organic pollutants by promoting Fe(III)/Fe(II) cycle with hydroxylamine. Environ. Sci. Technol. 2013, 47 (20), 11685−11691. (34) Zhu, G.; Bian, Y.; Hursthouse, A. S.; Wan, P.; Szymanska, K.; Ma, J.; Wang, X.; Zhao, Z. Application of 3-D Fluorescence: Characterization of Natural Organic Matter in Natural Water and Water Purification Systems. J. Fluoresc. 2017, 27 (6), 2069−2094. (35) Pikaar, I.; Likosova, E. M.; Freguia, S.; Keller, J.; Rabaey, K.; Yuan, Z. Electrochemical Abatement of Hydrogen Sulfide from Waste Streams. Crit. Rev. Environ. Sci. Technol. 2015, 45 (14), 1555−1578. (36) Koh, S.-H.; Shaw, A. Gaseous Emissions from Wastewater Facilities. Water Environ. Res. 2016, 88 (10), 1249−1260.
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DOI: 10.1021/acssuschemeng.8b02678 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
