- Email: [email protected]
Electrochemical regeneration of sulfur loaded electrodes
Electrochemistry Communications 11 (2009) 1437–1440
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Electrochemical regeneration of sulfur loaded electrodes Paritam K. Dutta, René A. Rozendal, Zhiguo Yuan, Korneel Rabaey, Jürg Keller * The University of Queensland, Advanced Water Management Centre (AWMC), St. Lucia, QLD 4072, Australia
a r t i c l e
i n f o
Article history: Received 12 April 2009 Received in revised form 8 May 2009 Accepted 12 May 2009 Available online 18 May 2009 Keywords: Sulfide Electrode Regeneration Polysulfide Wastewater treatment
a b s t r a c t Electrochemical oxidation of sulfide is a promising technique for its removal from wastewaters. Generally, the main product of this oxidation reaction is elemental sulfur. The latter deposits as a solid on the electrode and deactivates it. Therefore, an efficient and effective regeneration technique of sulfur loaded electrodes is required for the practical implementation of this technology. Here we demonstrate a method for in situ reduction of electrodeposited sulfur on carbon fibre electrodes to sulfide/polysulfides, at low energy input. The intermediary coulombic efficiency (CE) values strongly depend on pH and buffer capacity of the solution. These values were recorded up to 435 ± 5% due to simultaneous chemical dissolution of sulfur to polysulfides under alkaline conditions. This process demonstrates the potential for continuously removing dissolved sulfide from wastewaters at an anode as sulfur and recovering the deposited sulfur based on regular switching between anodic sulfide oxidation and cathodic sulfur reduction. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Sulfide is found in many domestic and industrial wastewaters and often needs to be removed because of its (eco)toxicity, corrosive properties, and unpleasant odour. Although several physicochemical and biological sulfide removal technologies are already available, there is a need for more cost-effective and efficient methods [1]. Electrochemical techniques for sulfide removal from wastewater might offer advantages over other methods in terms of selectivity and energy efficiency. Sulfide is an electro-active compound and can be removed by electrochemical oxidation either in fuel cell or electrolysis cell mode [2,3]. Elemental sulfur has been found to be the main final product from aqueous sulfide oxidation [3,4]. Unfortunately, the elemental sulfur produced deposits onto the electrode and deactivates it over time [2]. This is considered a major limitation of the process. A technique is needed to remove the solid elemental sulfur from the electrode surface in order to enable practical implementation of this technology [4]. Limited studies such as organic solvent extraction of sulfur with subsequent solvent evaporation, controlled sulfur precipitation in the bulk solution with alkali addition at elevated temperature were conducted to prevent sulfur induced electrode deactivation or to regenerate sulfur loaded electrodes [5,6]. However, these processes are not considered sustainable due to the involvement of toxic organic solvents, high energy requirements and restricted applications. * Corresponding author. Tel.: +61 7 3365 4727; fax: +61 7 3365 4726. E-mail address: [email protected] (J. Keller). 1388-2481/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2009.05.024
Here we propose an alternative approach for the regeneration of sulfur loaded electrodes, namely electrochemical regeneration. The narrow stability region of sulfur in its Pourbaix diagram [7] shows the theoretical possibility for further electrochemical oxidation or reduction of elemental sulfur, following a slight change of potential and pH. In a recent study, we demonstrated that sulfur electrodeposited on graphite granules is indeed electrochemically active [3]. Two main strategies can be considered for the electrochemical regeneration of sulfur loaded electrodes: (i) oxidizing the elemental sulfur to sulfate or (ii) reducing to sulfide and/or polysulfides. An inherent drawback of the first strategy is that it likely requires high anode potentials [3] on non-catalysed graphite/carbon electrodes, which are the preferred electrode materials for sulfide oxidation from an economic point of view. Furthermore, a high anode potential may also result in lower coulombic efficiencies due to competing reactions such as water oxidation and carbon corrosion. Additionally, sulfur oxidation to sulfate is a six-electron process and thus requires higher input of electrical current compared to sulfur reduction to sulfide, which involves transferring two electrons. Considering these drawbacks, it is hypothesized that sulfur reduction to sulfide is likely to be a more suitable regeneration strategy as it requires a lower current input, and consequently is expected to have a higher coulombic efficiency due to the simultaneous sulfur dissolution by the electrogenerated sulfide, leading to the formation of polysulfides in alkaline conditions [8]. Moreover, the cathodic reduction of sulfur can be coupled to the anodic oxidation (and thus removal) of sulfide from wastewaters. Such a process should allow for regular switching between anode and cathodic operations. In this case, the anode is removing sulfide
P.K. Dutta et al. / Electrochemistry Communications 11 (2009) 1437–1440
2. Experimental 2.1. Cell design and operation The cell was constructed as described elsewhere [3]. It consisted of identical rectangular anode and cathode chambers (volume 335 mL) separated by a cation exchange membrane (Ultrex, CM17000, MIC). Carbon fibre (SGL group) brush electrodes with a stainless steel core were used for the both chambers. A graphite rod (5 mm diameter) was used in the both chambers to connect the electrodes to the external circuit. The chamber contents were continuously recirculated over an external buffer flask (1 L) at a rate of 10 L h1. The cell was operated in batch mode and controlled galvanostatically using a potentiostat (VMP3, PAR). Each batch experiment lasted for about 40 h. In every alternate batch the cathode was used as anode and vice versa. The experiments were conducted at different buffer capacities (from 0 to 500 mM) of cathode solution. The cathode samples were collected at 2–4 h intervals to measure dissolved sulfur species. Before starting each experiment, the electrode used as cathode was loaded with an estimated sulfur amount of 12–15 kg m3 (of total anode/cathode volume) based on the observed conversion of known quantity of sulfide to sulfur [3]. Chemical polysulfides formation was examined by keeping the circuit open during a few batch experiments for 4–10 h with/without adding sulfide solution. For all experiments sulfide was used as the sole electron donor in the anode at varying concentrations (0.2–3.5 g S L1). The coulombic efficiency (CE) was calculated as the ratio between the amount of charge transfer theoretically required for obtaining the measured amount of sulfide and polysulfides (based on the two electron reduction of sulfur) and the measured amount of charge transferred in the process. All experiments were performed in triplicates.
a result, electrodeposited sulfur on the cathode was effectively reduced to sulfide and polysulfides at cathode potentials of 330 ± 5 mV (vs (standard hydrogen electrode, SHE)). As sulfide was used as the electron donor at the anode, the required cell voltage was low over the complete batch, i.e., between 0.2 and 0.35 V. Fig. 1A depicts the electrochemical production of sulfide/polysulfides from the reduction of electrodeposited sulfur at the cathode during a complete batch cycle. Polysulfides formation was observed due to the yellow coloration of the cathodic electrolyte. Sulfur reduction to sulfide most likely proceeded according to Eq. (1). This reaction consumed protons, causing an increased electrolyte pH, which stimulated the chemical formation of polysulfides according to Eq. (2) (Fig. 1). Polysulfides formation was confirmed during open circuit operation with sulfide and a sulfur loaded electrode at alkaline pH. During this experiment, the sulfide concentration and pH decreased and polysulfides were formed (data not shown). Polysulfides can also be formed as an intermediate during sulfur reduction to sulfide (Eq. (3)). However, all polysulfides were eventually reduced to sulfide upon completion of the batch cycle (Fig. 1 and Eq. (4)).
SðsÞ þ 2e þ Hþ ! HS ; HS þ
n1 S8 $ S2n þ Hþ 8
ð3Þ
5000
A 4000
3000
2000
1000
0 0
3. Results and discussion
ð1Þ ð2Þ
nSðsÞ þ 2e ! S2n ; E00 ¼ 0:319 ðn ¼ 5; S2n ¼ 1 M; pH ¼ 7Þ
2.2. Chemicals and chemical analyses
10
20
30
40
Time (hr) -0.2
B Cathode potential (V, SHE)
Na2S9H2O was used as a source of sulfide, Na2HPO4 and KH2PO4 as buffer and NaCl (1 g L1) as the supporting electrolyte. Sulfide, sulfate, thiosulfate and sulfite concentrations were measured by Ion Chromatography (IC), using a Dionex 2010i system [9]. To measure polysulfide concentrations, all dissolved sulfur species were oxidized to sulfate with H2O2 (after increasing the pH to around 12.5 with 0.4 M NaOH) [10]. The difference between the sulfate after H2O2 oxidation and sulfide measured before H2O2 oxidation was regarded as polysulfides. Sulfite and thiosulfate were detected (<10%) mainly in presence of polysulfides and these amounts taken as polysulfides assuming an artefact of sample preparation in the absence of any oxidant in the cathode. Microscopic images of carbon fibre were collected using a Scanning Electron Microscope (SEM) (JEOL JSM 6460LA) which was equipped with Energy Dispersion Spectrometry (EDS) used for the detection of elemental sulfur.
E00 ¼ 0:271 ðHS ¼ 1 M; pH ¼ 7Þ
-1
from wastewater and being loaded with sulfur, while the cathode, which was previously the anode, is regenerated. Therefore, the main purpose of this study is to examine the in-situ regeneration of sulfur loaded electrodes by reducing electrodeposited sulfur to sulfide/polysulfides. We also examined the influence of pH on the regeneration efficiency and the stability of an electrode during repetitive anode/cathode switches.
Concentration (mg-S L )
1438
-0.4
-0.6
-0.8
-1.0
-1.2 0
10
20
30
40
Time (hr)
3.1. Reduction of sulfur to sulfide/polysulfides The electrochemical cell was galvanostatically controlled at 200 mA (equivalent to 11.9 A m2 of membrane surface area). As
Fig. 1. (A) Sulfide-S ( ), polysulfide-S ( ), total-S (4) concentration and (B) evolved cathode potential over time during regeneration of sulfur loaded electrode at a current of 200 mA (11.9 A m2 of membrane surface area) and solution buffer capacity 5 mM.
1439
P.K. Dutta et al. / Electrochemistry Communications 11 (2009) 1437–1440 þ S2 n þ 2ðn 1Þe þ nH ! nHS ;
ðn ¼ 5;
E00 ¼ 0:255 S2 n
¼ HS ¼ 1; pH ¼ 7Þ
ð4Þ
During regeneration, the cathode potential slowly changed to more negative values, which was likely caused by the increase of sulfide/ polysulfides concentrations and pH. However, at the end of the regeneration cycle the potential suddenly dropped sharply (Fig. 1B), indicating almost complete reduction of all sulfur species (solid and dissolved) to sulfide as confirmed from the dissolved sulfur species analysis. This sudden potential drop was probably caused by a switch from sulfur/polysulfides reduction to hydrogen evolution. This rapid potential change clearly marks the end of a batch cycle and may be used for future monitoring of the process. For all batch tests, the total sulfur recovery as dissolved sulfur species from the known amount of deposited sulfur at the end of the batch cycle, i.e., after the sharp potential drop, was 91 ± 4%. The SEM images (Fig. 2) provide an additional indication of the regeneration of the electrode. EDS analysis (Fig. 2D) elucidated that the layer of solids deposited on the surface of the electrode (Fig. 2B) is sulfur. Visually, the carbon fibre surface after regeneration (Fig. 2A) looked similar as it was before sulfur deposition (Fig. 2C). This confirms for the almost complete removal of the deposited sulfur, but also demonstrates that the carbon fibre electrodes are not visually damaged in the process of sulfide oxidation and recovery. In addition to quantitative analysis and SEM pictures, the intermediary coulombic efficiency (CE) values as demonstrated in Fig. 3A and B provide more insight into the reaction mechanism of the regeneration process. The intermediary CE values depend
on the buffer capacity of the solution or pH values. CE values higher than 100% were likely caused by simultaneous chemical polysulfides formation (Eq. (2)) with the electrochemical reduction of sulfur to sulfide. The highest intermediary CE was found 435 ± 5% after four hours of operation in an experiment without phosphate buffer. This high intermediary CE is likely due to the rapid rise of the pH in absence of a pH buffer and the availability of large amounts of sulfur for polysulfides formation. However, at the end of each batch cycle, the generated polysulfides were further reduced to sulfide (Fig. 1) and the CE values approached 100%. The further reduction of polysulfides to sulfide was also confirmed by the disappearance of the yellow colour over the course of the batch experiment. The intermediary CE values at high buffer strength of 500 mM did not surpass 100% as the pH value only increased from 7 to 7.97 after 36 h of operation. Polysulfides exist in water only at pH > 6 and the average equilibrium constant of polysulfides, pKx at 21 °C has been determined as 8.82 for chemical sulfur [11]. For this reason, low polysulfides accumulation was observed for the 500 mM buffer experiment, with a maximum of 350 ± 40 mg polysulfide-S L1 compared to 3400 ± 100 mg polysulfide-S L1 for the experiment without buffer. 3.2. Polysulfide solutions as the preferred product of electrode regeneration Since chemical polysulfides formation can regenerate the electrode quickly with considerably higher CE, it is the preferred reaction. However, chemical polysulfides formation will not proceed if
A
B
C
S
30000
D
27000 24000
Counts
21000 18000 15000 12000 9000 6000 3000 0 0.00 0.30 0.60 0.90 1.20 1.50 1.80 2.10 2.40 2.70 3.00
keV Fig. 2. SEM pictures of carbon fibre electrodes used for electrochemical regeneration of sulfur: (A) before electro-deposition of sulfur, (B) after electro-deposition of sulfur, (C) after regeneration of the electrode, and (D) EDS spectrum of at the position indicated by the cross in Fig 1B.
1440
P.K. Dutta et al. / Electrochemistry Communications 11 (2009) 1437–1440
500
A % Coulombic Efficiency
400
300
200
100
0
0
10
20 Time (hr)
30
40
14
B
13
4. Conclusions
12
pH
anode from wastewater, when fully loaded with elemental sulfur, the anode can be switched over to cathode mode and be regenerated. In the presented work, so far the same electrode was used as both anode and cathode for over 15 cycles without any notable decrease of performance. The process performance needs to be further evaluated with real wastewater to assess the influence of other constituents. Although the cathodic regeneration process will not be affected as a clean freshwater solution will be used in the cathode, biofilm formation needs to be avoided in the anode to prevent biological sulfur reduction [12]. This may be achieved in the alternating regeneration process, as during this stage the pH in the cathode compartment may rise to inhibitory or toxic levels. In addition, continuous experiments on real wastewater in larger scale processes are needed to accurately assess the potential economic cost/benefit ratio of the electrochemical sulfide removal/recovery. Notably the conversion rate (and hence the reactor volume), the required input voltage, the current efficiency and the reactor design and lifetime will be critical parameters.
Sulfur loaded carbon fibre electrodes can be electrochemically regenerated by reducing elemental sulfur to sulfide/polysulfides solution, with an intermediary coulombic efficiency of up to 435 ± 5%. This process enables the development of technologies for sustainable electrochemical sulfide removal from wastewater. The technology is based on regular switching between anodic and cathodic operations which requires minimal energy input.
11 10 9 8 7
0
10
20 Time (hr)
30
40
Fig. 3. (A) Coulombic efficiency and (B) pH values during the regeneration of sulfur loaded electrodes at 200 mA (11.9 A m2 of membrane surface area) and at different buffer capacities: (4) without buffer ( ) 5 mM (h) 50 mM ( ) 500 mM.
there is no continuous sulfide and OH generation. The polysulfides formation is pH dependent and produces H+ during the reaction (Eq. (2)). Therefore, the regeneration of sulfur loaded electrodes over a longer period of time (by using lower current densities) might provide higher intermediary CE due to the longer time allowed for chemical polysulfides formation. Concentrated polysulfide solutions can be converted to elemental sulfur as a solid product either by adjusting the pH to near neutral or lightly aerating the solution. Moreover, in the pulp and paper industry, an industry with sulfide-rich wastewaters, polysulfides themselves have been reported to improve the yield and quality of paper [8]. Therefore, in a practical application the process needs to be optimized towards polysulfides formation, which implies that the cathode solution needs to be removed before further reduction to sulfur occurs. This would not allow using the sudden drop of cathode potential as end point of the regeneration reaction (Fig. 1B), since at that point all polysulfides have already been converted to sulfide. This process described here could allow for continuous sulfide removal from wastewater at ambient temperatures, with cheap carbon fibre electrodes. In the process sulfide is removed at the
Acknowledgements Paritam K. Dutta and Korneel Rabaey thanks the University of Queensland for scholarship (IPRS & UQILAS) and fellowship support (UQ Postdoctoral Research Fellowships), respectively. This work was funded by the Australian Research Council (Grant Nos. DP0666927, DP0879245, and LP0882016). References [1] L. Zhang, P. De Schryver, B. De Gusseme, W. De Muynck, N. Boon, W. Verstraete, Water Research 42 (2008) 1. [2] B.G. Ateya, F.M. Al-Kharafi, Electrochemistry Communications 4 (2002) 231. [3] P.K. Dutta, K. Rabaey, Z. Yuan, J. Keller, Water Research 42 (2008) 4965. [4] C.E. Reimers, P. Girguis, H.A. Stecher, L.M. Tender, N. Ryckelynck, P. Whaling, Geobiology 4 (2006) 123. [5] Y.S. Shih, J.L. Lee, Industrial & Engineering Chemistry Process Design and Development 25 (1986) 834. [6] Z. Mao, A. Anani, R.E. White, S. Srinivasan, A.J. Appleby, Journal of the Electrochemical Society 138 (1991) 1299. [7] M. Pourbaix, Atlas of Electrochemical Equilibrium in Aqueous Solutions, Pergamon, Oxford, London, 1966. [8] A. Chen, B. Miller, The Journal of Physical Chemistry B 108 (2004) 2245. [9] B. Keller-Lehmann, S. Corrie, R. Ravn, Z. Yuan, J. Keller, in: Second International IWA Conference on Sewer Operation and Maintenance, Vienna, Austria, 26–28 October, 2006. [10] P.L. Cloke, Geochimica et Cosmochimica Acta 27 (1963) 1265. [11] W.E. Kleinjan, A. de Keizer, A.J.H. Janssen, Colloids and Surfaces B: Biointerfaces 43 (2005) 228. [12] P.K. Dutta, J. Keller, Z. Yuan, R.A. Rozendal, K. Rabaey, Environmental Science and Technology 43 (2009) 3829.
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Electrochemical regeneration of sulfur loaded electrodes Paritam K. Dutta, René A. Rozendal, Zhiguo Yuan, Korneel Rabaey, Jürg Keller * The University of Queensland, Advanced Water Management Centre (AWMC), St. Lucia, QLD 4072, Australia
a r t i c l e
i n f o
Article history: Received 12 April 2009 Received in revised form 8 May 2009 Accepted 12 May 2009 Available online 18 May 2009 Keywords: Sulfide Electrode Regeneration Polysulfide Wastewater treatment
a b s t r a c t Electrochemical oxidation of sulfide is a promising technique for its removal from wastewaters. Generally, the main product of this oxidation reaction is elemental sulfur. The latter deposits as a solid on the electrode and deactivates it. Therefore, an efficient and effective regeneration technique of sulfur loaded electrodes is required for the practical implementation of this technology. Here we demonstrate a method for in situ reduction of electrodeposited sulfur on carbon fibre electrodes to sulfide/polysulfides, at low energy input. The intermediary coulombic efficiency (CE) values strongly depend on pH and buffer capacity of the solution. These values were recorded up to 435 ± 5% due to simultaneous chemical dissolution of sulfur to polysulfides under alkaline conditions. This process demonstrates the potential for continuously removing dissolved sulfide from wastewaters at an anode as sulfur and recovering the deposited sulfur based on regular switching between anodic sulfide oxidation and cathodic sulfur reduction. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Sulfide is found in many domestic and industrial wastewaters and often needs to be removed because of its (eco)toxicity, corrosive properties, and unpleasant odour. Although several physicochemical and biological sulfide removal technologies are already available, there is a need for more cost-effective and efficient methods [1]. Electrochemical techniques for sulfide removal from wastewater might offer advantages over other methods in terms of selectivity and energy efficiency. Sulfide is an electro-active compound and can be removed by electrochemical oxidation either in fuel cell or electrolysis cell mode [2,3]. Elemental sulfur has been found to be the main final product from aqueous sulfide oxidation [3,4]. Unfortunately, the elemental sulfur produced deposits onto the electrode and deactivates it over time [2]. This is considered a major limitation of the process. A technique is needed to remove the solid elemental sulfur from the electrode surface in order to enable practical implementation of this technology [4]. Limited studies such as organic solvent extraction of sulfur with subsequent solvent evaporation, controlled sulfur precipitation in the bulk solution with alkali addition at elevated temperature were conducted to prevent sulfur induced electrode deactivation or to regenerate sulfur loaded electrodes [5,6]. However, these processes are not considered sustainable due to the involvement of toxic organic solvents, high energy requirements and restricted applications. * Corresponding author. Tel.: +61 7 3365 4727; fax: +61 7 3365 4726. E-mail address: [email protected] (J. Keller). 1388-2481/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2009.05.024
Here we propose an alternative approach for the regeneration of sulfur loaded electrodes, namely electrochemical regeneration. The narrow stability region of sulfur in its Pourbaix diagram [7] shows the theoretical possibility for further electrochemical oxidation or reduction of elemental sulfur, following a slight change of potential and pH. In a recent study, we demonstrated that sulfur electrodeposited on graphite granules is indeed electrochemically active [3]. Two main strategies can be considered for the electrochemical regeneration of sulfur loaded electrodes: (i) oxidizing the elemental sulfur to sulfate or (ii) reducing to sulfide and/or polysulfides. An inherent drawback of the first strategy is that it likely requires high anode potentials [3] on non-catalysed graphite/carbon electrodes, which are the preferred electrode materials for sulfide oxidation from an economic point of view. Furthermore, a high anode potential may also result in lower coulombic efficiencies due to competing reactions such as water oxidation and carbon corrosion. Additionally, sulfur oxidation to sulfate is a six-electron process and thus requires higher input of electrical current compared to sulfur reduction to sulfide, which involves transferring two electrons. Considering these drawbacks, it is hypothesized that sulfur reduction to sulfide is likely to be a more suitable regeneration strategy as it requires a lower current input, and consequently is expected to have a higher coulombic efficiency due to the simultaneous sulfur dissolution by the electrogenerated sulfide, leading to the formation of polysulfides in alkaline conditions [8]. Moreover, the cathodic reduction of sulfur can be coupled to the anodic oxidation (and thus removal) of sulfide from wastewaters. Such a process should allow for regular switching between anode and cathodic operations. In this case, the anode is removing sulfide
P.K. Dutta et al. / Electrochemistry Communications 11 (2009) 1437–1440
2. Experimental 2.1. Cell design and operation The cell was constructed as described elsewhere [3]. It consisted of identical rectangular anode and cathode chambers (volume 335 mL) separated by a cation exchange membrane (Ultrex, CM17000, MIC). Carbon fibre (SGL group) brush electrodes with a stainless steel core were used for the both chambers. A graphite rod (5 mm diameter) was used in the both chambers to connect the electrodes to the external circuit. The chamber contents were continuously recirculated over an external buffer flask (1 L) at a rate of 10 L h1. The cell was operated in batch mode and controlled galvanostatically using a potentiostat (VMP3, PAR). Each batch experiment lasted for about 40 h. In every alternate batch the cathode was used as anode and vice versa. The experiments were conducted at different buffer capacities (from 0 to 500 mM) of cathode solution. The cathode samples were collected at 2–4 h intervals to measure dissolved sulfur species. Before starting each experiment, the electrode used as cathode was loaded with an estimated sulfur amount of 12–15 kg m3 (of total anode/cathode volume) based on the observed conversion of known quantity of sulfide to sulfur [3]. Chemical polysulfides formation was examined by keeping the circuit open during a few batch experiments for 4–10 h with/without adding sulfide solution. For all experiments sulfide was used as the sole electron donor in the anode at varying concentrations (0.2–3.5 g S L1). The coulombic efficiency (CE) was calculated as the ratio between the amount of charge transfer theoretically required for obtaining the measured amount of sulfide and polysulfides (based on the two electron reduction of sulfur) and the measured amount of charge transferred in the process. All experiments were performed in triplicates.
a result, electrodeposited sulfur on the cathode was effectively reduced to sulfide and polysulfides at cathode potentials of 330 ± 5 mV (vs (standard hydrogen electrode, SHE)). As sulfide was used as the electron donor at the anode, the required cell voltage was low over the complete batch, i.e., between 0.2 and 0.35 V. Fig. 1A depicts the electrochemical production of sulfide/polysulfides from the reduction of electrodeposited sulfur at the cathode during a complete batch cycle. Polysulfides formation was observed due to the yellow coloration of the cathodic electrolyte. Sulfur reduction to sulfide most likely proceeded according to Eq. (1). This reaction consumed protons, causing an increased electrolyte pH, which stimulated the chemical formation of polysulfides according to Eq. (2) (Fig. 1). Polysulfides formation was confirmed during open circuit operation with sulfide and a sulfur loaded electrode at alkaline pH. During this experiment, the sulfide concentration and pH decreased and polysulfides were formed (data not shown). Polysulfides can also be formed as an intermediate during sulfur reduction to sulfide (Eq. (3)). However, all polysulfides were eventually reduced to sulfide upon completion of the batch cycle (Fig. 1 and Eq. (4)).
SðsÞ þ 2e þ Hþ ! HS ; HS þ
n1 S8 $ S2n þ Hþ 8
ð3Þ
5000
A 4000
3000
2000
1000
0 0
3. Results and discussion
ð1Þ ð2Þ
nSðsÞ þ 2e ! S2n ; E00 ¼ 0:319 ðn ¼ 5; S2n ¼ 1 M; pH ¼ 7Þ
2.2. Chemicals and chemical analyses
10
20
30
40
Time (hr) -0.2
B Cathode potential (V, SHE)
Na2S9H2O was used as a source of sulfide, Na2HPO4 and KH2PO4 as buffer and NaCl (1 g L1) as the supporting electrolyte. Sulfide, sulfate, thiosulfate and sulfite concentrations were measured by Ion Chromatography (IC), using a Dionex 2010i system [9]. To measure polysulfide concentrations, all dissolved sulfur species were oxidized to sulfate with H2O2 (after increasing the pH to around 12.5 with 0.4 M NaOH) [10]. The difference between the sulfate after H2O2 oxidation and sulfide measured before H2O2 oxidation was regarded as polysulfides. Sulfite and thiosulfate were detected (<10%) mainly in presence of polysulfides and these amounts taken as polysulfides assuming an artefact of sample preparation in the absence of any oxidant in the cathode. Microscopic images of carbon fibre were collected using a Scanning Electron Microscope (SEM) (JEOL JSM 6460LA) which was equipped with Energy Dispersion Spectrometry (EDS) used for the detection of elemental sulfur.
E00 ¼ 0:271 ðHS ¼ 1 M; pH ¼ 7Þ
-1
from wastewater and being loaded with sulfur, while the cathode, which was previously the anode, is regenerated. Therefore, the main purpose of this study is to examine the in-situ regeneration of sulfur loaded electrodes by reducing electrodeposited sulfur to sulfide/polysulfides. We also examined the influence of pH on the regeneration efficiency and the stability of an electrode during repetitive anode/cathode switches.
Concentration (mg-S L )
1438
-0.4
-0.6
-0.8
-1.0
-1.2 0
10
20
30
40
Time (hr)
3.1. Reduction of sulfur to sulfide/polysulfides The electrochemical cell was galvanostatically controlled at 200 mA (equivalent to 11.9 A m2 of membrane surface area). As
Fig. 1. (A) Sulfide-S ( ), polysulfide-S ( ), total-S (4) concentration and (B) evolved cathode potential over time during regeneration of sulfur loaded electrode at a current of 200 mA (11.9 A m2 of membrane surface area) and solution buffer capacity 5 mM.
1439
P.K. Dutta et al. / Electrochemistry Communications 11 (2009) 1437–1440 þ S2 n þ 2ðn 1Þe þ nH ! nHS ;
ðn ¼ 5;
E00 ¼ 0:255 S2 n
¼ HS ¼ 1; pH ¼ 7Þ
ð4Þ
During regeneration, the cathode potential slowly changed to more negative values, which was likely caused by the increase of sulfide/ polysulfides concentrations and pH. However, at the end of the regeneration cycle the potential suddenly dropped sharply (Fig. 1B), indicating almost complete reduction of all sulfur species (solid and dissolved) to sulfide as confirmed from the dissolved sulfur species analysis. This sudden potential drop was probably caused by a switch from sulfur/polysulfides reduction to hydrogen evolution. This rapid potential change clearly marks the end of a batch cycle and may be used for future monitoring of the process. For all batch tests, the total sulfur recovery as dissolved sulfur species from the known amount of deposited sulfur at the end of the batch cycle, i.e., after the sharp potential drop, was 91 ± 4%. The SEM images (Fig. 2) provide an additional indication of the regeneration of the electrode. EDS analysis (Fig. 2D) elucidated that the layer of solids deposited on the surface of the electrode (Fig. 2B) is sulfur. Visually, the carbon fibre surface after regeneration (Fig. 2A) looked similar as it was before sulfur deposition (Fig. 2C). This confirms for the almost complete removal of the deposited sulfur, but also demonstrates that the carbon fibre electrodes are not visually damaged in the process of sulfide oxidation and recovery. In addition to quantitative analysis and SEM pictures, the intermediary coulombic efficiency (CE) values as demonstrated in Fig. 3A and B provide more insight into the reaction mechanism of the regeneration process. The intermediary CE values depend
on the buffer capacity of the solution or pH values. CE values higher than 100% were likely caused by simultaneous chemical polysulfides formation (Eq. (2)) with the electrochemical reduction of sulfur to sulfide. The highest intermediary CE was found 435 ± 5% after four hours of operation in an experiment without phosphate buffer. This high intermediary CE is likely due to the rapid rise of the pH in absence of a pH buffer and the availability of large amounts of sulfur for polysulfides formation. However, at the end of each batch cycle, the generated polysulfides were further reduced to sulfide (Fig. 1) and the CE values approached 100%. The further reduction of polysulfides to sulfide was also confirmed by the disappearance of the yellow colour over the course of the batch experiment. The intermediary CE values at high buffer strength of 500 mM did not surpass 100% as the pH value only increased from 7 to 7.97 after 36 h of operation. Polysulfides exist in water only at pH > 6 and the average equilibrium constant of polysulfides, pKx at 21 °C has been determined as 8.82 for chemical sulfur [11]. For this reason, low polysulfides accumulation was observed for the 500 mM buffer experiment, with a maximum of 350 ± 40 mg polysulfide-S L1 compared to 3400 ± 100 mg polysulfide-S L1 for the experiment without buffer. 3.2. Polysulfide solutions as the preferred product of electrode regeneration Since chemical polysulfides formation can regenerate the electrode quickly with considerably higher CE, it is the preferred reaction. However, chemical polysulfides formation will not proceed if
A
B
C
S
30000
D
27000 24000
Counts
21000 18000 15000 12000 9000 6000 3000 0 0.00 0.30 0.60 0.90 1.20 1.50 1.80 2.10 2.40 2.70 3.00
keV Fig. 2. SEM pictures of carbon fibre electrodes used for electrochemical regeneration of sulfur: (A) before electro-deposition of sulfur, (B) after electro-deposition of sulfur, (C) after regeneration of the electrode, and (D) EDS spectrum of at the position indicated by the cross in Fig 1B.
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P.K. Dutta et al. / Electrochemistry Communications 11 (2009) 1437–1440
500
A % Coulombic Efficiency
400
300
200
100
0
0
10
20 Time (hr)
30
40
14
B
13
4. Conclusions
12
pH
anode from wastewater, when fully loaded with elemental sulfur, the anode can be switched over to cathode mode and be regenerated. In the presented work, so far the same electrode was used as both anode and cathode for over 15 cycles without any notable decrease of performance. The process performance needs to be further evaluated with real wastewater to assess the influence of other constituents. Although the cathodic regeneration process will not be affected as a clean freshwater solution will be used in the cathode, biofilm formation needs to be avoided in the anode to prevent biological sulfur reduction [12]. This may be achieved in the alternating regeneration process, as during this stage the pH in the cathode compartment may rise to inhibitory or toxic levels. In addition, continuous experiments on real wastewater in larger scale processes are needed to accurately assess the potential economic cost/benefit ratio of the electrochemical sulfide removal/recovery. Notably the conversion rate (and hence the reactor volume), the required input voltage, the current efficiency and the reactor design and lifetime will be critical parameters.
Sulfur loaded carbon fibre electrodes can be electrochemically regenerated by reducing elemental sulfur to sulfide/polysulfides solution, with an intermediary coulombic efficiency of up to 435 ± 5%. This process enables the development of technologies for sustainable electrochemical sulfide removal from wastewater. The technology is based on regular switching between anodic and cathodic operations which requires minimal energy input.
11 10 9 8 7
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Fig. 3. (A) Coulombic efficiency and (B) pH values during the regeneration of sulfur loaded electrodes at 200 mA (11.9 A m2 of membrane surface area) and at different buffer capacities: (4) without buffer ( ) 5 mM (h) 50 mM ( ) 500 mM.
there is no continuous sulfide and OH generation. The polysulfides formation is pH dependent and produces H+ during the reaction (Eq. (2)). Therefore, the regeneration of sulfur loaded electrodes over a longer period of time (by using lower current densities) might provide higher intermediary CE due to the longer time allowed for chemical polysulfides formation. Concentrated polysulfide solutions can be converted to elemental sulfur as a solid product either by adjusting the pH to near neutral or lightly aerating the solution. Moreover, in the pulp and paper industry, an industry with sulfide-rich wastewaters, polysulfides themselves have been reported to improve the yield and quality of paper [8]. Therefore, in a practical application the process needs to be optimized towards polysulfides formation, which implies that the cathode solution needs to be removed before further reduction to sulfur occurs. This would not allow using the sudden drop of cathode potential as end point of the regeneration reaction (Fig. 1B), since at that point all polysulfides have already been converted to sulfide. This process described here could allow for continuous sulfide removal from wastewater at ambient temperatures, with cheap carbon fibre electrodes. In the process sulfide is removed at the
Acknowledgements Paritam K. Dutta and Korneel Rabaey thanks the University of Queensland for scholarship (IPRS & UQILAS) and fellowship support (UQ Postdoctoral Research Fellowships), respectively. This work was funded by the Australian Research Council (Grant Nos. DP0666927, DP0879245, and LP0882016). References [1] L. Zhang, P. De Schryver, B. De Gusseme, W. De Muynck, N. Boon, W. Verstraete, Water Research 42 (2008) 1. [2] B.G. Ateya, F.M. Al-Kharafi, Electrochemistry Communications 4 (2002) 231. [3] P.K. Dutta, K. Rabaey, Z. Yuan, J. Keller, Water Research 42 (2008) 4965. [4] C.E. Reimers, P. Girguis, H.A. Stecher, L.M. Tender, N. Ryckelynck, P. Whaling, Geobiology 4 (2006) 123. [5] Y.S. Shih, J.L. Lee, Industrial & Engineering Chemistry Process Design and Development 25 (1986) 834. [6] Z. Mao, A. Anani, R.E. White, S. Srinivasan, A.J. Appleby, Journal of the Electrochemical Society 138 (1991) 1299. [7] M. Pourbaix, Atlas of Electrochemical Equilibrium in Aqueous Solutions, Pergamon, Oxford, London, 1966. [8] A. Chen, B. Miller, The Journal of Physical Chemistry B 108 (2004) 2245. [9] B. Keller-Lehmann, S. Corrie, R. Ravn, Z. Yuan, J. Keller, in: Second International IWA Conference on Sewer Operation and Maintenance, Vienna, Austria, 26–28 October, 2006. [10] P.L. Cloke, Geochimica et Cosmochimica Acta 27 (1963) 1265. [11] W.E. Kleinjan, A. de Keizer, A.J.H. Janssen, Colloids and Surfaces B: Biointerfaces 43 (2005) 228. [12] P.K. Dutta, J. Keller, Z. Yuan, R.A. Rozendal, K. Rabaey, Environmental Science and Technology 43 (2009) 3829.