Electrochemical sulfide removal and recovery from paper mill anaerobic treatment effluent

water research 44 (2010) 2563–2571

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Electrochemical sulfide removal and recovery from paper mill anaerobic treatment effluent Paritam K. Dutta, Korneel Rabaey, Zhiguo Yuan, Rene´ A. Rozendal, Ju¨rg Keller* The University of Queensland, Advanced Water Management Centre (AWMC), St. Lucia, QLD 4072, Australia

article info

abstract

Article history:

Sulfide can be removed from wastewater and recovered as elemental sulfur using an

Received 8 August 2009

electrochemical process. Recently, we demonstrated this principle of product recovery on

Received in revised form

synthetic feeds. Here, we present a lab scale electrochemical reactor continuously

26 December 2009

removing sulfide from the effluent of an anaerobic treatment process operated on paper

Accepted 12 January 2010

mill wastewater. The effluent contained 44  7 mg of sulfide-S L1. Sulfide was reduced to

Available online 25 January 2010

8  2 mg-S L1, at a removal rate of 0.845  0.133 kg-S m3 of total anodic compartment (TAC) d1. The removed sulfide was recovered (75  4% recovery) as pure concentrated

Keywords:

alkaline sulfide/polysulfide solution, from which solid elemental sulfur was obtained. The

Sulfide removal

electrochemical sulfide removal was not affected by different soluble constituents or

Sulfur

particulate materials present in the wastewater. However, over time sulfide removal

Paper mill wastewater

decreased due to biological sulfur reduction using the organics present in the wastewater.

Electrochemical

Therefore, a periodic switching strategy between anode and cathode was developed.

Recovery

Biofilm formation was avoided as the pH of the cathode solution increased to inhibitory

Biofilm

levels during cathodic operation, while still allowing full recovery of the sulfur as end product. ª 2010 Elsevier Ltd. All rights reserved.

1.

Introduction

Sulfide is present in a wide range of industrial wastewaters such as tannery and paper mill wastewaters. It is toxic, corrosive and odorous and needs to be removed from wastewater before it is discharged into waterways. Sulfide discharges even to sewers are increasingly constraint due to the corrosive effects of sulfide in sewers, as well as due to the odour and occupational health impacts in the sewers and downstream wastewater treatment plants. In a recent review on sewage sulfide removal, high cost was reported as one of the main disadvantages of the existing sulfide removal processes (Zhang et al., 2008). Recently, an electrochemical approach was proposed for wastewater sulfide removal, in which sulfide can be directly oxidized at an anode (Rabaey

et al., 2006). Elemental sulfur is the key oxidation product when carbon/graphite materials are used as the electrode material (Ateya et al., 2003). Potential advantages of electrochemical sulfide removal are cost effectiveness, selectivity, and controllability (Rajeshwar et al., 1994; Chen, 2004). One of the main disadvantages of electrochemical sulfide removal is anode passivation by the precipitated elemental sulfur (Ateya et al., 2003; Reimers et al., 2006; Dutta et al., 2008). Extraction of sulfur by organic solvents and subsequent solvent evaporation or controlled sulfur precipitation in the bulk solution with alkali addition at elevated temperature were examined to prevent sulfur induced electrode passivation or to regenerate electrodes (Shih and Lee, 1986; Mao et al., 1991). These strategies involve toxic organics and/or high energy input, and therefore are not considered as

* Corresponding author. Tel.: þ61 7 3365 4727; fax: þ61 7 3365 4726. E-mail address: [email protected] (J. Keller). 0043-1354/$ – see front matter ª 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2010.01.008

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a sustainable approach. Recently, an electrochemical regeneration strategy was proposed that periodically switches between anode and cathode operation (Dutta et al., 2009a). This approach allows for sulfide removal from wastewater at the anode while at the cathode, elemental sulfur previously precipitated on the electrode, is reduced to sulfide/polysulfide. As a result, the electrode is regenerated (see equations (1)–(3)) and a concentrated sulfide/polysulfide solution can be obtained at the cathode with high coulombic efficiency. At high pH the coulombic efficiency can even increase to above 100% as under those conditions the electrochemical removal is assisted by the chemical dissolution of sulfur by sulfide to polysulfides (equation (4)). 0

SðsÞ þ 2e þ 2Hþ =Hþ !H2 S=HS ; E0 [-0:271 VðH2 S=HS ¼ 1 M; pH ¼ 7Þ 

(1) 0



nSðsÞ þ 2e !S2n ; E0 [-0:319 V n ¼ 5; S2n ¼ 1 M; pH ¼ 7


(2)

0





S2n þ 2ðn  1Þe þ nHþ !nHS ; E0 [-0:255 V n ¼ 5; S2n ¼ HS
¼ 1 M; pH ¼ 7 (3) HS þ

 n1 S8 !S2n þ Hþ 8

(4)

Sulfide containing domestic or industrial wastewater will often contain a wide range of organic and inorganic compounds, trace elements and suspended particulate materials. All these may significantly influence the electrochemical sulfide removal process, particularly during long time operation. The organics may stimulate biofilm formation at the anode. Micro-organisms in this biofilm can use electrodeposited sulfur as a preferred electron acceptor and release the sulfide irrespective of electrochemical conditions (Dutta et al., 2009b). This release can negatively affect the efficiency of the electrochemical sulfide removal process. To our knowledge, no studies have yet examined electrochemical sulfide removal from real wastewater considering the possible bacterial interaction with sulfide oxidation products, the effect of sulfur precipitation and electrode regeneration, as well as the influence of other constituents such as suspended particulate materials on the oxidation rate. Therefore, this study aimed to demonstrate electrochemical sulfide removal from the effluent of an anaerobic treatment plant operated on paper mill wastewater. In addition, an operating approach to avoid biofilm formation was developed. Finally, a strategy was derived for the recovery of wastewater sulfide as concentrated sulfide/polysulfide solution from which solid elemental sulfur can be obtained.

2.

Materials and methods

2.1.

Wastewater characteristics

The sulfide containing wastewater used in this study was collected at the exit of a high rate anaerobic treatment process

(VISY Paper Pty Ltd, Gibson Island, Queensland, Australia). The plant receives wastewater from the paper recycling plant and uses an Upflow Anaerobic Sludge Blanket (UASB) process to treat the wastewater before discharging the effluent into the sewer system. The typical characteristics of the effluent used in this study are shown in Table 1.

2.2.

Electrochemical cell design and operation

The electrochemical cell was constructed according to Dutta et al. (2008), consisting of two identical rectangular chambers used as anode and cathode (volume 335 mL each) and was separated by a cation exchange membrane (Ultrex, CM17000, Membranes International Inc.). Throughout this paper, the two identical chambers of the cell are referred to as side 1 and side 2. Two types of electrodes were used: carbon brushes and graphite granules. Carbon fibre (SGL Group) brushes were made with a twisted stainless steel core as described by Logan et al. (2007). These brushes were used as electrodes for both chambers. The electrodes were connected with the external circuit via the stainless steel core. All experiments were conducted with brush electrodes unless stated otherwise. Electrodes made with graphite granules (El Carb 100, Graphite Sales Inc.) were only used to examine sulfide removal rates at different loading rates and to compare the performance with carbon fibre brush electrodes. An Ag/AgCl (RE-5B, Bio-Analytical) electrode was used as the reference electrode (þ197 mV vs standard hydrogen electrode, SHE). The coulombic efficiency (CE) was calculated as the ratio between the measured amount of charge transferred in the process and the amount of charge transfer theoretically expected from the measured amount of sulfide removal (based on the two-electron sulfide oxidation to sulfur). Both the anodic and cathodic solutions were recirculated with a peristaltic pump (Watson Marlow) at a flow rate of

Table 1 – Typical characteristics of the anaerobic treatment effluent wastewater of VISY paper mill wastewater treatment plant. Parameters pH Conductivity (mS cm1) COD (mg L1) Volatile fatty acids (mg L1) acetic acid propionic acid butyric acid iso-valeric acid valeric acid Sulfide-S (mg L1) Sulfate-S (mg L1) TSS (mg L1) Caþ (mg L1) Cl (mg L1) Naþ(mg L1) Kþ (mg L1) Total N (mg L1) Total P (mg L1)

Values 7.3–7.6 1.5–4.5 1300–3000 250–680 450–1100 0–30 5–15 0–20 35–55 0.2–2.5 400–1200 200–400 220–280 900–1300 50–70 14–30 1–4

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12 L h1. A buffer flask of 1 L was included in the cathodic recirculation line. The cathodic regeneration solution contained only 1 g L1NaCl. Wastewater was pumped continuously into the anode chamber for all experiments using a peristaltic pump (Watson Marlow) with a flow rate of 8 L day1 unless otherwise mentioned. A balloon filled with nitrogen gas was connected to the top of the 25 L wastewater drum to compensate for the liquid drawn from the drum while avoiding leakage of oxygen into the drum. The cell was operated potentiostatically with a controlled anode potential of þ0.2 V vs SHE using a potentiostat (VMP3, PAR). Current, cathode potential and cell voltage data were monitored for all experiments. Anode inlet and outlet samples were collected daily to analyse dissolved sulfur species. First, the cell was operated for 2 days with synthetic wastewater containing only sulfide concentration of 50 mg-S L1 (no organics) in 50 mM phosphate buffer and 1 g L1NaCl to compare the sulfide removal rate with real wastewater. Subsequently, the cell was operated on the wastewater obtained from an anaerobic treatment effluent described in Table 1.

2.3.

Continuous wastewater experiments

All experiments for the removal and recovery of wastewater sulfide were divided into four different sets which are briefly described in Table 2. The table includes the objective and duration of the experiments, the switching time between anode and cathode and the frequency for the change of the cathode solution. During the first set of experiments, after 10 days, the cell was run with synthetic wastewater (composition as mentioned above, no organics) for one day containing

50 mg of S L1 to examine possible effects of different constituents present in real wastewater on the sulfide removal. The inlet and outlet sulfide and organics concentration were measured every three days. For these sets of experiments, when the sulfide removal stopped, carbon fibres were collected from the anode chamber and prepared for microscopic analysis via Scanning Electron Microscope (SEM). Afterwards 500 mL sodium hypochlorite solution (17.5 g L1 NaOCl, commercial laundry bleach) was recirculated through the anode chamber for about half an hour to inactivate a possibly formed biofilm. Then the cell was again operated for two more days with wastewater to compare the sulfide removal rate. For the second set of experiments, the cell switched over after every 72 h and 24 h interval. A NaOCl solution (17.5 g L1) was recirculated for about half an hour through both chambers to inactivate biofilm. Here, before switching over the chambers, all solutions were drained from the chambers, which were then washed 2 times with fresh water before refilling with the new solutions for the start of the next cycle. Samples were collected after 18  3 h of starting each cycle. The third set of experiments was repeated three times. Two identical reactors but with different electrodes were fed from the same wastewater feed tank at flow rates of 4, 8, 16, 24 and 32 L day1 for the fourth set of experiments.

2.4.

Chemical analyses

2Sulfide (H2S, HS and S2-), sulfite (SO23 ), sulfate (SO4 ) and 2thiosulfate (S2O3 ) and concentrations were measured by ion chromatography (IC), using a Dionex 2010i system, according to Keller-Lehmann et al. (2006). Samples collected from the

Table 2 – Overview of different sets of experiments for sulfide removal from paper mill wastewater. No. of experimental sets 1

Objective

i)

ii)

To verify probable impacts of different wastewater organic, inorganic materials and suspended particulates on sulfide removal rate Impact of biofilm formation and sulfur deposition on the cell performance

Duration of the Anode and experiment cathode (days) switching time

Changes in cathode solution

21

No switching operation

Every 3 days

2

To avoid biofilm formation in the anode

18 21

72 h 24 h

Every cycle started with 500 mL new solution

3

To obtain concentrated sulfide/polysulfide solution in the cathode To obtain polysulfide-rich sulfide solution in the cathode (i.e. to recover elemental sulfur from removed sulfide)

5

24 h

5

24 h

The same solution used in every cycle Two cathode solutions used: i) One for the first 8 h ii) The other for the rest 16 h; for both cases, the same solution used in 5 consecutive cycles

5

24 h (for each loading rate)

4

i) ii)

To evaluate the effects of loading rates (kg–S m3 of total anodic compartment (TAC)) To examine the difference between carbon fibre brushes and graphite granule electrodes

New solution in each cycle

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Electron microscopy and spectroscopy

The microscopic images of carbon fibres were collected using a Scanning Electron Microscope (SEM) (JEOL JSM 6400 & 6460 LA). The Secondary Electron (SE) detector provided the shape of the specimen whereas a Back Scattered Electron (BSE) detector produced images with compositional contrast. SEM images were taken to examine biofilm formation and deposition of any other constituents on the electrode surface. Before analysis, both fibre and sulfur samples were coated with platinum for 3 and 10 min respectively with a Sputter Coater. This was done to provide conductive coating of the SEM samples. The JEOL JSM 6460 was equipped with Energy Dispersion Spectrometry (EDS) which was used for detection of different elements.

3.

Results and discussion

3.1.

Continuous operation of the cell

At the anode potential of þ0.2 V vs SHE, sulfide present in the wastewater was oxidized to sulfur in the anode, which deposited on the electrode according to equation (1). This was confirmed by measuring the inlet and outlet concentrations of different sulfur species. Only sulfide concentration was decreased while the concentration of other sulfur oxyanions did not change. The concentrations of sulfite (SO23 ) and sulfate 1 (SO24 ) were negligible (<2% of sulfide-S mg L ) and approximately 5% of sulfide-S was found as thiosulfate (S2O23 ) both in the inlet and outlet which might be due to the oxidation of sulfide with oxygen during sample collection and preservation as reported earlier (Dutta et al., 2008). Sulfide oxidation to elemental sulfur was preferred for the anode reaction as it requires only two electrons, whereas sulfide oxidation to sulfate requires eight electrons. This means that a much higher input of electrical energy is required. Moreover, particularly for graphite/carbon electrodes, much higher anode potential are required for the oxidation of sulfide to sulfate compared to the oxidation of sulfide to elemental sulfur, which also results in lower current efficiencies due to competing reactions such as water oxidation and carbon corrosion. In addition, sulfate formation in wastewater always creates a risk for re-reduction to sulfide by micro-organisms downstream. Finally, unlike the coupling of cathodic sulfur

100

60

80

50

60

40

40

30

20

20

Current (mA)

2.5.

reduction to the anodic sulfide oxidation, sulfide oxidation to sulfate needs an additional cathodic process. In the cathode, hydrogen evolution from water reduction was preferred. The key other alternative was cathodic oxygen reduction which requires aeration and might cause imbalances at the anode due to oxygen diffusion. The another option was to use a chemical acceptor such as ferricyanide which might diffuse in the electrode and thus crossover between the cycles. Therefore, water reduction to hydrogen evolution was preferred in that stage of cell operation. Hydrogen evolution process is associated with a high overpotential at non catalysed carbon fibre electrodes, causing a cell voltage of 1.1–1.5 V for currents ranging from 14 to 50 mA (0.83–2.97 A m2 of membrane surface area). The obtained current from the anode can either be caused by sulfide oxidation or organics oxidation. No current was observed when the cell was operated with NaCl containing electrolyte. This confirms that the stainless steel core used in this experiment did not corrode at anode potential of þ0.2 V SHE. Fig. 1 shows a gradual decrease of the sulfide-S removal over time, decreasing from 85% to a negligible removal after about 15 days of continuous operation. During this period, the current increased from 14 to 50 mA. After 13 days, the outlet sulfide concentration was higher than the inlet concentration. Deactivation of the electrode due to the deposition of elemental sulfur or other wastewater constituents can cause a decrease of the sulfide removal rate (Dutta et al., 2008). To investigate this, the electrode surface coverage after 18 days of operation was evaluated by SEM. The electron microscopy images (Fig. 2A and B) show the presence of bacteria and particulate materials in many locations. In Fig. 2B, other particulate were seen on the carbon fibres. As no sulfur peak was found in the EDS spectrum (not shown), it is assumed that these are trapped wastewater particulates. However, substantial areas of the carbon fibres remained bare. EDS analysis on the bare areas showed only a carbon peak, confirming that they are the uncovered carbon fibres (Fig. 2C and D). This implies that the decrease of sulfide

% Sulfide-S Removal

reactors were immediately preserved in previously prepared Sulfide Antioxidant Buffer (SAOB) solution prior to ion chromatography analysis. SAOB solution was also used to dilute the samples where necessary. To measure total dissolved sulfur species including polysulfide concentrations, all sulfur species were oxidized to sulfate with H2O2 (after increasing the pH to around 12.5 with 0.4 M NaOH) (Cloke, 1963). The difference between the sulfate after H2O2 oxidation and other species measured before H2O2 oxidation was regarded as polysulfides. Volatile fatty acid (VFA) concentrations were determined using an LC-10ADVP HPLC system (Shimadzu). The VFA species analysed included acetic, propionic, butyric, isobutyric, valeric and isovaleric.

10

0 0

3

6

9

12

15

18

21 0

-20 No of Days

Fig. 1 – Percentage removal of sulfide and obtained current when the reactor operated continuously with the effluent of an anaerobic treatment process at a flow rate of 8 L dayL1 at controlled anode potential of D0.2 V vs SHE (inlet sulfide concentration 44 ± 7 mg-S LL1, sulfide loading rate 1.05 ± 0.18 kg-S mL3 of TAC dL1).

water research 44 (2010) 2563–2571

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Fig. 2 – SEM images of carbon fibre electrodes collected from the anode chamber after 18 days operated continuously for electrochemical sulfide removal. (A) Overall view of fibres at low resolution (4503 magnification) shows the presence of particulates (B) evidence of biofilm formation (8,0003 magnification) (C) image used for EDS spectrum (1,6003 magnification) (D) EDS spectrum of the position indicated by the cross in the centre of Fig. 2C.

removal was not caused by the deposition of elemental sulfur or other chemical constituents/particulate materials on the electrode fibres. Moreover, passivation of the electrode would have caused decreasing currents, but when the sulfide removal stopped, the highest currents of the experiment were observed. Analyses elucidated that at this time 160  25 mg of COD (calculated from VFA concentrations) was consumed, while in the initial phase no significant change in COD concentrations was observed. These results corroborate the observed formation of a biofilm on the electrode surface. The bacteria likely reduced elemental sulfur back to sulfide after it formed electrochemically and they were using organics as electron donor (Dutta et al., 2009b). This was also supported by the occurrence of a negative sulfide removal rate from day 13, which was likely the result of sulfide production from elemental sulfur previously deposited on the electrode. At that point the sulfide generation rate by bacteria might be higher than electrochemical sulfide oxidation rate. This finding was further confirmed when the sulfide removal rate was restored to about 82% of the initial value when the biofilm was killed with NaOCl after 18 days of operation. A higher sulfide removal rate was observed when synthetic wastewater (no organics) replaced the effluent of an anaerobic treatment process after 10 days. This suggests that biological

sulfur reduction did not occur in the absence of organics. It is important to note that sulfide was removed from synthetic wastewater at almost the same rate as those observed for real wastewater on Day 1, and for synthetic wastewater at the start of the experiment prior to any wastewater addition. This indicates that particulate materials and soluble constituents as such do not directly affect the electrochemical sulfide removal. The anaerobic process effluent used here contains high levels of COD (2100  800 mg L1) relative to only 44  7 mg of sulfide-S L1. The COD allows rapid biofilm development on the electrode surface, basically converting the cell into a microbial electrolysis cell for organics oxidation (Logan, 2009). The high COD/sulfur ratio (i.e. the abundance of electron donor with respect to electron acceptor, elemental sulfur) could be the reason for the net zero or even temporarily negative sulfide removal rate. Bacteria can also directly use the anode as electron acceptor, in parallel to using the electrodeposited sulfur. The current obtained in the first 1–3 days could be considered as the maximum current obtainable from sulfide oxidation to elemental sulfur for the used reactor, at the used loading rate. Higher currents than those obtained in the first 1–3 days indicate organics oxidation using the anode as electron

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3.2.

Regular switching between anode and cathode

In the second set of experiments, the anode and cathode polarity was reversed every 72 h and 24 h. During anodic oxidation, sulfide converted to sulfur which was deposited on the electrode; upon reversing the polarity of the electrode, the sulfur is again reduced to sulfide or polysulfide. Moreover, at high pH the released sulfide reacts with sulfur to form additional polysulfide (see equations (1)–(4)). Fig. 3 shows that over a period of 21 days the sulfide removal rate (81  4%) remained almost stable when operated at a flow rate of 8 L day1 and the electrode polarity reversed every 24 h. The wastewater sulfide decreased from 44  7 mg-S L1 to 8  2 mg-S L1 at a removal rate of 0.845  0.133 kg-S m3 TAC d1. The required cell voltage during this operation was between 0.52 and 1.3 V. This value varied as the cathodic reaction shifted from sulfur/ polysulfide reduction to hydrogen evolution. When all the sulfur/polysulfide was reduced to sulfide, the cathode potential suddenly dropped to a lower value as also observed and reported earlier (Dutta et al., 2009a). At the same time, the yellow color of the solution due to the presence of polysulfides had completely disappeared. The obtained current was in the range of 14–25 mA, corresponding to coulombic efficiencies (CE) of 75%–120% (considering two-electron sulfide oxidation to sulfur). The CE values for continuous sulfide oxidation from synthetic

Side 1

100

wastewater were less than 100% (i.e. 88  5%) due to autooxidation of sulfide to other dissolved sulfur species such as thiosulfate. In addition, bacteria and various chemical constituents present in real wastewater might also slightly contribute to such electron losses. CE values higher than 100% were observed during several cycles, notably in the last 5–6 h of a 24 h cycle (data not shown). This could be due to the initiation of biofilm development on the electrode surface and eventually for organics oxidation as here the CE values were defined considering only sulfide oxidation to elemental sulfur. Higher currents of up to 37 mA were noted at longer switching period of 72 h intervals which further supports organics oxidation. However, upon operating the cell at 72 h switching intervals over 18 days, the sulfide removal rate decreased from its initial value of 81  4% to around 57  3% (results not shown), again indicating biological sulfur reduction. The variation of current (14–25 mA) as described above resulted in a variation of the pH in the cathode solution. The pH of the cathode solution increased to 9.5–11.3 in individual 24 h cycles. In electrochemical systems that are operated on wastewater, cation exchange membranes predominantly transport cations such as Naþ, Kþ, Ca2þ etc. as they are more abundant in wastewater compared to protons (Rozendal et al., 2006). Therefore, the OH generated at the cathode from sulfur reduction or hydrogen evolution cause an increased pH due to the lack of balancing protons from the anode. Such rapidly changing, high pH levels very likely inhibit or kill micro-organisms that are attached to the electrode, therefore limiting the risk of sulfur loss due to biofilm formation. In a recent study, Gutierrez et al. (2009) also reported a 50% activity reduction of sulfate reducing bacteria due to the long term pH elevation from 7.6 to 9 in a sewer system. However, a key challenge for practical operation will be to ensure sufficient current densities to reach desirable pH levels, as well as using a non-buffered cathode electrolyte. Overall, periodic switching serves a double purpose: i) avoiding biofilm formation or inhibiting bacterial activities and ii) reducing electrodeposited sulfur to sulfide/polysulfide and thus

Side 2

Influent

Effluent

50

80

% Sulfide-S Removal

60

40 60 30 40 20 20

0

10

1

2

3

4

5

6

7

8

9 10 11 12 13 14 15 16 17 18 19 20 21

Concentration of Sulfide-S ( mg L-1)

acceptor, which eventually confirms biofilm formation on the anode surface. In this case, bacteria in parallel could also use electrodeposited elemental sulfur as electron acceptor. Therefore, to ensure efficient sulfide removal, biofilm formation must be avoided or regularly removed. We hypothesize that this can be achieved by periodic switching the polarity of the anode to cathodic mode. During cathodic reduction, the pH of the cathode electrolyte increases to levels toxic to most micro-organisms. Moreover, such periodic switching will be a very efficient way for the electrochemical regeneration of the sulfur loaded electrodes (Dutta et al., 2009a).

0

No of Days Fig. 3 – Percentage Sulfide-S removal, influent and effluent sulfide-S concentrations when the reactor operated continuously with periodic switching between side 1 and side 2 (anode and cathode) every 24 h at controlled anode potential of D0.2 V vs SHE (sulfide loading rate 1.05 ± 0.18 kg-S mL3 of TAC dL1).

water research 44 (2010) 2563–2571

regenerating the electrode, which will be important for the ongoing operation to remove sulfide from real wastewater streams.

3.3.

Recovery of wastewater sulfide

Instead of using a new cathode solution in every cycle, the same cathode solution was used for several cycles in an additional series of experiments. The repetitive use of the same cathode solution for 5 consecutive cycles allowed to reach higher pH values as well as more concentrated sulfide/ polysulfide solutions; i.e. the pH increased to 12.3 and about 890  50mg of different sulfur species (of which 74  5% of sulfide-S) were measured, corresponding to 75  4% recovery of the removed sulfide. Besides sulfide, thiosulfate and sulfate were found in the recovered solution, representing about 26% of dissolved sulfur species. These could be formed through sulfide/polysulfide oxidation, as the solution was exposed to air during manual periodic switching and washing the chambers. The reasons for the recovery loss of around 25% of the sulfide removed are difficult to determine. Inefficiencies can be caused by some (concentrated) solution remaining in the reactor during polarity switching, as well as some variability of the influent over a batch cycle. Higher levels of sulfide are known to be toxic for many anaerobic bacteria (Lens et al., 1998). Therefore, repetitive usage of the same cathode solution for several cycles might provide an additional advantage of inhibiting microbial processes due to a combination of high sulfide concentrations and higher pH. However, the concentrated sulfide solution obtained from the cathodic process needs further treatment to obtain solid elemental sulfur or sulfate that could be used for further applications. The further treatment could be done by chemical or biological oxidation preferably with oxygen. The oxidation of the (poly)sulfide to solid elemental sulfur is attractive as it generates a valuable product that can be easily harvested. Alternatively, the alkaline sulfide solution could be used for H2S absorption in an anaerobic treatment off-gas desulfurisation unit of a full scale wastewater treatment process as illustrated by Janssen et al. (2009). In that particular case, liquid phase sulfide removal by the proposed process could provide additional advantages of sulfide recovery and production of alkaline solutions needed for a biogas desulfurisation unit. However, the alkaline concentrated sulfide solution could also be used to produce hydrogen and solid sulfur in a photocatalytic process using visible light or sunlight (Jang et al., 2006). The photocatalyst CdS is well known to produce hydrogen from water under visible light, but it suffers from photocorrosion without sacrificial agent. Generally alkaline sulfide solution is used as sacrificial agent to overcome this (Kudo and Miseki, 2009). Polysulfide solutions are also promising electrolytes for electrochemical storage of energy and photoelectrochemical solar cells (Kamyshny et al., 2004). In addition, polysulfides may also improve the yield and quality of paper in the pulp and paper industry, an industry that generally produces sulfide-rich wastewaters (Chen and Miller, 2004). As intermediary towards sulfur recovery, polysulfide is preferred over sulfide. Concentrated polysulfide solutions can be converted to solid harvestable elemental sulfur by an easy step of either

2569

adjusting the pH to slightly acidic or near neutral or lightly aerating the solution. The abiotic oxidation kinetics of polysulfides is very fast relative to sulfide oxidation kinetics (Steudel, 2000; Van Den Bosch et al., 2008). Moreover, less electrons are needed to generate polysulfide. From the repeatedly used cathode solution, approximately 400 mg of polysulfide-S was recovered (8 h in each cycle over 5 cycles). When the pH of the polysulfide solution was adjusted to 5, solid elemental sulfur precipitated out immediately. Elemental sulfur was then collected, dried and analysed by EDS to examine its purity. The EDS analysis showed a dominant sulfur peak besides the small platinum peak caused by sputter coating (Fig. 4). Thus, the EDS analysis demonstrated that pure elemental sulfur could be recovered from real wastewater. To maximize elemental sulfur recovery by polysulfide formation, the process needs to be optimized towards polysulfide formation, which implies that the cathode solution needs to be removed before further reduction of polysulfide occurs. The required time for the removal of the polysulfide-rich concentrated solution will depend on the loading rate, reactor volume and importantly the anode potential or set current. After removal of the solution, fresh water could be added in the cathode chamber for water

Fig. 4 – (A) SEM image (20003 magnification) of elemental sulfur recovered from wastewater by periodic switching between anode and cathode and (B) corresponding EDS spectrum at the position indicated by the cross in Fig. 4A.

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60

A

B

50

50

40

40

Current (mA)

Concentration of Sulfide-S (mg L -1 )

60

30 20

30 20 10

10

0

0 0

1

2

3

4

5

Loading rate (kg-S m -3 of TAC d-1)

0

1

2

3

4

5

Loading rate (kg-s m -3 of TAC d-1)

Fig. 5 – (A) Influent and effluent sulfide-S concentration and (B) obtained current at different loading rates for carbon fibres and graphite granules electrodes during continuous electrochemical sulfide removal at controlled anode potential of D0.2 V vs. SHE. (C) influent sulfide-S, (,) graphite granules - effluent sulfide-S and current, (:) carbon fibres – effluent sulfide-S and current (TAC – total anodic compartment).

reduction to hydrogen production (or caustic production) and for the reduction of remaining sulfur/polysulfide to sulfide. Alternatively, two cathodes could be used for one anode in an electrochemical reactor. In that case, one cathode will be used to obtain polysulfide-rich solution and the other for the caustic production. Then, there will not be any interruption of the reactor operation.

3.4.

Influence of operational parameters

The results reported and discussed above were obtained with a fixed feed flow rate of 8 L d1,which corresponds to a sulfide loading rate of 1.05  0.18 kg-S m3 of TAC d1. When the reactor was operated at different loading rates by varying the feed flow rates, different effluent sulfide-S concentrations were observed (Fig. 5A). Graphite granules showed better performance in comparison to carbon fibre brush electrodes (Fig. 5B). Higher loading rates provide higher currents from the anodic process (Fig. 5B), allowing faster cathode regeneration and higher pH values. The downside of these increased loading rates was the elevated effluent sulfide-S concentrations (Fig. 5A). Therefore, an appropriate loading rate needs to be selected considering the benefits of operation at higher loading rates (i.e. either for higher feed sulfide concentrations or flow rates) and the discharge requirements for the effluent. Other critical factors involve the reactor size, hydrodynamic fluid flow patterns in the reactor, proper electrode materials, the input voltage etc. Larger scale operation will be necessary to accurately establish the life time and the economic cost/ benefit ratio of the electrochemical sulfide removal/recovery relative to other methods.

4.

treatment plant was evaluated in this study. Sulfide could be effectively removed by electrochemical process from real wastewater, although biofilm formation needs to be avoided. This was achieved by periodic anode and cathode switching as the pH of the cathode solution increased to inhibitory levels. The switching also allowed recovery of the sulfide as a concentrated alkaline sulfide/polysulfide solution, from which pure solid elemental sulfur could be obtained.

Conclusions

Electrochemical sulfide removal from the effluent of an anaerobic treatment process of a paper mill wastewater

Acknowledgements The authors sincerely thank Mr. Darren Ralston, VISY Paper Pty Ltd. (Australia) for his kind assistance and site access. PKD is supported by the University of Queensland (IPRS & UQILAS scholarship). KR is supported by the Australian Research Council (DPDP0879245). This work was funded by the Australian Research Council (Grant DP0666927, DP0879245 and LP0882016).

references

Ateya, B.G., AlKharafi, F.M., Al-Azab, A.S., 2003. Electrodeposition of sulfur from sulfide contaminated brines. Electrochem. Solid-State Lett. 6 (9), C137–C140. Chen, G., 2004. Electrochemical technologies in wastewater treatment. Separ. Purif. Technol. 38 (1), 11–41. Chen, A., Miller, B., 2004. Potential oscillations during the electrocatalytic oxidation of sulfide on a microstructured Ti/Ta2O5–IrO2 electrode. J. Phys. Chem. B 108 (7), 2245–2251. Cloke, P.L., 1963. The geologic role of polysulfides – part I the distribution of ionic species in aqueous sodium polysulfide solutions. Geochim. Cosmochim. Acta 27 (12), 1265–1298. Dutta, P.K., Rabaey, K., Yuan, Z., Keller, J., 2008. Spontaneous electrochemical removal of aqueous sulfide. Water Res. 42 (20), 4965–4975.

water research 44 (2010) 2563–2571

Dutta, P.K., Rozendal, R.A., Yuan, Z., Rabaey, K., Keller, J., 2009a. Electrochemical regeneration of sulfur loaded electrodes. Electrochem. Commun. 11 (7), 1437–1440. Dutta, P.K., Keller, J., Yuan, Z., Rozendal, R.A., Rabaey, K., 2009b. Role of sulfur during acetate oxidation in biological anodes. Environ. Sci. Technol. 43 (10), 3839–3845. Gutierrez, O., Park, D., Sharma, K.R., Yuan, Z., 2009. Effects of long-term pH elevation on the sulfate-reducing and methanogenic activities of anaerobic sewer biofilms. Water Res. 43 (9), 2549–2557. Jang, J.S., Li, W., Oh, S.H., Lee, J.S., 2006. Fabrication of CdS/TiO2 nano-bulk composite photocatalysts for hydrogen production from aqueous H2S solution under visible light. Chem. Phys. Lett. 425 (4–6), 278–282. Janssen, A.J.H., Lens, P.N.L., Stams, A.J.M., Plugge, C.M., Sorokin, D.Y., Muyzer, G., Dijkman, H., Van Zessen, E., Luimes, P., Buisman, C.J.N., 2009. Application of bacteria involved in the biological sulfur cycle for paper mill effluent purification. Sci. Total Environ. 407 (4), 1333–1343. Kamyshny, A., Goifman, A., Gun, J., Rizkov, D., Lev, O., 2004. Equilibrium distribution of polysulfide ions in aqueous solutions at 25 C: a new approach for the study of polysulfides’ equilibria. Environ. Sci. Technol. 38 (24), 6633–6644. Keller-Lehmann, B., Corrie, S., Ravn, R., Yuan, Z., Keller, J., 2006. Preservation and Simultaneous Analysis of Relevant Soluble Sulfur Species in Sewage Samples. BOKU-SIG, Vienna, Austria. Kudo, A., Miseki, Y., 2009. Heterogeneous photocatalyst materials for water splitting. Chem. Soc. Rev. 38 (1), 253–278. Lens, P.N.L., Visser, A., Janssen, A.J.H., Hulshoff Pol, L.W., Lettinga, G., 1998. Biotechnological treatment of sulfate-rich wastewaters. Crit. Rev. Environ. Sci. Technol. 28 (1), 41–88. Logan, B.E., 2009. Exoelectrogenic bacteria that power microbial fuel cells. Nat. Rev. Microbiol. 7 (5), 375–381.

2571

Logan, B., Cheng, S., Watson, V., Estadt, G., 2007. Graphite fiber brush anodes for increased power production in air-cathode microbial fuel cells. Environ. Sci. Technol. 41 (9), 3341–3346. Mao, Z., Anani, A., White, R.E., Srinivasan, S., Appleby, A.J., 1991. A modified electrochemical process for the decomposition of hydrogen sulfide in an aqueous alkaline solution. J. Electrochem. Soc. 138 (5), 1299–1303. Rabaey, K., VandeSompel, K., Maignien, L., Boon, N., Aelterman, P., Clauwaert, P., DeSchamphelaire, L., Pham, H.T., Vermeulen, J., Verhaege, M., Lens, P., Verstraete, W., 2006. Microbial fuel cells for sulfide removal. Environ. Sci. Technol. 40 (17), 5218–5224. Rajeshwar, K., Ibanez, J.G., Swain, G.M., 1994. Electrochemistry and the environment. J. Appl. Electrochem. 24 (11), 1077–1091. Reimers, C.E., Girguis, P., Stecher, H.A., Tender, L.M., Ryckelynck, N., Whaling, P., 2006. Microbial fuel cell energy from an ocean cold seep. Geobiology 4 (2), 123–136. Rozendal, R.A., Hamelers, H.V.M., Buisman, C.J.N., 2006. Effects of membrane cation transport on pH and microbial fuel cell performance. Environ. Sci. Technol. 40 (17), 5206–5211. Shih, Y.S., Lee, J.L., 1986. Continuous solvent extraction of sulfur from the electrochemical oxidation of a basic sulfide solution in the CSTER system. Ind. Eng. Chem. Process Des. Dev. 25 (3), 834–836. Steudel, R., 2000. The chemical sulfur cycle. In: Lens, P.N.L., Pol, L.H. (Eds.), Environmental Technologies to Treat Sulfur Pollution. IWA Publishing, London, pp. 1–31. Van Den Bosch, P.L.F., Sorokin, D.Y., Buisman, C.J.N., Janssen, A.J.H., 2008. The effect of pH on thiosulfate formation in a biotechnological process for the removal of hydrogen sulfide from gas streams. Environ. Sci. Technol. 42 (7), 2637–2642. Zhang, L., De Schryver, P., De Gusseme, B., De Muynck, W., Boon, N., Verstraete, W., 2008. Chemical and biological technologies for hydrogen sulfide emission control in sewer systems: a review. Water Res. 42 (1–2), 1–12.