Accepted Manuscript Electrochemical sulfide removal and caustic recovery from spent caustic streams Eleni Vaiopoulou, Thomas Provijn, Antonin Prévoteau, Ilje Pikaar, Korneel Rabaey PII:
S0043-1354(16)30038-0
DOI:
10.1016/j.watres.2016.01.039
Reference:
WR 11793
To appear in:
Water Research
Received Date: 6 October 2015 Revised Date:
30 December 2015
Accepted Date: 18 January 2016
Please cite this article as: Vaiopoulou, E., Provijn, T., Prévoteau, A., Pikaar, I., Rabaey, K., Electrochemical sulfide removal and caustic recovery from spent caustic streams, Water Research (2016), doi: 10.1016/j.watres.2016.01.039. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Electrochemical sulfide removal and caustic recovery from spent caustic
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streams
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Eleni Vaiopoulou1, Thomas Provijn1, Antonin Prévoteau1, Ilje Pikaar2, Korneel Rabaey1*
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Laboratory of Microbial Ecology & Technology, Faculty of Bioscience Engineering, University
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of Ghent; Coupure Links 653, 9000 Ghent, Belgium 2
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School of Civil Engineering, The University of Queensland, Brisbane QLD 4072, Australia *Corresponding author. Tel.: +32 9 264 5985; fax: +32 9 264 6248;
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E-mail address: [email protected]
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ABSTRACT
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Spent caustic streams (SCS) are produced during alkaline scrubbing of sulfide containing sour
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gases. Conventional methods mainly involve considerable chemical dosing or energy
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expenditures entailing high cost but limited benefits. Here we propose an electrochemical
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treatment approach involving anodic sulfide oxidation preferentially to sulfur coupled to
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cathodic caustic recovery using a two-compartment electrochemical system. Batch experiments
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showed sulfide removal efficiencies of 84 ± 4% with concomitant 57 ± 4% efficient caustic
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production in the catholyte at a final concentration of 6.4 ± 0.1 wt% NaOH (1.6 M) at an applied
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current density of 100 A m-2. Subsequent long-term continuous experiments showed that stable
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cell voltages (i.e. 2.7 ± 0.1 V) as well as constant sulfide removal efficiencies of 67 ± 5 % at a
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loading rate of 47 g(S) L-1 h-1 were achieved over a period of 77 days. Caustic was produced at
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industrially relevant strengths for scrubbing (i.e. 5.1 ± 0.9 wt% NaOH) at current efficiencies of
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96 ± 2 %. Current density between 0-200 A m-2 and sulfide loading rates of 50-200 g(S) L-1 d-1
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were tested. The higher the current density the more oxidized the sulfur species produced and the
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higher the sulfide oxidation. On the contrary, high loading rate resulted in a reduction of sulfide
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oxidation efficiency. The results obtained in this study together with engineering calculations
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show that that the proposed process could represent a cost-effective approach for sodium and
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sulfur recovery from SCS.
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Keywords: electrochemical treatment; spent caustic; sulfide; sodium hydroxide; recovery
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1. Introduction
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Hydrogen sulfide is a toxic, malodorous and corrosive compound. The removal of sulfide
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dissolved in wastewater and off-gases from chemical and petrochemical industrial activities
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represents a considerable cost (Maugans et al., 2010; Paulino and Alfonso, 2012; Veerabhadraiah
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et al., 2011). The resulting wastewater is known as spent caustic stream (SCS), named after the
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wasted or used caustic soda. A typical SCS contains 5-12 wt% NaOH and 0.1-4 wt% S2- and can
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be characterized as sulfidic, cresylic or naphthenic depending on their origin and composition
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(Alnaizy, R., 2008; Veerabhadraiah et al., 2011). The high pH and sulfide toxicity of SCS limit
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direct biological treatment, whereas neutralization and dilution may release H2S(g). SCS is a
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strong reducing agent and has a high oxygen demand (2 mol O2 per mol HS-) (Henshaw and Zhu,
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2001), resulting in dissolved oxygen depletion.
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The most commonly used methods to treat SCS involve physico-chemical processes including
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wet air oxidation and incineration (Alnaizy, R., 2008; Veerabhadraiah et al., 2011), oxidation
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with oxidant agents addition, precipitation and neutralization/acidification (Tanaka and
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Takenaka, 1995; Sheu and Weng, 2001), electrochemical (Hariz et al., 2013; Nuñez et al., 2009;
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Paulino and Alfonso, 2012), biological (De Graaf et al., 2012) or bio-electrochemical processes
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(Zhang et al., 2013).
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Despite the variety of available methods to treat SCS, the key limitations that restrict their
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application are cost, complexity, high consumption of chemicals, safety / handling issues
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(Alnaizy, R., 2008; Veerabhadraiah et al., 2011) and most importantly the lack of recovered
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product. Biological processes can alleviate some of these issues, as well as delivering
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hydrophilic sulfur as recovery product, but require SCS pre-treatment, biomass acclimation and
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sludge handling to overcome limitations imposed by high toxicity, pH and COD load.
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Therefore, there is a general interest in more cost-effective, energy efficient and chemical free
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methods such as (bio)electrochemical treatment. Several studies showed its feasibility via in situ
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production of e.g iron (Hariz et al., 2013), hypochlorous acid (Martinie et al., 2006), oxygen or
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other alike oxidizing agents, and possible coupling of sulfide removal to energy recovery (Kim
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and Han, 2014; Wei et al., 2012, 2013; Zhang et al., 2013). While these above mentioned studies
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revealed the potential and can be considered a step forward, they come with some disadvantages
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including sacrificial anodes, high-energy input to generate oxidizing agents and short life
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expectancy of materials.
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Here we propose a novel method that could avoid these concerns. The method relies on the
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simultaneous anodic oxidization of sulfide coupled to cathodic caustic generation in a two-
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compartment electrochemical cell. In the anode, sulfide is oxidized to elemental sulfur and other
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sulfur oxyanions, while in the cathode water is reduced to hydroxide anions. In order to maintain
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electroneutrality, sodium from the anode migrates through a cation exchange membrane (CEM)
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that separates the two chambers and allows the selective migration of sodium from the anode to
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the cathode chamber. Key advantages of this approach would be 1) elimination of chemical
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dosing for sulfide oxidation and thus, less operational, transport, handling and storage cost of
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potentially hazardous chemicals, which reduces occupational health and safety concerns, 2)
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recovery of sodium and oxidized sulfur species that can be re-used in situ or be sold, 3)
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straightforward process design, 4) potentially low energy demand that can be sourced from
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renewable supply and 5) a neutralized stream with lower salinity and sulfide is generated towards
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discharge. Therefore, the overall objective of this study is to investigate the feasibility of this
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approach and identify the key operational aspects.
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2. Materials and Methods
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2.1 Reactor setup and operation
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2.1.1 Batch-fed reactor
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The electrochemical cell consisted of two parallel Perspex frames with internal dimensions of 20
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× 5 × 2 cm separated by a CEM (Fumasep FKB-PK-130, Fumatech GmbH, Germany) according
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to (Pikaar et al., 2011). A reference electrode (Ag/AgCl (3M KCl), ALS, Japan, + 0.210 V vs.
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SHE at 25 °C) was placed in the anode compartment. A flattened mesh shaped tantalum-iridium
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mixed metal oxide (TaO2/IrO2 : 0.65/0.35) coated titanium electrode (Magneto Anodes BV, The
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Netherlands) with a projected surface area of 100 cm2 was used as anode material. Stainless steel
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fine mesh (projected surface area of 100 cm2) was used as cathode (mesh width 44 mµ, wire
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thickness: 33 mµ, Solana, Belgium) and a stainless steel frame was serving as current collector.
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A spacer (ElectroCell Europe A/S, Tarm, Denmark) was placed between the electrodes and the
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CEM to prevent membrane contact with the electrodes. The batch electrochemical cell was
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galvanostatically controlled using a power source (type PL-3003D, Protek) at a current density of
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100 A m-2.
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The anolyte consisted of 4 wt% NaOH and 1 wt% Na2S-S simulating a typical SCS. The
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catholyte was 4 wt% NaOH at the onset of the experiment which ensured sufficient initial
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conductivity and avoided putative non-electrically driven diffusion of sodium across the CEM. A
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recirculation flow of 6 L h-1 was applied to obtain sufficient mixing by a peristaltic pump
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(Watson-Marlow Inc., Massachusetts, US). Masterflex Norprene tubing with an internal diameter
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of 6 mm was used for both anolyte and catholyte and recirculation lines. H2 produced in the
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Figure 1 2.1.2 Continuous reactor operation
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The continuous-mode electrochemical cell (Fig. 1 red lines) was set up as a low-volume cell
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(internal dimensions of the cell compartments were 7 × 2 × 1 cm with 5 × 2 cm effective
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membrane area). SCS of the same composition as for batch mode was used as anolyte. The
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catholyte was initially 4 wt% sodium hydroxide and then distilled water was fed continuously at
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a flow rate of 82 ± 17 mL d-1 (HRT 4 h). Recirculation flow was set at 2 L h-1 to provide
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sufficient mixing in both compartments. Peristaltic pumps (Watson-Marlow Inc., Massachusetts,
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US) and flows were verified daily to assure accuracy and calculate standard deviations.
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Three different sets of experiments were performed. In the first one, the reactor was run in a
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continuous mode to determine a long-term operation performance at a fixed current density of
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100 A m-2. The operation time was run in four periods: 1) day 0-25 and 35-49; the anolyte flow
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rate was 128 ± 5 mL d-1 and sulfide loading rate (SLR) of 40 ± 3 g(S) L-1 d-1, 2) day 25-35; anode
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flow rate was decreased to 73 ± 6 mL d-1 and SLR was 26 ± 2 g(S) L-1d-1, 3) day 49-104; batch
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mode operation at 0.5 A m-2 (reactor remains assembled) and 4) day 104-132; anolyte flow rate
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was increased to 134 ± 6 mL d-1 and SLR was 47 ± 2 g(S) L-1d-1. The catholyte flow rate was kept
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constant at 82 ± 17 mL d-1. Along with flow rates, cell voltage, sulfur species, sodium and
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hydroxide concentrations were monitored on a daily basis. The second experiments assessed the
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impact of current density on reactor performance and sulfur speciation. Experiments were run at
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50, 100, 150 and 200 A m-2 and the values presented herein are the ones recorded in triplicates
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once a new steady state was reached (typically following the 5 times the hydraulic residence time
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thumb rule and as long as concentrations remained constant). Anolyte and catholyte flow rates
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were 121 ± 10 mL d-1 at SLR of 42 ± 4 g(S) L-1d-1 and 73 ± 3 mL d-1 respectively. The third
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experiments aimed to investigate the impact of SLR - by applying different flow rates ranging
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from 135 to 530 mL d-1. Experiments were run at 50, 100, 150 and 200 g(S) L-1d-1 at 100 A m-2
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and the values presented herein are the ones recorded in triplicates once a new steady state was
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reached. The ratio between the smallest SLR and smallest flow rate is slightly different than the
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ratio of the highest SLR and highest flow rate due to slight differences in HS- concentration
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when preparing feeding. An open circuit replicate was run to confirm that sulfide removal and
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NaOH recovery are only driven by the applied current. To assess whether sulfur species were
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crossing the membrane, catholyte samples were taken periodically.
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2.3 Chemical analysis
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Samples from the reactor were immediately preserved in previously prepared Sulfide
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Antioxidant Buffer solution prior to analysis as suggested by Keller-Lehmann et al. (2006).
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Sulfide, sulfite (SO32-) and thiosulfate (S2O32-) concentrations were measured by ion
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chromatography (IC), using an IC930 compact Metrohm IC system (Metrohm, Switzerland),
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according to Keller-Lehmann et al. (2006). The eluent consists of 3.5 mM Na2CO3 and 3mM
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NaHCO3 at a flow rate of 0.8 mL min-1. A 0.1 M NaOH solution is used to produce a pH
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gradient needed for thiosulfate detection in the IC system. Sulfate (SO42-) was determined on an
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IC761 compact Metrohm IC system (Metrohm, Switzerland) equipped with a Metrosep A Supp
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5-150 anion exchange column and a conductivity detector, according to Standard Methods
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(APHA, 1992). The eluent, consisting of 3.2 mM Na2CO3 and 1 mM NaHCO3, had a flow rate of
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0.7 mL min-1. To measure the polysulfide and elemental sulfur concentrations, all sulfur species
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were oxidized to sulfate with excess H2O2 as described elsewhere (Dutta et al., 2010). The
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difference in sulfur equivalent between the sulfate after H2O2 oxidation and other species
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measured before H2O2 oxidation (i.e. sulfide, sulfate, thiosulfate and sulfite) was regarded as the
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sum of polysulfides and elemental sulfur.
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Alkalinity, as indicator of NaOH concentration, is measured by titrating the cathode effluent with
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a 1 M HCl solution. Sodium was determined following procedures outlined in Standard Methods
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(APHA, 1992), using an IC761 compact Metrohm IC system (Metrohm, Switzerland) equipped
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with Metrosep C6-250/4.0 cation-exchange column and a conductivity meter. The eluent,
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consisting of 1.7 mM HNO3 and 1.7 mM dipicolinic acid, ran at a flow rate of 0.9 mL min-1.
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All samples were run in triplicates. Experimental values are provided as the mean +/- standard
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deviation.
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2.4 Electrochemical measurements and calculations
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During the continuous mode operation, cell voltage was monitored every 3 min with a VSP
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multichannel potentiostat (Princeton Applied Research, France). The electrical resistance of the
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cells was monitored by the current interrupt technique as described in the Supplementary
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Material. Current densities are reported with respect to the projected surface area of the anode.
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Depending on sulfide oxidation product, the oxidation process can involve different amount of
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electrons per sulfide consumed. Anodic reactions of sulfide oxidation are described elsewhere
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(Dutta et al., 2010). The coulombic efficiency (CE) for sulfide conversion was calculated
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assuming only the 2-electron conversion of sulfide to elemental sulfur as described elsewhere
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(Dutta et al., 2010). CE for sodium recovery is only restricted by membrane transport and was
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calculated as the ratio of sodium theoretically transferred due to current applied to hydroxyl
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anions produced (based on the assumption sodium cations and hydroxyl anions are equal, since
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no other cations are in solution).
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3. Results and Discussion
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3.1 Batch mode reactor
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At a fixed current density of 100 A m-2, 84 ± 4% of the sulfide was converted (Fig.2) at a
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coulombic efficiency (CE) of 75 ± 4%. A NaOH solution was recovered at an efficiency over the
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batch of 57 ± 4% at a CE of 91 ± 5%. The final concentration was 6.4 ± 0.1 wt% NaOH (1.6 M)
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after 8 h experiments. The pH of the anolyte was initially 13.7 but decreased to 13.2 ± 0.1 at the
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end of the experiment due to proton production from water splitting in the anode compartment
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(Fig.2). The remaining high pH cannot be directly discharged, but ensures that residual sulfide
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remains into solution rather than stripping off (pKa (H2S/HS-) = 6.9). In terms of energy input,
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cell voltage evolution shows a moderate increase over time from 1.79 to 2.47 V (Fig. S1). This
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increase is mainly because of sulfide depletion in the anolyte (low conductivity measured);
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eventually leading to more energy demanding O2 evolution once HS- mass transfer cannot
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sustain the current applied. This assumption is further based on the low internal resistance of the
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cell, which was 0.10 ± 0.03 Ω (SI). At this current (1 A), this implies an ohmic drop of 0.10 ±
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0.03 V accounting for about 4% of the operating voltage. The relatively low ohmic drop is
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attributed to the high conductivity of both electrolytes.
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Figure 2
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The effluent NaOH concentration of 1.6 M (at a fixed current density of 100 A m-2) is considered
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high, when compared with an electrodialysis SCS treatment that recovered up to 0.15 M NaOH
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at higher current densities (800 A m-2) (Wei et al., 2012). In the same study, experiments at fixed
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current density of 300 A m2 resulted in 0.05 M NaOH in 2 h batch experiments at CE of 100%.
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The higher CE in this case can be explained by the smaller electrode (7 cm2) and bigger
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catholyte volume leading to lower cathode concentrations ( > 500 cm2).
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3.2 Continuous mode reactor
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The electrochemical cell was operated in a continuous mode for 77 d exhibiting stable
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performance and reproducibility as confirmed by stable cell voltage, sulfide removal efficiency
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and restored values when disturbed (Fig.3). Cell voltage was constant at 2.74 ± 0.10 V for a
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sulfide loading rate (SLR) of 47 ± 2.2 g(S) L-1 h-1 and flow rate of 0.13 L d-1 in the anodic
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chamber. A SLR decrease from 40 ± 3.2 (day 0-25) to 26 ± 2.2 g(S) L-1 h-1 (day 25-34) resulted in
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higher cell voltage of 3.08 ± 0.26 V. The cell voltage was restored after the SLR increased again
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to 47 g(S) L-1 h-1 (day 35 - 45) (Fig.3). The same reproducible behavior was recorded after the
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batch mode operation period (day 49 - 104) and unexpected single day lab implications (day 47
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and day 124). Sulfide conversion efficiency for the overall 77 d of continuous operation was 67
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± 5 % at a CE of 54 ± 7 % for both SLR of 47 and 26 g(S) L-1 h-1. For the different SLR applied,
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sulfide conversion efficiency was 68 ± 5 % at a CE of 69 ± 3 % for a SLR of 47 g(S) L-1 h-1.
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When the SLR was decreased to 26 g(S) L-1 h-1, these efficiencies were 71 ± 8% and 45 ± 6%
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respectively,
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Figure 3
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Influent sulfide of 10.8 ± 0.3 g(S) L-1 was oxidized for 30% to thiosulfate (3.2 ± 0.3 g(S) L-1), then
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to sulfate (1.4 ± 0.1 g(S) L-1) for 13 %, to polysulfide and elemental sulfur (≈ 0.6 ± 0.7 g(S) L-1) for
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5.5 %, while 3.7 ± 0.1 g(S) L-1 of sulfide remained unconverted. These values come from analysis
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data and do not include the elemental sulfur that has been deposited on the electrode or other
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Clean NaOH solution was recovered in the catholyte via hydroxide electrogeneration and Na+
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migration across the CEM. As Na+ was the only cation present, alkalinity analysis results can be
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linked to NaOH concentrations. NaOH was produced at high CE (96 ± 2%) with a catholyte
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effluent concentration of 1.3 ± 0.1 M (5.1 ± 0.4 wt%).
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The iridium-tantalum oxide coated titanium anode showed stable performance through the whole
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experimental period of about 5 months, despite the harsh conditions of high sulfide
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concentrations and high pH. A study applying higher current densities than our range of 0-200 A
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m-2 reported corrosion and limited lifetime of a mixed iridium-tantalum oxide coated titanium
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electrode used as anode (Behm & Simonsson, 1997b). Corrosion in this case was attributed to
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the hydrodynamics, as it occurred mainly on points of turbulence, and at high applied potentials.
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Electrochemical sulfide oxidation at carbon electrodes can result in an elemental sulfur layer on
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the anode, causing electrode passivation (Dutta et al., 2008). Here the anode remained fully
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functional through the whole experimental period due to the different electrode material used
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(higher oxidizing power) and higher pH (neutral vs highly alkaline). In our study, possible
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mechanisms of continuous reactivation of the anode could be a de-flaking process by
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concomitant oxygen generation on the anode surface, sulfur dissolution by polysulfide anions in
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alkaline condition or further oxidation of sulfur in contact with the electrode to dissolved
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thiosulfate or sulfate. A joint mechanism could be possible if the inner layer of the elemental
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sulfur is oxidized to sulfur oxyanions while the remaining sulfur in the outer layer reacts with
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sulfides to form polysulphide (Behm & Simonsson, 1997a). This appears the most plausible
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mechanism as there was no observation of flakes or sulfur particles in the reactor or the effluent.
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3.3.Impact of current density and sulfide loading rate on sulfide oxidation products
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Current density affects anode potential and thus, further affects anode sulfide oxidation and the
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formation of more oxidized sulfur species. The higher the current density, the higher the
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proportion of sulfur oxyanions such as thiosulfate and sulfate became (Fig.4A). Controls in
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absence of current (OCV) showed no sulfide oxidation. At 50 A m-2 no sulfate but elemental
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sulfur and polysulfide were produced, at 100 and 150 A m-2 sulfide oxidation was gradually
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enhanced and concentrations of thiosulfate and sulfate increased, whereas at 200 A m-2 no steady
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state was achieved due to constantly increasing cell voltage (Fig.S2). Cell voltage attained the
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same values on the current density steps backwards (Fig.S2). The same trends were also
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confirmed for sulfur species production when repeating the experiments applying the reverse
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scheme of current densities, i.e. from 200 to 0 by steps of 50 A m-2 (Fig.4A, S2). The production
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of more oxidized sulfur species in higher current density is in agreement with the previously
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described sulfide oxidation mechanism. The same behaviour in high alkaline conditions has been
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observed elsewhere (Behm & Simonsson, 1997a; Kim and Han, 2014).
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Figure 4
Sulfide removal efficiency reached a maximum of 86 ± 3 % at 150 A m-2. The lowest recorded
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sulfide removal was 73 ± 1 % at 50 A m-2. The CE of sulfide oxidation decreased with increasing
253
current density, as expected. A CE higher than 100 % for sulfide oxidation at 50 A m-2 is likely
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related to polysulfide formation occurring at high pH at the electrode interface (Fig.4A) via the
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chemical reaction between previously formed S0 and dissolved sulfide (Behm & Simonsson,
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1997a). This reaction is thermodynamically and kinetically favored at high pH (Steudel and
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Eckert, 2003), which in our study is above 13. High sulfide loading rates (SLR) due to high
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influent flow rates resulted in lower overall sulfide removal and less oxidized sulfur species,
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which further induced elemental sulfur and polysulfide formation at high pH (Fig.4B). Higher
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SLR implied lower steady state voltage values and higher CE (Fig.S3). This is an apparent effect
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as it is mostly linked to the lower anode potential and low mass transfer limiting current for
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sulfide oxidation. Although it was not possible to differentiate analytically between elemental
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sulfur and polysulfide, at lower current densities the anode effluent had a characteristic intense
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yellow color that is indicative of polysulfide (Steudel and Eckert, 2003). Results regarding sulfur
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speciation, as far as polysulfide and elemental sulfur are concerned, were mostly qualitative
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based on the color and texture of the anode effluent.
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The NaOH production rate increased linearly with the current density increase at high CE (98 ± 3
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%). Effluent concentration of NaOH increased from 2.7 ± 0.1 to 9.8 ± 1.4 wt% at current
269
densities from 50 to 200 A m-2. Theoretically expected NaOH concentration values were
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calculated according to the current applied to drive the crossing of Na+ from anode to cathode
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compartment and catholyte flow rate, which was recorded daily (73 ± 3 mL d-1). Production
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efficiency of NaOH was 76 ± 2 % for current densities of 50 to 150 A m-2, based on the ratio of
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expected concentrations to actual sodium measurements. These findings imply, as expected, that
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the current density has within the range tested had no significant effect on CE, whereas caustic
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strength can have a greater impact as at higher NaOH concentrations, sodium and hydroxyl ion
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back diffusion might happen. Since the SLR experiments are based on flow rate changes in the
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anolyte, they have also no impact on the NaOH recovery. Effluent concentration of NaOH ranges
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from 5.2 ± 0.2 to 6.4 ± 0.8 wt% when SLR ranged from 50 to 150 g(S) L-1 h-1 and current density
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remained fixed at 100 A m-2. The fluctuations are explained by influent catholyte flow rate
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fluctuations ranging between 54 and 81 mL d-1. The measured values correspond well with the
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theoretically predicted values and they are well reproducible on the backwards step from 200 to
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50 g(S) L-1 h-1. CE is recorded close to 100 ±7 % for all experiments.
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3.5 Implications for practice
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Electrochemical SCS treatment was technologically feasible and resulted in recovery of sodium
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as a clean sodium hydroxide solution and different sulfide oxidation products, such as elemental
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sulfur, polysulfide, thiosulfate and sulfate. To implement such process into practice, there are a
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number of considerations to be addressed as a first step. These include design issues, economic
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viability, and optimization in terms of anode potential, higher caustic strengths, different
289
membranes to increase stability of the membrane and electrode over longer period of time etc.
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Further, the presence of other compounds such as organic pollutants need to be considered.
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Economics of the process would rely on the cost of investment, including materials and
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engineering cost, the operational cost and possible savings from recovery of chemicals and
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omission of other costly methods to remove sulfide. An estimation of this cost has been
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calculated as an indication only (Table S1). Further process optimization and testing with real
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wastewater is required before up-scaling this technology and calculating in more detail the
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process cost. Another preliminary techno-economical approach estimated electrochemical NaOH
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recovery cost from SCS at less than 1 USD per kg NaOH (Wei et al., 2012).
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4. Conclusions
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Electrochemical spent caustic steams treatment has been shown herein to potentially be
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an economically feasible and environmental friendly process that can recover valuable
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products from waste.
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Sulfide conversion can be driven towards elemental sulfur, polysulfide, thiosulfate and sulfate depending on the current density and sulfide loading rate.
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alleviate investment and operational costs. •
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Electrochemical sulfide oxidation and sodium recovery processes were robust and reactor materials remained unaffected during operation.
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Besides sulfur species, sodium can be recovered at high coulombic efficiencies and
Before this concept can be applied for more industrial applications and thus, promote implementation of sustainable technology and resource recovery, rigorous economic
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assessment, process optimization and testing in field conditions are required.
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Alnaizy, R., 2008. Economic Analysis for Wet Oxidation Processes for the Treatment of Mixed
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Refinery Spent Caustic. Environmental Progress 27 (3), 295–301.
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American Public Health Association (APHA), Greenberg, A.E., Clesceri, L.S. and Eaton, A.D
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1992. Standard Methods for the Examination of Water and Wastewater, 18th ed. Washington,
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DC, USA.
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Behm, M. & Simonsson, D. 1997a. Electrochemical production of polysulfides and sodium
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hydroxide from white liquor Part I: Experiments with rotating disc and ring-disc electrodes
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Behm, M. & Simonsson, D. 1997b. Electrochemical production of polysulfides and sodium
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hydroxide from white liquor Part II: Electrolysis in a laboratory scale flow cell. Journal of
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applied electrochemistry 27, 519-528.
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Buisman, C.J.N, Vellinga, S.H.J., Janssen, G.H.R. and Dijkman, H. 1999. Biological sulfide
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production for metal recovery. In TMS Congress 1999, Fundamentals of lead and zinc extraction
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for metal recovery 1–7.
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Hariz, I. Ben, Halleb, A., Adhoum, N., Monser, L. 2013. Treatment of petroleum refinery
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sulfidic spent caustic wastes by electrocoagulation. Separation and Purification Technology 107,
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aqueous sulfide. Water Research 42, 4965 – 4975.
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Dutta, P.K., Rabaey, K., Yuan, Z., Rozendal, R.A., Keller, J. 2010. Electrochemical sulfide
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removal and recovery from paper mill anaerobic treatment effluent Water Research 44, 2563 –
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Henshaw, P. F., Zhu, W. 2001. Biological conversion of hydrogen sulphide to elemental sulphur
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in a fixed-film continuous flow photo-reactor Water Research 35 (15), 3605-3610.
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Keller-Lehmann, B., Corrie, S., Ravn, R., Yuan, Z., Keller, J., 2006. Preservation and
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simultaneous analysis of relevant soluble sulfur species in sewage samples. In: 2nd International
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Kim, K. and Han, J.-I., 2014. Performance of direct alkaline sulfide fuel cell without sulfur
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deposition on anode International Journal of Hydrogen Energy 39, 7142 – 7146.
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Paulino, J.F. and Alfonso, J. 2012. New Strategies For Treatment and Reuse of Spent Sulfidic
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Caustic Streams from Petroleum Industry Quim. Nova 35 (7), 1447–1452
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Pikaar, I., Rozendal, R.A., Yuan, Z., Keller, J., Rabaey, K. 2011. Electrochemical sulfide
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removal from synthetic and real domestic wastewater at high current densities. Water Research
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Martinie, G.D., Shabrani, F.M.A., Dabrousi, B.O. 2006. Process for treating a Sulfur-containing
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spent caustic refinery stream using a membrane electrolyzer powered by a fuel cell. Pub. No.: US
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2006/0254930 A.
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Maugans, C., Howdeshell, M., De Haan, S. Apr 2010. Update: Spent caustic treatment.
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Hydrocarbon Processing, 89 (4), 61.
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Nuñez P., Hansen, H.K., Rodriguez, N., Guzman, J., Gutierrez C. 2009. Electrochemical
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Generation of Fenton's Reagent to Treat Spent Caustic Wastewater. Separation Science and
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Technology 44, 2223-2233.
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Sheu, S.H. and Weng H.S. 2001. Treatment of olefin plant spent caustic by combination of
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neutralization and Fenton reaction. Water Research 35 (8), 2017–2021.
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Steudel, R. and Eckert, B. 2003. Solid Sulfur Allotropes. Topics in Current Chemistry, 230, 1–
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79.
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Tanaka, N. and Takenaka, K., 1995. Control of hydrogen sulfide and degradation of organic
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matter by air injection into a wastewater force main. Water Science and Technology 31 (7), 273-
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282.
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Veerabhadraiah, G., Mallika, N., Jindal, S. 2011. Spent caustic management: Remediation
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review. Hydrocarbon Processing
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(http://www.hydrocarbonprocessing.com/Article/2925664/Spent-caustic-management-
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Remediation-review.html).
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Wei, Y. Li, C., Wang, Y., Zhang, X., Li, Q., Xu, T. 2012. Regenerating sodium hydroxide from
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the spent caustic by bipolar membrane electrodialysis (BMED) Separation and Purification
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Technology 86, 49–54.
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Wei, Y. Wang, Y., Zhang, X., Xu., T. 2013. Comparative study on regenerating sodium
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hydroxide from the spent caustic by bipolar membrane electrodialysis (BMED) and electro-
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electrodialysis (EED) Separation and Purification Technology 118, 1–5.
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Zhang, J. Zhang, B., Tian, C., Ye, Z., Liu, Y., Lei, Z., Huang, W., Feng, C. 2013. Simultaneous
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sulfide removal and electricity generation with corn stover biomass as co-substrate in microbial
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fuel cells. Bioresource technology 138, 198–203
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List of figure legends
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Fig.1 Schematic diagram of the electrochemical cell. The cell was run in batch (proof-of-concept
393
experiments) and continuous (long term experiments) mode. Anode influent consisted of 4 wt%
394
NaOH and 1 wt% Na2S-S, and cathode influent was 4 wt% NaOH for the batch experiments and
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distilled water for the continuous experiments.
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Fig. 2 Sulfide removal in the anode at high pH in 8h batch experiments and CE of 75± 4%.
397
Fig. 3 Cell voltage evolution during the long term experiment. Without considering the
398
interruption period (day 49 - 104), the decrease in loading rate (day 25 - 34), and other single day
399
lab implications (day 47 and day 124), a stable cell voltage of 2.74 ± 0.10 V was maintained for
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77 days of operation at 100 A m-2.
401
Fig. 4 (A) Impact of current density on sulfur speciation in the anolyte effluent. Influent SLR: 42
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± 4 g(S) L-1d-1, flow rate: 121 ± 10 mL d-1. Higher current densities result in more oxidized
403
species and higher sulfide oxidation. (B) Effect of different sulfide loading rates 50 - 200 g(S) L-1
404
h-1 by a step of 50 g(S) L-1 h-1 on sulfur speciation at 100 A m-2. Higher loading rates result in less
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sulfide removal, less oxidized sulfur species and induce polysulfide formation.
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Electrochemical spent caustic treatment allows economical product recovery
•
Current density and sulfide loading rate determine sulfide oxidation products
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Sodium is recovered at high coulombic efficiencies and alleviates costs
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Reactor materials remained unaffected during long term electrochemical operation
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S0043-1354(16)30038-0
DOI:
10.1016/j.watres.2016.01.039
Reference:
WR 11793
To appear in:
Water Research
Received Date: 6 October 2015 Revised Date:
30 December 2015
Accepted Date: 18 January 2016
Please cite this article as: Vaiopoulou, E., Provijn, T., Prévoteau, A., Pikaar, I., Rabaey, K., Electrochemical sulfide removal and caustic recovery from spent caustic streams, Water Research (2016), doi: 10.1016/j.watres.2016.01.039. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Electrochemical sulfide removal and caustic recovery from spent caustic
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streams
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Eleni Vaiopoulou1, Thomas Provijn1, Antonin Prévoteau1, Ilje Pikaar2, Korneel Rabaey1*
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Laboratory of Microbial Ecology & Technology, Faculty of Bioscience Engineering, University
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of Ghent; Coupure Links 653, 9000 Ghent, Belgium 2
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School of Civil Engineering, The University of Queensland, Brisbane QLD 4072, Australia *Corresponding author. Tel.: +32 9 264 5985; fax: +32 9 264 6248;
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E-mail address: [email protected]
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ABSTRACT
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Spent caustic streams (SCS) are produced during alkaline scrubbing of sulfide containing sour
18
gases. Conventional methods mainly involve considerable chemical dosing or energy
19
expenditures entailing high cost but limited benefits. Here we propose an electrochemical
20
treatment approach involving anodic sulfide oxidation preferentially to sulfur coupled to
21
cathodic caustic recovery using a two-compartment electrochemical system. Batch experiments
22
showed sulfide removal efficiencies of 84 ± 4% with concomitant 57 ± 4% efficient caustic
23
production in the catholyte at a final concentration of 6.4 ± 0.1 wt% NaOH (1.6 M) at an applied
24
current density of 100 A m-2. Subsequent long-term continuous experiments showed that stable
25
cell voltages (i.e. 2.7 ± 0.1 V) as well as constant sulfide removal efficiencies of 67 ± 5 % at a
26
loading rate of 47 g(S) L-1 h-1 were achieved over a period of 77 days. Caustic was produced at
27
industrially relevant strengths for scrubbing (i.e. 5.1 ± 0.9 wt% NaOH) at current efficiencies of
28
96 ± 2 %. Current density between 0-200 A m-2 and sulfide loading rates of 50-200 g(S) L-1 d-1
29
were tested. The higher the current density the more oxidized the sulfur species produced and the
30
higher the sulfide oxidation. On the contrary, high loading rate resulted in a reduction of sulfide
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oxidation efficiency. The results obtained in this study together with engineering calculations
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show that that the proposed process could represent a cost-effective approach for sodium and
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sulfur recovery from SCS.
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Keywords: electrochemical treatment; spent caustic; sulfide; sodium hydroxide; recovery
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1. Introduction
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Hydrogen sulfide is a toxic, malodorous and corrosive compound. The removal of sulfide
40
dissolved in wastewater and off-gases from chemical and petrochemical industrial activities
41
represents a considerable cost (Maugans et al., 2010; Paulino and Alfonso, 2012; Veerabhadraiah
42
et al., 2011). The resulting wastewater is known as spent caustic stream (SCS), named after the
43
wasted or used caustic soda. A typical SCS contains 5-12 wt% NaOH and 0.1-4 wt% S2- and can
44
be characterized as sulfidic, cresylic or naphthenic depending on their origin and composition
45
(Alnaizy, R., 2008; Veerabhadraiah et al., 2011). The high pH and sulfide toxicity of SCS limit
46
direct biological treatment, whereas neutralization and dilution may release H2S(g). SCS is a
47
strong reducing agent and has a high oxygen demand (2 mol O2 per mol HS-) (Henshaw and Zhu,
48
2001), resulting in dissolved oxygen depletion.
49
The most commonly used methods to treat SCS involve physico-chemical processes including
50
wet air oxidation and incineration (Alnaizy, R., 2008; Veerabhadraiah et al., 2011), oxidation
51
with oxidant agents addition, precipitation and neutralization/acidification (Tanaka and
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Takenaka, 1995; Sheu and Weng, 2001), electrochemical (Hariz et al., 2013; Nuñez et al., 2009;
53
Paulino and Alfonso, 2012), biological (De Graaf et al., 2012) or bio-electrochemical processes
54
(Zhang et al., 2013).
55
Despite the variety of available methods to treat SCS, the key limitations that restrict their
56
application are cost, complexity, high consumption of chemicals, safety / handling issues
57
(Alnaizy, R., 2008; Veerabhadraiah et al., 2011) and most importantly the lack of recovered
58
product. Biological processes can alleviate some of these issues, as well as delivering
59
hydrophilic sulfur as recovery product, but require SCS pre-treatment, biomass acclimation and
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sludge handling to overcome limitations imposed by high toxicity, pH and COD load.
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Therefore, there is a general interest in more cost-effective, energy efficient and chemical free
62
methods such as (bio)electrochemical treatment. Several studies showed its feasibility via in situ
63
production of e.g iron (Hariz et al., 2013), hypochlorous acid (Martinie et al., 2006), oxygen or
64
other alike oxidizing agents, and possible coupling of sulfide removal to energy recovery (Kim
65
and Han, 2014; Wei et al., 2012, 2013; Zhang et al., 2013). While these above mentioned studies
66
revealed the potential and can be considered a step forward, they come with some disadvantages
67
including sacrificial anodes, high-energy input to generate oxidizing agents and short life
68
expectancy of materials.
69
Here we propose a novel method that could avoid these concerns. The method relies on the
70
simultaneous anodic oxidization of sulfide coupled to cathodic caustic generation in a two-
71
compartment electrochemical cell. In the anode, sulfide is oxidized to elemental sulfur and other
72
sulfur oxyanions, while in the cathode water is reduced to hydroxide anions. In order to maintain
73
electroneutrality, sodium from the anode migrates through a cation exchange membrane (CEM)
74
that separates the two chambers and allows the selective migration of sodium from the anode to
75
the cathode chamber. Key advantages of this approach would be 1) elimination of chemical
76
dosing for sulfide oxidation and thus, less operational, transport, handling and storage cost of
77
potentially hazardous chemicals, which reduces occupational health and safety concerns, 2)
78
recovery of sodium and oxidized sulfur species that can be re-used in situ or be sold, 3)
79
straightforward process design, 4) potentially low energy demand that can be sourced from
80
renewable supply and 5) a neutralized stream with lower salinity and sulfide is generated towards
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discharge. Therefore, the overall objective of this study is to investigate the feasibility of this
82
approach and identify the key operational aspects.
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2. Materials and Methods
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2.1 Reactor setup and operation
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2.1.1 Batch-fed reactor
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The electrochemical cell consisted of two parallel Perspex frames with internal dimensions of 20
87
× 5 × 2 cm separated by a CEM (Fumasep FKB-PK-130, Fumatech GmbH, Germany) according
88
to (Pikaar et al., 2011). A reference electrode (Ag/AgCl (3M KCl), ALS, Japan, + 0.210 V vs.
89
SHE at 25 °C) was placed in the anode compartment. A flattened mesh shaped tantalum-iridium
90
mixed metal oxide (TaO2/IrO2 : 0.65/0.35) coated titanium electrode (Magneto Anodes BV, The
91
Netherlands) with a projected surface area of 100 cm2 was used as anode material. Stainless steel
92
fine mesh (projected surface area of 100 cm2) was used as cathode (mesh width 44 mµ, wire
93
thickness: 33 mµ, Solana, Belgium) and a stainless steel frame was serving as current collector.
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A spacer (ElectroCell Europe A/S, Tarm, Denmark) was placed between the electrodes and the
95
CEM to prevent membrane contact with the electrodes. The batch electrochemical cell was
96
galvanostatically controlled using a power source (type PL-3003D, Protek) at a current density of
97
100 A m-2.
98
The anolyte consisted of 4 wt% NaOH and 1 wt% Na2S-S simulating a typical SCS. The
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catholyte was 4 wt% NaOH at the onset of the experiment which ensured sufficient initial
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conductivity and avoided putative non-electrically driven diffusion of sodium across the CEM. A
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recirculation flow of 6 L h-1 was applied to obtain sufficient mixing by a peristaltic pump
102
(Watson-Marlow Inc., Massachusetts, US). Masterflex Norprene tubing with an internal diameter
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of 6 mm was used for both anolyte and catholyte and recirculation lines. H2 produced in the
104
cathode was collected in the cathode effluent bottle. 5
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Figure 1 2.1.2 Continuous reactor operation
107
The continuous-mode electrochemical cell (Fig. 1 red lines) was set up as a low-volume cell
108
(internal dimensions of the cell compartments were 7 × 2 × 1 cm with 5 × 2 cm effective
109
membrane area). SCS of the same composition as for batch mode was used as anolyte. The
110
catholyte was initially 4 wt% sodium hydroxide and then distilled water was fed continuously at
111
a flow rate of 82 ± 17 mL d-1 (HRT 4 h). Recirculation flow was set at 2 L h-1 to provide
112
sufficient mixing in both compartments. Peristaltic pumps (Watson-Marlow Inc., Massachusetts,
113
US) and flows were verified daily to assure accuracy and calculate standard deviations.
114
Three different sets of experiments were performed. In the first one, the reactor was run in a
115
continuous mode to determine a long-term operation performance at a fixed current density of
116
100 A m-2. The operation time was run in four periods: 1) day 0-25 and 35-49; the anolyte flow
117
rate was 128 ± 5 mL d-1 and sulfide loading rate (SLR) of 40 ± 3 g(S) L-1 d-1, 2) day 25-35; anode
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flow rate was decreased to 73 ± 6 mL d-1 and SLR was 26 ± 2 g(S) L-1d-1, 3) day 49-104; batch
119
mode operation at 0.5 A m-2 (reactor remains assembled) and 4) day 104-132; anolyte flow rate
120
was increased to 134 ± 6 mL d-1 and SLR was 47 ± 2 g(S) L-1d-1. The catholyte flow rate was kept
121
constant at 82 ± 17 mL d-1. Along with flow rates, cell voltage, sulfur species, sodium and
122
hydroxide concentrations were monitored on a daily basis. The second experiments assessed the
123
impact of current density on reactor performance and sulfur speciation. Experiments were run at
124
50, 100, 150 and 200 A m-2 and the values presented herein are the ones recorded in triplicates
125
once a new steady state was reached (typically following the 5 times the hydraulic residence time
126
thumb rule and as long as concentrations remained constant). Anolyte and catholyte flow rates
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were 121 ± 10 mL d-1 at SLR of 42 ± 4 g(S) L-1d-1 and 73 ± 3 mL d-1 respectively. The third
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experiments aimed to investigate the impact of SLR - by applying different flow rates ranging
129
from 135 to 530 mL d-1. Experiments were run at 50, 100, 150 and 200 g(S) L-1d-1 at 100 A m-2
130
and the values presented herein are the ones recorded in triplicates once a new steady state was
131
reached. The ratio between the smallest SLR and smallest flow rate is slightly different than the
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ratio of the highest SLR and highest flow rate due to slight differences in HS- concentration
133
when preparing feeding. An open circuit replicate was run to confirm that sulfide removal and
134
NaOH recovery are only driven by the applied current. To assess whether sulfur species were
135
crossing the membrane, catholyte samples were taken periodically.
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2.3 Chemical analysis
137
Samples from the reactor were immediately preserved in previously prepared Sulfide
138
Antioxidant Buffer solution prior to analysis as suggested by Keller-Lehmann et al. (2006).
139
Sulfide, sulfite (SO32-) and thiosulfate (S2O32-) concentrations were measured by ion
140
chromatography (IC), using an IC930 compact Metrohm IC system (Metrohm, Switzerland),
141
according to Keller-Lehmann et al. (2006). The eluent consists of 3.5 mM Na2CO3 and 3mM
142
NaHCO3 at a flow rate of 0.8 mL min-1. A 0.1 M NaOH solution is used to produce a pH
143
gradient needed for thiosulfate detection in the IC system. Sulfate (SO42-) was determined on an
144
IC761 compact Metrohm IC system (Metrohm, Switzerland) equipped with a Metrosep A Supp
145
5-150 anion exchange column and a conductivity detector, according to Standard Methods
146
(APHA, 1992). The eluent, consisting of 3.2 mM Na2CO3 and 1 mM NaHCO3, had a flow rate of
147
0.7 mL min-1. To measure the polysulfide and elemental sulfur concentrations, all sulfur species
148
were oxidized to sulfate with excess H2O2 as described elsewhere (Dutta et al., 2010). The
149
difference in sulfur equivalent between the sulfate after H2O2 oxidation and other species
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measured before H2O2 oxidation (i.e. sulfide, sulfate, thiosulfate and sulfite) was regarded as the
151
sum of polysulfides and elemental sulfur.
152
Alkalinity, as indicator of NaOH concentration, is measured by titrating the cathode effluent with
153
a 1 M HCl solution. Sodium was determined following procedures outlined in Standard Methods
154
(APHA, 1992), using an IC761 compact Metrohm IC system (Metrohm, Switzerland) equipped
155
with Metrosep C6-250/4.0 cation-exchange column and a conductivity meter. The eluent,
156
consisting of 1.7 mM HNO3 and 1.7 mM dipicolinic acid, ran at a flow rate of 0.9 mL min-1.
157
All samples were run in triplicates. Experimental values are provided as the mean +/- standard
158
deviation.
159
2.4 Electrochemical measurements and calculations
160
During the continuous mode operation, cell voltage was monitored every 3 min with a VSP
161
multichannel potentiostat (Princeton Applied Research, France). The electrical resistance of the
162
cells was monitored by the current interrupt technique as described in the Supplementary
163
Material. Current densities are reported with respect to the projected surface area of the anode.
164
Depending on sulfide oxidation product, the oxidation process can involve different amount of
165
electrons per sulfide consumed. Anodic reactions of sulfide oxidation are described elsewhere
166
(Dutta et al., 2010). The coulombic efficiency (CE) for sulfide conversion was calculated
167
assuming only the 2-electron conversion of sulfide to elemental sulfur as described elsewhere
168
(Dutta et al., 2010). CE for sodium recovery is only restricted by membrane transport and was
169
calculated as the ratio of sodium theoretically transferred due to current applied to hydroxyl
170
anions produced (based on the assumption sodium cations and hydroxyl anions are equal, since
171
no other cations are in solution).
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3. Results and Discussion
173
3.1 Batch mode reactor
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At a fixed current density of 100 A m-2, 84 ± 4% of the sulfide was converted (Fig.2) at a
175
coulombic efficiency (CE) of 75 ± 4%. A NaOH solution was recovered at an efficiency over the
176
batch of 57 ± 4% at a CE of 91 ± 5%. The final concentration was 6.4 ± 0.1 wt% NaOH (1.6 M)
177
after 8 h experiments. The pH of the anolyte was initially 13.7 but decreased to 13.2 ± 0.1 at the
178
end of the experiment due to proton production from water splitting in the anode compartment
179
(Fig.2). The remaining high pH cannot be directly discharged, but ensures that residual sulfide
180
remains into solution rather than stripping off (pKa (H2S/HS-) = 6.9). In terms of energy input,
181
cell voltage evolution shows a moderate increase over time from 1.79 to 2.47 V (Fig. S1). This
182
increase is mainly because of sulfide depletion in the anolyte (low conductivity measured);
183
eventually leading to more energy demanding O2 evolution once HS- mass transfer cannot
184
sustain the current applied. This assumption is further based on the low internal resistance of the
185
cell, which was 0.10 ± 0.03 Ω (SI). At this current (1 A), this implies an ohmic drop of 0.10 ±
186
0.03 V accounting for about 4% of the operating voltage. The relatively low ohmic drop is
187
attributed to the high conductivity of both electrolytes.
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Figure 2
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The effluent NaOH concentration of 1.6 M (at a fixed current density of 100 A m-2) is considered
190
high, when compared with an electrodialysis SCS treatment that recovered up to 0.15 M NaOH
191
at higher current densities (800 A m-2) (Wei et al., 2012). In the same study, experiments at fixed
192
current density of 300 A m2 resulted in 0.05 M NaOH in 2 h batch experiments at CE of 100%.
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The higher CE in this case can be explained by the smaller electrode (7 cm2) and bigger
194
catholyte volume leading to lower cathode concentrations ( > 500 cm2).
195
3.2 Continuous mode reactor
196
The electrochemical cell was operated in a continuous mode for 77 d exhibiting stable
197
performance and reproducibility as confirmed by stable cell voltage, sulfide removal efficiency
198
and restored values when disturbed (Fig.3). Cell voltage was constant at 2.74 ± 0.10 V for a
199
sulfide loading rate (SLR) of 47 ± 2.2 g(S) L-1 h-1 and flow rate of 0.13 L d-1 in the anodic
200
chamber. A SLR decrease from 40 ± 3.2 (day 0-25) to 26 ± 2.2 g(S) L-1 h-1 (day 25-34) resulted in
201
higher cell voltage of 3.08 ± 0.26 V. The cell voltage was restored after the SLR increased again
202
to 47 g(S) L-1 h-1 (day 35 - 45) (Fig.3). The same reproducible behavior was recorded after the
203
batch mode operation period (day 49 - 104) and unexpected single day lab implications (day 47
204
and day 124). Sulfide conversion efficiency for the overall 77 d of continuous operation was 67
205
± 5 % at a CE of 54 ± 7 % for both SLR of 47 and 26 g(S) L-1 h-1. For the different SLR applied,
206
sulfide conversion efficiency was 68 ± 5 % at a CE of 69 ± 3 % for a SLR of 47 g(S) L-1 h-1.
207
When the SLR was decreased to 26 g(S) L-1 h-1, these efficiencies were 71 ± 8% and 45 ± 6%
208
respectively,
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Figure 3
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Influent sulfide of 10.8 ± 0.3 g(S) L-1 was oxidized for 30% to thiosulfate (3.2 ± 0.3 g(S) L-1), then
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to sulfate (1.4 ± 0.1 g(S) L-1) for 13 %, to polysulfide and elemental sulfur (≈ 0.6 ± 0.7 g(S) L-1) for
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5.5 %, while 3.7 ± 0.1 g(S) L-1 of sulfide remained unconverted. These values come from analysis
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data and do not include the elemental sulfur that has been deposited on the electrode or other
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Clean NaOH solution was recovered in the catholyte via hydroxide electrogeneration and Na+
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migration across the CEM. As Na+ was the only cation present, alkalinity analysis results can be
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linked to NaOH concentrations. NaOH was produced at high CE (96 ± 2%) with a catholyte
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effluent concentration of 1.3 ± 0.1 M (5.1 ± 0.4 wt%).
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The iridium-tantalum oxide coated titanium anode showed stable performance through the whole
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experimental period of about 5 months, despite the harsh conditions of high sulfide
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concentrations and high pH. A study applying higher current densities than our range of 0-200 A
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m-2 reported corrosion and limited lifetime of a mixed iridium-tantalum oxide coated titanium
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electrode used as anode (Behm & Simonsson, 1997b). Corrosion in this case was attributed to
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the hydrodynamics, as it occurred mainly on points of turbulence, and at high applied potentials.
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Electrochemical sulfide oxidation at carbon electrodes can result in an elemental sulfur layer on
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the anode, causing electrode passivation (Dutta et al., 2008). Here the anode remained fully
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functional through the whole experimental period due to the different electrode material used
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(higher oxidizing power) and higher pH (neutral vs highly alkaline). In our study, possible
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mechanisms of continuous reactivation of the anode could be a de-flaking process by
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concomitant oxygen generation on the anode surface, sulfur dissolution by polysulfide anions in
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alkaline condition or further oxidation of sulfur in contact with the electrode to dissolved
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thiosulfate or sulfate. A joint mechanism could be possible if the inner layer of the elemental
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sulfur is oxidized to sulfur oxyanions while the remaining sulfur in the outer layer reacts with
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sulfides to form polysulphide (Behm & Simonsson, 1997a). This appears the most plausible
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mechanism as there was no observation of flakes or sulfur particles in the reactor or the effluent.
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3.3.Impact of current density and sulfide loading rate on sulfide oxidation products
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Current density affects anode potential and thus, further affects anode sulfide oxidation and the
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formation of more oxidized sulfur species. The higher the current density, the higher the
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proportion of sulfur oxyanions such as thiosulfate and sulfate became (Fig.4A). Controls in
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absence of current (OCV) showed no sulfide oxidation. At 50 A m-2 no sulfate but elemental
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sulfur and polysulfide were produced, at 100 and 150 A m-2 sulfide oxidation was gradually
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enhanced and concentrations of thiosulfate and sulfate increased, whereas at 200 A m-2 no steady
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state was achieved due to constantly increasing cell voltage (Fig.S2). Cell voltage attained the
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same values on the current density steps backwards (Fig.S2). The same trends were also
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confirmed for sulfur species production when repeating the experiments applying the reverse
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scheme of current densities, i.e. from 200 to 0 by steps of 50 A m-2 (Fig.4A, S2). The production
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of more oxidized sulfur species in higher current density is in agreement with the previously
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described sulfide oxidation mechanism. The same behaviour in high alkaline conditions has been
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observed elsewhere (Behm & Simonsson, 1997a; Kim and Han, 2014).
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Figure 4
Sulfide removal efficiency reached a maximum of 86 ± 3 % at 150 A m-2. The lowest recorded
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sulfide removal was 73 ± 1 % at 50 A m-2. The CE of sulfide oxidation decreased with increasing
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current density, as expected. A CE higher than 100 % for sulfide oxidation at 50 A m-2 is likely
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related to polysulfide formation occurring at high pH at the electrode interface (Fig.4A) via the
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chemical reaction between previously formed S0 and dissolved sulfide (Behm & Simonsson,
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1997a). This reaction is thermodynamically and kinetically favored at high pH (Steudel and
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Eckert, 2003), which in our study is above 13. High sulfide loading rates (SLR) due to high
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influent flow rates resulted in lower overall sulfide removal and less oxidized sulfur species,
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which further induced elemental sulfur and polysulfide formation at high pH (Fig.4B). Higher
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SLR implied lower steady state voltage values and higher CE (Fig.S3). This is an apparent effect
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as it is mostly linked to the lower anode potential and low mass transfer limiting current for
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sulfide oxidation. Although it was not possible to differentiate analytically between elemental
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sulfur and polysulfide, at lower current densities the anode effluent had a characteristic intense
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yellow color that is indicative of polysulfide (Steudel and Eckert, 2003). Results regarding sulfur
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speciation, as far as polysulfide and elemental sulfur are concerned, were mostly qualitative
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based on the color and texture of the anode effluent.
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The NaOH production rate increased linearly with the current density increase at high CE (98 ± 3
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%). Effluent concentration of NaOH increased from 2.7 ± 0.1 to 9.8 ± 1.4 wt% at current
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densities from 50 to 200 A m-2. Theoretically expected NaOH concentration values were
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calculated according to the current applied to drive the crossing of Na+ from anode to cathode
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compartment and catholyte flow rate, which was recorded daily (73 ± 3 mL d-1). Production
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efficiency of NaOH was 76 ± 2 % for current densities of 50 to 150 A m-2, based on the ratio of
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expected concentrations to actual sodium measurements. These findings imply, as expected, that
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the current density has within the range tested had no significant effect on CE, whereas caustic
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strength can have a greater impact as at higher NaOH concentrations, sodium and hydroxyl ion
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back diffusion might happen. Since the SLR experiments are based on flow rate changes in the
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anolyte, they have also no impact on the NaOH recovery. Effluent concentration of NaOH ranges
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from 5.2 ± 0.2 to 6.4 ± 0.8 wt% when SLR ranged from 50 to 150 g(S) L-1 h-1 and current density
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remained fixed at 100 A m-2. The fluctuations are explained by influent catholyte flow rate
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fluctuations ranging between 54 and 81 mL d-1. The measured values correspond well with the
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theoretically predicted values and they are well reproducible on the backwards step from 200 to
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50 g(S) L-1 h-1. CE is recorded close to 100 ±7 % for all experiments.
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3.5 Implications for practice
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Electrochemical SCS treatment was technologically feasible and resulted in recovery of sodium
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as a clean sodium hydroxide solution and different sulfide oxidation products, such as elemental
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sulfur, polysulfide, thiosulfate and sulfate. To implement such process into practice, there are a
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number of considerations to be addressed as a first step. These include design issues, economic
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viability, and optimization in terms of anode potential, higher caustic strengths, different
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membranes to increase stability of the membrane and electrode over longer period of time etc.
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Further, the presence of other compounds such as organic pollutants need to be considered.
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Economics of the process would rely on the cost of investment, including materials and
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engineering cost, the operational cost and possible savings from recovery of chemicals and
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omission of other costly methods to remove sulfide. An estimation of this cost has been
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calculated as an indication only (Table S1). Further process optimization and testing with real
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wastewater is required before up-scaling this technology and calculating in more detail the
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process cost. Another preliminary techno-economical approach estimated electrochemical NaOH
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recovery cost from SCS at less than 1 USD per kg NaOH (Wei et al., 2012).
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4. Conclusions
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Electrochemical spent caustic steams treatment has been shown herein to potentially be
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an economically feasible and environmental friendly process that can recover valuable
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products from waste.
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Sulfide conversion can be driven towards elemental sulfur, polysulfide, thiosulfate and sulfate depending on the current density and sulfide loading rate.
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alleviate investment and operational costs. •
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Electrochemical sulfide oxidation and sodium recovery processes were robust and reactor materials remained unaffected during operation.
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Besides sulfur species, sodium can be recovered at high coulombic efficiencies and
Before this concept can be applied for more industrial applications and thus, promote implementation of sustainable technology and resource recovery, rigorous economic
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assessment, process optimization and testing in field conditions are required.
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Alnaizy, R., 2008. Economic Analysis for Wet Oxidation Processes for the Treatment of Mixed
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List of figure legends
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Fig.1 Schematic diagram of the electrochemical cell. The cell was run in batch (proof-of-concept
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experiments) and continuous (long term experiments) mode. Anode influent consisted of 4 wt%
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NaOH and 1 wt% Na2S-S, and cathode influent was 4 wt% NaOH for the batch experiments and
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distilled water for the continuous experiments.
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Fig. 2 Sulfide removal in the anode at high pH in 8h batch experiments and CE of 75± 4%.
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Fig. 3 Cell voltage evolution during the long term experiment. Without considering the
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interruption period (day 49 - 104), the decrease in loading rate (day 25 - 34), and other single day
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lab implications (day 47 and day 124), a stable cell voltage of 2.74 ± 0.10 V was maintained for
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77 days of operation at 100 A m-2.
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Fig. 4 (A) Impact of current density on sulfur speciation in the anolyte effluent. Influent SLR: 42
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± 4 g(S) L-1d-1, flow rate: 121 ± 10 mL d-1. Higher current densities result in more oxidized
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species and higher sulfide oxidation. (B) Effect of different sulfide loading rates 50 - 200 g(S) L-1
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h-1 by a step of 50 g(S) L-1 h-1 on sulfur speciation at 100 A m-2. Higher loading rates result in less
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sulfide removal, less oxidized sulfur species and induce polysulfide formation.
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Electrochemical spent caustic treatment allows economical product recovery
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Current density and sulfide loading rate determine sulfide oxidation products
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Sodium is recovered at high coulombic efficiencies and alleviates costs
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Reactor materials remained unaffected during long term electrochemical operation
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