Electrochemical Production of Magnetite Nanoparticles for Sulfide

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Electrochemical production of magnetite nanoparticles for sulfide control in sewers Hui-Wen Lin, Kenny Couvreur, Bogdan C. Donose, Korneel Rabaey, Zhiguo Yuan, and Ilje Pikaar Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b01748 • Publication Date (Web): 12 Oct 2017 Downloaded from http://pubs.acs.org on October 12, 2017

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Environmental Science & Technology

Electrochemical production of magnetite nanoparticles for sulfide control in sewers

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Authors: Hui-Wen Lin1, Kenny Couvreur2, Bogdan C. Donose3, Korneel Rabaey1,2, Zhiguo

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Yuan1 and Ilje Pikaar1,4*

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Australia

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9000 Ghent, Belgium

The University of Queensland, Advanced Water Management Centre (AWMC), QLD 4072,

Center for Microbial Ecology and Technology (CMET), Ghent University, Coupure Links 653,

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The University of Queensland, The School of Chemical Engineering, QLD 4072, Australia

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The University of Queensland, The School of Civil Engineering, QLD 4072, Australia

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*Correspondence should be addressed to: Ilje Pikaar, The School of Civil Engineering, The

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University of Queensland, St. Lucia, QLD 4072, Australia

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Phone: +61 7 3345 1389; E-mail: [email protected]

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1 ACS Paragon Plus Environment


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Abstract

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Recently, naturally occurring magnetite (Fe3O4) has emerged as a new material for sulfide

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control in sewers. However, unrefined magnetite could have high heavy metal contents (e.g. Cr,

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Zn, Ni, Sn, etc.) and the capacity to remove dissolved sulfide is reasonably limited due to

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relatively large particle sizes. To overcome the drawbacks of unrefined magnetite we used an

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electrochemical system with mild steel as sacrificial electrodes to in-situ generate high strength

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solutions of plate-like magnetite nanoparticles (MNP). MNP with a size range between 120 to

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160 nm were electrochemically generated at 9.35±0.28 g Fe3O4-Fe/L, resulting in a Coulombic

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efficiency (CE) for iron oxidation of 93.5±2.8 %. The produced MNP were found to effectively

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reduce sulfide levels in sewage from 12.7±0.3 to 0.2±0.0 mg S/L at a sulfide-to-MNP ratio of

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0.26 g S/g Fe3O4-Fe. Subsequently, MNP were continuously generated with polarity switching at

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stable cell voltage for 31 days at 4.53±0.35 g Fe3O4-Fe/L with a CE for iron oxidation of

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92.4±7.2 %. The continuously produced MNP reduced sulfide at similar levels to around 0.2 mg

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S/L at a ratio of 0.28 g S/g Fe3O4-Fe.

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TOC art

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Introduction

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Generation of hydrogen sulfide (H2S) in sewers by sulfate reducing bacteria (SRB) is a notorious

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problem globally.1-2 H2S is responsible for concrete corrosion of the sewer pipes, releasing

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obnoxious odours and posing a threat to sewer workers due to its odorous and toxic nature.3-4

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Currently available sulfide control methods mainly involve chemical addition to sewers to either

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prevent H2S generation by SRB or mitigate its effect after H2S formation.5 Most commonly used

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chemicals for sulfide control are addition of sodium hydroxide, magnesium hydroxide, nitrate,

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iron salts and oxygen.6

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Magnetite, Fe2+(Fe3+)2O4 or in short Fe3O4, is a mixed-valence ferrimagnetic iron oxide

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which has been widely applied in several industrial processes, environmental and bio-medical

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applications.7-11 In a recent study, purchased magnetite powder (a particle size range of 45 to 60

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µm) reduced the sulfide level in synthetic sewage, thereby showing the feasibility of using

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magnetite for sulfide control in sewers.12 However, the study suggested that the metal impurities

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in raw magnetite should be limited to acceptable content in order to minimize the impact on

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sewage quality.12 Indeed, heavy metal contents (e.g. Cr, Zn, Ni, Sn, etc.) in raw magnetite are

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reported to be diverse, ranging from 0 to ~100000 ppm.13 Introducing raw and unrefined

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magnetite into sewers may result in contaminated sludge, reducing its reuse options. Therefore,

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purified magnetite is a necessity for sewer application. As high-purity magnetite is often high-

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priced, a new cost-effective method to produce purified magnetite for sulfide control in sewers is

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needed in order to become a viable and cost effective sulfide control method.

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Zhang et al.12 showed that with the reported optimal magnetite dosage of 40 mg/L,

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sulfide concentration was reduced from 6.5 mg/L to 1.3 mg/L (i.e. a 79% sulfide reduction;

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equivalent to a sulfide removal capacity of 197.3 mg S/g Fe3O4-Fe) over a period of 96 hours.



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Better sulfide control performance of magnetite is necessary as minor concrete corrosion may

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already occur at sulfide concentration of 0.1 to 0.5 mg S/L14, which indicates more input of

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magnetite is needed in order to further lower the sulfide concentration to less than 0.5 mg S/L.

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However, the study showed that the lowest attainable sulfide concentration using powdered

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magnetite was around 1.1 mg S/L at magnetite dosage of 60 mg/L, increasing magnetite dosage

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from 60 to 100 mg/L did not further reduce the sulfide concentration.12 In addition, many studies

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have indicated that nanoscale magnetite exhibits adsorbent properties and suggested that the

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smaller the particle size, the higher capacity of magnetite to reduce contaminants (e.g. arsenite,

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arsenate and phosphate).15-17 Hence, reducing the magnetite particle size from microscale (i.e.

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purchased magnetite powder; with a particle size range of 45 to 60 µm) to nanoscale is likely to

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improve its capability of sulfide control, resulting in lower sulfide concentration in sewage.

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While the majority of the current MNP applications (e.g. rotary shaft sealing, oscillation

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damping, medical diagnosis, etc.) require the particle size to be smaller than 20 nm7, the

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application of MNP for sulfide control in sewers is not limited by particle size. Many methods to

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synthetize MNP have been extensively studied, including electrochemical methods.8,

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Currently available studies on electrochemical generation of MNP in water were only conducted

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over a short period (i.e. < 12 hours) and low Fe3O4 strengths (i.e. < 0.71 g Fe3O4-Fe/L) were

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reported.20-23 In addition, these studies do not investigate the feasibility of the produced MNP for

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sulfide control. Therefore, this study aimed to demonstrate a cost-effective electrochemical

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method for production of MNP in water at practical relevant strength (i.e. 5 to 10 g Fe/L) for

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sulfide control.

18-19

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Here, short-term batch experiments were carried out to determine the product strength,

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structural characterisation, morphology, particle size of the produced MNP. Subsequently,


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sulfide removal tests were performed to confirm the ability of produced MNP to remove sulfide.

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In addition, a long-term experiment mimicking on-site operation was carried out, continuously

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producing MNP over a period of 31 days to determine the feasibility to continuously produce

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MNP and while remaining its sulfide control capability. Finally, based on the experimental

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results obtained in this study, we conducted a cost benefit analysis to assess the economic

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potential. Overall, this study showed the effective sulfide control using in-situ electrochemical

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generation of MNP.

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Material and methods

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Electrochemical cell and operation

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A schematic overview of the experimental setup is given in Figure S1 of the supporting

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information. Experiments were conducted using single-compartment electrochemical cells made

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of polycarbonate container (6.5 cm × 15.0 cm) with a volume of 500 mL. Experiments were

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operated in either batch mode (short-term experiments) or continuous mode (long-term

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experiment). As studies showed pure MNP could form in NaCl solution20-21, a 2 g/L NaCl

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solution prepared using tap water was used as the electrolyte in all experiments. Two iron plates

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(mild steel plat bar, Total Steel Australia) were placed in parallel, with an inter-electrode gap of

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either 0.4 cm (short-term experiments) or 1.0 cm (long-term experiment), and used as both anode

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and cathode electrodes. The dimensions of the iron plates were 15.0 cm × 1.4 cm × 0.3 cm in the

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short-term experiments and 10.0 cm × 3.6 cm × 0.4 cm in the long-term experiment. Electrodes

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were submerged at a depth of either 3.8 cm (short-term experiments) or 2.8 cm (long-term

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experiment) in the electrolyte, resulting in a geometric surface area submerged per electrode of

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12.5 and 23.5 cm2, respectively. Both electrodes were galvanostatically controlled using a



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Wenking potentiostat/galvanostat (KP07, Bank Elektronik, Germany). A data acquisition unit

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(Agilent Technologies, USA) was used to record electrode potentials every 2 minutes. Two

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Ag/AgCl reference electrodes (CH Instrument, Inc., USA) were equipped near anode and

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cathode surfaces (1 M KCl, assumed at + 0.235 V vs. SHE). Cell voltage, anode and cathode

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potentials were recorded throughout the duration of the experiments. All electrode potentials

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reported were adjusted versus SHE. The applied current density was calculated with respect to

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the geometric surface area of the corresponding electrode submerged in the electrolyte. All

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experiments were operated at a fixed current density of 8 mA/cm2.

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Experimental procedures

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Calculation of electron input

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The theoretical electron input for the formation of MNP in this study was calculated according to

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Dubrawski et al.24, which experimentally confirmed Fe2+ ions were firstly dissolved in water by

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electro-oxidation of iron anode and underwent further oxidation reaction with oxygen ( i.e.

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original oxygen content in water and atmospheric oxygen influx) to form to Fe3+ ions,

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immediately forming pure MNP from Fe3+. The oxidation state of Fe used in this study for

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electrochemical MNP production was therefore +2 (assuming electron inputs are 100% used for

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oxidation of iron to Fe2+) rather than the average value of 2.67 in Fe2+(Fe3+)2O4.19

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Short-term batch experiments

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Two sets of batch-mode experiments were conducted aiming to produce MNP solutions at a

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concentration of 10 g Fe/L for characterization and chemical analysis of MNP as well as for

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sulfide removal tests. In the first set of experiments (n=4), 200 mL electrolyte was used with a


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total applied current of 100 mA for 19.2 h. The pH values and dissolved oxygen (DO)

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concentrations were recorded throughout the course of the experiments. At the end of

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experiment, the electrochemical cell was stopped and both electrodes were rinsed with 1M

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hydrochloric acid (HCl) solution to dissolve the residual samples on the electrode surfaces.

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Subsequently, the electrolyte was acidified using 1M HCl solution to pH < 1 and analyzed to

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determine the total Fe concentration. The Coulombic Efficiency (CE) for iron oxidation was

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determined using total Fe concentration measured. The CE for iron oxidation was calculated

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according to Panizza et al.25 (see supporting information).

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In the second set of experiments (n=6), 250 mL electrolyte was used with a total applied

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current of 100 mA for 24 h. The pH of the electrolyte was measured at t = 0 h and 24 h. At the

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end of experiment, the electrochemical cell was stopped and 10 mL sample was taken to

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determine the MNP strength in terms of total Fe concentration. The produced MNP solution was

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immediately used for sulfide removal tests (a detailed description of the experimental protocol

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can be found in the section sulfide removal tests). A 100 mL sample was collected and analysed

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for structural characterisation of generated iron oxide solution using X-ray diffraction (XRD). A

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2 mL sample was collected and analyzed using scanning electron microscope (SEM) to

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determine the morphology of generated iron oxide solution. Another 2 mL sample was collected

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for the measurements of the particle size.

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Long-term continuous experiment

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The continuous experiment was carried out to determine the long-term cell performance in terms

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of CE for MNP generation (i.e. iron oxidation) as well as the energy consumption of the system.

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200 mL electrolyte was used with fresh electrolyte continuously added to the electrochemical



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cell at a flow rate of 0.96 L/day using a peristaltic pump (Watson-Marlow), resulting in a

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hydraulic retention time of 5 h. A total applied current of 188 mA was used to produce MNP at a

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concentration of 4.90 g Fe/L. Polarity switching of the electrodes was performed every 3.5 days

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in order to equally dissolve both electrodes. To compare the long-term synthesized MNP with

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short-term synthesized MNP, a 180 mL sample was collected on day 31 for XRD analysis and

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particle size measurement. SEM measurements using 2 mL sample were performed on day 9, 14,

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19, 22, 24, 28 and 31. Both total Fe concentrations and pH values of samples were determined on

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day 9, 14, 19, 22, 24, 28 and 31. Three sets of sulfide removal tests using the produced MNP

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solution were carried out on day 22, 24 and 28 (see section Sulfide removal tests).

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Sulfide removal tests

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Fresh sewage was collected from a local pumping station in Brisbane on a weekly basis and

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immediately stored at 4 °C. The typical domestic sewage contained 10 to 25 mg/L sulfate-S, < 3

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mg/L dissolved sulfide-S and negligible amount of sulfite and thiosulfate. The DO concentration

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in sewage was negligible and the average pH of the sewage was 7.36±0.11. Three 2 L Schott

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bottles were modified to be airtight, to prevent sulfide oxidation by oxygen from the atmosphere.

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Prior to sulfide removal tests, a volume of 1990 mL sewage was heated up to 20±1 °C using a

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water bath. Subsequently, the sewage was flushed with nitrogen gas for 15 minutes to ensure

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there was no residual oxygen in either sewage or the headspace of the reactor. 10 mL sulfide

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stock solution (3 g S/L) prepared by dissolving Na2S.9H2O (Sigma-Aldrich) in nitrogen flushed

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Milli-Q water was added to the sewage to obtain a desired sulfide concentration of

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approximately 15 mg S/L in sewage. As the sewage pH increased to > 8 after the addition of

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sulfide, a diluted HCl solution was used to lower the sewage pH to 7.2, which is more


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representative of sewage pH. Different amounts of the produced MNP solution added for each

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sulfide removal test are given in Table S1 of the supporting information. Each sulfide removal

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test was done in triplicate with an experimental duration of 6 h. To confirm sulfide is neither

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emitted to the atmosphere nor removed by sulfide oxidation, a control experiment without the

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addition of MNP was performed using the same experimental set-up and protocol. Sewage

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samples were taken before MNP addition at t = 0 min (at sewage pH of ~ 7.2) and after MNP

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addition at t= 3, 15, 30, 60, 90, 180 and 360 mins for the determination of dissolved sulfide

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concentration. Total metal concentration and pH value of the sewage samples were determined at

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t = 0 and 360 mins.

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Methods of chemical analysis and characteristics

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Total Fe concentration was measured using UV–Visible spectrophotometer (Varian Cary 50 Bio

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UV–Visible spectrophotometer) according to Liu et al.26 Total metal concentration (including

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total Fe) was measured using inductively coupled plasma (ICP) (ICP−OES Optima 7300DV).

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Dissolved sulfide and chloride concentrations were measured using Ion Chromatography (IC)

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equipped with a Dionex 2010i system according to Keller-Lehmann et al.27 XRD patterns was

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detected using a X-ray diffractometer (Bruker D8). Prior to XRD analysis, sample was dried in a

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stream of nitrogen at room temperature. Scanning electron microscope (SEM) images were

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analysed using a field emission gun SEM (JEOL JSM-7001F). Before SEM imaging, samples

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were deposited on aluminium foil and dried overnight in a vacuum cabinet. Particle size was

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measured using dynamic light scattering (Zetasizer Nano ZS, Malvern Instruments). DO

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concentrations and pH values were measured using an on-line DO sensor (Mettler Toledo,

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Switzerland) and pH sensor (Hanna Instrument, USA), respectively.



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Results

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Characterisations of MNP

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Figure 1 shows the results of characterisations of the produced black colloidal iron oxide

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solutions. Samples were taken from the second set of short-term experiment and the long-term

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experiment (day 31). Figure 1A shows the XRD patterns of the nitrogen dried iron oxide

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powders which have 7 characteristic peaks at 2 Theta values of: 30.5◦, 35.9◦, 37.0◦, 43.5◦, 53.6◦,

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57.3◦ and 63.1◦, matching the standard XRD patterns of Fe3O4. Therefore, both iron oxide

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solutions were identified as Fe3O4 by X-ray diffraction. For both types of samples presented in

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Figure 1A, diffractograms show traces of maghemite (Fe2O3) potentially resulted from the brief

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exposure to air and magnesioferrite, due to the present of Mg2+ in the tap water. Results of

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particle size distribution confirmed that MNP with a uniform particle size range between 120 and

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160 nm were generated in both short-term and long-term experiments (Figure 1B). In addition,

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the SEM images of short-term and long-term synthesized MNP are shown in Figure 1C and 1D

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(day 31), respectively, confirming that plate-like Fe3O4 was electrochemically generated at

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nanoscale (also see Figure S2 of the supporting information for an enlarged SEM image of

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hexagonal plate-like MNP). Other SEM images of long-term synthesized MNP on day 9, 14, 19,

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22, 24, 28 showed no obvious difference in the shape of MNP (data not shown).

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Short-term MNP production

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Four 19.2 h short-term experiments were conducted. Typical profiles of anode and cathode

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potentials and the cell voltage are shown in Figure S3 of the supporting information. The average

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anode potential and cell voltage were -0.16±0.05 V versus SHE and 1.84±0.12 V, respectively.


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Giving this low anode potential observed, several electrochemical reactions including oxidation

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of water to oxygen (E0= +1.23 V), oxidation of chloride to chlorine (E0= +1.36 V) and oxidation

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of Fe2+ to Fe3+ (E0= +0.77 V) were therefore excluded. Changes in pH values and DO

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concentrations in the electrolyte over the course of the experiment can be found in Figure S4of

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the supporting information. An overall increase in pH value of the electrolyte from 8.05±0.02 to

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9.70±0.31 was observed. The DO concentration in the electrolyte decreased from 9.18±0.34 to 0

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mg/L within 15 minutes and remained 0 mg/L throughout the experiment, which indicated MNP

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were electrochemically generated as a primary product (formation within seconds) as described

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by Dubrawski et al.24 The average total Fe concentration was determined as 9.35±0.28 g Fe/L

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using UV/Vis spectroscopy and further confirmed by ICP results (9.98±0.43 g Fe/L) as well as

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the weight loss of anode electrode (2.05±0.01 g Fe in 200 mL electrolyte). Taking the total Fe

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concentration into account, the CE for iron oxidation was 93.5±2.8%.

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MNP for sulfide control

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The results of sulfide removal using batch mode produced MNP are presented in Figure 2. For a

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theoretical MNP dosage of 26.1 mg Fe/L, the measured dissolved sulfide concentration in

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sewage before the addition of MNP was 12.0±0.3 mg S/L. After MNP addition, the dissolved

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sulfide concentration rapidly decreased to 4.5±0.1 mg S/L within 30 minutes and then slowly

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decreased to 2.8±0.3 mg S/L within 6 hours. This decrease in dissolved sulfide concentration

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was solely due to the addition of MNP, confirmed by the control experiment (Figure S5).

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Overall, 76.4±2.7 % of dissolved sulfide initially present was removed. The initial (t = 0 min)

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and end (t = 360 mins) pH values of sewage were 7.15±0.00 and 7.21±0.10, respectively. ICP

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results showed the dosage of MNP was 25.7±2.1 mg Fe/L, in agreement with the measured Fe



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content of the dosed MNP solution and the dosed amount. This gave a sulfide-to-MNP ratio of

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0.47 g S/g Fe3O4-Fe. The sulfide removal capacity of the MNP was 357.3 mg S/g Fe3O4-Fe.

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Hence, the minimum MNP dosage required for the removal of 15 mg S/L in sewage was 42.0 mg

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Fe/L (0.36 g S/g Fe3O4-Fe).

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In the second set of batch tests, MNP dosage of 52.3 mg Fe/L was chosen to ensure the

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dosage was higher than the minimum dosage of 42.0 mg Fe/L for the removal of 15 mg S/L in

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sewage. After the addition of MNP, the dissolved sulfide concentration rapidly decreased from

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12.7±0.3 to 1.5±0.3 mg S/L within 15 mins and then slowly decreased to 0.4±0.1 mg S/L within

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1 h. A dissolved sulfide concentration of 0.2±0.0 mg S/L was observed at 1.5 h after MNP

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addition, equalled to sulfide removal of 98.4±0.4 %. The initial (t = 0 min) and end (t = 360

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mins) pH values of sewage were 7.15±0.00 and 7.52±0.01, respectively. The dosage of MNP

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was later confirmed to be 48.8±1.5 mg Fe/L (0.26 g S/g Fe3O4-Fe) using ICP analyses. In

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addition, ICP results confirmed that concentrations of other metals in sewage such as As, Cr, Cd,

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Co, Ni, Pb, Se and Zn, etc. remained unchanged in sulfide removal tests (data not shown).

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Long-term MNP production

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A continuous electrochemical production of MNP was carried out for 31 days to determine the

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long-term cell performance. The average total Fe concentration obtained was 4.53±0.35 g Fe/L,

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resulting in a CE for iron oxidation of 92.4±7.2 %. The average pH value of the synthesized

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MNP solution was 8.94±0.04. Figure S6 of the supporting information shows the profile of

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anode and cathode potentials and cell voltage over the course of experiment. Overall, the anode

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potential and cell voltage were stable throughout 31 days, with the corresponding values of -

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0.23±0.19 V vs SHE and 3.74±0.23 V, respectively. Indeed, while performing polarity switching


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or sampling of the MNP solution for determination of total Fe concentration and sulfide removal

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tests, minor fluctuations in the cell voltage did occur.

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The results of sulfide removal tests conducted on day 22, 24 and 28 are presented in

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Figure 3. ICP results show the dosage of MNP for three tests was 42.5±0.3 mg Fe/L on day 22,

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43.5±0.7 mg Fe/L on day 24 and 47.0±0.5 mg Fe/L on day 28, with the corresponding initial

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sulfide level of 12.1±0.2, 12.4±0.3 and 13.0±0.6 mg S/L, respectively. The sulfide-to-MNP

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dosing ratio was approximately 0.28 g S/g Fe3O4-Fe in all three cases. At 6 h after MNP

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addition, sulfide concentrations were decreased to 0.2±0.1 (day 22), 0.1±0.0 (day 24) and

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0.1±0.0 (day 28) mg S/L, equalled to sulfide removal of 98.3±0.4 %, 99.2±0.1 % and 99.4±0.1

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%, respectively. Similar to the sulfide removal test using short-term produced MNP, the addition

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of MNP to sewage increased the sewage pH from 7.15±0.00 to 7.56±0.07 (day 22), 7.50±0.04

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(day 24) and 7.64±0.03 (day 28).

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Discussion

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Sulfide removal by MNP

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The reaction between MNP and dissolved sulfide is a surface controlled process. The first step

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involves chemical adsorption of hydrogen sulfide, generating new chemical bonds at the

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adsorbent surface. Subsequently, the adsorbed sulfide can react with the MNP, during which

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MNP is dissolved to form ferrous ions and sulfide oxidized to elemental sulfur.28 However,

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under circumneutral pH and ambient temperatures, the second step is a slow process with a time

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scale of days. As such, the oxidation process was likely negligible in our experiments. Similarly,

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it may not be a major contributor to sulfide control in a real sewer due to the relatively short

287

transport time (hours rather than days).



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A new cost-effective method for sulfide control in sewage

290

In this study, plate-like MNP were successfully generated in water using an electrochemical

291

system, with a uniform particle size range between 120 and 160 nm. In the short-term

292

experiments, the average MNP concentration was 9.35±0.28 g Fe/L, resulting in a CE for iron

293

oxidation of 93.5±2.8 %. Sulfide was removed from 12.0±0.3 to 2.8±0.3 mg S/L within 6 hours

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at a sulfide-to-MNP ratio of 0.47 g S/g Fe3O4-Fe, equalled to the sulfide removal capacity of

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357.3 mg S/g Fe3O4-Fe. A complete removal of sulfide from 12.7±0.3 to 0.2±0.0 mg S/L was

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observed at 1.5 h after MNP addition at dosage of 0.26 g S/g Fe3O4-Fe. In addition, a continuous

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long-term MNP production with polarity switching was carried out for 31 days with a stable

298

average cell voltage of 3.74±0.23 V. The average MNP concentration obtained in the long-term

299

production was 4.53±0.35 g Fe/L, resulting in the CE for iron oxidation of 92.4±7.2 %. The

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sulfide removal tests using long-term produced MNP on day 22, 24 and 28 also showed similar

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decreases in sulfide concentrations from around 12 mg S/L to less than 0.2 mg S/L within 6

302

hours at MNP dosage of 0.28 g S/g Fe3O4-Fe. In general, the addition of produced MNP in

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sewage achieved efficient sulfide removal (> 98%). A cost benefit analysis based on the results

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shows the economic potential of this new method with an estimated cost of around $36.2/ML

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sewage treated (see Table S2 of the supporting information), which is comparable to the

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chemical costs for current sulfide control practice (i.e. addition of sodium hydroxide, magnesium

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hydroxide, nitrate, iron salts and oxygen) ranging from 10.9 to 483.6 $/ML sewage treated.6

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Here, the results of long-term experiment mimicking on-site operation as well as the outcome of

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the cost benefit analysis clearly highlight the practical and economic feasibility of using MNP for

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sulfide control in sewers.


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Our results show that in-situ generation of MNP can be an interesting alternative to

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conventional dosing of iron salts or other chemicals such as NaOH, eliminates the need for

313

transport, handling and storage of bulky chemical solutions. The addition of MNP for sulfide

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control has several advantages over conventional iron salt dosing. First, the effectiveness of

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sulfide precipitation with Fe2+ depends on sulfide concentration. To reduce sulfide concentration

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to less than 0.2 mg S/L around the most prevalent circumneutral sewage pH levels, Firer et al.29

317

suggest that a Fe2+/S molar ratio of 1.3 and a Fe3+/S molar ratio of 0.9 to 1 should be applied

318

using programme predictions. However, it is difficult to get dissolved sulfide concentration to

319

very low level (i.e. less than 0.2 mg S/L) and as such excess iron salts are added to achieve

320

complete control.5 In practical situations, high Fe (Fe2+ or Fe3+) to S ratios are typically applied.

321

For example, Fe2+/S molar ratios ranging from 3.1 to 66.1 were reported.30 In this study, the

322

dosage of 0.28 g S/g Fe3O4-Fe is equal to a Fe3O4-Fe/S molar ratio of 2. The amount of Fe

323

required is well below the conventional iron salt dosing. Moreover, the magnetic property of

324

MNP may allow for the recovery of MNP at the inlet of the downstream wastewater treatment

325

plants. Second, the addition of MNP to sewage does not acidify sewage in contrast to the

326

conventional ferric salt dosing.31-33 Therefore, use of MNP for sulfide control could overcome

327

the drawbacks related to the decrease in sewage pH by ferric salts. These drawbacks include the

328

increase in the dosing requirements of iron salts29, as sulfide precipitation is less effective at

329

lower pH, and the release of more residual H2S to the sewer atmosphere in gravity sections.34

330

In comparison to unrefined magnetite powder, the results obtained in this study showed

331

remarkable improvements in terms of achievable sulfide concentrations (i.e. sewage between 0.1

332

and 0.2 mg S/L) as well as the sulfide removal capacity from 197.3 mg S/g Fe of purchased

333

magnetite powder to 357.3 mg S/g Fe of synthesized MNP. In addition, no increase in heavy



334

metal concentration in sewage was observed in the sulfide removal tests, thereby the concern

335

about heavy metal content of unrefined magnetite was eliminated by using refined iron plates

336

(i.e. > 98 % Fe) to produce MNP for sulfide control in sewers.

Page 16 of 24

337 338

Design and operation of the electrochemical cell

339

Currently available studies report that MNP can be electrochemically generated in water with

340

particle sizes ranging from 10 to 100 nm at various current densities (i.e. 0.2 to 20 mA/cm2) and

341

inter-electrode gaps (i.e. 0.3 to 8.0 cm).20, 22-23 However, the maximum attainable concentration

342

of MNP reported was 0.71 g Fe3O4-Fe/L with the experimental duration of 15.7 min.20 Further

343

research is warranted to investigate whether the particle size can be further reduced by means of

344

either decreasing the current density or increasing the inter-electrode gap.22 Achieving high

345

concentration of 10 g Fe/L in this study reduces not only the hydraulic load to the sewer pipes

346

but also the size of the reactor, which could become crucial for practical implementation when

347

targeting large size sewer sections. Long-term pilot plant studies are recommended to verify the

348

performance of sulfide control as well as scaling up of the system.

349 350

Acknowledgements

351

Hui-Wen Lin thanks The University of Queensland for scholarship support. This work was

352

funded by the Australian Research Council, District of Columbia Water and Sewer Authority,

353

ACTEW Corporation Limited, The City of Gold Coast, Queensland Urban Utilities, Yarra

354

Valley Water and Aquafin NV through ARC Linkage project LP120200238: “In-situ

355

electrochemical generation of caustic and oxygen from sewage for emission control in sewers”.

356

The authors acknowledge Dr. Beatrice Keller-Lehmann, Nathan Clayton and Ji Lu for their


Page 17 of 24


357

helpful assistance with the chemical analyses. The authors acknowledge the support of the

358

AMMRF at the Centre for Microscopy and Microanalysis at the University of Queensland.

359 360

Supporting Information Available: Detailed methodology, six figures and two tables. This

361

material is available free of charge via the Internet at http://pubs.acs.org.

362 363

Conflict of Interest Disclosure

364

The authors declare no competing financial interest.

365 366

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Page 22 of 24

451 452

Figure 1. (A) XRD patterns and (B) the particle size distribution of MNP obtained by

453

electrochemical synthesis in both short-term and long-term experiments. SEM micrographs of

454

(C) short-term and (D) long-term synthesized MNP. Short-term sample was taken from the

455

second set of short-term experiment while the long-term sample was taken on day 31.

456


Page 23 of 24


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Figure 2. Decrease in dissolved sulfide with different MNP dosages over the course of 6 h.

459



Page 24 of 24

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Figure 3. Decrease in dissolved sulfide with MNP dosage over the course of 6 hours.

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Experiments were conducted on day 22, 24 and 28.

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Electrochemical Production of Magnetite Nanoparticles for Sulfide

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