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HO production in microbial electrochemical cells fed with primary sludge Dongwon Ki, Sudeep C. Popat, Bruce E. Rittmann, and César Iván Torres Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 09 May 2017 Downloaded from http://pubs.acs.org on May 11, 2017

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Environmental Science & Technology

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H2O2 production in microbial electrochemical

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cells fed with primary sludge

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Dongwon Ki*†, Sudeep C. Popat‡, Bruce E. Rittmann†, and César I. Torres†,§

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†

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P.O. Box 875701, Tempe AZ 85287-5701, USA

Biodesign Swette Center for Environmental Biotechnology, Arizona State University,

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‡

Department of Environmental Engineering and Earth Sciences, Clemson University,

USA

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§

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E. Tyler Mall ECG 301, Tempe, AZ 85287, USA

School for Engineering of Matter Transport and Energy, Arizona State University, 501

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Corresponding Author

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*(D. Ki) Phone: +1 480 727 0848; fax: +1 480 727 0889

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Email: [email protected]

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ACS Paragon Plus Environment


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ABSTRACT: We developed an energy-efficient, flat-plate, dual-chambered microbial

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peroxide producing cell (MPPC) as an anaerobic energy-conversion technology for

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converting primary sludge (PS) at the anode and producing hydrogen peroxide (H2O2) at

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the cathode. We operated the MPPC with a 9-day HRT in the anode. A maximum H2O2

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concentration of ~230 mg/L was achieved in 6 hours of batch cathode operation. This is

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the first demonstration of H2O2 production using PS in an MPPC, and the energy

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requirement for H2O2 production was low (~0.87 kWh per kg H2O2), compared to

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previous studies using real wastewaters. The H2O2 gradually decayed with time due to

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the diffusion of H2O2-scavenging carbonate ions from the anode. We compared the

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anodic performance with a H2-producing microbial electrolysis cell (MEC). Both cells

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(MEC and MPPC) achieved ~30% Coulombic recovery. While similar microbial

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communities were present in the anode suspension and anode biofilm for the two

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operating modes, aerobic bacteria were significant only on the side of the anode facing

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the membrane in the MPPC. Coupled with a lack of methane production in the MPPC,

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the presence of aerobic bacteria suggests that H2O2 diffusion to the anode side caused

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inhibition of methanogens, which led to the decrease in COD removal. Thus, the

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Coulombic efficiency was ~16% higher in the MPPC than in the MEC (64% vs 48%,

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respectively).

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Keywords: Primary sludge; Hydrogen peroxide; Decomposition, Carbonate, Energy

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input

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TABLE OF CONTENTS/ABSTRACT ART:

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INTRODUCTION

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Hydrogen peroxide (H2O2) is a useful chemical in wastewater treatment due to its

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strong oxidizing properties. H2O2 can be used in the Fenton process or be combined with

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UV for generation of free radicals to achieve tertiary treatment and disinfection of

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wastewater.1,2 The conventional method for H2O2 production, the Anthraquinone

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Oxidation (AO) process, is energy-intensive. Moreover, anthraquinone and its

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derivatives are potential carcinogens.1,3 Electrochemical technologies have been

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proposed as alternatives for H2O2 production since the early 1990s.4-9

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Microbial electrochemical cells (MxCs) can produce H2O2 at the cathode.10 The

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H2O2 concentrations generated by an MxC can be well suited for on-site use within a

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wastewater-treatment facility: e.g., cleaning membranes, tertiary treatment, reducing

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odors, and pre-treating sludge or wastewater, or post-treating sludge. H2O2 may turn out

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to be a more useful MxC output in the wastewater-treatment setting than the more

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commonly studied MxC outputs: electrical power in a microbial fuel cell (MFC) or H2

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gas in a microbial electrolysis cell (MEC).

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We found ten studies addressing H2O2 production in MxCs.10-19 Depending on

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the type of MxC and its operation, the H2O2 concentration and production rate varied

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widely, from 8.5×10-5 to 9.7 g L-1 and from 7.9×10-4 to 18.6 g L-1 d-1 respectively.19

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Recent research on H2O2 production in MxCs has focused on materials for cathodes and

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membranes, as well as on improving oxygen diffusion to the cathode catalysts.17-18 Most

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of these studies were carried out with synthetic medium (e.g., acetate or glucose) fed to

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the anode.

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H2O2 production with real wastewater has been reported in only four studies.12-

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14,17

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H2O2 production with real wastewater was lower in terms of concentrations produced and

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the rates of production, and energy inputs were notably high (> 2 kWh per kg H2O2). The

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deficiencies in performance reflect on the importance of reactor design and operation to

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improve voltage efficiency.20,21 Also, cathodic H2O2 production efficiency (PPE), which

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is the fraction of electrons used and measured as H2O2 generation to the electrons

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captured as Coulombs, was variable, ranging from 5% to 70%.12-14,17

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Compared with synthetic medium (mostly acetate) as substrate for the MxC anode,

In previous studies, we optimized flat-plate MECs for H2 production.21-23 Here,

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we used the same flat-plate design for the anode, but we modified it for H2O2 collection,

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with passive air diffusion to the cathode. We used primary sludge (PS) to provide

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electron donor at the anode. The first goal of the study was to provide a proof-of-concept

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of H2O2 production with PS using an optimized design for a microbial peroxide-

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producing cell (MPPC).

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Past MPPC studies using domestic wastewater showed low anodic Coulombic

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efficiency (CE) of 6-22% with a 6-minute hydraulic retention time (HRT)13 and 2-4%

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with a 2~10-hour HRT,17 even though COD removal was 39-68% in the latter case.17 In

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a recent review,24 MEC studies using real wastewater at even higher HRTs showed better

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performance in terms of COD removal (67-90%) and CE (11-27%). Thus, the second

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goal of this study was to compare the anodic and cathodic performance -- including COD

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removal, anodic CE, and PPE -- between an MPPC and an MEC.

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Finally, we compared energy conversion and evaluated the microbial communities formed within MEC and MPPC. We used this information to gain a 5



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mechanistic understanding of the difference in performance between the two modes of

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operation.

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MATERIALS AND METHODS

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Primary sludge

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We collected PS from the Greenfield Water Reclamation Plant (GWRP), Gilbert,

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AZ, USA. We treated the PS with a pulsed electric field (PEF) unit (the FP alpha unit) in

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the laboratory, and we stored it in a temperature-controlled room at 4°C before use in

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order to minimize hydrolysis and fermentation during storage.22,25 The PS was diluted

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~3.8-fold with distilled water, and the influent PS fed to the MxCs had ~7.6 g/L of TCOD

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and ~3.8 g/L of VSS. Detail characteristics of PS are provided in Table S1 in Supporting

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Information.

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Design and operation of microbial electrochemical cells We operated a flat-plate microbial electrochemical cell with anode and cathode

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separated by an anion exchange membrane (AEM), AMI-7001 (Membranes

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International, Inc.). Following the same design as in Ki et al. (2016; 2017),21,23 one

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central anode chamber contained two anodes, and the anode chamber was flanked by two

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separate cathode chambers with one cathode each. The same electrochemical cell was

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used as an MEC and an MPPC. The anodes were exactly the same, but the electrodes

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used for cathodes and the cathode chambers were modified for the MPPC. Experiments

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were first conducted in MEC mode, followed by MPPC mode. Enough time was allowed

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for operation under each condition to reach a pseudo-steady state: ~30 days for MEC and

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~10 days for MPPC.

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The anodes were fabricated using carbon fibers (24K Carbon Tow, FibreGlast,

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OH, USA) woven as a mesh through a titanium plate current collector (Ultra-Corrosion-

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Resistant Titanium Grade 2, 0.035" of thickness, McMaster-Carr, USA). The geometric

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surface area of each anode was 100 cm2, and the anode chamber was 500 mL. The

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cathode for the MEC for H2 production was the same as before,21,23 but the cathode

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chamber for the MPPC for H2O2 production consisted of a serpentine flow field having a

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~120 mL volume between the AEM and cathode. Details on parts, configuration, and

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assembly of MPPC are provided in Figure S1 of the Supporting Information.

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We used stainless-steel meshes (Super-Corrosion-Resistance Type 316,

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McMaster-Carr, USA) as the cathodes for H2 production in the MEC. The cathodes for

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H2O2 production in the MPPC were prepared with a carbon-cloth support (GDL-CT, Fuel

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Cells Etc, TX, USA), coated with a slurry of Vulcan carbon powder on the liquid-

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exposed side, and with a 30% PTFE microporous layer (MPL) on the air-exposed side;

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this was the same as used in Young et al. (2016).19 The area of each cathode was ~79

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cm2 (8.8 cm x 8.8 cm).

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The distance between the anode and cathode was less than 0.5 cm in the MEC21

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and ~1 cm in the MPPC. We included a reference electrode (Ag/AgCl, MF-2052,

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Bioanalytical Systems, Inc., USA), which was at a ~2 cm distance from each of the

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anodes. We used a multi-channel potentiostat (VMP3, BioLogic Science Instruments,

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Knoxville, TN) to control the anode potential at -0.3 V (versus Ag/AgCl) and recorded

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current, and anode and cathode potential, every two minutes with the EC-Lab software

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(v. 10.37).

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We fed the MEC and the MPPC with PS semi-continuously at a 9-day HRT and

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with one feeding cycle per day; for example, we removed ~56 mL from the anode

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chamber and added the same volume of PS to the anode chamber every day. At each

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feeding, we maintained the pH of ~7.0 in the anode chamber with the addition of 5-M

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sodium hydroxide (NaOH) as needed.23 The electrolyte for the cathode (the catholyte)

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for MEC was 100-mM NaOH. The catholyte in the MPPC was 50-mM NaOH. We

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delivered the catholyte continuously in the serpentine cathode chamber of the MPPC with

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an ~1.5-h HRT for 18 days, and then we changed to batch mode with a one-day cycle.

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We collected catholyte samples of ~1 mL at 1.5, 3, 6, 12, and 24 hours during batch

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operation to measure pH, alkalinity, and H2O2 concentration.

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Analytical methods We characterized PS for total chemical oxygen demand (TCOD), semi-soluble

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COD (SSCOD), total suspended solids (TSS), and volatile suspended solids (VSS)

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according to Standard Methods.26 Semi-soluble COD was measured on filtrate passed

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through a 1.2-µm glass-fiber filter (WhatmanTM, UK).22 We used colorimetric methods

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to determine the H2O2 concentration in the catholyte of the cathode chamber.27 Briefly,

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the chemical solutions and samples were prepared in 1.5-mL cuvettes in the following

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order: 10 µL of sample (including H2O2 standard), 2 mL of HCl (50 mM), 0.2 mL of KI

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(1 M), 0.2 mL of ammonium molybdate (1M) in H2SO4 (0.5 M), and 0.2 mL of a starch

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solution (1%) as the indicator. H2O2 measurements were performed at a 570-nm

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wavelength using a Cary 50-Bio UV-Visible spectrophotometer (Varian, Palo Alto, CA).

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Total alkalinity was measured using spectrophotometric methods by HACH kit and

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spectrophotometer (DR2700, HACH, Loveland, CO).

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Calculations We calculated Coulombic recovery (CR), anodic Coulombic efficiency (CE), and

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H2O2 production efficiency (PPE). CR and CE were calculated as described in the

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previous study.22 The PPE was calculated for the batch-mode operation from the

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measured H2O2 concentration, the volume of the cathode chamber, and the recorded

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cumulative Coulombs as electrical current using Eq. (1), % =

× × ×

× 100 1 163

where n is the number of moles equivalent to moles of H2O2 (n = 2 here), F is the

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Faraday constant (96,485 Coulombs mol-1), CH2O2 is the measured concentration of H2O2

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(mol L-1), and V is the volume of cathode chamber. If all the cumulative Coulombs

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measured were utilized for production of H2O2, the PPE would equal 100%.

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Molecular microbial ecology We collected biomass samples for analysis of microbial community structure in

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the anodes of the MEC and the MPPC; the samples were from the anode suspension

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(AnS), biofilm from the anode side facing the chamber (BfC), and biofilm from the anode

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side facing the membrane (BfM) on the carbon fiber electrodes. A schematic showing 10


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the sampling locations is provided in Figure S2 of the Supporting Information. We

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centrifuged suspension samples to obtain pellets for DNA extraction. For the biofilm

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samples, we scraped off the anode surface using a pipette tip for at least 5 different

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positions. We prepared 0.17~0.27 gram of wet biomass by combining pellets or biofilm

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solids for each sample, and we added the combined biomass it to the bead tubes provided

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by a Power Soil DNA extraction kit (MoBio laboratories, Inc., Carlsbad, CA). We

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extracted DNA per the instructions provided with the kit, quantified DNA concentration

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with Nanodrop spectrophotometer, and stored DNA at -20 °C before sequencing.

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We sent the extracted DNA to the Microbiome Analysis Laboratory at Arizona

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State University for Illumina MiSeq sequencing. Amplicon sequencing of the V4 region

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of the 16S rRNA gene was performed with the barcoded primer set 515f/806r designed

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by Caporaso et al. (2012).28 Data received from the testing laboratory were analyzed

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using QIIME29 after discarding sequences shorter than 25 bp, longer than 450 bp, or

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labeled as chimeric sequences. After screening, primer sequences were trimmed off, and

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taxonomic classification was performed using RDP classifier at the 80%-confidence

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threshold.30 Operating taxonomic units (OTUs) were picked using the Greengenes

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database by UCLUST algorithm based on ≥97% identity.31 We aligned the

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representative sequences for each OTU to the Greengenes database using PYNAST.32,33

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Then, we made the OTU table and removed singletons. Lastly, the OTU table was

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rarefied to the minimum number of sequences among 6 samples (35,928 sequences); the

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total number of sequence reads for each sample after screenings were: MEC AnS =

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58,557, MEC BfC = 64,681, MEC BfM = 56,365, MPPC AnS = 35,928, MPPC BfC =

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58,902, and MPPC BfM = 55,570. 11



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RESULTS AND DISCUSSION

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H2O2 production and efficiency

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Figure 1a shows H2O2 concentrations (theoretical and measured) and H2O2

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production efficiency (PPE, based on Eq (1)) during batch operation of the cathode in the

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MPPC. The H2O2 concentration increased up to ~230 mg L-1 by 6 hours, but then

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decreased to ~121 mg L-1 by 24 hours. The theoretical H2O2 produced, based on the

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electrical current produced at the anode, increased linearly up ~2300 mg L-1, indicating

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that the PPE kept decreasing with time, from 72% at 1.5 hours to 5% at 24 hours. Clearly,

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H2O2 decomposition became increasingly important in the cathode chamber over 1 day

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of batch cathode operation.

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Catholyte pH in the MPPC decreased with time from ~12.6 (0 hour) to 10.4 (24

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hour), as shown in Figure 1b. This is contrary to the expected pH increase with time,

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since O2 reduction to H2O2 generates OH-:

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O2 + 2H2O + 2e- → H2O2 + 2OH-

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or, above the pKa of H2O2/HO2- (11.8),

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O2 + H2O + 2e- → HO2- + OH-

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Ion migration caused the decline in pH. Possible mechanisms of ion migration between

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the anode and cathode chambers are illustrated schematically in Figure S3 of Supporting

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Information. Specifically, OH- could have diffused from the cathode to the anode due to

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the large pH difference (~12.6 at the cathode and ~7 at the anode), and HCO3- could have

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diffused from the anode to the cathode due to its concentration gradient. Either transport

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mechanism would have contributed to a pH decrease at the cathode: loss of alkalinity

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due to OH- transport out of the cathode and pH buffering due to import of HCO3-. Based

(2)

(3)

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on the measured pH and total alkalinity of the catholyte (Figure 1b), OH- decreased

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significantly, while other alkalinity species (CO32- and HCO3-) increased with time. Thus,

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both ion-migration mechanisms appear to have been at work. During the first ~6 hours, the total alkalinity stayed approximately constant, while

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OH- and CO32- drastically decreased and increased, respectively, especially during the

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first 3 hours. The increase in CO32- indicates that HCO3- was moving from the anode to

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the cathode, and the stability of total alkalinity means that transport of OH- to the anode

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was off-setting the generation of OH- at the cathode and the input of HCO3- from the

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anode.

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After ~6 hours, the pH was ≤ 11, and HCO3- steadily increased due to a

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combination of an increase of the concentration of inorganic C and a shifting of the

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speciation of inorganic carbon towards, HCO3-, since the pKa of the HCO3-/CO32- couple

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is ~10.3. Due to continued import of HCO3- and generation of base, the CO32- alkalinity

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remained high. High CO32- is important, because CO32- can be responsible for H2O2

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decay (Lee et al., 2000); this is discussed more below.

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Figure 1. Results of cathode batch operation of a microbial peroxide producing cell

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(MPPC): (a) theoretical and measured H2O2 concentrations based on 100% conversion

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from cumulative coulombs and detection method, respectively, and H2O2 production

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efficiency (PPE) (number of measurements, N = 3). The inset zooms in on the first 6

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hours’ operation in the MPPC. (b) pH and alkalinity of catholyte over the 24-hour batch

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runs. 14


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The decrease in H2O2 concentration after 6 h means that the H2O2-decay rate was

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faster than the H2O2-generation rate at the cathode. H2O2 decay could have been

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accelerated by four mechanisms: 1) metal cations in PS diffusing from anode to the

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cathode and catalyzing H2O2 auto-decay, 2) membrane degradation with consumption of

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H2O2, 3) reaction of H2O2 with carbonate ions, and 4) diffusion of the HO2- ion to the

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anode. Direct movement of metal cations should not have been important, because we

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used an anion exchange membrane. As exemplified in Figure S4, we saw no evidence of

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membrane deterioration or chemical deposits after running 27 days in the MPPC fed with

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PS. Thus, the first two mechanisms were unlikely.

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Accelerated loss of H2O2 by reaction with carbonate is supported by prior studies

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and special tests we carried out. Lee et al. (2000) reported that the rate of decomposition

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of H2O2 loss was ~9-fold faster in a carbonate solution than in a caustic solution (NaOH)

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when the pH was 10~10.6, the temperature was 30~50 ˚C, and the ionic strength was

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1.5~3 M.34 Since the mechanism of H2O2 decay in the presence of carbonate ions has not

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been elucidated yet, we performed H2O2-decay tests to confirm that accelerated H2O2

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decay was linked to the carbonate concentration. Figure S5 in Supporting Information

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shows that H2O2 decomposed quickly during ~1 day in a sodium carbonate solution.

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Finally, produced H2O2 can dissociate to HO2- ion at high pH (pKa ~ 11.8)19, and

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the HO2- anions could have diffused to the anode chamber. We explore below evidence

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supporting this mechanism.

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Comparison of MEC and MPPC operation

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Figure 2 shows current densities and applied voltages over three one-day feeding

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cycles for the MEC and the MPPC pseudo-steady state operation periods. The pattern of

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current density was the same for both modes: high current at start of a one-day cycle and

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a decrease due to the consumption of PS-COD. The maximum current densities were

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similar, ~1 A m-2.

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Figure 2. Current densities and applied voltages in the MEC and MPPC operated with

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semi-continuous feeding (one-day cycle) and a 9-day HRT. Data are for the last 3 days

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of operation after the current stabilized for successive cycles.

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The applied voltage for the MPPC was ~0.2 V, a value much lower than that of

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the MEC, ~1.04V. The theoretical applied voltage for H2 production in MEC is 0.14 V,21

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which means that the total over-potential in the MEC was 0.9 V. Likewise, the

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theoretical potential for H2O2 production in MPPC is +0.56 V,10 making the total over-

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potential 0.76 V, a value smaller than with the MEC. In our previous study,21 we

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characterized the applied voltage of the same design of flat-plate MECs and categorized 16


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into anode over-potential (ηan), cathode over-potential (ηcat), Ohmic over-potential

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(ηOhmic), pH-related concentration over-potentials (ηpH), and theoretical voltage (Eth).

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The largest over-potentials were ηcat and ηpH, and that should be the same here for an

283

MEC and an MPPC. Since we poised the anode potential at -0.3 V (vs Ag/AgCl) in both

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cases and used the same substrate, ηan should be similar. ηOhmic should be small for

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current densities of ~1 A m-2 (in both reactors), although the distance between anode and

286

cathode was ~0.5 cm larger with the MPPC. Thus, the observed improvement of ~0.14 V

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of total over-potential with the MPPC should have been related to better cathodic

288

catalysis with H2O2 production, compared to H2 production in the MEC.19,21

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Figure 3 summarizes performance parameters for the effluent from the MPPC and

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MEC. Influent concentrations did not differ much between the two conditions (Table

291

S1), but the COD-removal in MPPC was 49 (±1)% and 62 (±5)% in the MEC. This

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difference was due to inhibition of methanogenesis in the MPPC, since the anode

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chamber of the MEC produced 27 ± 4 mL d-1 of biogas, versus only 2 ± 4 mL d-1 for the

294

MPPC. CH4 generation accounted for higher COD removal in the anode of the MEC.

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Although CRs were similar in the MEC and MPPC, ~30%, the CE was higher in the

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MPPC: 64% versus 48%. Higher CE in the MPCC was from lower COD removal

297

efficiency due to the decline in methanogenesis.

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Inhibition of methanogenesis could have been caused by H2O2 and O2 diffusion

299

into the anode chamber through the membrane from the air-cathode. Most MFC studies

300

using an air-cathode reported high COD-removal efficiency, but low CE, because O2

301

diffusing into the anode chamber allowed rapid aerobic biodegradation, which out-

302

competed anode respiration.35-37 Since the MPPC in our study had lower COD removal 17



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than the MEC, the difference in performance between the MPPC and MEC probably was

304

caused by diffusion of H2O2, not O2.

305 306

Figure 3. COD-removal efficiency, CR, and CE for the MEC and MPPC for a 9-day

307

HRT during semi-continuous operation with one-day feeding cycle of PS.

308 309 310

Microbial phylotypes relevant to community structure and function Figure 4 shows the taxonomy of the microbial communities, classified at the order

311

level, in 6 samples: anode suspension (AnS) and two sets of biofilm samples for the

312

MEC and the MPPC. Biofilm samples were taken from the chamber side (BfC) and

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membrane side (BfM) of the anode. The AnS and BfC communities were similar

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between the MEC and MPPC, but the BfM communities were significantly different.

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The dominant phylotype in AnS was Bacteroidales, which includes bacteria well-

316

known for carrying out hydrolysis and fermentation of complex organics.38,39 For the

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anode biofilm of the chamber side (BfC) in both systems, Desulfuromonadales and

318

Desulfobacterales were dominant orders. Geobacter, unclassified Pelobacteraceae, and

319

unclassified Desulfobulbaceae were dominant at the genus level. These genera contain

320

species that typically are found in anode biofilms.38,40-42

321

Despite being at the same anode surface, the BfM communities were different

322

from the BfC communities of each system, and the two BfM communities (MEC vs

323

MPPC) were very different from each other. Natranaerobiales was the most abundant

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phylotype in the MEC BfM, with Dethiobacter (Family Anaerobrancaceae) and

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unclassified Anaerobrancaceae being the largest fractions at the genus level, 23 and 25%,

326

respectively. Dethiobacter alkaliphilus, which is 96% similarity to the representative

327

sequence in the MEC BfM, is an obligate anaerobe that uses H2 as an electron donor and

328

thiosulfate, elemental sulfur, and polysulfide as electron acceptors in high-pH (~10) and

329

high-salt (~0.6 M of sodium) environments.43

330

In contrast, Oceanospirillales and Rhizobiales were predominant in the MPPC

331

BfM, and Halomonas and Parvibaculum were the largest fractions at the genus level, 17

332

and 16%, respectively. Halomonas species are aerobes living in saline conditions. Some

333

species, such as Halomonas salaria sp. and Halomonas denitrificans sp., have a yellow

334

or brown-yellow color44 that corresponded to the appearance of the membrane-side anode

335

at the end of the experiments with the MPPC, shown in Figure S6. Parvibaculum

336

lavamentivorans DS-1 also is an aerobe known for biodegrading synthetic laundry

337

detergents (e.g., linear alkylbenzenesulfonate, LAS).45 19



338

The different community structures of BfM from BfC in the MEC and MPPC

339

correlate to the very different environments of the cathode chamber. In particular, BfM

340

in the MPPC included a majority of aerobic bacteria, since O2 and H2O2, which can

341

degrade to O2, diffused through the air-cathode and AEM. Although H2O2 and O2

342

diffusion was not significant enough to change the microbial community structure of AnS

343

and BfC or divert electron flow away from anode respiration (similar CRs of ~30%,

344

Figure 3), it profoundly altered the BfM community.

345

346 347

Figure 4. Microbial community structure at the order level for anode suspension (AnS),

348

biofilm of chamber side (BfC), and biofilm of membrane side (BfM) in the MEC and

349

MPPC at the end of operation.

350 351 352 353

Low energy requirement With a 6-hour cathode HRT that produced ~230 mg H2O2 L-1 in the MPPC, the cathode potential was around -0.5 V (vs. Ag/AgCl). Since the anode potential was fixed 20


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354

at -0.3 V (vs. Ag/AgCl), the applied voltage was 0.2 V, resulting in ~0.87 kWh per

355

kgH2O2 for ~1 A m-2 current density. This value is consistent with the ~1 kWh per kg

356

H2O2 measured by Young et al. (2016)19 using the same reactor design (flat-plate MEC),

357

but fed with acetate as the electron donor. This energy requirement for H2O2 production

358

is relatively low compared to other published studies using actual wastewaters: 2.5-78

359

kWh per kg H2O2 using real wastewater,12-14,17 and similar to when using synthetic

360

wastewater (acetate) of ~0.93, 3, and 0.659 kWh per kg H2O2.10,13,15 The low energy

361

input can be attributed to the relatively small distance (~1 cm) between the anode and

362

cathode, which minimized the energy loss of ion transport.19,21

363

Modin and Fukushi (2013) demonstrated production of > 9 g L-1 of H2O2 (highest

364

concentration reported so far using MPPC fed with acetate) with a rate of > 11 g L-1 d-1.13

365

However, they needed a high energy input, ~3 kWh per kg H2O2. Even though we

366

demonstrate that we can produce H2O2 at ~1 kWh per kg H2O2, H2O2 can be theoretically

367

produced with the production of power or at least with little or no energy input.10,19 A

368

systematic investigation of cell design and materials (e.g., membranes, catalysts) may

369

lead to a higher concentration of H2O2 with lower or zero net energy input.19 The energy

370

input for H2O2 production using MPPC would be a significant improvement compared to

371

conventional H2O2 production methods: 2-10 kWh per kg H2O2 by anthraquinone

372

oxidation and 4-5 kWh per kg H2O2 by electrochemical technologies.46-49 However,

373

continuous H2O2 production in an MPPC using real wastewater needs to be studied and

374

optimized in the future.

375

In summary, we demonstrated H2O2 production using PS in a flat-plate, dual-

376

chambered MPPC. The maximum H2O2 concentration was achieved ~230 mg/L in 6 21



377

hours of batch catholyte operation, but H2O2 net decomposed for longer catholyte times;

378

the main cause of H2O2 loss was reaction with carbonate ions that moved from the anode

379

to the cathode and to diffusion of H2O2 from the cathode to the anode. The energy

380

requirement for H2O2 production was relatively low, ~0.87 kWh per kgH2O2, compared

381

to other H2O2-producing microbial electrochemical cells using real wastewater. While

382

the MPPC and an MEC achieved ~30% Coulombic recovery, the Coulombic efficiency

383

was ~12% higher in the MPPC than in the MEC, due to lower TCOD removal from

384

suppression of methanogenesis in the MPPC. Even though the microbial communities of

385

the anode suspension and anode biofilm for both cells were similar, aerobic bacteria were

386

significant only on the side of the anode facing the membrane in the MPPC. The

387

presence of aerobic bacteria and the suppression of methanogenesis in the MPPC support

388

that H2O2 diffused to the anode side. Transport of ions across the membranes was of importance in producing H2O2

389 390

when using real wastewater. When we use an AEM, carbonate/bicarbonate ions can

391

move from the anode to the cathode; if we were to use a cation exchange membranes

392

(CEM), other cations, such as K+, Na+, and metals, would diffuse from the anode to the

393

cathode to maintain electroneutrality. Transport of the cations would bring about two

394

possible negative effects: 1) lowering the pH below an acceptable threshold for ARB,

395

and 2) accelerated H2O2 decay with metal catalysis.

396 397

398

Supporting Information

ASSOCIATED CONTENT

22


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Page 23 of 30


399

MPPC design (Figure S1), schematic view for the sampling locations for microbial

400

community (Figure S2), possible mechanisms ion migration and diffusion, anion

401

exchange membrane used during MPPC (Figure S3), abiotic H2O2 decay tests in different

402

salt solutions (Figure S4), solid biomass on carbon fiber anodes (Figure S5), PS

403

characteristics (Table S1), a summary of H2O2-producing microbial electrochemical cell

404

studies using real wastewater (Table S2), are all available in the Supporting Information.

405 406

AUTHOR INFORMATION

407

Corresponding Author

408

*(D. Ki) Phone: +1 480 727 0848; fax: +1 480 727 0889

409

Email: [email protected]

410 411

Notes

412

The authors declare no competing financial interest.

413 414 415

ACKNOWLEDGEMENTS We acknowledge the Department of Defense’s SERDP program (project # ER-

416

2239) for providing the funding for this study. We also acknowledge Mike Davis for

417

assistance obtaining primary sludge samples at GWRP.

23



418

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