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Direct conversion of sewage sludge to electricity using polyoxomatelate catalyzed flow fuel cell
Accepted Manuscript Direct conversion of sewage sludge to electricity using polyoxomatelate catalyzed flow fuel cell Zhe Zhang, Congmin Liu, Wei Liu, Xu Du, Yong Cui, Jian Gong, Hua Guo, Yulin Deng PII:
S0360-5442(17)31664-X
DOI:
10.1016/j.energy.2017.09.143
Reference:
EGY 11637
To appear in:
Energy
Received Date: 8 May 2017 Revised Date:
5 September 2017
Accepted Date: 28 September 2017
Please cite this article as: Zhang Z, Liu C, Liu W, Du X, Cui Y, Gong J, Guo H, Deng Y, Direct conversion of sewage sludge to electricity using polyoxomatelate catalyzed flow fuel cell, Energy (2017), doi: 10.1016/j.energy.2017.09.143. 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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Direct conversion of sewage sludge to electricity using polyoxomatelate
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catalyzed flow fuel cell
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Zhe Zhanga,†, Congmin Liub,†, Wei Liua, Xu Dua, Yong Cuic, Jian Gonga, Hua Guob, Yulin
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Denga,*
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a.
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School of Chemical &Biomolecular Engineering and Renewable Bioproducts
Instituteat Georgia Tech, Georgia Institute of Technology, 500 10th Street N.W., Atlanta,
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GA 30332-0620, USA. b.
Guodian New Energy Technology Research Institute, 2 North Street, Future Science
and Technology Park, Changping District, Beijing, China, 102209.
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c.
Key Laboratory of Nuclear Materials and Safety Assessment, Division of Materials
for Special Environments, Institute of Metal Research, Chinese Academy of Sciences,
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Shenyang, Liaoning Province, 110016, China.
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Corresponding author: Dr. Yulin Deng; E-mail: [email protected]
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†
Z.Z. and C.L. contribute equally to this work
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Highlights
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A novel pathway of sludge treatment is successfully designed by flow fuel cell
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The power output reaches 50 mW/cm2 which is over 100 times than that of MFC
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The mechanisms are completely different from any reported sludge fuel cell
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Low-cost and stable POM is used as both the catalyst and charge carrier
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No complicated pretreatment or purification of sludge is needed
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Abstract
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The traditional treatment of sludge methods are high energy-consumption and expensive
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processes. Due to the increasing awareness considering risks for the global environment and
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human health, appropriate treatments of sewage sludge are urgently expected. A novel flow fuel
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cell technology was reported herein which could convert sewage sludge to electricity directly
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with high power output. In this flow fuel cell, chemically stable and completely regenerable
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polyoxometalates (POMs) were used as both catalysts and charge carriers. Thermal induced
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charge transfer from sludge organisms to POM was successfully used to power the flow fuel cell.
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The power density of the cell could achieve as high as 50 mW/cm2, which is 100 times higher
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than the output of microbial sludge fuel cell reported in literature. Catalyst recyclability was
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investigated and the POMs were demonstrated to be effective to degrade sludge and transfer
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electrons in the fuel cell system after four rounds.
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Keywords: fuel cells; polyoxomatelate; sewage sludge; green energy; chemical degradation
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1. Introduction
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Sewage sludge is a mixture produced in the wastewater treatment process, containing both
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organic and inorganic components, pathogens and water. The increasing awareness about the
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risks for the global environment as well as human health, finding appropriate treatments of
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sewage sludge is highly necessary and expected urgently. Traditionally, sludge treatment is an
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expensive practice, of which the cost is over 50% of the total cost in the water treatment
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process.[1] Therefore, innovative treatments are supposed to focus on the recovery of valuable
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products of sludge to make it economically acceptable.
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Traditionally, in the anaerobic digestion process, part of the sludge is hydrolyzed into
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biogas, a mixture of methane and carbon dioxide, which is applied in the production of
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energy.[2-5] However, this process always takes ~20 days with a low pathogens removal degree.
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Converting sludge into liquid or gaseous fuels is another option.[6-8] However, this always
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requires the complex selective separation system with high cost. Another strategy is incineration
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of the organic matters in sewage sludge either directly or in the coal-fired power plants.[9-12]
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Nevertheless, the potential issue is the severe emission of fly ashes[13] as well as toxic
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chemicals, like dioxins and furans[14], which may results in the pollution of particulate matters.
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The organic substances in sludge mainly include proteins, humic acids, lipids and
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polysaccharides.[15] A microbial fuel cell (MFC), typically consisting of an anaerobic anode
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chamber with microorganisms as catalysts and an aerobic cathode chamber, can convert the
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energy of organic matters in sludge into electricity directly.[16-18] A baffle-chamber
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membraneless MFC was applied by Hu[19] to generate electricity from wastewater and the
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resultant power density is 0.3mW/m2. Jiang et al[17] investigated the effect of ultrasound
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pretreatment on sludge and demonstrated it to be efficient on total chemical oxygen demand
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(TCOD). In his research, the power density was around 12 W/m3. Muhamad et al. used P.
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aeruginosa strain ZH1 in the MFC and reported a maximum power density of 451.26
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mW/m2.[20] Indeed, this sludge-fueled MFC technology has attracted significant scientific
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attentions[21-23] and detailed investigations have been conducted on different parameters, like
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chamber type[22, 24], sludge origin[23, 25], ionic conductivity[26], etc. However, it is
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noticeable that although MFC can convert part of the organic matters in sludge into electricity
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directly, the power density is very limited, which increases its cost and hinders its practical
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application.
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As a class of anionic metal-oxo polyhedral clusters formed with metal centers and
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oxygen atoms at the vertices, polyoxometalates (POMs) show great potentials in the area of
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energy storage since they can undergo highly reversible multi-electron redox reactions.[27]
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POMs have been demonstrated to be effective to degrade biomass and coal and capture electrons
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from these substrates.[27, 28] Recently, Liu et al[29] reported a POMs catalyzed flow fuel cell
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which converts biomass to electricity directly at low temperature. The power density of
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cellulose-based fuel cell was 100 times higher than the result of cellulose-based MFC
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reported.[30] Furthermore, a completely noble metal free liquid-catalyst flow fuel cell was then
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reported [31] where two POM solutions of different electrode potentials were applied on anode
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and cathode, respectively. Power densities reached 44mW/cm2 and 51mW/cm2 when switchgrass
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and bush allamanda were utilized as fuels with this technique.
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For the first time, we have developed a flow fuel cell technology that can directly convert
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sewage sludge to electricity with high power output. In this research, sewage sludge was found
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to generate electricity of high power output using POM solutions as catalysts and charge carrier
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which is completely different from traditional MFC. The power density of this fuel cell was
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demonstrated to be hundreds of times higher than that in the sludge-fueled MFC. Briefly, POM-I
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oxidizes the organic matters of sludge and captures electrons in the anode tanks and then the
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excess electrons transfer to POM-II in the cathode tank through external circuit. In the
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meanwhile, positive charges (H+) diffuse through proton-exchange-membrane from anode to
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cathode and, consequently, current is generated in the whole system. The reduced POM-II
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solution was re-oxidized by oxygen. Therefore, both POM-I and POM-II remain un-consumed in
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the process and the feeds are sludge and oxygen. Besides, it is well known that municipal sludge
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contains toxic inorganic and organic components, which can cause the deactivation of noble
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metal catalysts. However, POMs are tolerant to these contaminants and its strong oxidizing
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capability can destroy and remove most pathogens in the sludge.
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2. Experimental Section
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2.1. Materials
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The raw sewage sludge used in this study was obtained from the municipal wastewater treatment
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plant located in Beijing, China. The sewage sludge was collected after being concentrated by
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centrifugation in the plant. Phosphomolybdic acid (H3[PMo12O40]), H2O2, H3PO4 (85 wt%),
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MoO3, was purchased from Aladdin. V2O5 was purchased from Xiya. The high-density graphite
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plates (bipolar plate) and graphite felt were purchased from Guoyao. Nafion®115 (127
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micrometers thick) was purchased from DuPont and used as proton exchange membrane.
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2.2. Characterization of sewage sludge
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Raw sludge was freeze-dried to get the solid base and total organic carbon (TOC) of solid was
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measured by MULTI N/C ®2100, ANALYTIKJENA. Fourier Transform Infrared Spectroscopy
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(FTIR) was applied to study the functional groups in the sludge and the spectra were obtained by
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Bruker TENSOR II in the range of 500 cm-1 to 6000 cm-1 with 64 scans. Furthermore, X-ray
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fluorescence (XRF, ZSX Primus II, 4kW, 60kV-150mA at room temperature) was applied to
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investigate the elements contents and crystallization of sludge, respectively. Thermo stability of
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sewage sludge was measured by thermogravimetric analysis (TGA, Q50) in the range of 30-
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800oC. The morphology of sludge particles was observed by scanning electron microscopy
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(SEM, QUANTA F250). The sludge particles were dispersed in DI water at a concentration of
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0.1%. One drop of the mixture was dropped on the conductive glass and kept in the fume hood
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for overnight to evaporate all the moisture. The sample was sputtered with gold before SEM
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measurement. Reacted sludge was separated from POM solution by vacuum filtration and washed
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thoroughly with DI water until the water became colorless. Then the sludge was free-dried and
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characterized as the raw sludge.
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2.3. Preparation of POM solutions for anode (POM-I) and cathode (POM-II)
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Anode: A certain amount of H3[PMo12O40] was added to DI water to form solutions (POM-I)
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with a concentration of 0.3 mol/L. This solution was stirred at 60oC for 1 hour in order to
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dissolve H3[PMo12O40] completely. Cathode: H12[P3Mo18V7O85] solution (POM-II) was used as
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cathode electrolyte and was synthesized according to Odyakov’s work[32]. A typical synthesis
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process of H12[P3Mo18V7O85] can be found in supporting information.
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2.4. Redox reactions between sewage sludge and POM solutions
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Both dried sludge powder and wet sludge were used as substrates.
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2.4.1. Heating induced reaction with dried sludge powders
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Sewage sludge samples were first freeze-dried to remove water to form sludge powders. Then
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the redox reaction was conducted by mixing POM-I solution (0.3 M, 50mL) with sludge power
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(2.5g) and heating to required temperature (80, 100 and 150 oC) for certain periods (6, 12, 24 and
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36h). Reactions at 80 oC and 100 oC underwent in glass vessels with reflex condenser, while
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reaction at 150 oC was conducted in an autoclave reactor.
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2.4.2. Heating induced reaction with wet sludge
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Sewage sludge was also used directly in the redox reaction without freeze-drying pretreatment.
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Since the water content was 87 wt % in the original sludge, 20 grams of sewage sludge was
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mixed with 27.35 grams of H3[PMo12O40] and 25 mL DI water was added to the mixture to keep
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the content of each component the same as the case with sludge powders. Afterwards, the
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mixture was kept at 100 oC for certain periods (6, 12, 24 and 36h).
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2.5. Assembly of fuel cell
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The graphite felt was soaked in a of H2SO4 and HNO3 solution (volumetric ratio equaled to 3:1)
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at 50oC for 30 minutes and then washed with DI water to make it neutral. The resultant graphite
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felt was then filled into the pre-carved flow channel with a geometry function area of 1cm2 on
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each bipolar plate. Nafion 115 membrane was sandwiched between two bipolar plates. Two
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acrylic plastic plates were applied outside of the carbon plates to fix them with PTFE gaskets
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included to prevent leakage. A pump connected by PTFE pipelines was used to transport
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electrolyte solutions into and out of the fuel cell at each electrode. Anode electrolyte (POM-I
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solution) was pumped from the anode tank after the filtration of sludge residue with a 0.2
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micrometer filter head into the anode plate at a flow rate of 30mL min-1. In the meanwhile,
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POM-II was pumped into the cathode of the fuel cell from the cathode tank at the same flow rate
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(30mL/min). Anode tank was kept at 80oC during the discharging process. Oxygen was pumped
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into the cathode tank under high-speed homogenizing (20,000 rpm) at 80oC in order to re-oxidize
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POM-II solution. I-V curves were obtained with a Versa Stat Electrochemical Working Station
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(Princeton Applied Research) through the controlled potentiostatic method.
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2.6. Characterization of (NH4)3[PMo12O40]
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Sludge powers (2.5g) were reacted with H3[PMo12O40] solution (0.3 M, 50 mL) at 100 oC for 6h.
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Then sludge was removed by vacuum filtration. 1 mL of H2O2 (30%) was added to the reduced
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POM-I solution and its color changed to yellow with white precipitate ((NH4)3[PMo12O40]) at
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the bottom. The white precipitate was separated with centrifugation at 7000 rpm for 10 min.
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Then the powers were dried at room temperature before X-Ray Diffraction measurement (XRD,
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D8 Advance, Cu Kα radiation from 5o to 75o).
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3. Results and discussion
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3.1. Mechanism of sewage sludge flow fuel cell
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Figure 1. (A) Schematic illustration of the sewage sludge fuel cell; (B) illustration of cell stack
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(left) and inner side of bipolar plate (right)
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The reactions in the electrode tanks and the transferring of electrons in the flow fuel cell
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could be expressed as:
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(1) In the anode tank, sludge is oxidized by POM-I under heating:
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sludge PMo O H PMo CO degraded products (1) Mo O
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(2) On the anode, extra electrons of POM-I captured from sludge particles were
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transferred to the graphite felt and the reduced POM-I solution which was re-oxidized
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(MoV to MoVI):
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H PMo → PMo O n e n H ( (2) Mo O
(3) On the cathode, POM-II is the acceptor of electrons where VV was reduced to VIV:P V* Mo + O+, n e n H( → H P V V* Mo + O+, (3)
(4) In the cathode tank, reduced POM-II was then re-oxidized by oxygen (VIV to VV):
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H P V V* Mo + O+, O → P V* Mo + O+, H O (4)
Overall reaction in the fuel cell is the summation of reactions (1) to (4), 4567 8677
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The mechanism and system set-up of the flow fuel cell are shown in Figure 1. Organic
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components in the sludge (carbohydrate, humic acid, lipids et.) were oxidized by POM-I in
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anode tank and the electrons in these organics were transferred to POM molecules, which caused
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the decrease of anode potential. Due to the higher potential of cathode electrolyte POM-II than
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that of POM-I in the anode cell, the reduced POM-I could be re-oxidized by POM-II, as shown
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in Figure 1B, therefore, the as-obtained electrons transferred onto anode electrode and further
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transferred to cathode through external circuit. Finally, they were captured by POM-II molecules.
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In the meanwhile, protons (hydrogen ions) penetrated through the Nafion membrane which was
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sandwiched between two electrodes. As a result, current was generated in the fuel cell system.
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For the entire flow cell process, POM-I was reduced by sewage sludge and re-oxidized by POM-
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II through the fuel cell discharge process. At meantime, reduced POM-II was re-oxidized by
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oxygen. As a result, both POM-I and POM-II were completely regenerated. These processes are
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schematically shown in Figure 1. Clearly, both POM-I and POM-II were used as catalyst without
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net consumption.
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Sewage sludge without POM could not generate electricity in this fuel cell. Besides,
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when sludge was treated with POM at room temperature, no reaction occurred and no power was
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generated. However, if we increased reaction temperature, sludge was oxidized by POM-I
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successfully, meanwhile, POM-I molecules were reduced via obtaining extra electrons. As
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indicated in Figure 2(A), after 6 hours’ reaction at 80 oC, the reduced POM-I solution could
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generate electricity with an open-circuit voltage (Vmax) of 0.3V and a maximum power density
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(Pmax) of 13.56 mW/cm2. With the increase in reaction time, more electrons were captured by
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POM-I solution (as shown in Figure S2, supporting information) leading to a better cell
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performance, e.g. Vmax was 0.32 and 0.33V while Pmax was 16.07 and 17.91 mW/cm2 for
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different reaction time of sludge degradation for 12 and 24h, respectively. However, further
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increasing reaction time to 36h could not further enhance its electrical performance due to the
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complete conversion of reactive components in sludge.
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Temperature was another important factor that could affect the reaction extent kinetically.
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When the degradation temperature was 100 oC, the maximum power density, Pmax, for 6, 12 and
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24 hours’ reactions were 39.7, 47.6 and 49.8 mW/cm2 (Figure 2(B)), respectively. Higher
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temperature resulted in higher reaction rates between sludge and POM-I. The fast reaction and
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high reaction degree leaded to a lower reduction degree of POM-I molecules, which was
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favorable to the electricity generation. Therefore, when reaction temperature was raised to 150
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respectively, which was better than that reacted at 80 or 100 oC, as expected.
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C in an autoclave, as shown in Figure 2(C), Pmax was improved to 48.8, 54.0 and 61.4 mW/cm2,
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Figure 2. Performance of sewage sludge flow fuel cell with sewage sludge degradation at
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different conditions: (A) dry sludge powder reacted with POM-I at 80 oC; (B) dry sludge powder
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reacted with POM-I at 100 oC; (C) dry sludge powder reacted with POM-I at 150 oC. (D) Wet
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sewage sludge (without drying) was used directly in the fuel cell and redox reaction temperature
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was 100 oC. All the experiments were conducted three times and the values shown on the figure
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are the average of the results from three experiments with the standard deviations less than 5%.
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To investigate this technique in a more practical aspect, sewage sludge without drying
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was used directly to react with POM-I and the corresponding electrical behaviors were studied,
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as shown in Figure 2(D). It was noted that when the temperature was kept the same (100 oC
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herein), electrical performances expressed similar tendency using either dried sludge or wet
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sludge. The power density increased from 37.7 to 52.0mW/cm2 as wet sludge was pre-degraded
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by POM-I for 6 to 24h. Further degradation of sludge to 36h showed little improvement on
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electrical output.
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A batch test of the flow fuel cell was operated at a constant current density of 100
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mA/cm2. The discharge process was kept running until all reduced POM-I was re-oxidized. The
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initial power density was 45 mW/cm2, and then decreased gradually during the discharging
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process due to the consumption of energy stored in reduced POM-I. The power output reduced to
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0 after 25h. In this process, the reduced POM-I molecules from the chemical reaction between
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sludge and POM-I were oxidized (MoV ->MoVI) on anode and the electrical potential increased
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accordingly. After the POM-I was completely discharged, the sludge was added into the POM-I
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solution and heated again for the second circle of the test. As shown in Figure 3(A), the
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degradation and discharging cycle was repeated three times, and the power density was used as a
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function of the repeat circles of the degradation-discharge. Obviously, energy released during the
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second and third rounds was much less than that of the first round. This was mainly caused by
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the lower reduction degree of POM-I in the second and third round, as most reactive materials in
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the sludge were reacted during the first degradation process and fewer organisms were available
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in the solution for the following rounds. In order to investigate the conversion of sludge in each
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degradation round, solid weight loss and total organic carbon (TOC) of the sludge residuals were
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analyzed. As shown in Figure 3(B), over 60 wt % of sludge was either reacted or dissolved after
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cooking with POM-I during the first round of the reaction and this number increased to 85%
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after the second round. Actually, in the first round, the reacted carbon content was around ~38%
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(see supporting information). In the meanwhile, TOC of solid sludge residual after drying at
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room temperature decreased from 170 to 32 g/kg and further decreased to 23 g/kg after the 1st
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and 2nd round, respectively. This result was consistent with the electrical performance and
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indicated that most organic components could be oxidized in 24h with H3[PMo12O40] at 100 oC.
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Figure 3. (A) Performance of fuel cell in discharging process: POM-I solution was used to
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degrade sludge and discharged three time without adding fresh sludge. (B) Corresponding mass
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loss in each run and total organic carbon content in sludge residuals. (C), (D) Performance of
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fuel cell in discharging process: recycled POM-I solution was reacted with same amount of raw
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sludge (5g) before each discharging process. All redox reactions were carried out at 100 oC and
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fuel cell was operated at 80 oC. (C) Power density curve in the first cycle. (D) Power densities at
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different discharging time (0, 10, 20h) in each cycle, the values shown on the figure are the
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average of the results from three experiments with the standard deviations less than 5%.
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In order to investigate the catalysis recyclability of POM-I, new sewage sludge powder
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was added to react with recycled POM-I solution in each round. Stable performance was
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obtained, as shown in Figure 3(C), which is the power density curve in the first cycle. Actually,
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in all four discharging cycles, the power density curves were similar, with the initial power
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density around 44 mW/cm2, a power density of 37 mW/cm2 at 10h and a power density of 25
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mW/cm2 at 20h, as shown in Figure 3(D). All discharging processes took around 22-25h (Power
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density curves for all four cycles are shown in Figure S3). This demonstrated the excellent
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stability of POM-I in this technology, and also demonstrated that the electricity could be
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generated continuously with the continuous consumption of sewage sludge.
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3.2. Characterization of sewage sludge
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Figure 4. Characterization of original sludge and reacted sludge (100 oC for 12h):
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(A) TGA; (B) DSC; (C) SEM images; (D) FTIR; (E) Solid-state 13C NMR.
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In order to investigate the degradation reaction, experiments were performed on both
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original sewage sludge and sludge residues after reaction to study the structure changes. The
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results of TGA (Figure 4(A)) shows that original sludge lost most of its organic components in
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the range of 200-550 oC sharply and around 45 wt % were left at 1000 oC. However, after the
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degradation process by POM, sludge lost its weight gradually in the range of 200-700 oC and the
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weight percent of remained residues was over 80 wt %. The residue content increased as the
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degradation time extended. This was consistent with the electrical results: with the extending of
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reaction time, more sludge was degraded and more electrons were captured by POM molecules
292
which leaded to better fuel cell performances. The results of differential scanning calorimetry
293
(DSC) (Figure 4(B)) of sludge samples further confirmed the conclusion since the heat flow of
294
reacted sludge reduced dramatically which was resulted from the degradation of organic matter
295
by POM. Detailed TGA and DSC spectra for reacted sludge of different reaction time periods
296
were provided in Figure S4 and Figure S5 (supporting information).
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Scanning electron microscope (SEM) images (Figure 4(C)) show the morphology of
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sludge particles. As expected, particle shapes are non-uniform due to the heterogeneity of sewage
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sludge. However, the image of original sludge sample shows some fibrous structures, as marked
300
in Figures 4 (E), which represents the presence of microorganisms.[33] In reacted sludge, these
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fibrous structures are not presented due to the degradation of organic matters.
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FTIR spectra of sludge in Figure 4(D) indicate the change of functional groups of sludge
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caused by POM-I solution degradation. Compared with original sludge, the intensity of bands for
304
inorganic components increases significantly, which is consistent with our conclusion that most
305
organic parts in sludge were degraded or dissolved by POM-I solution. In the spectra, the broad
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band in the region of 1200 – 950 cm-1 is attributed to the overlaps of aromatic ethers, Si-O and C-
307
O stretching vibration.[34-38] After the redox reaction, the intensity of band at 1090 cm-1, which
308
is assigned to the mineral compounds[39], increases dramatically. Additionally, a band at 803
309
cm-1 appears in the reacted sludge sample, which is assigned to the symmetric stretching
310
vibration of Si-O-Si.[40] On the spectrum of original sludge, the band at 2375 cm-1 is attributed
311
to CO2[41] and this band disappears in degraded sludge since CO2 released during the
312
degradation process. The
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three different regions: region of 0-110 ppm is assigned to aliphatic region; region of 110-160
314
ppm is assigned to aromatic carbons; region of 160-190 ppm is assigned to carboxyl, amide and
315
ester in lipids or proteins.[42-45] As shown in Figure 4(E), only some long chain aliphatic
316
carbons were left after reaction with POM at 100 oC for 24h. Most amides were removed from
317
sludge residue, which is consistent with the result of elemental analysis.
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C NMR spectra of sewage sludge could be roughly separated into
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Besides C, O and H elements, N is another major element in sewage sludge as it is an
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essential constituent of nucleic acids and proteins.[46-48] The content of nitrogen is about 4
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wt % in reported sewage sludge[49, 50] and is 3.75 wt % for the sewage sludge used in this
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study as shown in Figure 5(A). Nitrogen content decreased to 2.28, 1.55 and 0.73wt %,
322
respectively after the first, second and third degradation round. This might be mainly caused by
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the formation of ammonium phosphomolybdate ((NH4)3[PMo12O40]), which is less soluble in
324
water at oxidative state (MoVI) compared with phosphomolybdate acid (H3[PMo12O40]):
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3NH( PMo O → (NH ) PMo O
It is a concern that the POM may not be reused if it forms ammonia salt precipitate. However,
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because the molybdenum salt in the (NH4)3[PMo12O40] precipitate has a similar oxidation state as
327
its acid molecules, i.e. (MoVI) in both (NH4)3[PMo12O40] and H3[PMo12O40], it is expected the
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POM ammonia salt precipitate can still oxidize the sludge organisms in the solution. This
329
assumption was confirmed by mixing the sludge with the ammonia POM salt precipitates. As
330
shown in Figure 5(B), the color of POM-I solution changed from yellow to blue (heteropoly
331
blue) by the redox reaction with sludge. One drop of H2O2 was added to the solution after the
332
removal of sludge residual by filtration and this resulted in the mixture color changing back to
333
yellow with some precipitates at the bottom, which was determined to be (NH4)3[PMo12O40] by
334
X-ray powder diffraction (XRD)[51], as shown in Figure 5(C). The same amount of sewage
335
sludge was then added and reacted with (NH4)3[PMo12O40] suspension and the blue color of the
336
mixture indicated the reduction of (NH4)3[PMo12O40]. According to XRF results (Table S2,
337
supporting information), similar element contents, especially Mo contents, were found in the
338
solid residual suggesting most (NH4)3[PMo12O40] was dissolving in aqueous solution after
339
reaction with sewage sludge. Besides, the reduction degrees of the two heteropoly blue solutions
340
(one from H3[PMo12O40] reduction and the other from (NH4)3[PMo12O40] reduction) were close
341
indicating the similar oxidative capacities of (NH4)3[PMo12O40] and H3[PMo12O40]. Additionally,
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the formation of POM ammonium salts in the process can precipitate the POM, which can be
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easily separated from the solution if it is needed, which avoids the secondary pollution to
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environment. In conclusion, POM solutions were applied herein as both electron carrier and
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catalysts without net consumption.
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Figure 5. (A) N elemental contents in raw sludge and reacted sludge; (B) Colors and status of
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POM-I mixtures at different conditions: (I) fresh POM-I solution (H3[PMo12O40], 0.3M); (II)
349
Reacted POM-I with sewage sludge, ammonium phosphomolybdic formed; (III) Re-oxidized
350
POM-I by H2O2 after the filtration of sludge residual, ammonium salt precipitated at the bottom;
351
(IV) POM-I mixture (with ammonium phosphomolybdic precipitate) reacted with sludge again,
352
ammonium salt dissolved. (C) XRD pattern for precipitated (NH4)3[PMo12O40].
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4. Conclusion
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For the first time, we have developed a flow fuel cell technology that can directly convert
356
sewage sludge to electricity with high power output. Completely different from traditional MFC,
357
POMs were exploited as both catalyst and charge carrier. The power density reaches as high as
358
50 mW/cm2 which is over 100 times than that of traditional MFC and, in the meanwhile, over 80
359
wt% of organic components in the sludge was degraded in 24h. In one batch, the power density
360
of this fuel cell was over 25 mW/cm2 after 20 hours’ discharging. POM was very stable without
361
any consumption in the process and it was demonstrated that POMs kept effective to degrade
362
sludge and transfer electrons after four discharging cycles, which greatly reduces the cost. This
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flow fuel cell is a promising technology to turn sewage sludge into green energy.
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Acknowledgements
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Z.Z., W.L. and X.D. thank the PSE scholarships supported by RBI at Georgia Tech
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Highlights A novel pathway of sludge treatment is successfully designed by flow fuel cell The power output reaches 50 mW/cm2 which is over 100 times than that of MFC
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The mechanisms are completely different from any reported sludge fuel cell Low-cost and stable POM is used as both the catalyst and charge carrier
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No complicated pretreatment or purification of sludge is needed
S0360-5442(17)31664-X
DOI:
10.1016/j.energy.2017.09.143
Reference:
EGY 11637
To appear in:
Energy
Received Date: 8 May 2017 Revised Date:
5 September 2017
Accepted Date: 28 September 2017
Please cite this article as: Zhang Z, Liu C, Liu W, Du X, Cui Y, Gong J, Guo H, Deng Y, Direct conversion of sewage sludge to electricity using polyoxomatelate catalyzed flow fuel cell, Energy (2017), doi: 10.1016/j.energy.2017.09.143. 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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Direct conversion of sewage sludge to electricity using polyoxomatelate
2
catalyzed flow fuel cell
3
Zhe Zhanga,†, Congmin Liub,†, Wei Liua, Xu Dua, Yong Cuic, Jian Gonga, Hua Guob, Yulin
4
Denga,*
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a.
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1
School of Chemical &Biomolecular Engineering and Renewable Bioproducts
Instituteat Georgia Tech, Georgia Institute of Technology, 500 10th Street N.W., Atlanta,
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GA 30332-0620, USA. b.
Guodian New Energy Technology Research Institute, 2 North Street, Future Science
and Technology Park, Changping District, Beijing, China, 102209.
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c.
Key Laboratory of Nuclear Materials and Safety Assessment, Division of Materials
for Special Environments, Institute of Metal Research, Chinese Academy of Sciences,
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Shenyang, Liaoning Province, 110016, China.
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*
Corresponding author: Dr. Yulin Deng; E-mail: [email protected]
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†
Z.Z. and C.L. contribute equally to this work
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Highlights
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A novel pathway of sludge treatment is successfully designed by flow fuel cell
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The power output reaches 50 mW/cm2 which is over 100 times than that of MFC
19
The mechanisms are completely different from any reported sludge fuel cell
20
Low-cost and stable POM is used as both the catalyst and charge carrier
21
No complicated pretreatment or purification of sludge is needed
22
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Abstract
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The traditional treatment of sludge methods are high energy-consumption and expensive
25
processes. Due to the increasing awareness considering risks for the global environment and
26
human health, appropriate treatments of sewage sludge are urgently expected. A novel flow fuel
27
cell technology was reported herein which could convert sewage sludge to electricity directly
28
with high power output. In this flow fuel cell, chemically stable and completely regenerable
29
polyoxometalates (POMs) were used as both catalysts and charge carriers. Thermal induced
30
charge transfer from sludge organisms to POM was successfully used to power the flow fuel cell.
31
The power density of the cell could achieve as high as 50 mW/cm2, which is 100 times higher
32
than the output of microbial sludge fuel cell reported in literature. Catalyst recyclability was
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investigated and the POMs were demonstrated to be effective to degrade sludge and transfer
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electrons in the fuel cell system after four rounds.
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Keywords: fuel cells; polyoxomatelate; sewage sludge; green energy; chemical degradation
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1. Introduction
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Sewage sludge is a mixture produced in the wastewater treatment process, containing both
40
organic and inorganic components, pathogens and water. The increasing awareness about the
41
risks for the global environment as well as human health, finding appropriate treatments of
42
sewage sludge is highly necessary and expected urgently. Traditionally, sludge treatment is an
43
expensive practice, of which the cost is over 50% of the total cost in the water treatment
44
process.[1] Therefore, innovative treatments are supposed to focus on the recovery of valuable
45
products of sludge to make it economically acceptable.
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Traditionally, in the anaerobic digestion process, part of the sludge is hydrolyzed into
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biogas, a mixture of methane and carbon dioxide, which is applied in the production of
48
energy.[2-5] However, this process always takes ~20 days with a low pathogens removal degree.
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Converting sludge into liquid or gaseous fuels is another option.[6-8] However, this always
50
requires the complex selective separation system with high cost. Another strategy is incineration
51
of the organic matters in sewage sludge either directly or in the coal-fired power plants.[9-12]
52
Nevertheless, the potential issue is the severe emission of fly ashes[13] as well as toxic
53
chemicals, like dioxins and furans[14], which may results in the pollution of particulate matters.
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The organic substances in sludge mainly include proteins, humic acids, lipids and
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polysaccharides.[15] A microbial fuel cell (MFC), typically consisting of an anaerobic anode
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chamber with microorganisms as catalysts and an aerobic cathode chamber, can convert the
57
energy of organic matters in sludge into electricity directly.[16-18] A baffle-chamber
58
membraneless MFC was applied by Hu[19] to generate electricity from wastewater and the
59
resultant power density is 0.3mW/m2. Jiang et al[17] investigated the effect of ultrasound
60
pretreatment on sludge and demonstrated it to be efficient on total chemical oxygen demand
61
(TCOD). In his research, the power density was around 12 W/m3. Muhamad et al. used P.
62
aeruginosa strain ZH1 in the MFC and reported a maximum power density of 451.26
63
mW/m2.[20] Indeed, this sludge-fueled MFC technology has attracted significant scientific
64
attentions[21-23] and detailed investigations have been conducted on different parameters, like
65
chamber type[22, 24], sludge origin[23, 25], ionic conductivity[26], etc. However, it is
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noticeable that although MFC can convert part of the organic matters in sludge into electricity
67
directly, the power density is very limited, which increases its cost and hinders its practical
68
application.
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As a class of anionic metal-oxo polyhedral clusters formed with metal centers and
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oxygen atoms at the vertices, polyoxometalates (POMs) show great potentials in the area of
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energy storage since they can undergo highly reversible multi-electron redox reactions.[27]
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POMs have been demonstrated to be effective to degrade biomass and coal and capture electrons
73
from these substrates.[27, 28] Recently, Liu et al[29] reported a POMs catalyzed flow fuel cell
74
which converts biomass to electricity directly at low temperature. The power density of
75
cellulose-based fuel cell was 100 times higher than the result of cellulose-based MFC
76
reported.[30] Furthermore, a completely noble metal free liquid-catalyst flow fuel cell was then
77
reported [31] where two POM solutions of different electrode potentials were applied on anode
78
and cathode, respectively. Power densities reached 44mW/cm2 and 51mW/cm2 when switchgrass
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and bush allamanda were utilized as fuels with this technique.
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For the first time, we have developed a flow fuel cell technology that can directly convert
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sewage sludge to electricity with high power output. In this research, sewage sludge was found
82
to generate electricity of high power output using POM solutions as catalysts and charge carrier
83
which is completely different from traditional MFC. The power density of this fuel cell was
84
demonstrated to be hundreds of times higher than that in the sludge-fueled MFC. Briefly, POM-I
85
oxidizes the organic matters of sludge and captures electrons in the anode tanks and then the
86
excess electrons transfer to POM-II in the cathode tank through external circuit. In the
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meanwhile, positive charges (H+) diffuse through proton-exchange-membrane from anode to
88
cathode and, consequently, current is generated in the whole system. The reduced POM-II
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solution was re-oxidized by oxygen. Therefore, both POM-I and POM-II remain un-consumed in
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the process and the feeds are sludge and oxygen. Besides, it is well known that municipal sludge
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contains toxic inorganic and organic components, which can cause the deactivation of noble
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metal catalysts. However, POMs are tolerant to these contaminants and its strong oxidizing
93
capability can destroy and remove most pathogens in the sludge.
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2. Experimental Section
96
2.1. Materials
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The raw sewage sludge used in this study was obtained from the municipal wastewater treatment
98
plant located in Beijing, China. The sewage sludge was collected after being concentrated by
99
centrifugation in the plant. Phosphomolybdic acid (H3[PMo12O40]), H2O2, H3PO4 (85 wt%),
100
MoO3, was purchased from Aladdin. V2O5 was purchased from Xiya. The high-density graphite
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plates (bipolar plate) and graphite felt were purchased from Guoyao. Nafion®115 (127
102
micrometers thick) was purchased from DuPont and used as proton exchange membrane.
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2.2. Characterization of sewage sludge
104
Raw sludge was freeze-dried to get the solid base and total organic carbon (TOC) of solid was
105
measured by MULTI N/C ®2100, ANALYTIKJENA. Fourier Transform Infrared Spectroscopy
106
(FTIR) was applied to study the functional groups in the sludge and the spectra were obtained by
107
Bruker TENSOR II in the range of 500 cm-1 to 6000 cm-1 with 64 scans. Furthermore, X-ray
108
fluorescence (XRF, ZSX Primus II, 4kW, 60kV-150mA at room temperature) was applied to
109
investigate the elements contents and crystallization of sludge, respectively. Thermo stability of
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sewage sludge was measured by thermogravimetric analysis (TGA, Q50) in the range of 30-
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800oC. The morphology of sludge particles was observed by scanning electron microscopy
112
(SEM, QUANTA F250). The sludge particles were dispersed in DI water at a concentration of
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0.1%. One drop of the mixture was dropped on the conductive glass and kept in the fume hood
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for overnight to evaporate all the moisture. The sample was sputtered with gold before SEM
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measurement. Reacted sludge was separated from POM solution by vacuum filtration and washed
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thoroughly with DI water until the water became colorless. Then the sludge was free-dried and
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characterized as the raw sludge.
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2.3. Preparation of POM solutions for anode (POM-I) and cathode (POM-II)
120
Anode: A certain amount of H3[PMo12O40] was added to DI water to form solutions (POM-I)
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with a concentration of 0.3 mol/L. This solution was stirred at 60oC for 1 hour in order to
122
dissolve H3[PMo12O40] completely. Cathode: H12[P3Mo18V7O85] solution (POM-II) was used as
123
cathode electrolyte and was synthesized according to Odyakov’s work[32]. A typical synthesis
124
process of H12[P3Mo18V7O85] can be found in supporting information.
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2.4. Redox reactions between sewage sludge and POM solutions
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Both dried sludge powder and wet sludge were used as substrates.
127
2.4.1. Heating induced reaction with dried sludge powders
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Sewage sludge samples were first freeze-dried to remove water to form sludge powders. Then
129
the redox reaction was conducted by mixing POM-I solution (0.3 M, 50mL) with sludge power
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(2.5g) and heating to required temperature (80, 100 and 150 oC) for certain periods (6, 12, 24 and
131
36h). Reactions at 80 oC and 100 oC underwent in glass vessels with reflex condenser, while
132
reaction at 150 oC was conducted in an autoclave reactor.
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2.4.2. Heating induced reaction with wet sludge
134
Sewage sludge was also used directly in the redox reaction without freeze-drying pretreatment.
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Since the water content was 87 wt % in the original sludge, 20 grams of sewage sludge was
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mixed with 27.35 grams of H3[PMo12O40] and 25 mL DI water was added to the mixture to keep
137
the content of each component the same as the case with sludge powders. Afterwards, the
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mixture was kept at 100 oC for certain periods (6, 12, 24 and 36h).
139
2.5. Assembly of fuel cell
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The graphite felt was soaked in a of H2SO4 and HNO3 solution (volumetric ratio equaled to 3:1)
141
at 50oC for 30 minutes and then washed with DI water to make it neutral. The resultant graphite
142
felt was then filled into the pre-carved flow channel with a geometry function area of 1cm2 on
143
each bipolar plate. Nafion 115 membrane was sandwiched between two bipolar plates. Two
144
acrylic plastic plates were applied outside of the carbon plates to fix them with PTFE gaskets
145
included to prevent leakage. A pump connected by PTFE pipelines was used to transport
146
electrolyte solutions into and out of the fuel cell at each electrode. Anode electrolyte (POM-I
147
solution) was pumped from the anode tank after the filtration of sludge residue with a 0.2
148
micrometer filter head into the anode plate at a flow rate of 30mL min-1. In the meanwhile,
149
POM-II was pumped into the cathode of the fuel cell from the cathode tank at the same flow rate
150
(30mL/min). Anode tank was kept at 80oC during the discharging process. Oxygen was pumped
151
into the cathode tank under high-speed homogenizing (20,000 rpm) at 80oC in order to re-oxidize
152
POM-II solution. I-V curves were obtained with a Versa Stat Electrochemical Working Station
153
(Princeton Applied Research) through the controlled potentiostatic method.
154
2.6. Characterization of (NH4)3[PMo12O40]
155
Sludge powers (2.5g) were reacted with H3[PMo12O40] solution (0.3 M, 50 mL) at 100 oC for 6h.
156
Then sludge was removed by vacuum filtration. 1 mL of H2O2 (30%) was added to the reduced
157
POM-I solution and its color changed to yellow with white precipitate ((NH4)3[PMo12O40]) at
158
the bottom. The white precipitate was separated with centrifugation at 7000 rpm for 10 min.
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Then the powers were dried at room temperature before X-Ray Diffraction measurement (XRD,
160
D8 Advance, Cu Kα radiation from 5o to 75o).
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3. Results and discussion
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3.1. Mechanism of sewage sludge flow fuel cell
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Figure 1. (A) Schematic illustration of the sewage sludge fuel cell; (B) illustration of cell stack
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(left) and inner side of bipolar plate (right)
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The reactions in the electrode tanks and the transferring of electrons in the flow fuel cell
169
could be expressed as:
170
(1) In the anode tank, sludge is oxidized by POM-I under heating:
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sludge PMo O H PMo CO degraded products (1) Mo O
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(2) On the anode, extra electrons of POM-I captured from sludge particles were
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transferred to the graphite felt and the reduced POM-I solution which was re-oxidized
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(MoV to MoVI):
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H PMo → PMo O n e n H ( (2) Mo O
(3) On the cathode, POM-II is the acceptor of electrons where VV was reduced to VIV:P V* Mo + O+, n e n H( → H P V V* Mo + O+, (3)
(4) In the cathode tank, reduced POM-II was then re-oxidized by oxygen (VIV to VV):
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H P V V* Mo + O+, O → P V* Mo + O+, H O (4)
Overall reaction in the fuel cell is the summation of reactions (1) to (4), 4567 8677
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The mechanism and system set-up of the flow fuel cell are shown in Figure 1. Organic
183
components in the sludge (carbohydrate, humic acid, lipids et.) were oxidized by POM-I in
184
anode tank and the electrons in these organics were transferred to POM molecules, which caused
185
the decrease of anode potential. Due to the higher potential of cathode electrolyte POM-II than
186
that of POM-I in the anode cell, the reduced POM-I could be re-oxidized by POM-II, as shown
187
in Figure 1B, therefore, the as-obtained electrons transferred onto anode electrode and further
188
transferred to cathode through external circuit. Finally, they were captured by POM-II molecules.
189
In the meanwhile, protons (hydrogen ions) penetrated through the Nafion membrane which was
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sandwiched between two electrodes. As a result, current was generated in the fuel cell system.
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For the entire flow cell process, POM-I was reduced by sewage sludge and re-oxidized by POM-
192
II through the fuel cell discharge process. At meantime, reduced POM-II was re-oxidized by
193
oxygen. As a result, both POM-I and POM-II were completely regenerated. These processes are
194
schematically shown in Figure 1. Clearly, both POM-I and POM-II were used as catalyst without
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net consumption.
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Sewage sludge without POM could not generate electricity in this fuel cell. Besides,
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when sludge was treated with POM at room temperature, no reaction occurred and no power was
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generated. However, if we increased reaction temperature, sludge was oxidized by POM-I
199
successfully, meanwhile, POM-I molecules were reduced via obtaining extra electrons. As
200
indicated in Figure 2(A), after 6 hours’ reaction at 80 oC, the reduced POM-I solution could
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generate electricity with an open-circuit voltage (Vmax) of 0.3V and a maximum power density
202
(Pmax) of 13.56 mW/cm2. With the increase in reaction time, more electrons were captured by
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POM-I solution (as shown in Figure S2, supporting information) leading to a better cell
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performance, e.g. Vmax was 0.32 and 0.33V while Pmax was 16.07 and 17.91 mW/cm2 for
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different reaction time of sludge degradation for 12 and 24h, respectively. However, further
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increasing reaction time to 36h could not further enhance its electrical performance due to the
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complete conversion of reactive components in sludge.
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Temperature was another important factor that could affect the reaction extent kinetically.
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When the degradation temperature was 100 oC, the maximum power density, Pmax, for 6, 12 and
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24 hours’ reactions were 39.7, 47.6 and 49.8 mW/cm2 (Figure 2(B)), respectively. Higher
211
temperature resulted in higher reaction rates between sludge and POM-I. The fast reaction and
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high reaction degree leaded to a lower reduction degree of POM-I molecules, which was
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favorable to the electricity generation. Therefore, when reaction temperature was raised to 150
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o
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respectively, which was better than that reacted at 80 or 100 oC, as expected.
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C in an autoclave, as shown in Figure 2(C), Pmax was improved to 48.8, 54.0 and 61.4 mW/cm2,
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Degradation at 80 C
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Degradation at 100 C
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Degradation at 150 C
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6h 12h 24h 36h
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I (mA/cm )
(C)
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Figure 2. Performance of sewage sludge flow fuel cell with sewage sludge degradation at
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different conditions: (A) dry sludge powder reacted with POM-I at 80 oC; (B) dry sludge powder
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reacted with POM-I at 100 oC; (C) dry sludge powder reacted with POM-I at 150 oC. (D) Wet
222
sewage sludge (without drying) was used directly in the fuel cell and redox reaction temperature
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was 100 oC. All the experiments were conducted three times and the values shown on the figure
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are the average of the results from three experiments with the standard deviations less than 5%.
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To investigate this technique in a more practical aspect, sewage sludge without drying
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was used directly to react with POM-I and the corresponding electrical behaviors were studied,
228
as shown in Figure 2(D). It was noted that when the temperature was kept the same (100 oC
229
herein), electrical performances expressed similar tendency using either dried sludge or wet
230
sludge. The power density increased from 37.7 to 52.0mW/cm2 as wet sludge was pre-degraded
231
by POM-I for 6 to 24h. Further degradation of sludge to 36h showed little improvement on
232
electrical output.
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A batch test of the flow fuel cell was operated at a constant current density of 100
234
mA/cm2. The discharge process was kept running until all reduced POM-I was re-oxidized. The
235
initial power density was 45 mW/cm2, and then decreased gradually during the discharging
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process due to the consumption of energy stored in reduced POM-I. The power output reduced to
237
0 after 25h. In this process, the reduced POM-I molecules from the chemical reaction between
238
sludge and POM-I were oxidized (MoV ->MoVI) on anode and the electrical potential increased
239
accordingly. After the POM-I was completely discharged, the sludge was added into the POM-I
240
solution and heated again for the second circle of the test. As shown in Figure 3(A), the
241
degradation and discharging cycle was repeated three times, and the power density was used as a
242
function of the repeat circles of the degradation-discharge. Obviously, energy released during the
243
second and third rounds was much less than that of the first round. This was mainly caused by
244
the lower reduction degree of POM-I in the second and third round, as most reactive materials in
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the sludge were reacted during the first degradation process and fewer organisms were available
246
in the solution for the following rounds. In order to investigate the conversion of sludge in each
247
degradation round, solid weight loss and total organic carbon (TOC) of the sludge residuals were
248
analyzed. As shown in Figure 3(B), over 60 wt % of sludge was either reacted or dissolved after
249
cooking with POM-I during the first round of the reaction and this number increased to 85%
250
after the second round. Actually, in the first round, the reacted carbon content was around ~38%
251
(see supporting information). In the meanwhile, TOC of solid sludge residual after drying at
252
room temperature decreased from 170 to 32 g/kg and further decreased to 23 g/kg after the 1st
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and 2nd round, respectively. This result was consistent with the electrical performance and
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indicated that most organic components could be oxidized in 24h with H3[PMo12O40] at 100 oC.
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(A) 60
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Time (h)
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Weight percent TOC
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Figure 3. (A) Performance of fuel cell in discharging process: POM-I solution was used to
259
degrade sludge and discharged three time without adding fresh sludge. (B) Corresponding mass
260
loss in each run and total organic carbon content in sludge residuals. (C), (D) Performance of
261
fuel cell in discharging process: recycled POM-I solution was reacted with same amount of raw
262
sludge (5g) before each discharging process. All redox reactions were carried out at 100 oC and
263
fuel cell was operated at 80 oC. (C) Power density curve in the first cycle. (D) Power densities at
264
different discharging time (0, 10, 20h) in each cycle, the values shown on the figure are the
265
average of the results from three experiments with the standard deviations less than 5%.
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In order to investigate the catalysis recyclability of POM-I, new sewage sludge powder
268
was added to react with recycled POM-I solution in each round. Stable performance was
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obtained, as shown in Figure 3(C), which is the power density curve in the first cycle. Actually,
270
in all four discharging cycles, the power density curves were similar, with the initial power
271
density around 44 mW/cm2, a power density of 37 mW/cm2 at 10h and a power density of 25
272
mW/cm2 at 20h, as shown in Figure 3(D). All discharging processes took around 22-25h (Power
273
density curves for all four cycles are shown in Figure S3). This demonstrated the excellent
274
stability of POM-I in this technology, and also demonstrated that the electricity could be
275
generated continuously with the continuous consumption of sewage sludge.
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3.2. Characterization of sewage sludge
(A)
90 80
60 50 40
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Temperature (oC) Raw sludge
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(E) Raw sludge Heating 200
Reacted sludge Original sludge
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Figure 4. Characterization of original sludge and reacted sludge (100 oC for 12h):
282
(A) TGA; (B) DSC; (C) SEM images; (D) FTIR; (E) Solid-state 13C NMR.
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In order to investigate the degradation reaction, experiments were performed on both
285
original sewage sludge and sludge residues after reaction to study the structure changes. The
286
results of TGA (Figure 4(A)) shows that original sludge lost most of its organic components in
287
the range of 200-550 oC sharply and around 45 wt % were left at 1000 oC. However, after the
288
degradation process by POM, sludge lost its weight gradually in the range of 200-700 oC and the
289
weight percent of remained residues was over 80 wt %. The residue content increased as the
290
degradation time extended. This was consistent with the electrical results: with the extending of
291
reaction time, more sludge was degraded and more electrons were captured by POM molecules
292
which leaded to better fuel cell performances. The results of differential scanning calorimetry
293
(DSC) (Figure 4(B)) of sludge samples further confirmed the conclusion since the heat flow of
294
reacted sludge reduced dramatically which was resulted from the degradation of organic matter
295
by POM. Detailed TGA and DSC spectra for reacted sludge of different reaction time periods
296
were provided in Figure S4 and Figure S5 (supporting information).
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Scanning electron microscope (SEM) images (Figure 4(C)) show the morphology of
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sludge particles. As expected, particle shapes are non-uniform due to the heterogeneity of sewage
299
sludge. However, the image of original sludge sample shows some fibrous structures, as marked
300
in Figures 4 (E), which represents the presence of microorganisms.[33] In reacted sludge, these
301
fibrous structures are not presented due to the degradation of organic matters.
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FTIR spectra of sludge in Figure 4(D) indicate the change of functional groups of sludge
303
caused by POM-I solution degradation. Compared with original sludge, the intensity of bands for
304
inorganic components increases significantly, which is consistent with our conclusion that most
305
organic parts in sludge were degraded or dissolved by POM-I solution. In the spectra, the broad
306
band in the region of 1200 – 950 cm-1 is attributed to the overlaps of aromatic ethers, Si-O and C-
307
O stretching vibration.[34-38] After the redox reaction, the intensity of band at 1090 cm-1, which
308
is assigned to the mineral compounds[39], increases dramatically. Additionally, a band at 803
309
cm-1 appears in the reacted sludge sample, which is assigned to the symmetric stretching
310
vibration of Si-O-Si.[40] On the spectrum of original sludge, the band at 2375 cm-1 is attributed
311
to CO2[41] and this band disappears in degraded sludge since CO2 released during the
312
degradation process. The
313
three different regions: region of 0-110 ppm is assigned to aliphatic region; region of 110-160
314
ppm is assigned to aromatic carbons; region of 160-190 ppm is assigned to carboxyl, amide and
315
ester in lipids or proteins.[42-45] As shown in Figure 4(E), only some long chain aliphatic
316
carbons were left after reaction with POM at 100 oC for 24h. Most amides were removed from
317
sludge residue, which is consistent with the result of elemental analysis.
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Besides C, O and H elements, N is another major element in sewage sludge as it is an
319
essential constituent of nucleic acids and proteins.[46-48] The content of nitrogen is about 4
320
wt % in reported sewage sludge[49, 50] and is 3.75 wt % for the sewage sludge used in this
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study as shown in Figure 5(A). Nitrogen content decreased to 2.28, 1.55 and 0.73wt %,
322
respectively after the first, second and third degradation round. This might be mainly caused by
323
the formation of ammonium phosphomolybdate ((NH4)3[PMo12O40]), which is less soluble in
324
water at oxidative state (MoVI) compared with phosphomolybdate acid (H3[PMo12O40]):
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3NH( PMo O → (NH ) PMo O
It is a concern that the POM may not be reused if it forms ammonia salt precipitate. However,
326
because the molybdenum salt in the (NH4)3[PMo12O40] precipitate has a similar oxidation state as
327
its acid molecules, i.e. (MoVI) in both (NH4)3[PMo12O40] and H3[PMo12O40], it is expected the
328
POM ammonia salt precipitate can still oxidize the sludge organisms in the solution. This
329
assumption was confirmed by mixing the sludge with the ammonia POM salt precipitates. As
330
shown in Figure 5(B), the color of POM-I solution changed from yellow to blue (heteropoly
331
blue) by the redox reaction with sludge. One drop of H2O2 was added to the solution after the
332
removal of sludge residual by filtration and this resulted in the mixture color changing back to
333
yellow with some precipitates at the bottom, which was determined to be (NH4)3[PMo12O40] by
334
X-ray powder diffraction (XRD)[51], as shown in Figure 5(C). The same amount of sewage
335
sludge was then added and reacted with (NH4)3[PMo12O40] suspension and the blue color of the
336
mixture indicated the reduction of (NH4)3[PMo12O40]. According to XRF results (Table S2,
337
supporting information), similar element contents, especially Mo contents, were found in the
338
solid residual suggesting most (NH4)3[PMo12O40] was dissolving in aqueous solution after
339
reaction with sewage sludge. Besides, the reduction degrees of the two heteropoly blue solutions
340
(one from H3[PMo12O40] reduction and the other from (NH4)3[PMo12O40] reduction) were close
341
indicating the similar oxidative capacities of (NH4)3[PMo12O40] and H3[PMo12O40]. Additionally,
342
the formation of POM ammonium salts in the process can precipitate the POM, which can be
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easily separated from the solution if it is needed, which avoids the secondary pollution to
344
environment. In conclusion, POM solutions were applied herein as both electron carrier and
345
catalysts without net consumption.
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Figure 5. (A) N elemental contents in raw sludge and reacted sludge; (B) Colors and status of
348
POM-I mixtures at different conditions: (I) fresh POM-I solution (H3[PMo12O40], 0.3M); (II)
349
Reacted POM-I with sewage sludge, ammonium phosphomolybdic formed; (III) Re-oxidized
350
POM-I by H2O2 after the filtration of sludge residual, ammonium salt precipitated at the bottom;
351
(IV) POM-I mixture (with ammonium phosphomolybdic precipitate) reacted with sludge again,
352
ammonium salt dissolved. (C) XRD pattern for precipitated (NH4)3[PMo12O40].
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4. Conclusion
355
For the first time, we have developed a flow fuel cell technology that can directly convert
356
sewage sludge to electricity with high power output. Completely different from traditional MFC,
357
POMs were exploited as both catalyst and charge carrier. The power density reaches as high as
358
50 mW/cm2 which is over 100 times than that of traditional MFC and, in the meanwhile, over 80
359
wt% of organic components in the sludge was degraded in 24h. In one batch, the power density
360
of this fuel cell was over 25 mW/cm2 after 20 hours’ discharging. POM was very stable without
361
any consumption in the process and it was demonstrated that POMs kept effective to degrade
362
sludge and transfer electrons after four discharging cycles, which greatly reduces the cost. This
363
flow fuel cell is a promising technology to turn sewage sludge into green energy.
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Acknowledgements
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Z.Z., W.L. and X.D. thank the PSE scholarships supported by RBI at Georgia Tech
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Reference:
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[1] Rulkens W. Sewage sludge as a biomass resource for the production of energy: overview and
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assessment of the various options†. Energy Fuels. 2007;22(1):9-15.
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[2] Camacho P, Deleris S, Geaugey V, Ginestet P, Paul E. A comparative study between
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mechanical, thermal and oxidative disintegration techniques of waste activated sludge. Water
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Science and Technology. 2002;46(10):79-87.
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[3] Müller J, Winter A, Strünkmann G. Investigation and assessment of sludge pre-treatment
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processes. Water Science and Technology. 2004;49(10):97-104.
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[4] Neis U, Nickel K, Tiehm A. Enhancement of anaerobic sludge digestion by ultrasonic
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Highlights A novel pathway of sludge treatment is successfully designed by flow fuel cell The power output reaches 50 mW/cm2 which is over 100 times than that of MFC
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The mechanisms are completely different from any reported sludge fuel cell Low-cost and stable POM is used as both the catalyst and charge carrier
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No complicated pretreatment or purification of sludge is needed