学习化妆的技巧Sulfate reduction with electrons directly derived from electrodes in bioelectrochemical systems
Wentao Su,Lixia Zhang,Yong Tao,Guoqiang Zhan,Dongxun Li,Daping Li ⁎
Key Laboratory of Environmental and Applied Microbiology,Chengdu Institute of Biology,Chine Academy of Sciences,Chengdu 610041,China Environmental Microbiology Key Laboratory of Sichuan Province,Chengdu 610041,China
a b s t r a c t
a r t i c l e i n f o Article history:
Received 2March 2012
Received in revid form 1April 2012Accepted 23April 2012
Available online 18May 2012Keywords:Biocathode
Sulfate reduction
Direct electron transfer Electrochemical activity
In this paper,we reported microbially catalyzed sulfate reduction with polarized electrode (−400mV vs.Ag/AgCl)as the sole electron donor.In potentiostatic batch assays,sulfate was reduced to sul fide,at rates falling in the range of 6.70–12.16equiv./l d.Cyclic voltammetry tests revealed that the sulfate-reducing bio films could accept elec-trons from electrodes directly without via electron shuttles or hydrogen production.Scanning electron microscope revealed that the electrode was colonized by veral ellip-shaped and short rod-shaped microorganisms,which cloly related to Desulfobulbus propionicus and Geobacter species .
©2012Elvier B.V.All rights rerved.
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1.Introduction
Bioelectrochemical systems (BESs)have been recently propod as a new and sustainable technology for bioremediation of contami-nated environments [1].In the device,microorganisms were ud as biocatalysts to accept electrons from the cathode [2]for the reduction of oxidized contaminants including nitrate [3],U (VI)[4]and chlori-nated organic compounds [5–7].The propod mechanisms include:(i)direct electron transfer via redox components located on the outer surface of the microorganism (hromes);(ii)mediated electron transfer bad on diffusible redox mediators [8].
To date,only two papers reported that sulfate reduction could be achieved in BESs,which were catalyzed by Desulfovibrio caledoniensis [9]and Desulfovibrio desulfuricans [10],respectively.The former car-ried out with the consumption of lactate for hydrogen production,and the later was conducted with more negative potentials (−0.81V vs.Ag/AgCl)for hydrogen evolution as the mediator for the electron trans-fer.Potential disadvantages of producing hydrogen as an electron donor carrier included high energy costs due to the low electrode potentials and stimulating growth of unwanted non –sulfate reducing microorgan-isms [11].Therefore,exploring an approach of using electrons directly derived from electrodes for sulfate reduction will be bene ficial to the effective electron transfer and practical application.
In this study,sulfate reduction was investigated in a BES with polar-ized biocathode (−400mV vs.Ag/AgCl)as the sole electron donor.By employing a combination of chemical,electrochemical,and microscopy
techniques,as well as of molecular tools,a direct extracellular electron transfer pathway was proved for sulfate reduction without hydrogen evolution or electron shuttles as the mediator.2.Experimental
2.1.BESs tup and operation
The BESs ud in this study consisted of two compartment (made of polymethyl methacrylate),physically parated by a cation exchange membrane (CEM,AMI7001).Both anode (counter electrode)and cath-ode (working electrode)electrodes were carbon felt of 4.0cm×4.0cm.Both chambers were flushed with sterile,Argon gas,filled with buffer medium and connected to a potentiostat.Ag/AgCl (sat.KCl,0.197V vs.SHE)electrode was ud as reference electrode.All connections be-tween the electrodes and the potentiostat have been described previ-ously [12].The buffer medium contained the following (per liter of distilled water):0.1g of KCl,0.04g of MgCl 2,10.9g of Na 2HPO 4·12H 2O,3.0g of NaH 2PO 4,2.0g of NaHCO 3,1.0ml of trace mineral mix [13].The pH of the medium was 7.0.In parallel,veral control batch experi-ments were also performed under identical conditions,but for the ab-nce of the microbial culture (abiotic control tests)or for the abnce of cathode polarization (open circuit voltage tests).
The mixed culture ud in bioelectrochemical experiments was originated from a wastewater treatment plant of Chengdu,China.Prior to bioelectrochemical experiments,the mixed culture was enriched with H 2described by Aulenta [6].At the end of this pha,growth medium was replaced with buffer medium containing sodium sulfate (1.91mmol).Both chambers were flushed with argon gas to eliminate volatile compounds (i.e.,sul fide and residual hydrogen).Thereafter,a
Electrochemistry Communications 22(2012)37–40
⁎Corresponding author at:Key Laboratory of Environmental and Applied Microbiol-ogy,Chengdu Institute of Biology,Chine Academy of Sciences,Chengdu 610041,
China.
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ries of batch experiments,with the cathode potential t to −400mV (vs.Ag/AgCl),were performed.The temperature for tests was 30±1°CC.
2.2.Analytical techniques
SO 4big wolf
2−
and S 2−were taken regularly and analyzed according to the standard methods for the examination of water and wastewater [14].Total H 2production was measured using gas chromatography (thermal
conductivity detection)as previously described [6].The minimum detection limit for H 2was 0.05ml/l.D
enaturing gradient gel electropho-resis (DGGE)was ud to analyze the microbial community of the cathode.DNA extraction,PCR ampli fication and DGGE construction were carried out as described [15].Micrographs were taken by the scanning electron microscope (JSW –5900LV,Japan)as described [16].Electrochemical potentiostatic measurements and monitoring were per-formed using a CHI 1000B (CHI instrument,USA).The current of work-ing ‐electrode was collected every 50s by CHI 1000B with a Power Laboratory 8SP unit connected to a computer.CV measurements were performed with an electrochemical working station (CS120,China)at a scan rate of 5mV s −1in the potential range from −0.48V to 0.4V.陕西师范大学研究生院
Cumulative electric charge (eq i )that was transferred at the electrodes was calculated by integrating current (A)over the period of electrode polarization,by using the following conversion factor:eq i .=C/F.Cumu-lative reducing equivalents (eq p )that were ud for sulfate reduction was calculated from the formation of sul fide,considering the corresponding molar conversion factor of 8eq mol −1.Coulombic ef ficiency for reduced products was accordingly calculated as ηP =(eq p /eq i )×100.3.Results and discussion
3.1.Batches of sulfate reduction experiments
In order to verify whether the polarized carbon felt electrode could rve as the sole electron donor for the microbial reductive reduction of sulfate,batch assays were performed,as reported in Table 1.Fig.1A showed the time cour of sulfate reduction during a typical batch assay (batch 3,Table 1)with the microbial culture and the cathode po-larized to −400mV (vs.Ag/AgCl).In contrast,there was no loss of
Table 1
Summary of results of batch sulfate reduction assays.
Microbial culture
Cathode potential (vs.Ag/AgCl)Maximum sulfate reduction rate a (equiv./l d)H 2production rate (equiv./l d)
Batch 1Yes −400mV 6.70N.D.d Batch 2Yes −400mV 10.60N.D.Batch 3Yes −400mV 12.16N.D.Control 1No −400mV 0.01N.D.Control 2Yes OCV c 0N.D.Control 3
Yes b
−400mV
N.D.
a Maximum sulfate reduction rates are bad on formed sul fide.
b The microorganisms on the working electrode were killed after experiments.
c OCV:open circuit voltage.d
N.D.:no detected.
-0.2
-0.1
0.0
S u l f a t e , s u l f i d e (m m o l L -1)
t/d
I /A .m -2hfg
0.0
0.4
0.8
1.2
1.6
2.0
500
美籍华人英语1000
1500
2000
e q /L
t/d
B
A
Fig.1.(A)Time cour of microbial sulfate reduction at −400mV (vs.Ag/AgCl).Error bars reprent the standard deviation of replicated analys.(B)Cumulative electric charge transferred and sul fide formed.
-2
-1
1
2 Control Batch 1 Batch 3
lumia怎么读
-0.4
-0.2
0.0
0.2
0.4
0.6
I (m A /m 2)
-2
-1
1
2
I (m A /m 2)
V (vs. Ag/AgCl)
-0.4-0.20.00.20.40.6
V (vs. Ag/AgCl)
Before replacement of mixed-culture medium After replacement of mixed-culture medium Mixed-culture filtered medium
B
A
Fig.2.(A)CV for batch sulfate reduction assays.(B)CV for cathode filter-sterilized su-pernatant (dot line),before (solid line)and after the replacement (dash line)of growth medium.
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W.Su et al./Electrochemistry Communications 22(2012)37–40
sulfate or any sulfide production if no current was supplied to the elec-trode(control2).Also,no loss of sulfate if no mixed culture was sup-plied(control1),or if the microorganisms on the electrodes were killed with heat(control3).
建工网校In our experimental conditions,a significant current enhance was achieved with time(Fig.1A).The variation regularity of current in-cread gradually to the maximum current of236.39mA m−2(stable f
or24h),then it decread as follows.Simultaneously,sulfate deplet-ed from1.91mmol l−1to0.36mmol l−1and sulfide accumulated to 1.52mmol l−1.Fig.1B showed the cumulative electric charge trans-ferred during the test.On day10,thefinal coulombic efficiency for the sulfate reduction was71.83%,thereby indicating a very efficient utiliza-tion of the electric charge by the sulfate-reducing microorganisms. Moreover,no H2production obrved in biotic control experiments (controls1,2and3,Table1),as well as in all the other tested conditions, suggesting that H2was not responsible for sulfate reduction.
3.2.Electrochemical activities of the working electrode
Cyclic voltammetry(CV)tests were carried out on the biocathode at two different ,clo to the beginning(batch1)and the end (batch3)of the batch sulfate reduction experiments.Fig.2A showed the voltammetric respon of biofilm in different time point.Oxidation–reduction peaks appeared at about0.10and−0.2V(vs.Ag/AgCl),respec-tively and the peak potentials were very similar to that of Klebsiella pneumoniae[16]and that of Shewanella putrefaciens IR-1[17].It was plau-sible that the cathodic extracellular electron transfer from electrodes to biofilms was mediated by the cell surface cytochrome(s)[18],which need further characterization of the involved type of redox protein. None of the signals was obrved in the control without cathode polarization.Furthermore,the negative pot
ential(−0.4V vs.Ag/AgCl) applied in prent study was too high for significant H2production,for which the potential needed was−0.81V(vs.Ag/AgCl)in previous report [10].Together with the measured rate of H2production in Table1,the role of H2as an electron donor in the sulfate reduction process could be ruled out.
In order to further investigate the involved electron transfer mecha-nism of biofilms,the electrochemical activities of biofilms before the re-placement,after the replacement and in thefilter-sterilized effluent (0.2μm)with fresh carbon felt electrode were evaluated by CV(Fig. 2B).Fig.2B showed that the electrochemical activity of sulfate-reducing biofilm was not decrea but incread after the replacement of the me-dium,whereas the oxidation–reduction peak disappeared suddenly after filtering effluent of the cathodic chamber.The results suggested that the electron transfer process was unlikely mediated by soluble shuttle but rather by microbial components developing at electrode interface during continuous polarization at−0.40V vs.Ag/AgCl.
3.3.Morphological and molecular characterization of the working electrode
Scanning electron microscopy(SEM)micrographs of electrode sur-faces were showed before and after the potentiostatic experiments. SEM micrograph of enrichment electrode surface revealed that
manage
elec-trode was covered with ellip-shaped and rod-shaped cells formed biofilms with a more complex architecture,whereas control electrode that was not connected to a potentiostat(and thus not providing any current)was not heavily colonized(Fig.3A and B).
To determine electro-active microbial community respon for sulfate reduction,DGGE was conducted with time.At the beginning,a
stable
Fig.3.SEM images of electrode not connected to a potentiostat(A)and sulfate-reducing biocathode(B).(C)DGGE profiles of16S rRNA fragments amplified from batch sulfate reduction assays.(D)Neighbor-joining phylogenetic trees generated from16S rRNA quences derived from the excid DGGE bands.
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W.Su et al./Electrochemistry Communications22(2012)37–40
community pattern containing bands A-1,A-2,A-3,A-4,A-7,A-8and A-9 formed with H2rving as electron donors(Fig.3C,control).Blastn arches showed that the band quences were similar to Pudomonadaceae,Clostridiaceae and Desulfobulbaceae(Fig.3D).Sub-quently,the carbon felt electrode was polarized to−400mV(vs.Ag/ AgCl)for direct electron transfer,instead of H2.The bacterial populations appeared to change considerably since bands from H2-enriched culture decread.Interestingly,the new bands(A-5and A-6)appeared during the biofilm development.The quence of band A-5was similar to Geobacter species,with a97%similarity to that of Geobacter sulfurreducens.Geobacter species were well known for the capacity to directly accept electrons from
polarized electrode[19]and distributed sparly on electrode surface in short rod–shaped[5,7].The quence of band A-6was100%similar to that of Desulfobulbus propionicus, which was ellip-shaped bacteria[20]and could grow via electron transfer to electrodes[21].The results from DGGE and the obrvation with SEM suggested that the corresponding bacterium were responsible for sulfate reduction using electrons from negatively poid electrode.
4.Conclusion
In the prented study,microbially catalyzed sulfate reduction was achieved using a polarized electrode(−400mV vs.Ag/AgCl)as the sole electron donor.The sulfate-reducing biofilms were largely domi-nated by phylotypes cloly related to Desulfobulbus propionicus and Geobacter species.
Acknowledgment
This work was supported by the National Natural Science Founda-tion of China(No.51074149and No.31000070).References
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