SUPPLEMENTARY MATERIALS
High current densities enable exoelectrogens to outcompete aerobic
heterotrophs for substrate
Lijiao Rena, Xiaoyuan Zhanga, Weihua Heb, Bruce E. Logan*a
a
Department of Civil and Environmental Engineering, 212 Sackett Building, The Pennsylvania
State University, University Park, PA 16802, USA
b
State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of
Technology, 73 Huanghe Road, Harbin 150090, P.R. China
*Corresponding author. Tel.: +1 814 863 7908; fax: +1 814 863 7304.
E-mail address: blogan@psu.edu (B.E. Logan).
1.2
Open
1000
300
100
sCOD/sCODin
1.0
0.8
fit-Open
fit-1000
fit-300
fit-100
0.6
0.4
0.2
0.0
0
2
4
6
8
10
Time (h)
Figure S1 sCOD concentrations over time in fed-batch cycles at different external resistances
(open circuit, 1000, 300, and 100 Ω). The data were fitted assuming first-order kinetics, with
sCOD concentrations normalized by the initial sCOD (394.7 ± 22.5 mg/L).
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sCOD removal (%)
100
54
36 53
54
51
80
28
60
40
52
42
23
20
0
2
5
9
Time (h)
Figure S2 sCOD removals (fraction of initial substrate concentration) and CEs over time in fedbatch cycles at different external resistances [open circuit (green), 1000 Ω (purple), 300 Ω
(orange), and 100 Ω (blue)]. The shaded areas are the amount of sCOD accounted for by current,
and the filled areas are that lost to oxygen leakage. The numbers above the stacked columns are
the CE values.
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0.6
A
Power (mW)
0.5
0.4
0.3
0.2
0.1
1
2
3
4
5
0
0.6
0
1
1.5
2
2.5
3
2.5
3
2.5
3
Current (mA)
B
0.5
Voltage (V)
0.5
0.4
0.3
0.2
0.1
0
0.2 0
Potential (V)
0.1
0.5
1
1.5
2
Current (mA)
C
0
-0.1
-0.2
-0.3
-0.4
-0.5
0
0.5
1
1.5
2
1-C
2-C
3-C
4-C
5-C
1-A
2-A
3-A
4-A
5-A
Current (mA)
Figure S3 Power, voltage and electrode potentials of the MFCs at different current: (A) power,
(B) voltage, and (C) electrode potentials. The five duplicates were indicated by numbers “1” to
“5”, and in (C) the letter “C” indicated the cathodes and “A” indicated the anodes.
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Carbon cloth anode
Anion exchange
membrane
Air cathode
Potassium ferrocyanide
50mM PBS buffer
Figure S4 Set-up of the abiotic reactor for the measurement of oxygen mass transfer coefficient
of air-cathode.
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Oxygen transfer calculation
In order to better understand substrate consumption by aerobic heterotrophs at open circuit
compared to that with current production, we calculated the amount of oxygen that could be
transferred into the bulk solution under these two different conditions based on the oxygen mass
transfer coefficients measured in abiotic reactors.
With an open circuit, the oxygen mass transfer coefficient is k = 2.5 ×10–3 cm/s, and
assuming that the saturated oxygen concentration is DOs = 7.7 mg/L and the oxygen
concentration in the bulk solution is DO = 1 mg/L, we can get the oxygen flux as
J = r × (DOs – DO)
= 2.5 ×10–3 (cm/s) × (7.7 – 1) (mg/L)
= 2.5 × 10–3 (cm/s) × 6.7 (mg/1000 cm3)
= 1.675 × 10–5 (mg/s-cm2)
We know the cathode cross sectional area is A = 7 cm2, and thus the total mass of oxygen
that could be transferred into the bulk solution in t = 9 h is
m=J×A×t
= 1.675 × 10–5 (mg/s cm2) × 7 (cm2) × 9 (h)
= 1.675 × 10–5 (mg/s cm2) × 7 (cm2) × 9 × 3600 (s)
= 3.80 (mg)
COD equivalent = 3.7989 (mg) / 14 (mL) = 3.7989 (mg) / 14 × 10–3 (L) = 271.4 (mg/L)
We know the influent COD concentration is averaged of 394.7 mg/L, and thus the percent
of COD lost to oxygen leakage is therefore
271.4 (mg/L) / 394.7 (mg/L) = 68.7%
This is consistent with that measured in the open circuit tests, as we calculated that 67% of
the substrate in the biotic MFCs was removed in 9 h. Thus, it is reasonable to assume that all of
the substrate that removed was lost to heterotrophs using oxygen.
Similarly, we can calculate the COD lost to oxygen leakage at high current density, using
the oxygen mass transfer coefficient measured when the current was 4.3 A/m2, where k = 2.2
×10–3 cm/s, as
COD equivalent = 3.34 (mg) / 14 (mL) = 238.8 mg/L
The fraction of COD is therefore 238.80 (mg/L) / 394.7 (mg/L) = 60.5%. This is higher than
the 41% calculated from the COD balance, indicating that aerobic heterotrophs could not oxidize
the same amount of substrate using oxygen when there was current generation. With current
generation, the faster reduction in the COD concentration reduced the rate of substrate consumed
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by aerobic heterotrophs as it was shown in open circuit tests that substrate removal was firstorder with respect to COD concentration. Thus, a decrease in COD with current generation
reduces the rate that substrate is lost to aerobic heterotrophs.
The reduction in COD lost to oxygen leakage at high current density compared to open
circuit reactors is
[271.4 (mg/L) – 238.8 (mg/L)] / 271.4 (mg/L) = 32.6 (mg/L) / 271.4 (mg/L) = 12 %
Based on these calculations, it is concluded that part of the reduction in substrate
consumption by aerobic heterotrophs (from 67% at open circuit to 35% at high current density)
could partly be due to less oxygen transfer (12%) into the reactor, but the main reason was the
faster COD depletion with current generation due to the increased consumption by
exoelectrogens.
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