Supplementary information for the manuscirpt
Application of Gas Diffusion Biocathode in Microbial Electrosynthesis from Carbon dioxide
Suman Bajracharya1,2, Karolien Vanbroekhoven1, Cees J.N. Buisman2, , Deepak Pant1*, David P. B. T. B. Strik2#
1Separation & Conversion Technologies, Flemish Institute for Technological Research (VITO), Mol, Belgium
2Sub-department of Environmental Technology, Wageningen University, Wageningen, The Netherlands
*# Corresponding authors.
Tel.: +3214336969; fax: +3214335599,
E-mail address: *, (D. Pant); #
Summary
· Figure SI1 Biocatalyst development and enrichment steps followed in inoculum preparation for CO2 reducing MES
· Figure SI2 scheme for the start-up and operation of MES for CO2 reduction with GDE
· Table SI3 Estimated mass balance of CO2 in MES system based on the production rates observed in this work with GDE at various composition of N2:CO2 gas mixture feed
· SI4 CO2 flushing in aqueous solution maintained acidic pH and CO2 bioavailability
Fig SI4 pH change observed during CO2 flushing in buffer solution and demineralized water
Figure SI1 Biocatalyst development and enrichment steps followed in inoculum preparation for CO2 reducing MES
Figure SI2 scheme for the start-up and operation of MES for CO2 reduction with GDE
Table SI3 Estimated mass balance of CO2 in MES system based on the production rates observed in this study with GDE at various composition of N2:CO2 gas mixture feed
Feed gas composition (N2:CO2) / CO2 saturation concentration C*(g CO2 /L) / Highest mass transfer rate with GDE
(g CO2 /L/d)$ / Highest CO2 assimilation rate measured with GDE
(g CO2 /L/d) § / Remark / Maximum acetate production rate supported at CO2 limitingcondition
(g acetate/L/d)
90:10 / 0.14 / 13.19 / 0.088£ / CO2 is not depleted / 8.98
80:20 / 0.28 / 26.36 / 0.36 / CO2 is not depleted / 17.97
20:80 / 1.12 / 105.43 / 1.001 / CO2 is not depleted / 71.88
$calculated as kLa x C* and kLa =3.9 /h
§calculated from highest production rate + 5% to biomass
£production rate from (Patil et al., 2015a)
SI4: CO2 flushing in aqueous solution maintained acidic pH and CO2 bioavailability
Dissolution of gases in aqueous solution is governed by Henry’s law which states that the saturation concentration of a gas in aqueous solution at equilibrium only depends on the partial pressures when other gas/liquid properties and temperature are constant. The saturation concentration of CO2 at 25 °C with 1 atm partial pressure is 1.4gCO2/L. When CO2 dissolves in the aqueous solution, the pH decreases as it forms carbonic acid with water which will be neutralized if OH- is available otherwise it will increase the acidity of the solution. The dissolved CO2 is converted to HCO3-/CO32- according to the equilibrium condition
CO2 + H2O <--> H2CO3 <--> HCO3- + H+ [pKa1 = 6.35 (25 °C)]
HCO3- <--> CO32- + H+ [pKa2 = 10.32 (25 °C)]
At pH > 10, CO2 + OH- <--> HCO3- and HCO3- + OH- <--> CO32- + H2O
For the biological assimilation of CO2, carbonic acid and bicarbonate forms are necessary since carbonic acid can readily diffuse across cell-membranes whereas the bicarbonate ions cross the membranes via the magnesium/calcium trans-membrane channels (Gutknecht et al., 1977; Missner et al., 2008). Buffer solutions control the final equilibrium pH of the medium on CO2 sparging but the amount of dissolved CO2 (mainly H2CO3) at saturation remains independent of the pH. When CO2 was supplied to the aqueous solution at pH > 10, CO2 reacts with OH- to produce CO32- and thus, the pH decreases. Neutralization and dissolution of CO2 in alkaline aqueous solution captures CO2 in the form of carbonate ions rather than dissolved form. At acidic pH, the carbonate/bicarbonate species shifted to readily bioavailable carbonic acid form.
In the CO2 dissolving tests, when there was no buffering capacity in the aqueous solution (demineralized water) at pH 10, a rapid fall of pH was observed as shown in Fig SI4. and after CO2 saturation, the solution maintained buffering at pH 4.5-5 and the dissolved CO2 measured by titration with 1 M NaOH after 1 h test was 1.17 g/L. In case of CO2 dissolution in phosphate buffer of pH 7, the decrease in pH was not as drastic as in demineralized water. The final pH remained stabilized at 6 while CO2 sparging in phosphate buffer solution of pH 7. The dissolved CO2 measured after 1 h sparging was 1.24 g/L which was fairly close to the measured dissolve CO2 in demineralized water. Buffer solutions control the equilibrium pH but the concentration of dissolved CO2 attained under CO2 flushing was independent of the equilibrium pH. At aAcidic pH, attained during CO2 flushing, shifted the carbonate/bicarbonate species shifted to readily bioavailable carbonic acid form.
Fig SI4 pH change observed during CO2 flushing in buffer solution and demineralized water
References
Gutknecht, J., Bisson, M.A., Tosteson, F.C., 1977. Diffusion of carbon dioxide through lipid bilayer membranes. Effects of carbonic anhydrase, bicarbonate, and unstirred layers. J. Gen. Physiol. 69, 779–794.
Missner, A., Kugler, P., Saparov, S.M., Sommer, K., Mathai, J.C., Zeidel, M.L., Pohl, P., 2008. Carbon dioxide transport through membranes. J. Biol. Chem. 283, 25340–25347.
Patil, S.A., Arends, J.B.A., Vanwonterghem, I., van Meerbergen, J., Guo, K., Tyson, G.W., Rabaey, K., 2015a. Selective Enrichment Establishes a Stable Performing Community for Microbial Electrosynthesis of Acetate from CO2. Environ. Sci. Technol. 150701101446004.