The theoretical maximum absorption of CO2 by the MDEA amine was based on the base-catalyzed hydration reaction of CO2 (reaction 1), which allows the binding of 1 mole of CO2 per mole of MDEA and a minor fraction of CO2 dissolved in the PBS buffer. The concentration of available CO2, which was consumed to produce CH4, was quantified in batch reactors with excessive amounts of H2 (Figure ). Not all CO2 is converted to CH4 as a small fraction of the CO2 would also be used for anabolic processes of cell growth, which would underestimate the concentration of bioavailable CO2. However, Martin et al. estimated that only 1.4% of the CO2 would be diverged towards biomass growth in a CSTR for biomethanation , and would therefore be very minor compared to the catabolic methanogenesis reaction. which was bioavailable for the methanogenic culture, began to decrease until 200 mM, where only 0.09 mole of CO2 per mole of MDEA could be achieved. This tendency correlated with the reduced CH4 productivity rates at MDEA concentrations above 130 mM (figure ). The inhibition of the methanogenic activity is partly thought to originate directly from the presence of the amine and indirectly from the gradual increase in pH when the CO2 is consumed. However, since the amine solution without captured CO2 has a pH of >10, the issue of pH increase can be solved operationally by only converting a part of the captured CO2 by the aqueous amine.