Role of hydrogen (H ₂ ) mass transfer in microbiological H ₂ -threshold studies

Gas-to-liquid mass transfer of hydrogen (H ₂ ) was investigated in a gas–liquid reactor with a continuous gas phase, a batch liquid phase, and liquid mixing regimes relevant to assessing kinetics of microbial H ₂ consumption. H ₂ transfer was quantified in real-time with a H ₂ microsensor for no mixing, moderate mixing [100 rotations per minute (rpm)], and rapid mixing (200 rpm). The experimental results were simulated by mathematical models to find best-fit values of volumetric mass transfer coefficients—k _L a—for H ₂ , which were 1.6/day for no mixing, 7/day for 100 rpm, and 30/day for 200 rpm. Microbiological H ₂ -consumption experiments were conducted with Methanobacterium bryantii M.o.H. to assess effects of H ₂ mass transfer on microbiological H ₂ -threshold studies. The results illustrate that slow mixing reduced the gas-to-liquid H ₂ transfer rate, which fell behind the rate of microbiological H ₂ consumption in the liquid phase. As a result, the liquid-phase H ₂ concentration remained much lower than the liquid-phase H ₂ concentration that would be in equilibrium with the gas-phase H ₂ concentration. Direct measurements of the liquid-phase H ₂ concentration by an in situ probe demonstrated the problems associated with slow H ₂ transfer in past H ₂ threshold studies. The findings indicate that some of the previously reported H ₂ -thresholds most likely were over-estimates due to slow gas-to-liquid H ₂ transfer. Essential requirements to conduct microbiological H ₂ threshold experiments are to have vigorous mixing, large gas-to-liquid volume, large interfacial area, and low initial biomass concentration.