Role of hydrogen (H₂) mass transfer in microbiological H₂-threshold studies

Gas-to-liquid mass transfer of hydrogen (H₂) 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 H₂ consumption. H₂ transfer was quantified in real-time with a H₂ 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_La-for H₂, which were 1.6/day for no mixing, 7/day for 100 rpm, and 30/day for 200 rpm. Microbiological H₂-consumption experiments were conducted with Methanobacterium bryantii M.o.H. to assess effects of H₂ mass transfer on microbiological H₂-threshold studies. The results illustrate that slow mixing reduced the gas-to-liquid H₂ transfer rate, which fell behind the rate of microbiological H₂ consumption in the liquid phase. As a result, the liquid-phase H₂ concentration remained much lower than the liquid-phase H₂ concentration that would be in equilibrium with the gas-phase H₂ concentration. Direct measurements of the liquid-phase H₂ concentration by an in situ probe demonstrated the problems associated with slow H₂ transfer in past H₂ threshold studies. The findings indicate that some of the previously reported H₂-thresholds most likely were over-estimates due to slow gas-to-liquid H₂ transfer. Essential requirements to conduct microbiological H₂ threshold experiments are to have vigorous mixing, large gas-to-liquid volume, large interfacial area, and low initial biomass concentration.