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 (200rpm). The experimental results were simulated by mathematical models to find best-fit values of volumetric mass transfer coefficientsk(L)afor H-2, which were 1.6/day for no mixing, 7/day for 100rpm, and 30/day for 200rpm. 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.