Evaluation of the influence of CO_2 on hydrogen production by Caldicellulosiruptor saccharolyticus

摘要

Stripping gas is generally used to improve hydrogen yields in fermentations. Since CO_2 is relatively easy to separate from hydrogen it could be an interesting stripping gas. However, a higher partial CO_2 pressure is accompanied with an increased CO_2 uptake in the liquid, where it hydrolyses and induces an increased requirement of NaOH to maintain the pH. This enhances the osmotic pressure in the culture by 30%, which inhibited the growth of Caldicellulosiruptor saccharolyticus. Indications for this conclusion are: ⅰ) Inhibition could almost completely be circumvented by reducing the bicarbonate through decreasing the pH (from 6.5 to 5.5), ⅱ) Growth rates were reduced by 60 ± 10% at an osmotic pressure of 0.218 ± 0.005 osm/kg H_2O independently of the stripping gas, ⅲ) Increased extracellular DNA and protein concentrations were observed as a function of the osmotic pressure. In addition to growth inhibition, the increased sodium bicarbonate in the effluent will contribute to a negative environmental impact when applied at industrial scale.