Enhancement of Escherichia coli bacterial biomass and hydrogen production by some heavy metal ions and their mixtures during glycerol vs glucose fermentation at a relatively wide range of pH

Escherichia coli growth and H-2 production were followed in the presence of heavy metal ions and their mixtures during glycerol or glucose fermentation at pH 5.5-7.5. Ni2+ (50 mu M) with Fe2+ (50 mu M) but not sole metals stimulated bacterial biomass during glycerol fermentation at pH 6.5. Ni2++Fe3+ (50 mu M), Ni-2 +Fe3++Mo6+ (20 mu M) and Fe3++Mo6+ (20 mu M) but not sole metals enhanced up to 3-fold H-2 yield but Cu+ or Cu2+ (100 mu M) inhibited it. At pH 7.5 stimulating effect on biomass was observed by Ni2++Fe2++Mo6+. H-2 production was enhanced 2.7 fold particularly by Ni2++Fe3++Mo6+ at the late stationary growth phase. Whereas at pH 5.5 increased biomass was when Fe2++Mo6+ or Mo6+ were added. H-2 yield was decreased compared with that at pH 6.5, but metal ions again enhanced it. During glucose fermentation at pH 6.5 biomass was increased by the mixtures of metal ions, and 1.2 fold increased H-2 yield was observed. At pH 7.5 Ni2++Fe2+ increased biomass but Cu+ or Cu2+ had suppressing effect; Fe3++Mo6+ stimulated H-2 production. At pH 5.5 biomass also was raised by Ni2++Fe2++Mo6+; H-2 yield was increased upon Mo6+ and Mo6++Fe2+ or Mo6++Fe3+ additions. The results point out the importance of Ni2+, Fe2+, Fe3+ and Mo6+ and some of their combinations for E. coli bacterial growth and H-2 production mostly during glycerol but not glucose fermentation and at acidic conditions (pH 5.5 and 6.5). They can be used for optimizing fermentation processes on glycerol, controlling bacterial biomass and developing H-2 production biotechnology. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.