Hydrogen production by Escherichia coli without nitrogen sparging and subsequent use of the waste culture for fast mass scale one-pot green synthesis of silver nanoparticles

Abstract

Abstract In view of the transition to hydrogen as a major energy carrier in the future, new routes for bringing down the cost of biological hydrogen production need to be explored. The current study was devoted to optimizing the dark fermentation by Escherichia coli HD701 for hydrogen production from an acid-hydrolyzed potato starch residue stream without nitrogen sparging to reduce the cost. To further increase the economic feasibility of hydrogen production by E. coli, this study explores the use of the waste culture after hydrogen production in mass scale one-pot green synthesis of silver nanoparticles. Hydrogen gas production by E. coli was conducted from an optimum 10 g/L reducing sugars of acid-hydrolyzed potato starch residue stream. At low initial bacterial optical density OD600 of 0.02, the optimum air space was 3% of the fermentor for hydrogen production without nitrogen sparging. However, the cumulative hydrogen gas produced was 17% lower than that of the hydrogen produced after nitrogen sparging. Increasing the initial bacterial optical density OD600 increased the cumulative hydrogen produced up to that of the nitrogen sparged control. The initial bacterial optical density of 0.04 was optimum for hydrogen production without nitrogen sparging producing 0.444 mol/mole glucose compared to 0.448 mol/mole glucose obtained after nitrogen sparging. Eliminating the nitrogen sparging step extended the lag phase of the hydrogen production profile, but the slight increase in the initial bacterial optical density enhanced the rate of hydrogen formation at the log phase. The waste culture after hydrogen production could be efficiently used in one-pot mass scale green synthesis of silver nanoparticles. The waste culture after hydrogen production was autoclaved and centrifuged. The supernatant containing the bacterial cell extracts after autoclaving and the fermentation residual reducing sugars was used for bioreduction of silver ions into nanoparticles at 15 psi and 121 °C for only 5 min. Silver nanoparticles with a size range of 5–25 nm and an average nanoparticles size of 15.6 ± 2.46 nm, as analyzed using transmission electron microscopy, were obtained at an optimum of pH 7 and 10 mM silver nitrate. The synthesized nanoparticles had a reddish-brown color in aqueous solution and showed the typical wavelength spectrum of silver nanoparticles with a maximum wavelength of 430 nm. The use of the waste culture after hydrogen production for fast one-pot mass scale green synthesis of silver nanoparticles suggests interlinking the two biotechnologies in future applications to increase the economic feasibility of both biotechnologies.