A cost-effective, temperature dependent, control of H2 production period by Escherichia coli and using waste culture to detoxify the carcinogenic Cr6+

Abstract

This study was conducted for investigating a cost-effective temperature dependent control of the fermentation period through controlling the H2 production rate and efficiency of the hydrogen over-producing Escherichia coli HD701 from acid hydrolyzed barley starch. A temperature range from 30 to 40 °C can be used for H2 production by E. coli and the most apparent optimum was 35 °C where rising the fermentation temperature up to 40 °C increased the rate of H2 production while decreasing the temperature down to 30 °C decreased the rate without significantly reducing the efficiency in both cases. Through such temperature dependency of the rate, the fermentation process can be controlled without affecting the hydrogen production efficiency. Controlling the rate of hydrogen production is often important for adjusting the gas concomitant collection and purification systems. As it was not affecting significantly the efficiency, the increase in the rate of E. coli H2 production by such slight increase in the fermentation temperature from 35 to 40 °C would be cost-effective as it shorten the total fermentation period and hence save the energy consumed in prolonged heating and operation of the fermenter. For further enhancement of the feasibility of bacterial hydrogen production, the second part of the study was devoted for inspecting the use of the waste bacterial culture after H2 production for bioremediation of the toxic chromium (VI) through its bioreduction to nontoxic Cr3+. After H2 production, the waste bacterial E. coli culture could efficiently detoxify concentrations of Cr6+ significantly higher than minimum toxic level determined using human cell lines. The addition of palladium nanoparticles (PdNPs) to the waste E. coli culture significantly increased the bioreduction rate of Cr6+. The mechanism might occur through catalyzing Cr6+ reduction by the formate in the waste fermentation culture accumulated at the late phase of fermentation.This is the first study investigating the control of the fermentation period without affecting the hydrogen production efficiency to decrease the cost, save time and hence increase the fermentation industrial rounds. Besides, the use of waste fermentation culture in Cr6+ bioremediation would increase the feasibility of both biotechnologies.