Microbial Pb(II) precipitation : yeast extract autolyzed from Saccharomyces cerevisiae as a sustainable growth substrate

Abstract

This study aimed to quantify the effectiveness of yeast extract, autolyzed from commercial baker’s yeast (Saccharomyces cerevisiae), as a more economical growth medium for the Pb(II) removing consortium. Three parameters were tested, i.e. the amount Pb(II) removed, metabolic activity, and a measurement of precipitate formed (optical density). The experiments were conducted over a 48 h period, under anaerobic conditions, spiked with 80 ppm Pb(II) and with the produced yeast extract as sole carbon- and nitrogen source. Most of the Pb(II) was removed (50 %) within the first 7 minutes of experimentation indicating rapid Pb(II) removal. From there it was observed that 100% of the Pb(II) was removed from solution within 24 h. Metabolic activity initially increased slowly to 5.36 ± 0.0692 absorbance units from 0 h to 8 h, followed by a growth spurt from 8 h to 48 h with a measured metabolic activity of 18.9 ± 0.0604 absorbance units. The optical density exhibited a slow increase to 3.60 ± 0.0453 absorbance unit up to 16 h, with a dramatic increase to 6.37 ± 0.0782 absorbance units after 24 h. A dark precipitate was observed in all the batch reactors as seen in previous studies conducted by this team. The initial rapid drop of Pb(II) concentration within the first few minutes before sampling may indicate a biosorption mechanism due to the high amount of yeast biomass present. The remaining Pb(II) was subsequently removed during the period of slow metabolic activity and optical density increase, likely as a result of an initial detoxification mechanism (biosorption). The large metabolic activity and optical density increase appear to correspond to low Pb(II) concentrations, most probably due to a Pb(II) inhibitory effect or slow access to more complex components in the yeast hydrolysate. It can be concluded that yeast extract, autolyzed from commercial baker’s yeast can successfully be used as substrate in the bioremediation of Pb(II) from contaminated wastewater. However, more research is required to determine the inhibitory effect of Pb(II) and/or the effect of the complex substrate on the microbial consortium.