The kinetics of ferrous ion oxidation by Leptospirillum ferriphilum in continuous culture: The effect of pH

The kinetics of ferrous ion oxidation by Leptospirillum ferriphilum were studied in continuous culture with a focus on the effect of solution pH (pH 0.8–2.0), assuming that the effect of pH on cell metabolism can be independently studied of reactor context and other reactions common in bioleach heaps. A simplified competitive ferric ion inhibition model and the Pirt Equation were used to analyze the experimental data. The results showed that the maximum specific activity of L. ferriphilum has a symmetrical bell-shaped curve relationship with pH. The maximum specific ferrous-iron oxidation rate, qFe2 +maxgave a highest value of 14.54 mmol Fe2+(mmol C h)− 1 at pH 1.3, and was described by a quadratic function. The steady state carbon biomass in the reactor and the apparent affinity constant, K′Fe2 +, also increased with increase in pH; however, a slight increase in the carbon biomass was observed beyond pH 1.6.The results also showed that ferric ion precipitation is significant beyond pH 1.3 and about 13% total iron from the feed was lost at pH 2.0. The maximum biomass yield increased linearly with pH, while the culture maintenance coefficient was significantly small in all experiments and was minimum at pH 1.3. The values are indicative of actively growing chemostat cultures.This study shows that microbial ferrous ion oxidation by L. ferriphilum may be sustained at pH lower than pH 0.8 as the microbial activity is much higher than reported values for common mesophilic acidophiles. This may have implications on how bioleach heap operations can be started-up to improve metal recovery.

Research Highlights
► L. ferriphilum activity, qFe2 +max, has a symemetrical bell-shaped curves relationship with pH.
► qFe2 +max,versus pH gave a quadratic equation and it is 14.54 molC(molFe2+h)-1 at pH 1.3
► Carbon biomass and apparent affinity constant, K′Fe2 +, increased with increase in pH.
► 13% total iron from the feed was lost at pH 2.0
► Biomass yield increases linearly with pH, while a negligible maintenance was obtained.