The ferrous-iron oxidation kinetics of a bacterial culture consisting predominantly of Leptospirillum ferrooxidans were studied in continuous-flow bioreac- tors. The bacterial culture was fed with a salts solution containing 12 g/L ferrous-iron, at dilution rates ranging from 0.01 to 0.06 l/h, and temperatures ranging from 30 to 40°C, at a pH of 1.75. The growth rate, and the oxygen and ferrous-iron utilization rates of the bacteria, were monitored by means of off-gas analysis and redoxpotential measurement. The degree-of-reduction balance was used to compare the theoretical and experimental values of -r CO 2 , -r O 2 and -r Fe +2, and the correlation found to be good.
The maximum bacterial yield on ferrous-iron and the maintenance coefficient on ferrous-iron, were determined using the Pirt equation. An increase in the temperature from 30 to 40°C did not appear to have an effect on either the maximum yield or maintenance coefficient on ferrous-iron. The average maximum bacterial yield and maintenance coefficient on ferrous-iron were found to be 0.0059 mmol C/mmol Fe 2+ and 0.7970 mmol Fe 2+ / mmol C)/h, respectively. The maximum specific growth rate was found to be 0.077 l/h.
The maximum specific ferrous-iron utilization rate increased from 8.65 to 13.58 mmol Fe 2+ /mmol C/h across the range from 30 to 40°C, and could be described using the Arrhenius equation. The kinetic constant in bacterial ferrous-iron oxidation increased linearly with increasing temperature. The ferrous-iron kinetics could be accurately described in terms of the ferric/ferrous-iron ratio by means of a Michaelis-Menten-based model modified to account for the effect of temperature.
A threshold ferrous-iron level, below which no further ferrous-iron utilization occurred, was found at a ferric/ ferrous-iron ratio of about 2500. At an overall iron concentration of 12 g/L, this value corresponds to a threshold ferrous-iron concentration of 78.5 ×10 -3 mM.