Below about pH 3.5, ferrous iron oxidizes in streams according to the overall reaction Fe²⁺+1/4O₂​+H₂​O→Fe³⁺+H₂O.
The rate law for the inorganic oxidation of ferrous iron under these conditions is given by:
d(Fe(II))/dt​=-k₊(Fe(II)) PO₂
at 20°C, where k₊10⁻³.²/bar day.
Nordstrom 1985) measured the oxidation rate of ferrous iron in an acid mine drainage stream in which the initial Fe²⁺ concentration was 300 mg/L. The stream had a practically constant pH of about 2.5. The fer rous iron concentration dropped to about 5 mg/L after the stream had flowed for about 24 hours at about 0.2 m/s. Nordstrom concluded that the oxidation process was independent of the ferrous iron concentration but was instead proportional to the concentration of the iron-oxidizing bacteria, T. ferrooxidans, in the stream
Calculate the reduction in ferrous iron concentration expected in the stream during this same 24-hour period due only to inorganic oxidation as expressed in the above rate law and compare it to the reduc- tion in Fe2- reported by Nordstrom.