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Contact time for iron oxidation by oxygen and manganese by permanganate

Iron oxidation by air is a hundred times faster per pH unit. Manganese needs a stronger oxidant or a very high pH. Both rates turn into reactor volumes directly.

Flowm³/d

Iron inmg/L

Iron targetmg/L

Temperature°C

Water ionisation constantThe rate depends on hydroxide squared, so this choice matters an order of magnitude in cold water.

Oxygen partial pressureatm

Manganese inmg/L

Manganese targetmg/L

Permanganate dosemg/L

Manganese dioxide presentmg/LSeeds the autocatalytic path.

Iron by oxygen

Iron half-life

47.4min

Plug-flow contact time

192.3min

Completely mixed contact time

1,070.7min

Plug-flow volume

1,335m³

Completely mixed volume

7,435m³

pKw used

14.53

Manganese by permanganate

Manganese plug-flow time

1.7s

Manganese completely mixed time

10.8s

How it works

Ferrous iron oxidation by oxygen is first order in iron and second order in hydroxide:

- \frac{d [{Fe}^{2 +}]}{d t} = k [{Fe}^{2 +}] [{OH}^{-}]^{2} P_{O_{2}} k_{r} = k [{OH}^{-}]^{2} P_{O_{2}} k = 8.0 \times 10^{13} \frac{L^{2}}{{mol}^{2} min atm}

(1)

τ_{P F R} = \frac{1}{k_{r}} \ln \frac{C_{0}}{C} τ_{C M F R} = \frac{C_{0} - C}{k_{r} C}

(2)

Manganese by permanganate has an oxidative path and an autocatalytic path on existing dioxide:

k' = 9.55 \times 10^{12} [{KMnO}_{4}] [{OH}^{-}]^{1.1} + 8.7 \times 10^{3} [{MnO}_{2}]

(3)

where

$[{Fe}^{2 +}]$: ferrous iron concentration
$k$: rate constant, 8.0 × 10¹³ L²/mol²·min·atm at 20 °C
$[{OH}^{-}]$: hydroxide concentration, mol/L
$P_{O_{2}}$: oxygen partial pressure, atm
$k_{r}$: pseudo first-order rate constant, 1/min
$τ_{P F R}, τ_{C M F R}$: contact time in plug flow and in a completely mixed reactor, min
$C_{0}, C$: iron in and target, mg/L
$k'$: pseudo first-order constant for manganese, 1/s
$[{KMnO}_{4}]$: permanganate concentration, mol/L
$[{MnO}_{2}]$: manganese dioxide present, mol/L

Valid above pH 5.5 without organic complexation; humic iron oxidises far slower. Ionic strength cuts the rate several-fold in brackish water. Plug flow gives the minimum time; baffled basins of 15 to 30 minutes after the aerator are usual for iron, and pilot tests are needed because colloidal iron does not follow the kinetics. Manganese by oxygen is impractical below pH 9.

Contact time for iron oxidation by oxygen and manganese by permanganate

How it works

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