The removal of Iron and ManganeseBack
Iron and manganese, that are present in reduced form or in complexes with organic substances, humic and fulvic acids in particular, are oxidized into hydroxides which precipitate since they have little solubility.
Chlorine dioxide reduces to chlorite ion in this reaction (Knocke et al., 1993). About 1.2 mg/L of chlorine dioxide is required to remove 1.0 mg/L of iron, and 2.5 mg/L of chlorine dioxide are required to remove 1.0 mg/L of manganese. For high concentrations of iron and manganese, the use of chlorine dioxide is limited to the 1.0 mg/L chlorite/chlorate ion byproduct. Ferrous iron may be added prior to conventional coagulation to chemically reduce chlorite ion (to chloride ion) and improve the overall flocculation process.
Chlorine dioxide is more effective than chlorine, above all, in manganese removal because the reaction rate of chlorine dioxide is faster at pH > 7. Furthermore, in contrast with chlorine, the oxidation reaction does not involve significant reduction in alkalinity and the consequent alteration of the calcium-carbonate balance in the treated water.
The chemical reactions of chlorine dioxide with iron and manganese have been described in the chapter “Reactivity with Inorganics” of ISIA Chlorine dioxide Handbook .