Iron (Fe) and Manganese (Mn) are responsible for a number of problems with water supplies. Above 0.3 mg/L iron and 0.05 mg/L manganese, these contaminants cause aesthetic problems such as discolouration of water, turbidity, staining and unpleasant taste.
The presence of iron and manganese can also accelerate biological activity further exacerbating taste, odour and colour problems. Chlorine dioxide selectively oxidises the relatively soluble Fe (II) and Mn (II) to insoluble Fe (III) and Mn (III or IV) species.
The resulting precipitate of insoluble iron and manganese compounds are removed by ﬁltration or sedimentation followed by ﬁltration. Any remaining Fe (II) and Mn (II) is removed by hydrated iron and manganese species coated on the ﬁlter media where the trapped ions are oxidised to the insoluble form.
In addition, chlorine dioxide can oxidise organic complexing agents which assist in keeping iron and manganese in solution. In some cases, the chlorine dioxide works by destroying bioﬁ lms that encapsulate organically bound metal ions preventing them from being oxidised by conventional oxidising biocides.spread of disease-causing bacteria, fungi and viruses.
Many industrial processes cannot tolerate the presence of iron in the feed water. The problem may be unacceptable colour or taste (in food related industries) or it may be the presence of iron promotes unacceptable growth of bacteria.
Chlorine dioxide rapidly oxidises Fe(II) to Fe(III) which then precipitates as iron hydroxide:
ClO2 + 5Fe(HCO3)2 + 3H2O → 5Fe(OH)3 + 10CO2 + H+ + Cl
Optimum reaction conditions are neutral to slightly alkaline. Above pH 5, approximately 1.2mg/L chlorine dioxide is required to remove 1.0mg/L iron.
The resulting precipitate is reported to be 99% removed by a 45 micron sieve above pH 5.
Above a concentration of 0.05mg/L, manganese causes “black” water, discolouration of laundry, encrustation of water mains, deposits on taps and industrial machinery as well as adversely affecting the taste of drinking water and beverages.
Chlorine has been used to control these problems but the reaction rate is very slow and manganese can still be present at unacceptable levels even after 24 hours contact with chlorine.
Chlorine dioxide rapidly oxidises manganese to manganese dioxide which can then b e removed by filtration at the treatment plant. Reaction is complete in less than 5 minutes under favourable conditions.
Optimum reaction conditions are neutral to slightly alkaline:
Above pH 7: Mn2+ + 2СЮ2 + 40H- → Mn02 + 2СЮ2- + 2H2O
Below pH7: 5Mn2+ + 2СЮ2 + 6H20 → 5Mn02 + 12H+ + 2CI-
Above pH 7, it requires approximately 2.45mg/L chlorine dioxide to remove 1.0mg/L manganese.