Iron and manganese sometimes occur as dissolved pollutants in water. Which of the following treatment methods alone cannot effectively remove these dissolved iron and manganese ions from water?

Introduction / Context:
Water treatment processes are designed to remove different types of impurities, including dissolved metals such as iron and manganese. These metals can cause staining, bad taste, and operational problems in distribution systems. This question asks which water treatment method, when used alone, is not effective for removing dissolved iron and manganese ions. Understanding this helps students differentiate between processes used for disinfection and those used for metal removal.

Given Data / Assumptions:

• The pollutants of concern are dissolved iron and manganese ions in water.
• Several treatment methods are listed, including physical, chemical, and ion exchange processes.
• The question asks which method alone cannot remove these metals effectively.
• We assume typical concentration ranges seen in groundwater or raw water sources.

Concept / Approach:
Dissolved iron and manganese in water are usually present as ferrous (Fe2+) and manganous (Mn2+) ions. To remove them, they are often oxidised to ferric (Fe3+) and higher oxidation states, which form insoluble hydroxides or oxides that can be removed by settling and filtration. Processes such as aeration, chemical oxidation, lime soda treatment, manganese zeolite filtration, and ion exchange can all be used for this purpose. Chlorination, while excellent for disinfection and oxidation of some substances, is not designed on its own to precipitate and remove iron and manganese unless combined with filtration processes. As a simple stand alone disinfection step, chlorination does not remove dissolved metal ions from the water.

Step-by-Step Solution:

Verification / Alternative check:
Standard water treatment schemes often show a sequence of processes: aeration or chemical oxidation, sedimentation, filtration, and then disinfection by chlorination. Iron and manganese removal steps are placed before chlorination. Technical references emphasise that chlorination is primarily for disinfection and, although chlorine is an oxidant, its main use is not to precipitate metals unless specifically engineered with other steps. Moreover, many iron and manganese removal units operate without chlorine, using aeration and filtration instead. This confirms that chlorination alone is not considered an iron and manganese removal method.

Why Other Options Are Wrong:

• Oxidation followed by settling and filtration: This is a standard approach for iron and manganese removal, as it converts dissolved ions into insoluble forms that can be removed.
• Ion exchange process: Can selectively remove dissolved metal ions by exchanging them with other ions on a resin.
• Lime soda process or manganese zeolite process: Both are known for treating hard water and for helping remove iron and manganese by precipitation and filtration.
• Aeration followed by filtration: Aeration can oxidise iron and manganese, and filtration then removes the precipitated solids.

Common Pitfalls:
Students sometimes think of chlorination as a universal solution for all water quality problems because it is widely used in treatment plants. This leads to the mistaken belief that chlorination removes any pollutant, including metals. It is important to remember that chlorination is primarily for disinfection and sometimes taste and odour control. Physical removal of metals usually requires oxidation combined with settling, filtration, or ion exchange. Keeping this distinction clear helps to choose the correct answer.

Final Answer:
Dissolved iron and manganese present as pollutants in water cannot be effectively removed by Chlorination alone.