The oxidation of manganese in surface water is not effective until the pH goes to or surpasses which value?

• A. 9
• B. 9.2
• C. 9.5
• D. 9.8

Answer: (C)

Manganese removal by oxidation hinges on pH. As pH rises, Mn(II) becomes easier to convert to MnO2 and form solid particles that can be filtered out. In surface water, this oxidation and subsequent precipitation happen efficiently only when the water is quite alkaline, around 9.5 or higher. Below that, Mn2+ tends to remain dissolved and isn’t removed effectively by filtration.

So, the value around 9.5 is the practical threshold at which oxidation becomes reliably effective for manganese removal, which is why that option is best. Values lower than that may not achieve good removal, while a higher figure like 9.8 is not typically necessary.