What is the purpose of greensand treated with potassium permanganate?

The purpose of greensand treated with potassium permanganate is primarily to oxidize both iron and manganese to their insoluble oxides. Greensand itself is a type of sand that is coated with a green mineral called glauconite, and it is particularly effective in removing iron and manganese from water.

When potassium permanganate is applied to greensand, it serves as a strong oxidizing agent that converts dissolved forms of iron and manganese into their respective solid oxides. This conversion is crucial because these oxides can then be easily filtered out of the water. The treatment enhances the greensand's capability to effectively clarify water by removing these unwanted minerals, which can cause staining and other aesthetic issues.

The other options do not reflect the primary function of greensand treated with potassium permanganate. While lighter tints in water may occur due to various contaminants, this is not the intended purpose. Similarly, while increased filter run times may be a potential benefit of effective filtration, the main goal focuses on oxidation of specific contaminants rather than simply enhancing filter performance or duration.