White button mushrooms (Agaricus bisporus) and their spent compost can remove toxic metals like cobalt and manganese from water. In lab tests, both the fresh mushrooms and the leftover compost bound and trapped the metals through natural chemical groups like amines and carboxyls. The compost worked even better than the mushrooms themselves, and both materials could be reused for up to seven cycles, showing real promise for low-cost, sustainable water treatment.