"What he found was that banana peels contain nitrogen, sulfur and organic compounds such as carboxylic acids. Castro was especially pleased to find these acids because they can bind with positively charged metals that leach into rivers from industrial operations.
To test the peels, Castro and his colleagues chopped up and dried a bunch of them and mixed them with water from Brazil's Paraná River, which was polluted with copper and lead. The peels performed as well or better than many typical filtering materials, like silica or carbon. What's more, the peels are a cheap waste material. Castro found they could be used up to 11 times before they stopped working as purifiers."
But what do you do with them when they are used up? No good for compost anymore.