Don't stew lotus root in iron pot. Nutrients of lotus root will be oxidized when heated.

Lotus root turns black after heating because it is rich in reducing substances such as vitamin C, which oxidizes with iron when heating, and the color becomes darker. It is best to use ceramic or stainless steel utensils, avoid using iron pots and aluminum pot, and try not to cut lotus roots with iron knives to reduce oxidation.

In other words, iron oxide is produced in the soup. No toxicity. It just affects the appearance and taste.

It's equivalent to eating some rust. . . . . . . . .