Iron (specifically iron oxide, or rust) is a great thing to add to biochar. If you get rusty water to infuse your feedstock, then the biochar will have tiny crystals of iron oxide scattered all over its surface area and this helps with many of the attributes that we want in our soil in particular: cation exchange, adsorption, oxidation and even catalytic properties. Iron-enhanced biochar even
destroys "forever" chemicals and can be reused without wearing out.