I assume the bentonite is part of the substrate? It is a clay, so it has a high CEC. CEC means the particles can adsorb lots of positive ions (cations), like Fe+++. That could remove some of the iron from the water, but it would then be even more available to the plants through their roots. That is the reason why high CEC is a desirable attribute of substrates.
But, from what I have been reading for a few years, when you dose iron the level in the water doesn't stay there very long, because the plants quickly take it in, apparently storing it for future use.