 Iron (II) is abundant in anoxic sediment, and the Ksp for iron (II) sulfide is 10−22.39 (with a comparable value for manganese (II) sulfide), so effectively all the sulfide in an uncontaminated sediment will be bound to Fe or Mn.Several toxic heavy metals, however, have Ksp values substantially lower than those of the sulfides of iron and manganese - for example, cadmium (II) sulfide (CdS) has a Ksp equal to 10−32.85.This means cadmium binds sulfide with a much higher affinity than does iron.