In anoxic sediments uncontaminated by heavy metals, the associated metal (M in equation 2) is usually iron (Fe) or manganese (Mn).[6] 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.