Seasonal iron cycling in the salt-marsh sedimentary environment: the importance of ligand complexes with Fe(II) and Fe(III) in the dissolution of Fe(III) minerals and pyrite, respectively

A biogeochemical cycle is proposed for the reactivity of iron in salt-marsh sediments. The main reactions of the iron cycle are: (1) solubilization of Fe(III) by organic ligands; (2) reduction of soluble Fe(III) to Fe(II) by these ligands, soluble reduced sulfur or solid phase reduced sulfur; (3) the oxidation of the resulting Fe(II) (complexed to organic chelates) by Fe(III) minerals; (4) the formation of iron sulfide minerals when dissolved sulfide is in excess. The cycle of iron solubilization will continue as long as bacteria and/or plants produce organic ligands. The cycle will stop when sulfate reduction rates are high and organic ligand production is low. At this point soluble hydrogen sulfide reacts with Fe(II) and Fe(III) to form sulfide minerals. Penetration of O2 into the surface sediments will also oxidize Fe(II) to Fe(III) with subsequent formation of Fe(III) (oxy)hydroxide minerals. The reactions which represent the iron cycle indicate that the iron mineral system has substantial acid/base buffering capacity. The ligands responsible for the cycling of iron are weak field anionic ligands containing oxygen as the ligating atom. The electron transfer from Fe(II) complexes to Fe(III) minerals is discussed using the molecular orbital approach. An outer sphere electron transfer is possible. Laboratory evidence is presented for the reaction of Fe(III) complexes with pyrite over the pH range of 4–6.5. The Fe(II) production rate and pH decrease are consistent with field data from Great Marsh, Delaware. The direct oxidant for the oxidation of pyrite and other reduced sulfur compounds in salt-marsh sediments is Fe(III) rather than oxygen based on the cycle and data presented. Oxygen is not present in any pore waters sampled in this work. This is consistent with the microelectrode work of other researchers. Manganese oxides are not likely oxidants in salt-marsh sediments in Great Marsh, Delaware because they are not as abundant as Fe(III) minerals. The iron cycle presented may occur in other marine and freshwater sedimentary systems and in aquatic systems with oxic/anoxic interfaces.