Iron is Earth’s fourth most widespread element (6.2% in mass), behind oxygen, silicon, and aluminum. It exists mostly as ferric oxide and oxyhydroxide (Fig. 7.1a) and to a lesser extent as sulfide (pyrite), carbonate (siderite), and silicate (fayalite). Iron oxides are largely used in technological areas such as metallurgy, colored pigments, magnetic materials, and catalysts. They also play an important role in the environment because the dissolution of ferric oxides in natural waters, promoted by acid–base, redox, photochemical phenomena, and also microbial mediation, allows iron to be involved in many biogeochemical processes. Iron is present in many living organisms such as plants, bacteria, mollusks, animals, and humans in various forms: . . . Porphyrinic complexes of iron, which are active centers of hemoglobin and several ferredoxins involved in biological functions, especially respiration mechanism and photosynthesis. Nanoparticles of amorphous ferric oxyhydroxides in animal and human organisms as ferritin, which allows regulation and storage of iron and in various nanophases present in plants as phytoferritin. Crystalline iron oxy(hydroxi)des produced by biomineralization processes. Goethite, lepidocrocite, and magnetite are the main constituents of radulas and the teeth of mollusks (limpets, chitons). Magnetite nanoparticles produced by magnetotactic bacteria (Fig. 7.1b), as well as by bees and pigeons, are used for purposes of orientation and guiding along the lines of force of the Earth’s magnetic field. Green rusts are also ferric- ferrous compounds belonging to the biogeochemical cycle of iron. . . . The crystal chemistry of iron oxy(hydroxi)des is very rich. The ferric, ferrous, and mixed ferric- ferrous oxygenated compounds correspond to around a dozen crystal structural types (Fig. 7.2). Most of these crystal phases can be synthesized from solutions in the laboratory, giving rise to a most diversified chemistry. They are also formed in nature because of the large variability of physicochemical conditions: an acidity range from around pH 0 to 13; redox conditions from oxic to totally anoxic media; bacterial activity that can be extremely intense; salinity largely varying from almost pure waters to real brines; presence of many organic and inorganic ligands; and various photochemical processes.
Nanosized Iron Oxide Colloids Strongly Enhance Microbial Iron Reduction
