Iron polymers

Environmental Geochemistry of Ferric Polymers in Aqueous Solutions and Precipitates

Iron, one of the most abundant elements on the Earth’s surface, plays a critical role in chemical reactivity at the solid-water interface. This element is particularly interesting to geochemists because of its stability in multiple valence forms, its importance as an essential nutrient for organisms, and its formation of insoluble mineral phases [1]. Insoluble minerals exist as oxides, hydroxides and oxyhydroxides (referred to collectively as the "iron oxides") and are ubiquitous in many natural soil systems. They are known to mediate many environmental reactions, as suggested in the following examples: 1) they have the ability to preferentially scavenge heavy metals and other contaminants from water and entrain them in the soil column, 2) they can act as catalysts in interfacial reactions and 3) they can coat mineral surfaces, such as silica, thereby completely changing the reactivity of a particular system.

 Figure 1
Figure 1: Fate of free iron in the environment.

Surface weathering of rocks results in the release of free iron into the environment, which undergoes hydrolysis reactions in aqueous systems to form iron oxide species [2]. The formation of iron oxides occurs through an intermediate step in which there is the transient formation of complex ferric polymers [3]. Despite their relevance to the geochemical cycling of elements and nutrients in natural systems and their abundance in soils and sediments, amorphous iron oxides and ferric polymers are not well understood from a chemical standpoint.

 

Figure 2
Figure 2: Mechanism of polymer formation.

 

Metal polymerization is a common result of hydrolysis of such metals as aluminum, silicon, chromium, and iron. Previous studies of ferric polymers have suggested their typical size ranges (15-30 Å) and approximate structures, which depend on the hydrolysis precursors [4,5]. The formation of ferric polymers has been described by the following polymerization mechanism [3]: 1) formation of low molecular weight iron monomers and oligomers, 2) aggregation of smaller units to form polymers, and 3) polymer aging and structural reorganization as they assume a more crystalline structure. As a result of their inherent complexity and the inadequacy of previous analytical tools, there are still many unknowns regarding how ferric polymers behave in the environment. The key to their reactivity lies in the nature of bonding between iron centers and bridging oxygen or terminal hydroxyl moieties [2]. Recent advances in molecular techniques are extremely promising in their ability to define more accurately this bonding relationship [4], which will allow for a more thorough investigation of the iron chemistry in amorphous oxides and polymers.

Research Goals

This research will focus on clarifying the structure and reactivity of ferric polymers because of their potential implications on various environmental processes, including contaminant transport, catalysis of oxidation-reduction reactions in natural systems, and the formation of stable oxide minerals. In order to achieve this goal, various synthetic iron oxides and polymers will be characterized using spectroscopic techniques in order to answer the following questions:

  1. What is the chemical environment of Fe-OH bonds in crystalline Fe (III) (oxy)hydroxides and related amorphous polymers?
  2. What is the molecular mechanism of iron polymer formation in aqueous media via hydrolysis of Fe (III)?
  3. How does the presence of organic and inorganic ligands affect the formation and chemistry of ferric polymers?

 

Figure 3
Figure 3: Crystal structures of FeOOH Polymorphs: 1) Goethite a-FeOOH, 2) Akaganeite b-FeOOH•Cl, 3) Lepidocrocite g-FeOOH, and 4) Feroxyhite d '-FeOOH.

 

Because the crystalline structures and the nature of linkages between iron atoms and hydroxyl moieties are known for the iron oxides with some certainty, the first step in addressing the above questions is to establish a relationship between the mineral lattice structures and the electronic structure and local coordination environment of Fe-bound hydroxyls. A combination of analytical techniques involving infrared (IR) and X-ray absorption spectroscopy hopefully will contribute to this quantitative correlation. Once completed, these results can be applied to ferric polymers, explored with similar analytical techniques, to understand better the chemical bonding and long-range molecular structure within the polymers. Finally, a more complete description of polymer structure will lead potentially to a comprehensive understanding of the mechanisms of formation of ferric polymers, and how their chemistry and reactivity affects their environmental impact.

References

  1. Murad E., Fischer W.R. (1988) The Geochemical Cycle of Iron. In Iron in Soils and Clay Minerals (eds. J.W. Stuck, B.A. Goodman, and U. Schwertmann), NATO ASI series C 217, 1-18.
  2. Bigham J. M., Fitzpatrick R. W., Schulze D. G. (2002) Iron Oxides. In Soil Mineralogy with Environmental Applications (eds. J.B. Dixon and D.G. Schulze), SSSA Book Series 7, 323-366.
  3. Brinker C. J., Scherer G. W. (1990) Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing. Academic Press, New York.
  4. Bottero J. Y., Manceau A., Villieras F., Tchoubar D. (1994) Langmuir 10, 316-319.
  5. Combes J. M., Manceau A., Calas G., Bottero J. Y. (1989) Geochim. Cosmochim. Acta53, 583-594.