A Study of the Initially-Formed Hydrolysis Species and Intermediate Polymers and their Role in Determining the Product Iron Oxides Formed in the Weathering of Iron

Iron oxides and hydrous oxides (oxyhydroxides and hydroxides) are ubiquitous in soils and weathered iron-containing material. In the weathering process iron is initially released to form aquated iron(II) and/or iron(III) ions in Solution. Under appropriate pH and redox conditions aquated iron(II) is readily oxidized to form iron(III) ions which may then lose proton(s) to form FeOH2+ and/or Fe(OH)+₂ species as well as the dimer, Fe₂(OH)₂4+. The true nature of these species are probably [Fe₄₊(H₂O)₆]3+, [Fe(H₂O)₅ OH)]2+, [Fe(H₂O)₄ (OH)₂] , and [(H₂O)₄ Fe(OH)₂ Fe(H₂O)₄]4+, respectively, but they are usually abbreviated to the former. Such species have been identified in Solution by spectrophotometric and Potentiometrie methods [1–9], and by magnetic methods [10]. However, in other than very acidic Solution, hydrolysis continues beyond these simple products and based upon ultracentrifuge analyses [11] large polymers form. Pathways for polymer formation have been suggested by Dousma and deBruyn [7]. With time, and a continued supply of monomeric species such polymers continue to grow and ultimately yield precipitates of iron oxides or hydrous oxides, [12,13] the nature of which can vary depending on the conditions in the hydrolyzing medium. Factors influencing the product distribution include pH, temperature, iron concentration, rate of hydrolysis and the nature and concentration of other solutes [12]. Possible pathways for the formation of the product iron oxides and hydrous oxides are shown in Figure 1 and the general characteristics of these oxides [14] are presented in Table I. In some cases previous workers have used the term iron oxides to include collectively both the oxides and hydrous oxides [14]. For simplicity this approach will be adopted here.