Quantitative Dissolution of Environmentally Accessible Iron Residing in Iron-Rich Minerals: A Review

Classifying iron residing in iron-bearing minerals is an important process across numerous disciplines. Iron exists in many forms and with varying degrees of accessibility for reactions. A wide range of procedures exist for identifying and quantifying iron in various forms, but they are scattered throughout numerous journals and typically have several variations. Methods for selective mineral dissolution and iron quantification are reviewed here to improve clarity and accessibility. The methods are described from least aggressive to most aggressive and include cation exchange with salt solutions and dissolutions by acetic acid to target carbonates, hydroxylamine-HCl to target ferrihydrite and lepidocrocite, sodium dithionite to target select iron (oxyhydr)oxides, oxidizing reagents to target organic matter, oxalic acid to target magnetite, concentrated HCl to target iron (oxyhydr)oxides, HF to target silicates, and acid-Cr to target iron sulfides. The dissolution methods using acetic acid, hydroxylamine-HCl, oxalic acid under darkness, and concentrated HCl were critically analyzed for quantifying iron residing in several natural and synthetic minerals. Iron quantification by UV-visible spectroscopy is reviewed, including the use of common colorimetric reagents, such as potassium thiocyanate, substituted ortho-dihydroxybenzenes, and ferroin-bearing reagents. Iron detection using ferrozine in the presence of oxalate was also critically evaluated and results indicate that oxalate competes with the formation of the Fe(II)-ferrozine complex but also efficiently photoreduces Fe(III). Natural samples present several challenges and the presence of mineral salts and redox-active species must be considered. Finally, the importance of standard reporting protocols for fostering accessibility and facilitating comparisons between data sets is discussed.