Estimation of forms of Fe and Al - a review, and analysis of contrasting soils by dissolution and Mossbauer methods

Abstract

Three methods have been used to extract iron (Fe) and aluminium (Al) from a wide range of soils. The three extractants were pyrophosphate reagent (p), acid-oxalate reagent (o), and dithionite-citrate reagent (d), and each reagent is thought to extract different forms of Fe and Al. The forms of Fe in the soils were studied before and after extraction using Moessbauer spectroscopy. Pyrophosphate-extractable Fe (Fep) does not specifically relate to any particular form of Fe in soils and it should not be used to estimate Fe in humus complexes. In some soils (e.g., Spodosol, Ochrept) Fep arises from Fe in ferrihydrite. In other soils (Ultisol, Hydrandept) Fep arises from goethite and ferrihydrite which is dispersed by pyrophosphate. Alp corresponds to Al in humus complexes and, in most soils, can be used to estimate Al in such complexes. Al and Si in allophane and imogolite were extracted in acid-oxalate reagent, and could be estimated from Alo-Alp and Sio, provided large amounts of ferrihydrite were not present. Ferrihydrite was also extracted in acid-oxalate reagent and, assuming complete extraction, could be estimated by multiplying Feo by the factor 1.7. Dithionite-citrate reagent dissolved goethite and hematite in addition to ferrihydrite. In some soils repeated extractions were required to dissoive most of the goethite and hematite. Quantitative estimates of Fe in goethite and hematite can be made from Fed -Feo. Other techniques, however, such as X-ray diffraction and Moessbauer spectroscopy, were necessary to identify iron oxides positively in most soils.

Immobilization of Cu and As in two contaminated soils with zero-valent iron – Long-term performance and mechanisms

“…In AsZVI, the fraction of oxalate-extractable iron was lower, corresponding to 30% of added ZVI. This indicates that some of the added ZVI had been converted to more crystalline forms such as goethite in this soil (Parfitt and Childs, 1988). A likely explanation for the smaller fraction of poorly crystalline iron (hydr)oxides in this soil compared to CuZVI is the longer time since application (15 years).…”

“…77% of the added ZVI remained oxalate-extractable 6 years after application in this soil. As oxalate/oxalic acid primarily extracts poorly crystalline iron (hydr)oxides (Parfitt and Childs, 1988) and this means that a majority of added ZVI was present in poorly crystalline forms such as ferrihydrite. In AsZVI, the fraction of oxalate-extractable iron was lower, corresponding to 30% of added ZVI.…”

Modified rothamsted carbon model for andosols and its validation: changing humus decomposition rate constant with pyrophosphate-extractable Al

“…Parfitt and Childs (1988) summarized the process as follows: pyrophosphate-extractable Al (Alp) corresponds to Al in humus complexes; Al content in allophane and imogolite is estimated from the content of acid-oxalate-extractable Al (Alo) minus Alp; Si content in allophane and imogolite corresponds to acid-oxalate-extractable Si (Sio); and the ferrihydrite content can be estimated from that of acid-oxalate-extractable Fe (Feo).…”

Surface complexation model of boron adsorption by calcareous soils

“…The oxalate reagent allows allophanes, iron and aluminum organic complexes, hydrated oxides of iron, and aluminum to be dissolved, whereas the dithionite-citrate-bicarbonate (DCB) reagent method (Mehra and Jackson 1960) extracts iron from the amorphous and crystalline compounds without significant modification of aluminosilicates or lithogenic hematite. Well-crystallized Al hydroxides are not extracted by the DCB method, and they are only partly extracted by the oxalate reagent (Parfitt and Childs 1988).…”