Kinetics and mechanisms of soil iron oxide dissolution in EDTA, oxalate and dithioniate

Poorly crystalline soil iron oxides are dissolved by 3-7 months' extraction with EDTA at pH 8-10.5 and by 2 hours' extraction in the dark with oxalate at pH 3 ; likely by a surface complexation mechanism. The great difference in dissolution rates may be due to pH. However, oxalate but not EDTA decomposes easily forming radicals and thus iron(II), which promote the dissolution particularly as oxalate is a good electron conductor, whereas tetranuclear surface complexes possibly formed by EDTA inhibit dissolution. Dithionite readily dissolves pedogenic crystalline iron oxides, presumably by a radical mechanism, where S02 radicals mainly reduce surface iron (III), which are released by means of the added ligand (citrate or EDTA). Nonreducible structural impurities such as aluminium decrease the dissolution rate due to surface enrichment with the impurity.