Selectivity of reagents used to extract iron from soil

Abstract

The selectivity of the Tamm and Mehra-Jackson reagents to iron (hydr)oxides was verified chemically by comparing the efficiencies of the two methods of iron extraction: parallel (commonly adopted) and sequential, i.e., by calculating the difference ΔFe = [Fedit − (Feox(paral) + Fedit(seq))]. The expected equality of the extracted iron (i.e., ΔFe ∼ 0) is rarely observed. A positive balance reaching ΔFe = (0.5−0.8)% predominates upon iron extraction from forest soils of the Cis-Ural region. This is probably due to the stable Fe(II)-oxalate forming in the course of the Tamm extraction, which is incompletely dissolved by the dithionite-citrate-bicarbonate (DCB). On the contrary, a negative balance with ΔFe reaching −0.4% predominates in the steppe soils of the Stavropol region. This may be caused by Fe(II) minerals (pyrite and siderite) acting as catalyzers in Tamm’s extract, while being weakly soluble in the DCB. To follow the additivity principle for the Tamm and Mehra-Jackson extracts, we suggest a modification of the Schwertmann criterion KSch = (Feox: Fedit(paral)), which sometimes exceeds 1, to K Sch m = Feox: (Feox + Fedit(seq)), which is always less than 1. The values of the modified Schwertmann criterion better agree with the color of the Cis-Ural forest soils expressed in the CIE-L*a*b* optical system as compared with the initial Schwertmann criterion. In the steppe soils, the use of the modified criterion makes it possible to replace the illogical values of KSch = 1.11−1.37 by the quite acceptable K Sch m = 0.68−0.83.