Solid-phase iron characterization during common selective sequential extractions

Selective chemical extractions provide semiquantitative information on elemental partitioning within operationally defined soil fractions. In this study, the efficiency of common extraction steps was determined for a mining-impacted soil by analyzing Fe transformations in the solid phase using x-ray diffraction, scanning electron microscopy, and x-ray absorption near edge structure (XANES) spectroscopy. Extractions involve the isolation of operationally defined double-deionized water (soluble), magnesium chloride (exchangeable), sodium hypochlorite (organic matter), sodium acetate-acetic acid (carbonate), hydroxylamine hydrochloride-nitric acid (Mn-oxides), ammonium oxalate in the dark (AOD) (noncrystalline material), hydroxylamine-hydrochloride-acetic acid (Fe oxides), potassium perchlorate-hydrochloric-nitric acid (sulfidic), and hydrochloric-nitric-hydrofluoric acid (residual) fractions of the solid phase. Ferric Pe remained in the solid phase throughout the extraction sequence until its removal by hydrochloric-nitric-hydrofluoric acid (residual fraction). The hydroxylamine-hydrochloride (1.0 ni in 25% [v/v] HOAc) extraction may underestimate Fe associated with crystalline materials. Thus, selective sequential extractions need to be optimized for a given soil prior to implementation and should be used in conjunction with spectroscopic techniques, when possible, to fully ascertain elemental partitioning within the solid phase.