Extraction Techniques for Selective Dissolution of Amorphous Iron Oxides from Soils and Sediments

Abstract

Abstract A comparative study of extraction techniques for the selective dissolution of amorphous iron oxides from soils and sediments was conducted using nine iron oxide samples characterized by x‐ray diffraction as model mineral substrates. Several extracting solutions were evaluated as to their efficiency and specificity for dissolving amorphous iron oxides. Solutions examined included (i) 0.175 M (NH 4 ) 2 C 2 O 4 ‐0.100 M H 2 C 2 O 4 (Tamm's reagent) in the dark, (ii) HCl of different concentrations at both room and boiling water temperatures, (iii) 3% H 2 C 2 O 4 at boiling water temperature, (iv) 0.25 M NH 2 OH·HCl‐25% HOAc at 70°C in a shaker hath, and (v) 0.25 M NH 2 OH·HCl‐0.25 M HCl at 70°C in a shaker hath. Some solutions were found inadequate because of attack on other mineral species, dissolution of crystalline iron oxides such as magnetite, lack of specificity, or lengthy reaction time. Using results obtained on amorphous iron oxides by extraction with Tamm's reagent as a standard reference, the 0.25 M NH 2 OH·HCl‐0.25 M HCl combined solution, modified to extract at 50°C for 30 min, was chosen as the most desirable extractant for amorphous iron oxides based on four considerations: (i) only 30 min required for the extraction, (ii) minor dissolution of crystalline iron oxides (< 1% of the total iron), (iii) low slope of the time series dissolution curve, and (iv) results in close agreement with the 0.175 M (NH 4 ) 2 C 2 O 4 ‐0.100 M H 2 C 2 O 4 extraction in the dark. The modified procedure was applied to six geochemical exploration reference samples consisting of soil, sediment, and weathered rock. Results obtained on amorphous iron oxides were in close agreement with those achieved by the acid ammonium oxalate extraction. The application of the extractant to studies of exploration geochemistry and soil active iron ratios is discussed.