Reactive iron

Reactive iron is an operationally defined quantity of solid phase iron oxides that are thought to be available for bacterial iron reduction and/or sulfide oxidation. Iron oxides range from amorphous iron oxides (e.g., ferrihydrite) to more crystalline iron oxides such as goethite, magnetite, and hematite. Furthermore, iron may exist in the solid phase in its reduced, ferrous form adsorbed to surfaces, as sulfides (e.g., FeS, Fe₃S₄, FeS₂), mixed oxides (e.g., magnetite), and in clays (e.g., in chlorite). Over the past thirty years a number of chemical extractions have been designed to extract and speciate one or more of these phases. Here we describe a protocol, based on the work of Kostka and Luther (1994), that attempts to distinguish amorphous iron oxides, magnetite, total reducible oxide iron, and readily extractable iron bound in silicates.

Dissolved iron in the extracts is measured using the Ferrozine method (Stookey, 1970).

a)

Two-step Ascorbate-Anoxic Oxalate Extraction:

In this reaction we distinguish between readily reducible, amorphous iron oxides and iron contained in magnetite. The ascorbate step extracts the amorphous iron oxides and the oxalate step will attack magnetite (with some contributions from silicate bound iron).

Reagents:

Ascorbate Solution Prepare fresh. Add 10 g sodium citrate and 10 g sodium bicarbonate to 200 mL distilled water. De-gas with nitrogen. Add 5 g Ascorbic acid (pH should be ca. 8).

0.2 M Ammonium Oxalate Solution Dissolve 16. 2 g ammonium oxalate and 10.8 g oxalic acid in 1 L distilled water. Adjust the pH to 3.0 with HCl or NaOH. Store cool and in dark. De-gas before using.

Step 1: Ascorbate Extraction:

Add 10 mL of Ascorbate solution to a 50 mL screw-top, plastic centrifuge tube. Weigh 0.1 to 0.3 g of fresh sediment and add to solution. Extract for 24 hours with gentle shaking. Centrifuge (4500 rpm for 5 minutes) and save supernatant for Fe analysis.

Step 2. Anaerobic Oxalate Extraction:

Wrap centrifuge vial with remaining sediment with aluminum foil. De-gas Oxalate solution. Add 10 mL of oxalate solution to Corning vial and de-gas vial. Re-suspend sample and shake for 24 hours. Centrifuge (4500 rpm for 5 minutes) and measure Fe(II) and total Fe immediately.

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