MAGNETITE DISSOLUTION RATES IN ACIDIC SOLUTIONS

The magnetic properties of soils and sediments are used as indicators of paleoclimate and environmental factors. Although not found in high concentrations, small amounts of magnetite contribute the majority of the magnetic signature to these materials while other Fe oxyhydroxides contribute much less to the magnetic properties. In order to better understand how magnetite reacts in near surface environments, we investigated the rates of dissolution under various acidic conditions. Two dissolution methods were used in this study, and dissolved Fe was determined using AAS. The magnetite used was separated from heavy sands, sieved, and the final product determined to have a median particle size of 0.312 mm using a laser diffraction particle size analyzer. Assuming a spherical particle and no surface roughness, the specific surface area is 37.3 cm² g^-1. Batch dissolution experiments were used to evaluate five different solutions to determine the most effective dissolution reagent. Batch experiments were carried out for 10,000 min with solutions changed at designated intervals. Results of the batch experiments indicated that the acidified oxalate at pH 4 was most the effective at dissolving the magnetite. Cumulative Fe removed was 0.046 mg Fe removed per gram of magnetite with oxalate while citrate removed 0.012 mg g^-1 and the hydroxylamine HCl removed 0.024 mg g^-1. The remaining solutions removed 0.01 mg Fe g^-1 magnetite or less. The oxalate solution was chosen for use in the continuous flow-through column experiments. The column was 1.26 cm in diameter and 10 cm in length, and filled with 32 g of magnetite resulting in a bulk density of 2.53 g cm^-3 and a pore volume of 6.46 ml. Solutions were pumped through the columns using a peristaltic pump for 14,505 min with flows rates averaging 0.43 ml min^-1. Cumulative Fe released was determined to be 9.33 mg at pH 4 and 16.79 mg at pH 3. Calculated release rates averaged 1.20 * 10^-11 mol cm^-2 sec^-1 and 1.98 * 10^-11 mol cm^-2 sec^-1 at pH 4 and 3, respectively. X-ray diffraction and magnetic susceptibility measurements do not indicate the presence of a significant amount of another mineral phase. This suggests that dissolution is a simple process with little or no secondary precipitate is forming on mineral grain surfaces and no alteration is occurring to the remaining magnetite grains.