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Neutralization Potential Determination of Siderite (FeCO₃) Using Selected Oxidants

^a Railroad Commission of Texas, Surface Mining and Reclamation Division, P.O. Box 12967 Austin, TX 78711-2967
^b USDA-ARS, 808 East Blackland Road, Temple, TX 76502
^c Department of Soil and Crop Sciences, MS 2474, Texas A&M University, College Station, TX 77843-2474

^* Corresponding author (gn-white{at}tamu.edu)

Received for publication May 12, 2005. Siderite (FeCO₃) is commonly found in coal overburden and, when present, can cause interference in the determination of neutralization potential (NP). Under acidic testing conditions, FeCO₃ reacts to neutralize acid, which contributes to the NP. However, continued weathering of FeCO₃ (oxidation of Fe²⁺ and hydrolysis of Fe³⁺) produces a neutral to slightly acidic solution. The effects of hydrogen peroxide (H₂O₂), potassium permanganate (KMnO₄), and O₂ on the laboratory measurement of NP of siderite samples taken from overburden were examined. All oxidation treatments lowered the NP values of the siderite samples as compared with the standard USEPA method. However, oxidation with H₂O₂ produced variable results depending on the amount of H₂O₂ added. Neutralization potential values obtained after oxidation treatments were highly correlated with Mn concentration. Reaction products (i.e., 2-line ferrihydrite) of siderite samples with H₂O₂ and KMnO₄ were not representative of natural siderite weathering. Oxidation with O₂ produced the lowest NP values for siderite samples. The reaction products produced by oxidation with O₂ most closely represent those intermediate products formed when siderite is exposed to atmospheric weathering conditions. Oxidation with O₂ also proved to be the most reproducible method for accurately assessing NP when siderite is present in overburden samples.

Abbreviations: AAS, atomic adsorption spectroscopy • ABA, acid–base account • AFM, acid-forming materials • EA, exchangeable acidity • GR1(CO₃), green rust • NP, neutralization potential • PA, potential acidity • XRD, X-ray diffraction analysis

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