Abstract. Surface mine reclamation laws in Texas permit the use of mixed overburden as a topsoil substitute. Overburden material must have a positive acid/base account (ABa) to remain within 4 feet of the reclaimed surface. Acid/base account is calculated by subtracting potential acidity (PA) plus exchangeable acidity (EA) from the inherent acid neutralization potential (NP) of the material. Material with a negative ABa may be limed to increase the AB a value. Two separate research studies were conducted to determine 1) the effects of siderite (FeC03), a frequently identified mineral in mixed overburden, on NP determination and 2) the long-term pH stability of potentially acidic mixed overburden limed to an AB a of zero. Siderite stoichiometrically weathers to produce acidity upon complete oxidation and hydrolysis of released ferrous iron. Siderite contributes to NP because of incomplete iron hydrolysis under the conditions used to determine NP in the laboratory. This "siderite error" can lead to a false positive AB a value when the current NP procedures are used on samples containing significant quantities of PA and siderite. The long-term pH stability of potentially acidic mixed overburden limed with CaC03 was evaluated using a simulated weathering study. The liming rates ranged from Oto 125% of the theoretical amount of lime needed to neutralize the measured AB a deficit. The data showed the addition of CaC0:3 significantly slowed the oxidation of FeS2-however, the dissolution of the applied lime proceeded at a steady rate with each percolating water front. The dissolution of applied lime was measured to be significantly faster than the release of PA in all treatments, thus the ABa values of all the treatments were found to decrease with time. Treatments which received less than a 50% lime rate became highly acidic within one year. The results indicate that all treatments would eventually become highly acidic.
Siderite (FeCO3) is found in coal overburden, and is commonly subjected to acidic environments in reclaimed minelands. Manganese commonly substitutes for Fe, and can be present in significant concentrations in the mineral. Siderite was incubated in a solution of either potassium hydrogen phthalate (KHP) or sulfuric acid with an initial pH of 3.0 in environments of CO2, air, or O2. Iron and Mn release from siderite under varying gaseous environments decreased in the following order: CO2 > air > O2. Weathering zones developed that were from 0.39 to 0.85 mm thick. Iron oxides precipitated in the weathering zone and slowed iron release from siderite samples incubated in H2SO4 under all three gaseous atmospheres. Iron oxides did not precipitate in the weathering zone for samples reacted in KHP solution. The release of Fe and Mn to solution followed a zero order reaction rate.
Abstract. Within the area that is now the Jewett Mine, there were several acres of abandoned underground lignite mines.
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