Neutralization Potential of Overburden Samples Containing Siderite

Abstract

Acid-base accounting (ABA) is a common procedure to predict the alkaline or acid-producing potential of overburdens. Neutralization potential (NP) as currently written in ABA overestimates alkalinity when siderite (FeCO 3 ) is present in the overburden. Siderite initially yields alkalinity upon digestion, but with time the alkalinity is neutralized by aridity from ferric iron (Fe 3+ ) hydrolysis and precipitation. Thirty-one overburden samples containing varying amounts of siderite, calcite, pyrite, and quartz were analyzed by four NP digestion methods and titrated either by hand or by autotitration. The NP methods were: (i) standard Sobek method (Sobek); (ii) a method that boils the sample for 5 min (BOIL); (iii) a method similar to BOIL but it includes filtering and treating the sample with hydrogen peroxide before back-titrating (H 2 O 2 ); and (iv) a modified Sobek method that adds H 2 O 2 after the first hand titration (SobPer). For samples containing primarily calcite, quartz, or clays, the NP values for a particular sample were similar among digestion methods. For samples containing pyrite, the SobPer method (no filtering) produced the lowest NP values. Siderite-containing samples showed wide variation in NP values among methods. The H 2 O 2 method decreased NP values of siderite samples compared to Sobek and BOIL methods. Lower NP values were generally obtained with autotitration vs. hand titration because autotitration added the base slowly, which allowed concurrent oxidation and hydrolysis of iron. Hand-titration of siderite samples requires H 2 O 2 treatment to accelerate iron oxidation. Variation in NP values for a particular sample was high among three laboratories using the Sobek hand titration method, but the average variation in NP values among labs decreased by 66% when using the H 2 O 2 hand method. Variation in NP values among labs was also due to the same samples being assigned different fizz ratings by laboratory technicians, which changed the concentration of acid added in the digestion procedure. With more acid, NP values generally increased, especially for siderite samples. A more quantitative approach is needed to determine the amount of acid to add for NP digestion, and the percent insoluble residue of the sample used in this study may be a good alternative but requires more testing and multilaboratory screening. The ABA values (using %S and NP from the various methods) were compared with soxhlet leachate pH and cumulative alkalinity. The ABA values with H 2 O 2 digestion were consistent with soxhlet leachate quality in 13 out of 13 samples. It is suggested that laboratories conducting NP in the ABA procedure use the H 2 O 2 method.