NEUTRALIZATION POTENTIAL OF OVERBURDEN SAMPLES CONTAINING SIDERITE

Acid-base accounting (ABA) is a common procedure to predict the alkali ne or acid-producing potential of overburdens. Neutralization potentia l (NP) as currently written in ABA overestimates alkalinity when sider ite (FeCO3) is present in the overburden. Siderite initially yields al kalinity upon digestion, but with time the alkalinity is neutralized b y acidity from ferric iron (Fe3+) hydrolysis and precipitation. Thirty -one overburden samples containing varying amounts of siderite, calcit e, pyrite, and quartz were analyzed by four NP digestion methods and t itrated either by hand or by autotitration. The NP methods were: (i) s tandard 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 (H2O2); and (iv) a modified Sobek method that adds H2O2 after the firs t hand titration (SobPer). For samples containing primarily calcite, q uartz, or clays, the NP values for a particular sample were similar am ong digestion methods. For samples containing pyrite, the SobPer metho d (no filtering) produced the lowest NP values. Siderite-containing sa mples showed wide variation in NP values among methods. The H2O2 metho d decreased NP values of siderite samples compared to Sobek and BOIL m ethods. Lower NP values were generally obtained with autotitration vs. hand titration because autotitration added the base slowly, which all owed concurrent oxidation and hydrolysis of iron. Hand-titration of si derite samples requires H2O2 treatment to accelerate iron oxidation. V ariation in NP values for a particular sample was high among three lab oratories using the Sobek hand titration method, but the average varia tion in NP values among labs decreased by 66% when using the H2O2 hand method. Variation in NP values among labs was also due to the same sa mples being assigned different fizz ratings by laboratory technicians, which changed the concentration of acid added in the digestion proced ure. With more acid, NP values generally increased, especially for sid erite samples. A more quantitative approach is needed to determine the amount of acid to add for NP digestion, and the percent insoluble res idue of the sample used in this study may be a good alternative but re quires more testing and multilaboratory screening. The ABA values (usi ng %S and NP from the various methods) were compared with soxhlet leac hate pH and cumulative alkalinity. The ABA values with H2O2 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 H2O2 method.