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Problems in alkalinity and acidity measurements in mine drainage

, : Problems in alkalinity and acidity measurements in mine drainage. Abstracts with Programs - Geological Society of America 32(7): 339

Not all mine drainage is acidic, and problems in measurements and interpretations of alkalinity and acidity arise especially in mine drainage containing alkalinity. Synthetic and field samples of mine drainage were analyzed using seven titration methods, and the results were compared to theoretical concentrations of alkalinity and acidity and to geochemical computer modeling using PHREEQC. Synthetic and field mine drainage with no alkalinity and pH values less than 4.5 gave consistent results for all titration methods, thus there seems to be no problem in the interpretation of acidity in such samples. In synthetic samples containing alkalinity, standard methods for alkalinity returned the same values as calculated alkalinity. However, acidity values varied significantly among titration methods, and the most commonly accepted titration method significantly underestimated acidity in samples containing alkalinity. This behavior is due to the addition of hydrogen peroxide before titration which causes the oxidation. The hydrolysis of iron in turn allows hydrogen ions to react with the alkalinity present in the sample. Modeling of synthetic samples with PHREEQC also supports this conclusion. Metal-laden natural waters such as some wetland ground waters suffer from the same problems with alkalinity and acidity interpretations. Many workers commonly use the calculated "net alkalinity" (measured alkalinity minus measured acidity) to determine whether mine water has sufficient alkalinity to neutralize acidity during treatment. Using the measured alkalinity and acidity values in this fashion can lead to design of treatment systems with insufficient alkalinity to neutralize acidity due to metals and hydrogen ion. This study suggests that the most reliable way to obtain a "net alkalinity" is to add hydrogen peroxide and store samples open to the atmosphere to allow oxygen to enter the sample and carbon dioxide to escape, then perform both alkalinity and acidity titrations after several days.


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