Net alkalinity and net acidity 2: Practical considerations

The pH, alkalinity, and acidity of mine drainage and associated waters can be misinterpreted because of the chemical instability of samples and possible misunderstandings of standard analytical method results. Synthetic and field samples of mine drainage having various initial pH values and concentrations of dissolved metals and alkalinity were titrated by several methods, and the results were compared to alkalinity and acidity calculated based on dissolved solutes. The pH, alkalinity, and acidity were compared between fresh, unoxidized and aged, oxidized samples. Data for Pennsylvania coal mine drainage indicates that the pH of fresh samples was predominantly acidic (pH 2.5– 4) or near neutral (pH 6–7); 25% of the samples had pH values between 5 and 6. Following oxidation, no samples had pH values between 5 and 6. The Standard Method Alkalinity titration is constrained to yield values >0. Most calculated and measured alkalinities for samples with positive alkalinities were in close agreement. However, for low-pH samples, the calculated alkalinity can be negative due to negative contributions by dissolved metals that may oxidize and hydrolyze. The Standard Method hot peroxide treatment titration for acidity determination (Hot Acidity) accurately indicates the potential for pH to decrease to acidic values after complete degassing of CO2 and oxidation of Fe and Mn, and it indicates either the excess alkalinity or that required for neutralization of the sample. The Hot Acidity directly measures net acidity (= net alkalinity). Samples that had near-neutral pH after oxidation had negative Hot Acidity; samples that had pH < 6.3 after oxidation had positive Hot Acidity. Samples with similar pH values before oxidation had dissimilar Hot Acidities due to variations in their alkalinities and dissolved Fe, Mn, and Al concentrations. Hot Acidity was approximately equal to net acidity calculated based on initial pH and dissolved concentrations of Fe, Mn, and Al minus the initial alkalinity. Acidity calculated from the pH and dissolved metals concentrations, assuming equivalents of 2 per mole of Fe and Mn and 3 per mole of Al, was equivalent to that calculated based on complete aqueous speciation of Fe/Fe. Despite changes in the pH, alkalinity, and metals concentrations, the Hot Acidities were comparable for fresh and most aged samples. A meaningful ‘‘net’’ acidity can be determined from a measured Hot Acidity or by calculation from the pH, alkalinity, and dissolved metals concentrations. The use of net alkalinity = (Alkalinitymeasured Hot Aciditymeasured) to design mine drainage treatment can lead to systems with insufficient Alkalinity to neutralize metal and H acidity and is not recommended. The use of net alkalinity = Hot Acidity titration is recommended for the planning of mine 0883-2927/$ see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2005.07.003 * Corresponding author. Tel.: +1 570 577 1385/717 524 1385; fax: +1 570 577 3031. E-mail addresses: [email protected] (C.S. Kirby), [email protected] (C.A. Cravotta). 1 Tel.: +1 717 730 6963; fax: +1 717 730 6997. 1942 C.S. Kirby, C.A. Cravotta / Applied Geochemistry 20 (2005) 1941–1964 drainage treatment. The use of net alkalinity = (Alkalinitymeasured Aciditycalculated) is recommended with some cautions. 2005 Elsevier Ltd. All rights reserved.