Environ. Sci. Technol. 2006, 40, 1984-1990
Chemical Stability of Acid Rock Drainage Treatment Sludge and Implications for Sludge Management DANNY M. MCDONALD* AND JOHN A. WEBB Environmental Geoscience, La Trobe University, Victoria 3086, Australia JEFF TAYLOR Earth Systems, Suite 507, 1 Princess Street Kew, Victoria 3101, Australia
To assess the chemical stability of sludges generated by neutralizing acid rock drainage (ARD) with alkaline reagents, synthetic ARD was treated with hydrated lime (batch and high-density sludge process), limestone, and two proprietary reagents (KB-1 and Bauxsol). The amorphous metal hydroxide sludge produced was leached using deionized water, U.S. EPA methods (toxicity characteristic leaching procedure, synthetic precipitation leaching procedure), and the new strong acid leach test (SALT), which leaches the sludge with a series of sulfuric acid extractant solutions; the pH decreases by ∼1 pH unit with each test, until the final pH is ∼2. Sludges precipitated by all reagents had very similar leachabilities except for KB-1 and Bauxsol, which released more aluminum. SALT showed that lowering the pH of the leaching solution mobilized more metals from the sludges. Iron, aluminum, copper, and zinc began to leach at pH 2.5-3, ∼4.5, ∼5.5, and 6-6.5, respectively. The leachability of ARD treatment sludges is determined by the final pH of the leachate. A higher neutralization potential (e.g., a greater content of unreacted neutralizing agent) makes sludges inherently more chemically stable. Thus, when ARD or any acidic metalliferous wastewater is treated, a choice must be made between efficient reagent use and resistance to acid attack.
Introduction Acid rock drainage (ARD) is one of the most costly and longlived environmental issues facing metal and coal mines worldwide (1, 2). Oxidation of sulfide minerals (usually pyrite) by exposure to air and water produces acidic waters (pH often
