Attenuation of Acid Mine Drainage in a Coal Waste Deposit in Southern Brazil and the Prospect of Transitioning from Active to Passive Treatment

Capão da Roça, located in the municipality of Charqueadas, is one of the few areas of coal tailing deposits at the surface within the State of Rio Grande do Sul, Brazil that generates acid mine drainage (AMD). Over the course of 2007, the landfill was characterised in detail, and an active treatment plant involving pH neutralisation and metal precipitation operations was implemented to meet emission standards for mine water. In that year, based on the sulphur mass balance, it was estimated that the process of AMD generation would last for approximately two decades. The objective of this work was to study the temporal evolution of the parameters of the raw AMD. The effluent was analysed for 17 years on a monthly basis in regard to pH, acidity, metals (Fe, Al, and Mn), and sulphates. The results indicated an increase in pH (from 2.1 to 4.7), a decay in the concentration of metals (from 177.8 to 0.1 mg L−1 for iron, 29.0 to 0.1 mg L−1 for aluminium, and 3.1 to 0.6 mg L−1 for manganese), sulphates (from 2023 to 307 mg L−1), and acidity (from 539.5 mg CaCO3 L−1 to 3.96 mg CaCO3 L−1), which were adjusted to a first-order kinetic model in agreement with observations at some other mining sites. Over the years, the active lime neutralisation–precipitation treatment system has been efficient in treating the effluent. Today, most water quality parameters already meet emissions standards; however, the AMD treatment plant is still necessary to prevent pH fluctuations and to reduce the concentrations of manganese. For this reason, a transition from an active to a passive treatment system was considered. Pilot scale studies confirmed that channels filled with gravel-size limestone or slag enable the neutralisation/increase in the pH of the effluent and remove residual amounts of some metals, resulting in an effluent with no level of toxicity to the microcrustacean Daphnia magna.