Metal sulfide tailings of the Sardinian (Italy) abandoned Baccu Locci arsenic mine show high concentrations of aluminum, arsenic, cadmium, copper, manganese, lead, and zinc in acid percolate (pH 5 4) and have been classified as ''dangerous waste.'' This paper shows that the release of toxic metals can be strongly reduced when the tailings are placed on a reactive permeable bed (7 wt %) of porous, alkaline pellets of transformed red mud (TRM). During a laboratory percolation test, two columns with 80 kg of waste, of which one contained a bottom layer of TRM pellets, were each alimented with 600 L of de-ionized water. Comparing pH, electroconductivity, metal, and sulfate concentrations of collected percolate from both columns demonstrates efficient neutralization (pH 5 7.4) and removal of metals (80 to 99%) for the column with the permeable reactive bottom layer. Water Environ. Res., 82, 319 (2010).KEYWORDS: acid rock drainage, column leaching, transformed red mud pellet, permeable reactive barrier.