Five types of forestry wastes (coniferous and mixed coniferous-deciduous barks, dating back to 1965), inoculated with sulphate reducing bacteria from sediments or sewage sludge, were placed into closed vessels with a synthetic solution of acid mine drainage, with or without dolostone, and allowed to react, in order to assess their importance in the initiation and the evolution of sulphate reduction reactions. pH, alkalinity, and Fe2+ and SO42- concentrations were measured at intervals, over 70 days.
Increase of pH and decrease of Fe2+ and SO42- were observed in all reacting cells. In samples without dolostone, pH increased from 1.6 to 5-6, Fe2+ decreased from 400 to 42- decreased from 2500 to 750-1250 mg/L. Samples with sediments inocula gave a slightly better performance. Those containing dolostone showed pH reaching 6.5-7.0, and Fe2+ and SO42- dropping to <5 and 20-40 mg/L, respectively. Reactivity was not a simple inverse function of the age of the forestry wastes. Despite being one of the oldest (1980's), the barks that best performed were those which contained a significant component of deciduous bark, and little sawdust. Samples with high proportions of coniferous bark and/or sawdust were associated to sluggish sulphate reduction rates.
Notwithstanding the huge improvement conferred by dolostone in early acid neutralisation, it follows that the type of forestry wastes eventually used in a reactive barrier is of prime importance for the efficiency of the reduction process. A treatment system using forestry waste to sustain sulphate reduction should use bark (rather than wood) from deciduous (rather than coniferous) trees.