Assessment of locally available reactive materials for use in permeable reactive barriers (PRBs) in remediating acid mine drainage

Abstract

The management and treatment of contaminated mine water is one of the most urgent problems facing the South African mining industry. The cost advantage of permeable reactive barriers (PRBs) has seen their increased application as means of passively treating mine drainage. A PRB is built by placing a reactive material in the path of polluted groundwater. As the contaminant moves through the material, reactions occur that transform it into an environmentally acceptable form. Batch tests were carried out on limestone, dolomite, fly ash, concrete and wood chips to find a reactive material and/or reactive mixture, for use in a PRB, which can neutralise acidity, remove metals and is locally abundant. Batch tests involved the leaching of the materials in deionised water to determine the leachable component of each reactive material and the pH that each material could achieve in deionised water. The materials were also tested in acidic water (pH 5.54) to determine the effectiveness of each reactive material in removing contaminants. In terms of the ability to increase the pH, the top-performing reactive materials were limestone and fly ash as they both achieved a pH above 11. Limestone, concrete, fly ash and dolomite successfully removed at least 99% of the iron (Fe) from the mine water. Limestone and fly ash removed at least 99% of manganese (Mn) and magnesium (Mg) from the mine water. While the other reactive materials were ineffective in removing sulphate (SO42-), limestone and fly ash, respectively, removed 72% and 99.9 % of SO42- from the mine water. The study of 3 reactive material mixtures, namely: (i) limestone-wood chips-concrete, (ii) limestone-fly ash, and (iii) fly ash-concrete, showed that all 3 systems were effective in removing heavy metals present in the mine water. All of the mixtures increased the pH to above 11, increased the alkalinity and decreased Fe, Mn, and Mg concentrations to below the prevailing South African discharge criteria of wastewater into a water resource. Reactive Mixtures 2 and 3 successfully removed 99% of SO42- within 14 days. This study found that the most suitable reactive material for remediating acid mine groundwater was fly ash because it was able to neutralise acidity and remove Fe, Mn, and Mg and SO42-.

Keywords: acid mine drainage, permeable reactive barrier, batch tests, remediation