Selection of potential reactive materials for a permeable reactive barrier for remediating acidic groundwater in acid sulphate soil terrains

A permeable reactive barrier is being designed to remediate leachate from acid sulphate soils. The current research relates to testing of alkaline materials for use in the barrier, with an emphasis on waste materials. Thirteen alkaline materials including recycled concrete, limestone, calcite-bearing zeolitic breccia, blast furnace slag, lime and fly ash were tested. The batch tests involved several phases, such as leaching in deionised water to characterise the soluble components of the materials and the pH that each material could achieve. Another phase involved testing with acidic water (pH 3) to determine the acid leachable components of the materials and the pH after neutralisation. The pH achieved by each reactive material was controlled by the reaction kinetics of the dominant alkaline mineral. The concretes, fly ash, and air-cooled blast furnace slag (ACBFS) all achieved a pH that is consistent with the dissolution of lime (pH 11 to 12). The limestone and zeolitic breccias all achieved a pH consistent with the dissolution of calcite (pH ~7.4). Based on the results of the batch tests, a short-list of materials was selected that included a recycled concrete, ACBFS, three zeolitic breccias and limestone. The short-listed materials were examined for exhaustion of neutralising ability by repeatedly replacing the acidic water and monitoring the resultant pH. The precipitates that formed during this process were analysed to characterise the chemical reactions that occurred during the tests. Based on the results, the recycled concrete was selected for testing in columns that will simulate flow conditions through the barrier.