No abstract

Contaminated groundwater resulting from pyrite oxidation of acid sulphate soils (ASS) is a major environmental problem in coastal Australia. A column test was carried out for an extended period with recycled concrete to study the efficiency of the reactive materials for neutralising acidic groundwater. Results show that the actual acid neutralisation capacity of the recycled concrete could decrease to less than 50% of the theoretical value due to armouring effects. Nevertheless, the performance is good as a spot treatment in Acid Sulphate Soil Terrain utilising a near-zero cost waste product. Based on the test results and site characterisation, a PRB with recycled concrete was designed and installed in ASS terrain on the Shoalhaven River floodplain, southeastern, Australia in October 2006. The performance of the PRB was studied over two and half years to assess the potential of recycled concrete (i) to neutralise the groundwater acidity and (ii) to remove the dissolved heavy metals such as iron and aluminium from in situ acidic groundwater. To date, performance monitoring of the PRB shows that recycled concrete can successfully improve the pH of groundwater from acidic to mildly alkaline. In addition, it successfully removes groundwater iron and aluminium. Results reported here also reveal a slow decrease in the performance of the PRB due to armouring effects probably caused by precipitation of iron and aluminium on the surface of the reactive recycled concrete materials.

Acidic groundwater generated from pyrite oxidation in acid sulphate soil (ASS) is a major geoenvironmental problem in Australia. This study aims to evaluate recycled concrete as a reactive material in permeable reactive barriers (PRBs) for the remediation of acidic groundwater in lowlying ASS floodplains. Laboratory experiments were systematically conducted to investigate the acid neutralisation behaviour of recycled concrete and its potential to remove dissolved Al and Fe. The results confirmed that recycled concrete could effectively treat acidic groundwater from an ASS terrain, resulting in near-neutral effluent over a long period with complete removal of Al and Fe. The major mechanisms involved in neutralising acidic groundwater are thought to be the precipitation of Al and Fe as oxides, oxy-hydroxides and hydroxides. However, the accumulation of secondary minerals could decrease the reactivity of the recycled concrete. Indeed, chemical armouring could decrease the neutralising capacity of recycled concrete by up to 50% compared to the theoretical acid neutralisation capacity of this material. The results reported here also show that the neutralisation capacity and reactive efficiency of recycled concrete are dependent on not only the initial pH value but also the concentration of both Al and Fe in the acidic groundwater.

Deep drainage technique utilised for flood mitigation in low-land coastal areas of Australia during the late 1960s has resulted in the generation of sulphuric acid in soil by the oxidation of pyritic materials.Further degradation of the sub-surface environment with widespread contamination of the underlying soil and groundwater presents a major and challenging environmental issue in acid sulphate soil removal efficiency (> 95%) over this study period indicates that recycled concrete could successfully treat acidic groundwater. However, the overall pH neutralising capacity of the materials within the barrier declined with time from an initial pH 10.2 to pH 7.3. The decline in the performance with time was possibly due to the armouring of the reactive material surface by the mineral precipitates in the form of iron and aluminium hydroxides and oxyhydroxides as indicated by geochemical modelling.