Gyanendra Regmi

Performance of a prb for the remediation of acidic groundwater in acid sulfate soil terrain

Contaminated groundwater resulting from pyrite oxidation of acid sulfate soils (ASSs) 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 neutralizing acidic groundwater. Results show that the actual acid neutralization capacity of the recycled concrete could decrease to less than 50% of the theoretical value due to armoring effects. Nevertheless, the performance is good as a spot treatment in ASS Terrain using a near-zero cost waste product. Based on the test results and site characterization, a permeable reactive barrier (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 (1) to neutralize the groundwater acidity and (2) to remove the dissolved heavy metals such as iron and aluminum 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 aluminum. Results reported here also reveal a slow decrease in the performance of the PRB due to armoring effects probably caused by precipitation of iron and aluminum on the surface of the reactive recycled concrete materials.

Treatment of Acidic Groundwater in Acid Sulfate Soil Terrain Using Recycled Concrete: Column Experiments

Acidic groundwater generated from pyrite oxidation in acid sulfate (AS) soil 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 low-lying AS soil floodplains. Laboratory experiments were systematically conducted to investigate the acid neutralization behavior 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 AS soil terrain, resulting in near neutral effluent over a long period with complete removal of Al and Fe. The major mechanisms involved in neutralizing acidic groundwater are thought to be the precipitation of Al and Fe as oxides, oxyhydroxides, and hydroxides. However, the accumulation of secondary minerals could decrease the reactivity of the recycled concrete. For example, chemical armoring could decrease the neutralizing capacity of recycled concrete by up to 50% compared with the theoretical acid neutralization capacity of this material. The results reported here also show that the neutralization capacity and reactive efficiency of recycled concrete are dependent on the initial pH value and also the concentration of Al and Fe in acidic groundwater.

Geo-environmental Approaches for the Remediation of Acid Sulphate Soil in Low-lying Floodplains

Acidity generated from the oxidation of pyrite and other sulphidic compounds that exist at shallow depths in acid sulphate soils (ASS) presents a challenging environmental problem in coastal Australia. The generated acidic groundwater can adversely impact coastal ecosystems, aquaculture and agriculture. Groundwater manipulation using weirs and modified floodgates in creeks and flood mitigation drains in ASS-affected farmland, which has been practiced for over a decade for preventing pyrite oxidation, is not effective in low-lying floodplains due to the high risk of flooding. In this paper, the authors present an overview of their experience in coastal Australia, a critical evaluation of currently practiced geo-environmental remediation methods as well as a demonstration of a pilot permeable reactive barrier (PRB) to control acidic groundwater pollution. The selection of recycled concrete, a commonly available alkaline waste material, and the systematic investigation of its longevity are highlighted through a series of batch and column experiments. In addition, the improvement of the groundwater quality by a pilot PRB using recycled concrete in ASS terrain within the Shoalhaven region of NSW, Australia will be elucidated based on field data collected over the last 3.5 years.

Characterisation and assessment of recycled concrete aggregates used in a permeable reactive barrier for the treatment of acidic groundwater

The acidification of coastal waterways because of acid sulphate soil is an environmental, economic and social problem within Australia. A pilot-scale permeable reactive barrier (PRB), using recycled concrete aggregates as reactive material, was installed in low-lying acid sulphate soil terrain for acidic groundwater remediation. Column experiments were previously undertaken with synthetic groundwater to ascertain the dominant reactions occurring within the PRB. Results showed that armouring of the reactive material surface by precipitated Al- and Fe-bearing minerals significantly reduced its acid neutralisation capacity (ANC). The purpose of this current study was to validate this decline in ANC through characterisation of the virgin and armoured concrete aggregates, and precipitates that formed on the concrete. Samples of concrete aggregates and precipitates were analysed using X-ray diffraction (XRD), X-ray fluorescence (XRF), Fourier transform infrared spectroscopy (FTIR), Scanning electron microscopy-Energy dispersive spectroscopy (SEM-EDS) and X-ray micro-computed tomography (μCT). The conclusions drawn from these analyses are that Al-bearing (gibbsite 14.3%, boehmite 10.9%) and Fe-bearing (goethite 38.2%) mineral precipitates of diverse morphology form as a thin layer coating the aggregate surfaces. A reduction of CaO in the armoured concrete aggregates by 47% correlates with the reduction in ANC of the virgin concrete by 50% due to armouring.

A neural network approach to predict the performance of recycled concrete used in permeable reactive barriers for the treatment of acidic groundwater

Abstract This study developed a neural network model for examining the performance of recycled concrete for the treatment of acidic groundwater. Concentrations of Al, Fe and Ca and alkalinity of the effluent were selected as the output parameters to simulate the performance of recycled concrete for neutralizing acidic groundwater. The input variables were the number of pore volumes, pH, oxidation reduction potential and the average hydraulic conductivity. Of the 658 experimental datasets available, 409 datasets were used for training, 184 datasets were used for validation, and the remaining datasets were used for cross-validation. The reported results indicate that the neural model is a valuable tool to assess and simulate the performance of recycled concrete. The sensitivity study confirmed that the selected input signals of the output estimate were equally important. A similar model could also be used for full-scale permeable reactive barrier installation provided that up-scaling issues such as the possible non-homogeneous nature of the recycled concrete and variation in groundwater quality can be effectively resolved.