Bacterial Impacts on Permeable Reactive Barrier Performance: Role of Acidithiobacillus ferrooxidans in Acid Sulphate Soil Environments

Permeable Reactive Barriers (PRBs) are widely used for passive in situ treatment of acidic groundwater using alkaline media in acid sulphate soil regions. However, proliferation of acidophilic iron-oxidizing bacteria can significantly alter the geochemical and hydraulic performance of PRBs by accelerating iron oxidation, acid generation, and secondary mineral precipitation, ultimately leading to bio-clogging and permeability loss. This study investigates the coupled biogeochemical and hydraulic impacts of Acidithiobacillus ferrooxidans on PRB performance under environmentally relevant pH and temperature conditions. Controlled laboratory experiments were conducted to quantify bacterial growth, iron oxidation behaviour, redox evolution, and porosity changes under varying pH and temperature regimes, followed by column-scale PRB tests to assess spatial patterns of clogging. Results indicate that A. ferrooxidans activity was strongly pH- and temperature-dependent, with growth and iron oxidation suppressed at pH values above 2 and optimal activity observed at 32-33 °C. Column experiments showed pronounced bacterial colonization at the influent zone, where rapid acidification, elevated redox potential, and Fe³⁺ precipitation were observed, resulting in progressive permeability loss. A porosity reduction of up to 15% was measured at the inlet zone over the test period, corresponding to a substantial decline in hydraulic conductivity. Field monitoring of two PRB systems in the Shoalhaven floodplain demonstrated that seasonal groundwater fluctuations modulated microbial activity, redox conditions, and treatment efficiency, with limestone-based PRBs exhibiting greater buffering capacity and resistance to bacterial acidification than recycled concrete aggregate systems. Comparison between laboratory and field observations indicates that fully saturated laboratory columns packed with finer media experienced approximately 30-fold higher contaminant exposure, leading to accelerated iron oxidation and bio-clogging rates relative to field conditions. Overall, the study quantitatively demonstrates how pH evolution, temperature, and redox gradients govern iron oxidation kinetics, hydraulic degradation, and long-term PRB effectiveness.