Microbial and mineral evolution in zero valent iron-based permeable reactive barriers during long-term operations

Impacts of subsurface biogeochemical processes over time have always been a concern for the long-term performance of zero valent iron (Fe(0))-based permeable reactive barriers (PRBs). To evaluate the biogeochemical impacts, laboratory experiments were performed using flow-through glass columns for 210 days at controlled temperature (20 °C). Two different particle sizes of Fe(0) were used in the columns, and to simulate indigenous microbial activity, extra carbon source was provided in the two columns (biotic columns) and the remaining two columns were kept abiotic using gamma radiations. Heavy metals (Zn, As) were removed efficiently in all the columns, and no exhaustion of treatment capability or clogging was observed during our experimental duration. Newly formed Fe mineral phases and precipitates were characterized using X-ray diffraction (XRD), scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX), and micro-XRF techniques in solid phase at the end of the experiment. In addition, 16S rRNA gene extraction was used for microbial community identification in biotic columns. During the incubation, microbial population shifted in favor of Desulfosporosinus species (sulfate-reducing bacteria) from initial dominance of Acidithiobacillus ferrooxidans in sediments. Dominant mineral phases detected in biotic columns were mackinawite (FeS) and sulfate green rust, while in abiotic columns, magnetite/maghemite phases were more prevalent.