In situ and ex situ bioremediation of seleniferous soils from northwestern India

Purpose: Selenium (Se) toxicity or deficiency disorders are chiefly associated with Se concentration and speciation in soils. Elevated soil Se content may lead to contamination of water bodies and groundwaters due to the leaching caused by rainfall and irrigation. This study is focused on Se removal by in situ (biostimulation and bioaugmentation) and ex situ (soil washing) bioremediation as well as on its recovery. Materials and methods: In this research, in situ bioremediation of Se-rich soil collected from rice fields in Ludhiana, Northwest India was studied in microcosms. The effect of biostimulation was determined by amending soil with different organic sources (fermentable, non-fermentable, and non-hydrolysable electron donors). The effect of bioaugmentation was determined by adding anaerobic granular sludge to the microcosms. With regard to ex situ bioremediation, the Se-rich soil was leached with water and the resulting leachate was biologically treated in an upflow anaerobic sludge blanket (UASB) reactor using lactate as electron donor. The UASB reactor was operated for 78 days in different conditions of lactate (electron donor) dosing to achieve maximum Se removal and recovery as elemental Se(0) on the granular sludge. The effluent of the UASB reactor was regularly analyzed to determine Se removal efficiencies. Results and discussion: The effect of biostimulation and bioaugmentation showed no significant difference in terms of Se reduction profiles in the microcosms. This suggested that the indigenous Se-reducing microorganisms and oxidizable organic carbon present in the soil are sufficient for in situ soil bioremediation. During treatment of soil leachate in the UASB reactor, 90% Se removal was achieved irrespective of the lactate dosing and mineral salt medium composition of the reactor influent. Analysis of the granular sludge using scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDS) and powder X-ray diffraction (P-XRD) confirmed the presence of elemental Se on the granular sludge. The total Se concentration in the anaerobic granular sludge amounted to 43.5 (± 0.7) μg Se per gram of granular sludge. Conclusions: In situ bioremediation achieved Se reduction in the Se-rich soil investigated. However, risk of Se re-oxidation and leaching into groundwater after in situ remediation cannot be disregarded. In contrast, during ex situ treatment, effluent from the UASB reactor contained less than the USEPA guideline value 5 μg L−1 Se. This study showed biological treatment of Se-rich soils is suitable for cleaning the soil, Se recovery, and environmentally acceptable effluent discharge of the soil washing leachate treatment.