Abstract
A 30-month pilot study was conducted to evaluate the potential of in-situ metal(loid) removal through biostimulation of sulfate-reducing processes. The study took place at an industrial site in Flanders, Belgium, known for metal(loid) contamination in soil and groundwater. Biostimulation involved two incorporations of an organic substrate (emulsified vegetable oil) as electron donor and potassium bicarbonate to raise the pH of the groundwater by 1–1.5 units. The study focused on the most impacted permeable fine sand aquifer (8–9 m below groundwater level) confined by layers of non-permeable clay. The fine sands exhibited initially oxic conditions (50–200 mV), an acidic pH of 4.5 and sulfate concentrations ranging from 600 to 800 mg/L. At the central monitoring well, anoxic conditions (−200 to −400 mV) and a pH of 5.9 established shortly after the second substrate and reagent injection. Over the course of 12 months, there was a significant decrease in the concentration of arsenic (from 2500 to 12 μg/L), nickel (from 360 to <2 μg/L), zinc (from 78,000 to <2 μg/L), and sulfate (from 930 to 450 mg/L). Low levels of metal(loid)s were still present after 34 months (end of study). Mineralogical analysis indicated that the precipitates formed were amorphous in nature. Evidence for biologically driven metal(loid) precipitation was provided by compound specific stable isotope analysis of sulfate. In addition, changes in microbial populations were assessed using next-generation sequencing, revealing stimulation of native sulfate-reducing bacteria. These results highlight the potential of biostimulation for long-term in situ metal(loid) plume treatment/containment.
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•Native sulfate reducing bacteria were biostimulated in the presence of high sulfate levels•Change from iron-oxidizing to strictly anoxic conditions by the addition of organic substrate triggered sulfate reduction•In-situ metal(loids) removal by (co)precipitation as biogenic mineral sulfides was confirmed by stable isotope analysis and mineralogy•Microbial amplicon sequencing and shotgun metagenomics revealed the genus Desulfosporosinus as the major sulfate reducer