Abstract
Recovery of selenium (Se) from wastewater provides a solution for both securing Se supply and preventing Se pollution. Here, we developed a high-rate process for biological selenate reduction to elemental selenium. Distinctive from other studies, we aimed for a process with selenate as the main biological electron sink, with minimal formation of methane or sulfide. A sequencing batch reactor, fed with an influent containing 120 mgSe L−1 selenate and ethanol as electron donor and carbon source, was operated for 495 days. The high rates (419 ± 17 mgSe L−1 day−1) were recorded between day 446 and day 495 for a hydraulic retention time of 6 h. The maximum conversion efficiency of selenate amounted to 96% with a volumetric conversion rate of 444 mgSe L−1 day−1, which is 6 times higher than the rates reported in the literature thus far. At the end of the experiment, a highly enriched selenate reducing biomass had developed, with a specific activity of 856 ± 26 mgSe−1day−1gbiomass−1, which was nearly 1000-fold higher than that of the inoculum. No evidence was found for the formation of methane, sulfide, or volatile reduced selenium compounds like dimethyl-selenide or H2Se, revealing a high selectivity. Ethanol was incompletely oxidized to acetate. The produced elemental selenium partially accumulated in the reactor as pure (≥80% Se of the total mixture of biomass sludge flocs and flaky aggregates, and ~100% of the specific flaky aggregates) selenium black hexagonal needles, with cluster sizes between 20 and 200 µm. The new process may serve as the basis for a high-rate technology to remove and recover pure selenium from wastewater or process streams with high selectivity.
Highlights
Anthropogenic activities have dramatically increased selenium emissions, especially in the mining and metallurgical industries (Etteieb et al, 2020)
This study presented a high-rate process for the recovery of selenium based on biological selenate reduction to elemental selenium without the formation of highly toxic hydrogen selenide or organic selenium species
Pure crystalline hexagonal Se0 was identified as selenium solid accumulating in the reactor in clusters and was easy to separate
Summary
Anthropogenic activities have dramatically increased selenium emissions, especially in the mining and metallurgical industries (Etteieb et al, 2020). Selenium (Se) is an essential micronutrient for animals and humans, it is toxic at slightly higher intake levels than metabolically needed (Rayman, 2012; Ullah et al, 2018). A promising method to eliminate selenate from industrial effluents is the microbiological reduction to elemental selenium (Se0) under anaerobic conditions. Technologies based on this concept can successfully remove selenate down to low ppb levels (Lenz et al, 2008a; Staicu and Barton, 2017; Tan, 2018).
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