How sulfur species can accelerate the biological immobilization of the toxic selenium oxyanions and promote stable hexagonal Se0 formation.

Toxic selenium oxyanions and sulfur species are often jointly present in contaminated waters and soils. This study investigated the effect on kinetics and resulting products for bio-reduction of selenium oxyanions in the presence of biologically produced sulfur resulting from bio-oxidation of sulfide in (bio)gas-desulfurization (bio-S0) and of sulfate. Selenite and selenate (~2 mmol L-1) bio-reduction was studied in batch up to 28 days at 30 oC and pH 7 using lactic acid and a sulfate-reducing sludge, 'Emmtec'. Bio-S0 addition increased the selenite removal rate, but initially slightly decreased selenate reduction rates. Selenite reacted with biologically generated sulfide resulting in selenium-sulfur, which upon further bio-reduction creates a sulfur bio-reduction cycle. Sulfate addition increased the bio-reduction rate for both selenite and sulfate. Bio-S0 or sulfate promoted hexagonal selenium formation, whereas without these, mostly amorphous Se0 resulted. With another inoculum, 'Eerbeek', bio-S0 accelerated the selenite reduction rate less than for 'Emmtec' because of lower sulfur and higher selenite bio-reduction rates. Bio-S0 addition increased the selenate reduction rate slightly and accelerated hexagonal selenium formation. Hexagonal selenium formation is advantageous because it facilitates separation and recovery and is less mobile and toxic than amorphous Se0. Insights into the interaction between selenium and sulfur bio-reduction are valuable for understanding environmental pathways and considerations regarding remediation and recovery.

High-rate biological selenate reduction in a sequencing batch reactor for recovery of hexagonal selenium.

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.

Simultaneous recovery of calcium phosphate granules and methane in anaerobic treatment of black water: Effect of bicarbonate and calcium fluctuations.

Calcium phosphate (CaP) granules were discovered in the anaerobic treatment of vacuum collected black water (BW), using upflow anaerobic sludge blanket (UASB) technology. This allows simultaneous recovery of CaP granules and methane in the UASB reactor. However, the role of BW composition on CaP granulation is not yet understood. Moreover, CaP granulation was not observed in previous research on anaerobic treatment of BW, although similar treatment conditions were applied. Therefore, this study shows specifically the influence of bicarbonate and calcium fluctuations in BW on the phosphorus accumulation in the UASB reactor, which directly affects CaP granulation. Without calcium addition, 5% of the total phosphorus (P) fed was found as CaP granules in the reactor (61 mgP g-1dried matter), after 260 days of operation. Simultaneously, 65% of the COD in BW was efficiently converted into methane at 25 °C. Variations of bicarbonate and calcium concentrations in raw BW showed a significant influence on phosphorus accumulation in the UASB reactor. Geochemical modelling showed that the increase of soluble calcium from 39 to 54 mg L-1 in BW triggers supersaturation for calcium phosphate precursors (Cax(PO4)y). Concurrently, bicarbonate decreased from 2.7 to 1.2 g L-1, increasing further the ionic activity of calcium. Formation and accumulation of seed particles possibly enhanced CaP granulation. Preliminary results showed that addition of calcium (Ca2+/PO43- molar ratio of 3) increased the accumulation of total P in the UASB reactor to more than 85%. This further increases the granulation rate and consequently, the process feasibility.

Microbial selenium sulfide reduction for selenium recovery from wastewater.

Microbial reduction of selenium sulfide (SeS2) is a key step in a new treatment process to recover selenium from selenate and selenite streams. In this process, selenate is first reduced to selenite, and subsequently selenite is reduced by sulfide and precipitates from the solution as SeS2. The latter is bio-reduced to elemental selenium and sulfide. Two anaerobic granular sludges (Eerbeek and Emmtec) were tested for their efficiency to reduce commercial crystalline SeS2. Emmtec sludge had the highest reducing capacity with commercial SeS2 and was therefore also used for the bioreduction of laboratory synthesized amorphous SeS2. Synthesized SeS2 was formed mixing a sulfide solution and effluent containing selenite. With both SeS2 solids (commercial and synthesized SeS2), Emmtec sludge produced sulfide and a solid consisting of hexagonal elemental selenium. The crystalline hexagonal structure suggests the absence of biomolecules, which stabilize amorphous selenium bio-particles under comparable process conditions (T=30°C and a pH between 6 and 7). Selenium particles were not attached to the biomass, suggesting an extracellular formation. The results support the feasibility of the bio-reduction process using sulfur for recovering selenium from water.