Search

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-S 0 ) and of sulfate. Selenite and selenate (~2 mmol L -1 ) bio-reduction was studied in batch up to 28 days at 30  o C and pH 7 using lactic acid and a sulfate-reducing sludge, 'Emmtec'. Bio-S 0 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-S 0 or sulfate promoted hexagonal selenium formation, whereas without these, mostly amorphous Se 0 resulted. With another inoculum, 'Eerbeek', bio-S 0 accelerated the selenite reduction rate less than for 'Emmtec' because of lower sulfur and higher selenite bio-reduction rates. Bio-S 0 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 Se 0 . Insights into the interaction between selenium and sulfur bio-reduction are valuable for understanding environmental pathways and considerations regarding remediation and recovery., (Copyright © 2022 The Authors. Published by Elsevier B.V. All rights reserved.)

Microbial reduction of selenium sulfide (SeS 2 ) 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 SeS 2 . 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 SeS 2 . Emmtec sludge had the highest reducing capacity with commercial SeS 2 and was therefore also used for the bioreduction of laboratory synthesized amorphous SeS 2 . Synthesized SeS 2 was formed mixing a sulfide solution and effluent containing selenite. With both SeS 2 solids (commercial and synthesized SeS 2 ), 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., (Copyright © 2017 Elsevier B.V. All rights reserved.)

Reduction of the amount of waste sludge from waste water treatment plants (WWTPs) can be achieved with the aquatic worm Lumbriculus variegatus in a new reactor concept. In addition to reducing the amount of waste sludge, further processing of produced worm faeces and released nutrients should also be considered. This study gives the mass balances for sludge consumed by L. variegatus, showing the fate of the consumed organic material, nutrients and heavy metals associated with the sludge. A distinction is made between conversion into worm biomass, release as dissolved metabolites and what remains in the worm faeces. The results showed that 39% of the nitrogen and 12% of the phosphorus in the sludge digested by the worms are used in the formation of new worm biomass, which has potential for reuse. Experiments showed that settling of the worm faeces leads to a factor 2.5 higher solids concentration, compared to settling of waste sludge. This could lead to a 67% reduction of the volumetric load on thickening equipment. The worm reactor is expected to be most interesting for smaller WWTPs where a decrease on the volumetric load on sludge handling operations will have most impact., (Copyright (c) 2009 Elsevier B.V. All rights reserved.)