Biological Alkylation and Colloid Formation of Selenium in Methanogenic UASB Reactors

Abstract

Bioalkylation and colloid formation of selenium during selenate removal in upflow anaerobic sludge bed (UASB) bioreactors was investigated. The mesophilic (30 degrees C) UASB reactor (pH = 7.0) was operated for 175 d with lactate as electron donor at an organic loading rate of 2 g COD L(-1) d(-1) and a selenium loading rate of 3.16 mg Se L(-1) d(-1). Combining sequential filtration with ion chromatographic analysis for selenium oxyanions and solid phase micro extraction gas chromatography mass spectrometry (SPME-GC-MS) for alkylated selenium compounds allowed to entirely close the selenium mass balance in the liquid phase for most of the UASB operational runtime. Although selenate was removed to more than 98.6% from the liquid phase, a less efficient removal of dissolved selenium was observed due to the presence of dissolved alkylated selenium species (dimethylselenide and dimethyldiselenide) and colloidal selenium particles in the effluent. The alkylated and the colloidal fractions contributed up to 15 and 31%, respectively, to the dissolved selenium concentration. The size fractions of the colloidal dispersion were: 4 to 0.45 mum: up to 21%, 0.45 to 0.2 mum: up to 11%, and particles smaller than 0.2 mum: up to 8%. Particles of 4 to 0.45 mum were formed in the external settler, but did not settle. SEM-EDX analysis showed that microorganisms form these selenium containing colloidal particles extracellularly on their surface. Lowering the temperature by 10 degrees C for 6 h resulted in drastically reduced selenate removal efficiencies (after a delay of 1.5 d), accompanied by the temporary formation of an unknown, soluble, organic selenium species. This study shows that a careful process control is a prerequisite for selenium treatment in UASB bioreactors, as disturbances in the operational conditions induce elevated selenium effluent concentrations by alkylation and colloid formation.

Bioaugmentation of UASB reactors with immobilized Sulfurospirillum barnesii for simultaneous selenate and nitrate removal

“…3c) compared to previous studies (Lenz et al 2008a;Oremland et al 2004) using suspended cultures. As not subject to sheer forces, selenium precipitates formed by a membrane bound enzyme are not sloughed from the cell, and larger particle sizes can thus develop due to the cell immobilization in the gel.…”

Selenium environmental cycling and bioavailability: a structural chemist point of view

“…Several bacteria strains have been identified to reduce toxic selenium oxyanions using lactate as electron donor, and producing elemental selenium nanospheres that accumulated inside and outside the cell wall (Oremland et al 2004). This process has been observed even at very low selenium concentrations (lM) (Lenz et al 2008a). Other researchers have reported the reduction of selenite to intracellular granules of elemental monoclinic selenium, Se(0), by Ralstonia metallidurans CH34 (Sarret et al 2005).…”

Ecology and Biotechnology of Selenium-Respiring Bacteria

“…Removal of SeO 4 2Ϫ in UASB reactors was investigated under methanogenic, denitrifying, or sulfate-reducing conditions (6). Anaerobic granular sludge from a UASB reactor removed SeO 4 2Ϫ from synthetic wastewater under methanogenic (118,119), sulfate-reducing (119,120), and denitrifying (120,121) conditions. SeO 4 2Ϫ was most likely removed by sulfate-reducing bacteria under sulfate-reducing conditions, while enrichment of selenium-respiring bacteria enabled reduction of SeO 4 2Ϫ under methanogenic conditions (118).…”