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TECHNICAL REPORTS

Bioremediation and Biodegradation

Biological Alkylation and Colloid Formation of Selenium in Methanogenic UASB Reactors

^a Sub-Department of Environmental Technology, Wageningen Univ., Bomenweg 2, 6700 EV Wageningen, The Netherlands
^b Institute for Ecopreneurship, Univ. of Applied Sciences Northwestern Switzerland (FHNW), Gründenstrasse 40, 4132 Muttenz, Switzerland
^c Lab. of Plant Cell Biology, Wageningen Univ., Arboretumlaan 4, 6703 BD Wageningen, The Netherlands
^d UNESCO-IHE Inst. for Water Education, Westvest 7, 2611 AX Delft, The Netherlands

^* Corresponding author (Piet.Lens{at}wur.nl).

Received for publication December 3, 2007. Bioalkylation and colloid formation of selenium during selenate removal in upflow anaerobic sludge bed (UASB) bioreactors was investigated. The mesophilic (30°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 µm: up to 21%, 0.45 to 0.2 µm: up to 11%, and particles smaller than 0.2 µm: up to 8%. Particles of 4 to 0.45 µm 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°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.

Abbreviations: COD, chemical oxygen demand • DMSe, dimethylselenide (CAS 593–79–3) • DMDSe, dimethyldiselenide (CAS 7101–31–7) • DMDS, dimethyldisulfide (CAS 624–92–0) • DMSeO₂, dimethylselenone (selenonylbismethane, CAS 22089–69–6) • DMSeS, dimethylselenenylsulfide (methanesulfenoselenoic acid methyl ester, CAS 41884–42–8) • HRT, hydraulic retention time • OLR, organic loading rate • Se_dis, total selenium dissolved • Se_in, total selenium in influent • Se_undet, selenium undetermined • UASB, upflow anaerobic sludge bed • VFA, volatile fatty acids • VSS, volatile suspended solids • v_up, superficial liquid upflow velocity