Master Thesis

Transformation of metal(loid) species during bio gas production

Johannes Besold (10/2013-02/2015)

Support: Britta Planer-Friedrich, Frank Thomas

Metal(loid)s enter biogas plants by raw material and fermentation additives and can be accumulated in fermenters. Because of the anaerobic milieu in biogas plants Methanoarchaea are able to methylate and volatize metal(loid)s. Extent and possible consequences on methane production are yet unknown. The aim of this study was to analyze the amounts of metal(loid)s in the solid, liquid and gaseous phase of three biogas plants around the city of Bayreuth and to study the influence of arsenic and sulfide upon the methane production under controlled laboratory conditions with mini biogas reactors. Concentrations of metal(loid)s in the solid phase of fermentation sludges differed from few mg/g dry matter (Al, Fe) via single digit µg/g concentration range (Ni, Mo, Pb) to the range of 100s (Se, As) and 10s (Sb, Bi, Te) ng/g. Depending on the plant, different raw materials were used for biogas production among others grain, grass and corn silage, cattle dung and cattle slurry as well as primary sludge and thickened sludge from municipal wastewater treatment. Furthermore, fermentation additives (Se, Co, Mo, Ni) and precipitating agents (Fe, Al) were added depending on the biogas plant. Metal(loid)s accumulated in every biogas plant whereas concentrations varied strongly between a factor of 1,06 for Cu (cattle slurry) and 102 for Co (grain), dependent on element, raw material and biogas plant. Besides the expected soluble arsenic compounds arsenite (4,2 %) and arsenate (21 %) in the fermenter of one biogas plant (12 µg/L As), soluble methylated (4,3 %) and for the first time thiolated (1,9 %) and methylthiolated (21 %) arsenic species could be detected. The volatile arsenic species (CH3)AsH2 and (CH3)2AsH were detected in the ng/m³ range. Methodic problems with NaOCl traps prevented quantification of the volatile metal(loid)s Se, As, Sb, Te, Pb and Bi. The adjustment of 10-25 µmol/L arsenic in the mini biogas reactors reduced the production of methane between 37 and 61 % (no methane reduction at 1 µmol/L). In the presence of 25,5-510 µmol/L sulfide and 25 µmol/L arsenic, soluble methylated arsenic compounds and methane production tended to increase with adjusted sulfide concentration whereas formation of the volatile arsenic species (CH3)AsH2 and (CH3)2AsH decreased. On the one hand, these findings showed the strong relation between methanogenesis and arsenic methylation. On the other hand, they illustrated how sulfide counteracts the toxic influence of arsenite through precipitation of As-S minerals, adsorption of arsenic on organic matter with sulfide bridges and through the formation of lesser toxic monothioarsenate. It remained unclear why the formation of volatile As species decreased with increasing adjusted sulfide concentration. The occurrence of the same arsenic species in real biogas plants as well as in the laboratory experiments showed that under real conditions the same formation processes of soluble and volatile methylated arsenic species took place, even though total soluble arsenic concentrations in real biogas plants were around 160 times lower than in the laboratory experiments.