Chemical speciation of sulfur and metals in biogas reactors – Implications for cobalt and nickel bio-uptake processes

Abstract

A balanced supply of micronutrients, including metals such as iron (Fe), cobalt (Co), and nickel (Ni), is required for the efficient and stable production of biogas. During biogas formation, the uptake of micronutrient metals by microorganisms is controlled by a complex network of biological and chemical reactions, in which reduced sulfur (S) compounds play a central role. This thesis addresses the interrelationship between the overall chemical speciation of S, Fe, Co, and Ni in relation to the metals bio-uptake processes. Laboratory continuous stirred tank biogas reactors (CSTBR) treating S-rich grain stillage, as well as a number full-scale CSTBRs treating sewage sludge and various combinations of organic wastes, termed codigestion, were considered. Sulfur speciation was evaluated using acid volatile sulfide (AVS) extraction and S X-ray absorption near edge structure (XANES). The chemical speciation of Fe, Co, and Ni was evaluated through the determination of aqueous metals and metal fractions pertaining to solid phases, as well as kinetic and thermodynamic analyses (chemical speciation modelling).The relative Fe to S content in biogas reactors, which in practice is regulated through the addition of Fe for the purpose of sulfide removal or prior to the anaerobic digestion of sewage sludge, is identified as a critical factor for the chemical speciation and bio-uptake of metals. In the reactors treating sewage sludge, the quantity of Fe exceeds that of S, inducing Fe(II)-dominated conditions under anaerobic conditions, while sulfide dominates in the co-digestion and laboratory reactors due to an excess of S over Fe. Under sulfide-dominated conditions, chemical speciation of the metals is regulated by hydrogen sulfide and the formation of metal sulfide precipitates, which in turn restrict the availability of metals for microorganisms. However, despite the limitations set by sulfide, aqueous concentrations of different Co and Ni species were shown to be sufficient to support metal acquisition by the microorganisms under sulfidic conditions. Comparatively, the concentrations of free metal ions and labile metalphosphate and -carbonate complexes in aqueous phase, which directly participate in bio-uptake processes, are higher under Fe-dominated conditions. This results in an enhanced metal adsorption on cell surfaces and faster bio-uptake rates. It is therefore suggested that the chemical speciation and potential bioavailability of metals may be controlled through adjustments of the influent Fe concentration in relation to S content. The results also indicated that the pool of metal sulfides in the biogas reactors could be regarded as a source of metals for microbial activities. Thus, the recovery and utilisation of this fraction of metals may be considered as a measure with which to minimise the metal dosing concentrations to CSTBRs.Keywords: Biogas, Anaerobic digestion, Chemical speciation, bio-uptake, Sulfur, Iron, Cobalt, Nickel SammanfattningFör att en effektiv och stabil biogasproduktion från organiskt a...

Enhanced Anaerobic Biogas Production From Wheat Straw by Herbal-Extraction Process Residues Supplementation

“…Sulfide chemistry is the main factor that causes these reactions, which had been claimed as the main regulator of the trace elements in the digesters (Fermoso et al, 2009 ). For instance, the metal sulfide of FeS is well-known to potentially and or co-precipitate Ni and Co, and then form the complexes of Fe-Ni/Co-S (Gustavsson et al, 2013 ; Shakeri Yekta et al, 2016 ).…”

Biogeochemistry of trace elements in anaerobic digesters

“…Free cobalt ion (Co 2+ ) is present in natural and engineered environments in extremely low concentrations, i.e. at the level of pmol L −1 (Yekta et al, 2017 Specifically in anaerobic environments, S 2− concentration controls the speciation of Co (Gustavsson et al, 2013b) and only very strong complexing agents such as EDTA can keep Co in solution .…”

Mathematical modelling of trace element dynamics in anaerobic digestion environments

“…It is widely accepted that metal geochemistry and environmental conditions in an engineered system affect the uptake of TEs by microorganisms (Hudson, 1998;Jansen, 2004;Sunda & Huntsman, 1998;Worms et al, 2006). The bioavailability of metals in the case of AD has been recently studied (Gustavsson, 2012;Jansen, 2004;Yekta et al, 2016). Bioavailability is thought to depend on: (i) the internalization pathway; (ii) specificity of metals to the transporters; (iii) physicochemistry of the bulk phase; (iv) size and nature of microorganisms;…”

“…proteins, humic acids, fulvic acids, yeast extract, EDTA, NTA, EDDS; Section 5.3.3) (Gonzalez-Gil et al, 2003; and species distribution according to the element speciation (e.g. poly hydroxyl species) (Yekta et al, 2016). Alternatively, as a rough estimate of the effective concentration (dose) it would be appropriate to use the soluble fraction of the TE present in the reactor liquid phase.…”

“…However even under this convention, metal complexes with organic macromolecules could still be in non-bioavailable form. Thus, it is of primary concern to determine which chemical species of the TE are actually considered directly bioavailable to enhanced biogas production and system stability (Yekta et al, 2016).…”