Bioavailability and chemical forms of Co and Ni in the biogasprocess : an evaluation based on sequential and acid volatile sulfide extractions

• Several previous studies report stimulatory effects on biogas process performance after trace element supplementation. However, the regulation of the bioavailability in relation to chemical speciation (e.g. the role of sulfide) is not fully understood. The objective of the present study was to determine the effect of sulfide on the chemical speciation and bioavailability of Co and Ni in lab-scale semi-continuously fed biogas tank reactors, digesting grain stillage. The chemical forms and potential bioavailability of Co and Ni in the reactors were determined by sequential extraction (SE), and analysis of acid volatile sulfide (AVS) together with simultaneously extracted metals (AVS-Me). The results for metal speciation analysis demonstrated that Ni was completely associated to the organic  matter/sulfide fraction and AVS, suggesting low potential Ni-bioavailability. Cobalt was predominantly associated to organic matter/sulfide and AVS, but also to more soluble fractions which are considered to be more bioavailable. Process performance data showed that both Co and Ni were available for microbial uptake. Although the actual bioavailability of Co could be explained by association to more bioavailable chemical fractions as determined by SE, AVS and AVS-Me analysis, the complete association of Ni with organic matter/sulfides and AVS shows that Ni was taken up despite its expected low bioavailability. Thus, the results of the present study imply that Ni-sulfide precipitation does not prevent microbial uptake in the studied biogas reactors.

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