Biological CH4 production from H2/CO2 streams: Influence of trace metals concentration on the hydrogenotrophic process

Hydrogenotrophic methanogens require trace metals to perform their metabolic activity, the implementation of gas-phase bioreactors targeting methane production from CO2 and H2 streams will therefore require the addition of these trace metals. In the present study, a kinetic study was performed in batch experiments testing six concentration levels of iron, zinc, molybdenum, cobalt, and nickel, which are cofactors of the enzymes involved in the hydrogenotrophic methane production. The occurrence of acetoclastic methanogenesis or homoaceto- genesis were ruled in the present kinetic characterization. The predominant archaea of the culture belonged to the Methanolinea and Methanobacterium genera, with relative abundances of 67.16% and 28.15%, respectively, considering the total archaeal community identified. A mineral salt medium (MSM) with basal trace metals concentration denominated as MSM-1X was used as the reference culture medium in the kinetic studies. Then, 0X, 0.5X, 2X, 10X and 20X concentrations relative the MSM-1X were tested. Experimental CH4 production data were fitted to the modified Gompertz model, yielding 96-99% correlation coefficients. Optimum concentrations of 1.479, 0.063, 0.011, 0.032, and 0.061 mg gVSS-1 of Fe, Zn, Mo, Co, and Ni, respectively, were found. The Haldane-Andrews kinetic model showed that MSM provided with trace metals concentrations 10 and 20 times higher compared with the basal MSM-1X led to an inhibited performance in terms of (i) the duration of the lag phase, (ii) the maximum methane production rate, and (iii) the maximum volume of methane produced.