Inhibitory Effects of the Divalent Metal Ions on Biomethanation by Isolated Mesophilic Methanogen in AC21 Medium in Presence or Absence of Juices from Water Hyacinth

Abstract

The effect of divalent metal ions on biomethanation was studied using pre-reduced methanogenic AC21 medium in absence or in presence of leaf or root extracts of Eichornia grassipes (water hyacinth) in vitro. The metal ions like Cu2+, Fe2+, Ni2+, and Co2+ were found to be effective in increasing methane production in vitro at low concentrations, but toxic inhibitory effects were observed at relatively high concentrations. Zn2+ ion was inhibitory in character with exceptions. The bioavailability of metal ions in AC21 medium alone or with juices from leaf or root extracts of plant (water hyacinth in the present case) was limited due to complex formation of metal ions with different constituents in the medium, precipitation, etc. However, the results of the microscopic experiments (in vials) with metal ions for methane production by isolated methanogens in the methanogenic medium should not be compared with the conversion of biomass to methane in biodigesters. In such cases, not only a large number of methanogenic bacteria of different strains but also a consortium of bacteria of different specificities is usually involved. Though the biomass may contain sufficient concentrations of metal ions, it is advisable to add metal ions like Cu2+, Fe2+ , Zn 2+ ,Co2+, etc. in the biomass, but the concentrations of the metal ions should be limited to 10–20 µM of the biomass to make methane generation successful.