Heavy metal effects on fermentative hydrogen production using natural mixed microflora

The effects of ionic Cr, Cu and Zn on fermentative hydrogen production were examined using serum vial assays. The hydrogen-producing natural mixed microflora (dominated by the Clostridium   species) was cultivated from sewage sludge with sucrose. The relative heavy metal toxicity to fermentative hydrogen production was Zn>Cu>CrZn>Cu>Cr. The microflora hydrogen production activity was reduced by 50% for a biomass that came into contact with 4.5 mg Zn/L, 6.5 mg Cu/L and 60 mg Cr/L. However, low concentrations of 3 mg Cu/L and 15 mg Cr/L resulted in peak 10–20% in hydrogen production stimulation. The threshold concentrations were 4 mg Cu/L and 25 mg Cr/L, over which declining stimulation occurred. Heavy metals affected the hydrogen fermentation in hydrogen production potential, hydrogen production rate, lag-phase time and soluble microbial products; their influences were dependent on metal kind and concentration. Metabolic pathway shift occurred when the metal concentrations varied. Based on the maximum specific hydrogen production rate, the inhibition patterns of Cu and Zn dosages were kinetically competitive with inhibition coefficients of 2.9 and 4.5 mg/L, respectively. The metal-toxicity to the hydrogenesis, acidogenesis and methanogenesis in anaerobic digestion was compared. Strategies based on heavy metal concentration control in the substrate influent for optimal hydrogen production were proposed.