Regulatory function of divalent cations in controlling the acidogenic biohydrogen production process

Abstract

The functional role of divalent cations viz., iron (Fe²⁺), magnesium (Mg²⁺), nickel (Ni²⁺), zinc (Zn²⁺) and manganese (Mn²⁺) in regulating the biohydrogen (H₂) production potential of a biocatalyst was evaluated under varying concentrations. The optimum conditions were evaluated in detail and compared with the principle conditions for enhanced conversion efficiency. Higher concentration of Fe²⁺ showed an enhanced H₂ production efficiency of the biocatalyst (61.94 mmol) due to its role as a component of hydrogenase and ferredoxin. On the contrary, a higher concentration of Mg⁺² showed a higher substrate utilization capability (76.46%) of the biocatalyst. The optimum metal concentration showed a higher H₂ yield (18.23 mol kg⁻¹ COD_R; 3.5 mol mol⁻¹ glucose) over principle operation conditions (14.76 mol kg⁻¹ COD_R; 2.84 mol mol⁻¹ glucose). Higher concentrations of acetate (78 ± 6%) as the metabolic intermediate over butyrate (12 ± 2%) supported the observed higher H₂ yields. Higher reductive catalytic current over oxidative current along with the enhanced dehydrogenase activity supported the higher H₂ production efficiency. Shift in the redox Tafel slope was witnessed near zero in the case of Fe²⁺ and optimum conditions indicating the simultaneous oxidation and reduction reactions facilitating the availability of higher number of protons through metabolite inter-conversions that can make H₂. The presence of divalent cations at an optimum concentration will enhance the H₂ production capability of the biocatalyst. However, elevated levels of metal concentration showed a negative influence on the biocatalyst efficiency.