Opinion
The lower H2 production yield by organism to the theoretical values of 4 mol/mol of hexose and high cost of the feed are the major limiting factors for the large scale production under dark-fermentative conditions [9, 31]. Therefore, to enhance the H2 production, various strategies has been adapted to overcome these problems, including optimization of process parameters, screening of potential H2 producers and use of low cost feed such as biowaste substrate [15, 16, 34]. The hydrogenases are key enzymes involved in the BHP and their activity was significantly influenced with the Fe2? and Ni2? metal ions. Recently, the NPs forms of these metals along with others including, activated carbon, Ag, Au, CNTs, Pd, Si and Ti also resulted in the profound effect of up to 6.7-fold on the H2 production yield [27, 29, 45, 48, 49, 51, 55, 58, 62]. Here, the enhancement in the H2 production is associated with NPs concentration and their properties. In the contrast, CuNPs exhibited the negative effect on the H2 production yield at lower concentration of 2.5 mg/l [30]. The variable influences of NPs or their mixtures on the BHP was observed using pure and mixed culture from both sugars and biowastes as a feed. These NPs mostly enhance the H2 production yield through their significant positive effects on the organism growth, feed degradation efficiency and intermediate metabolites profile. Primarily, in the presence of NPs H2 producers shift intermediate metabolites towards higher ratio of acetate to butyrate and inhibition in the production of ethanol and propionate production.
