Rederiving kinetics to model biohydrogen production from immobilized microalgae alginate beads at various polymerization degrees of alginate under dark fermentative environment

The performance of immobilized microalgae-alginate beads on biohydrogen production and its stability across several dark fermentation cycles is influenced by the sodium alginate concentrations. Thus, it is vital to determine the optimal condition for immobilization to achieve maximum encapsulation efficiency, stability and biohydrogen production. In this work, different sodium alginate concentrations (2 to 8 w/v%) were used to immobilize microalgae in influencing the dark fermentative biohydrogen productions from municipal wastewater, and its reusability was also investigated. The immobilized microalgae-alginate beads with sodium alginate concentration of 2 % showed the lowest stability and biohydrogen production in all cycles. The highest biohydrogen production was achieved by immobilized microalgal-alginate beads prepared from 8 % sodium alginate, followed by 6 % and 4 % in the first and second cycles. However, 8 % of immobilized microalgae-alginate beads generated the lowest biohydrogen volume during the third cycle. Besides, a model was rederived from the modified Gompertz model and Fick's law of diffusion equation to describe the relationship between polymeric viscosity and biohydrogen production from immobilized microalgae-alginate beads with different sodium alginate concentrations. As the sodium alginate concentrations increased, the viscosity also increased which significantly affected the properties of immobilization matrix formed such as encapsulation efficiency, growth of microalgae, diffusion of substrate and biohydrogen yield. The rederived model managed to fit the experimental data with a coefficient of determination values of >0.95 for all the polymerization degrees of alginate. The kinetic parameters, namely, yield of biohydrogen and specific microalgae growth rate of sodium alginate concentration of 6 % were 129.80 L kg⁻¹ and 4.69 h⁻¹, respectively, which were considered as maximum results as compared with other sodium alginate concentrations. Besides, the difference in biohydrogen productions for all cycles and kinetic data of biohydrogen yields obtained from the rederived model between sodium alginate concentration of 6 % and 8 % only exhibited a slight variance (<3 %). Thus, sodium alginate concentration of 6 % was considered to be an optimal for immobilizing microalgae in performing the dark fermentation. Overall, the results revealed the significance of suitable sodium alginate concentration in maximizing the immobilization of microalgae for performing the dark fermentative biohydrogen production.