Produção de hidrogênio por fotofermentação empregando diodos emissores de luz visível e infravermelha próxima

Abstract

Biohydrogen is considered the versatile fuel of the future, with the potential to replace fossil fuels in several industrial and commercial sectors. Among the options available for biohydrogen generation, photofermentation is one of the most attractive approaches due to its versatility in using raw materials, including industrial and agricultural residues, cost-effectiveness, and low environmental impact. However, efficient use of light in photofermentative hydrogen production remains a significant challenge to make this promising biological process a viable industrial technology. In the current work, biohydrogen production was evaluated through photofermentation using light-emitting diodes (LEDs) in different ranges of the electromagnetic spectrum (visible and near-infrared), employing the photosynthetic bacteria Rhodobacter capsulatus and Rhodospirillum rubrum. The carbon sources used were glucose and glycerol of analytical grade, both at a concentration of 10 g/L. Illumination in the visible spectrum was provided by cold white LEDs and in the near-infrared spectrum (NIR) by LEDs at a wavelength of 850 nm. The experiments were conducted in 50 mL reactors (small-scale) and a 1.5 L stirred-tank reactor (large-scale). In the small-scale experiments, promising results were obtained with the photosynthetic bacterium R. rubrum inoculated in a medium containing glucose as a substrate and under simultaneous illumination of the LED system - cool white and NIR light. The maximum values for productivity and conversion were 39.67 mmol H2/L.d and 9.46 mol H2/mol of glucose consumed, respectively. Productivity increased by over 3,000% compared to the illumination provided exclusively by NIR LED. Furthermore, the obtained conversion was highly efficient, corresponding to approximately 80% of the theoretical maximum conversion. When glycerol was used as a substrate, there was a substantial drop in results. However, biohydrogen production proved feasible, particularly standing out when conducted with the R. rubrum strain under NIR illumination. The maximum productivity obtained was 5.71 ± 0,3 mmol H2/L.d, with a conversion of 3.14 ± 0,08 mol H2/mol of glycerol consumed. On an enlarged scale, the effect of the initial cell concentration (1 and 0.5 g/L) was evaluated, replicating the lighting and substrate conditions of the assay that showed the best performance on a reduced scale. In this configuration, the highest hydrogen production was promoted by the bacterium R. rubrum at a concentration of 1 g/L, reaching a maximum productivity value of 78.36 mmol H₂/L.d. With an initial concentration of 0.5 g/L, the maximum productivities obtained were 4.17 and 31.8 mmol H₂/L.d for the bacteria R. rubrum and R. capsulatus, respectively.