Enhancing phototropic hydrogen production by solid-carrier assisted fermentation and internal optical-fiber illumination

Three strategies were applied to promote the phototrphic H₂ production of an indigenous purple nonsulfur bacterium Rhodopseudomonas palustris WP3-5 using acetate as the sole carbon substrate. First, a small amount of solid carriers (e.g., activated carbon, silica gel, and clay) was supplemented to fermentation broth to stimulate cell growth and H₂ production. Second, the acetate concentration leading to optimal production of H₂ was identified. Finally, an innovative optical-fiber illuminating system was designed to facilitate the efficiency of the photobioreactor. The results show that addition of clay and silica gel was effective in promoting H₂ production, resulting in 67.2-50.9% and 37.2-32.5% increases in H₂ production rate (v_H2) and H₂ yield (Y_H2), respectively. For clay-supplemented batch cultures, the optimal acetate concentration was 1000 mg COD/l, leading to a v_H2 and Y_H2 value of 28.5 ml/h/l and 2.97 mol H₂/mol acetate, respectively. Moreover, combination of internal optical-fiber illumination system, clay addition, and optimal acetate concentration further elevated the v_H2 and Y_H2 to a maximum level of 43.8 ml/h/l and 3.63 mol H₂/mol acetate, respectively. These values are considerably higher than most reported results from relevant studies. Meanwhile, the results of continuous cultures operated at 36 h HRT (hydraulic retention time) show that the high phototrophic H₂ production efficiency was stably maintained for over 17 days with a steady-state v_H2 and Y_H2 of 44.0 ml/h/l and 3.57 mol H₂/mol acetate, respectively.