Upgrading continuous H₂ gas recovery from rice straw hydrolysate via fermentation process amended with magnetite nanoparticles

Continuous H₂ production from alkali-hydrolyzed rice straw in two mesophillic anaerobic baffled reactors (ABRs) was investigated in parallel. ABR₁ was inoculated with thermally pretreated sludge while the sludge in ABR₂ was amended with magnetite nanoparticles (MNPs). Both reactors were operated at organic loading rates (OLRs) ranged from 0.8 to 4.8 g_COD/L/day. The addition of MNPs significantly improved the H₂ production and yield, ie, maximum H₂ yield was 21.4 ± 3.2 and 60.6 ± 5.7 mL/g COD_removed for both ABR₁ and ABR₂, respectively, at OLR of 1.6 g_COD/L/day. Likewise, substrate degradation efficiency (SDE) was augmented from 48.4 ± 3.9% (ABR₁) to 55.8 ± 4.2% (ABR₂). The enhancement effect of MNPs can be emphasized by hydrogenase enzyme activity increase from 0.097 ± 0.013 to 0.205 ± 0.019 mg MB_reduced/min at ABR₁ and ABR₂, respectively. Besides, ABR₂ witnessed higher concentrations of acetate (HAc) and butyrate (HBu) coupled with lower values of propionate (HPr). Additionally, HAc/HBu ratio for ABR₂ (3.03 ± 0.31) was higher than that for ABR₁ (1.95 ± 0.29). Microbial analysis indicated that Bacillus and Clostridium species became dominant in mixed culture bacteria supplemented with MNPs. Furthermore, economic analysis revealed that the minimum payback period of 10.5 years at OLR of 3.2 g_COD/L/day was recorded for ABR₂ compared with 14.6 years at OLR of 4.8 g_COD/L/day for ABR₁.