Comparative evaluation of metal oxide nanoparticles (Mn3O4, NiO, Mn-doped NiO, and SnO2) on biogas and methane production from cow dung

• SnO 2 nanoparticles increased biogas yield 1.87-fold over control. • Smaller nanoparticle size resulted in higher catalytic activity. • EDS and ICPMS confirmed nanoparticle stability and retention in digestate. • Metal oxide NPs enhance sustainable biogas production efficiency. The impacts of metal oxide nanoparticles (NPs) on biogas and methane (CH 4 ) production were investigated by anaerobic digestion (AD) experiments using cow dung as the raw material. This study aims to enhance the biogas production using metal oxide NPs and a comparative analysis of their effects on it. Spherical Mn 3 O 4 , NiO, Mn-doped NiO, and SnO 2 NPs, with sizes of 58, 27, 26, and 8 nm, respectively, were synthesized and integrated into the AD process to accelerate slurry breakdown and stimulate methanogenic activity, leading to increased biogas and CH 4 production over a 60-day hydraulic retention time (HRT). The addition of the Mn 3 O 4 , NiO, Mn-doped NiO, and SnO 2 NPs to the AD process significantly increased the biogas volume by 1.47, 1.77, 1.76, and 1.87 times more than the control (p < 0.05), respectively. Correspondingly, CH 4 volume was amplified by factors of 1.61, 2.00, 1.97, and 2.15 (p < 0.05) from the control. Notably, SnO 2 NPs exhibited the highest efficacy, achieving the maximum specific biogas and CH 4 production (p < 0.05), with 517.93 mL biogas per g volatile solids (VS) and 334.73 mL CH 4 per g VS in comparison to the control, which produced only 277.53 mL biogas per g VS and 155.73 mL CH 4 per g VS. Moreover, the existence of NPs after completing the AD process was confirmed by analyzing the elemental composition of the post-residue of each treatment. Post-digestion analysis confirmed NPs retention in the solid residue, underscoring their stability within the system. These findings highlight the catalytic potential of metal oxide NPs in optimizing AD and CH 4 generation.