Effect of La₂ O₃ and CeO₂ on anaerobic methanogenesis process: Kinetics analysis and process mechanism study

La₂ O₃ and CeO₂ , as main rare earth oxides, with unique physical and chemical properties have been widely used in catalyst and grinding industry. In this study, the effects of La₂ O₃ and CeO₂ on the anaerobic process were investigated. The biological methane production tests showed that 0-0.05 g/L La₂ O₃ and 0-0.05 g/L CeO₂ enhanced anaerobic methanogenesis process. The result showed maximum specific methanogenic rates of La₂ O₃ and CeO₂ were 56.26 mL/(h·gVSS) and 49.43 mL/(h·gVSS) and, compared with the control, increased 4% and 3%, respectively. La₂ O₃ significantly reduced the accumulation of volatile fatty acids (VFAs), whereas CeO₂ had no similar effect. Dissolution experiments demonstrated that the content of extracellular La in the anaerobic granular sludge reached 404 μg-La/g volatile suspended solid (VSS), which was 134 times higher than that of extracellular Ce (3 μg-Ce/gVSS). The content of intracellular La reached 206 μg-La/gVSS, which was 19 times higher than that of intracellular Ce (11 μg-Ce/gVSS). The different stimulation between La³⁺ and Ce³⁺ could be attributed to the different dissolution of La₂ O₃ and CeO₂ . The result of this work is helpful to optimize anaerobic processes and to develop novel additives. PRACTITIONER POINTS: Novel anaerobic additives were developed. La2O3 and CeO2 in 0-0.05 g/L enhanced organics degradation and methane production. The addition of La2O3 significantly reduced the accumulation of volatile fatty acids. The solubilization of La2O3 was stronger than CeO2. The promoting effects of low concentrations of La2O3 and CeO2 were derived from dissolved La and Ce.