Iron-coated biochar alleviates acid accumulation and improves methane production under ammonium enrichment conditions

The high stress of ammonia-nitrogen in swine manure anaerobic digestion (SMAD) negatively impacts methane yields. Here, the effects of iron-coated biochar in SMAD under different ammonium stresses were investigated. Iron-coated biochar prepared at 500 °C (500BC@Fe) had a large specific surface area (123.2 cm³/g) and an acceptable ammonium adsorption capacity (5.25 mg/g). In SMAD, 500BC@Fe addition effectively broke the thermodynamic barrier from butyrate to acetate and accelerated propionate degradation. It acted as a temporary electron acceptor to promote direct interspecies electron transfer in the initial SMAD stage. As the ammonium stress sharply increased from 400 mg/L to 4000 mg/L, the methanogenesis efficiency decreased from 94.3% to 94.0% and the biochemical methane potential decreased from 189.7 NmL/g VS to 176.1 NmL/g VS. A kinetic analysis showed that the predictive value of SMAD may be calculated more accurately using the Logistic function than the Modified Gompertz equation. This study provides basic theoretical data and important kinetic parameters for the intensive production of iron-coated biochar and its large-scale application in SMAD.