Interaction between magnetite and inoculum characteristics in accelerating methane production kinetics

Magnetite nanoparticles can boost methane production via direct interspecies electron transfer. However, the combined effect of inoculum and particle characteristics on magnetite's methanogenesis stimulation is poorly understood. Here, the influence of inoculum type, particle size, and particle concentration on the ability of magnetite to accelerate methanogenesis was studied in batch anaerobic digestion experiments. Fresh and degassed mesophilic digester sludge was used as inoculum, representing methanogenic communities in the exponential or stationary growth and endogenous decay phases, respectively. Three magnetite particle size ranges, small (50–150 nm), medium (168–490 nm), and large (800 nm–4.5 μm), at two different concentrations (2 and 7 mM) were used. With degassed sludge, the effect of magnetite on the methane production rate was weak and depended on the particle size and concentration. Only magnetite of medium size at both 2 and 7 mM significantly increased the methane production rate by 12% compared to the control with no magnetite. The lag phase was reduced by 17% compared to the control, only with 2 mM of both small and medium size magnetite. Conversely, adding magnetite into fresh sludge significantly increased the methane production rate by an average of 32% while simultaneously decreasing the lag phase by 15%–40%, as compared to the control, independently of the magnetite's size and concentration. The stimulation of methane production depends on magnetite and inoculum characteristics.