Biogas production and metal passivation analysis during anaerobic digestion of pig manure: effects of a magnetic Fe3O4/FA composite supplement

Abstract

Anaerobic digestion has been widely used to produce biogas renewable energy and stabilize fecal manure. In this work, magnetic fly ash composites (Fe₃O₄/FA) were synthesized and mixed with pig manure in different ratios to study their effects on biogas production and metal passivation during anaerobic digestion. The results showed that the use of 0.5% Fe₃O₄/FA presented the most positive impact on biogas production compared to anaerobic digestion without Fe₃O₄/FA, i.e., the total biogas and methane content increased by 13.81% and 35.13%, respectively. Variations in the concentration and speciation of heavy metals (i.e., Cu and Zn) with and without Fe₃O₄/FA during anaerobic digestion were also analyzed. The concentrations of Cu and Zn increased after anaerobic digestion, showing a significant “relative concentration effect”. Additionally, sequential fractionation suggested that Cu was mainly present in organic matter, whereas Zn was mainly distributed in the oxidation states of iron and manganese. The addition of Fe₃O₄/FA enhanced the passivation of Cu and Zn in the solid digested residues, i.e., the residual states of Cu and Zn increased by 10.73% to 45.78% and 33.49% to 42.14% compared to the control, respectively. Moreover, better performance was found for the treatment with 2.5% Fe₃O₄/FA. X-ray diffraction (XRD) and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX) analysis demonstrated that Fe₃O₄/FA deactivated heavy metals mainly via physical adsorption during anaerobic digestion, which can convert them into stable mineral precipitates and thus decrease the solubility and mobility of these metals.