Effect of different charged Fe3O4 nanoparticles on methane production for anaerobic digestion of wheat straw

Anaerobic digestion (AD) is an effective measure to deal with organic waste, and biogas can be recovered from waste as clean energy. China is mainly an agricultural country, and a large amount of wheat straw is produced every year, which needs to be treated innocuously and resourcefully. In this context, the purpose of this study was to evaluate the effect of exogenous conductive material Fe3O4 nanoparticles on AD of wheat straw and to further evaluate the effect of the charge of nanoparticles on AD. The effects of different charged Fe3O4 nanoparticles on AD of wheat straw were studied. Fe3O4 nanoparticles with positive, negative, and no charge were successfully prepared and used them in the AD of wheat straw. The experiments were carried out in a 500-mL glass bottle with an effective volume of 350 mL and a total solid (TS) concentration of 6%. The results showed that Fe3O4 nanoparticles with positive, negative, and no charge could increase the cumulative methane production of AD, and the pure ligands used to modify Fe3O4 nanoparticles had no considerable effect on AD of wheat straw. The negatively charged nanoparticles (NNPs) had the greatest positive effects on the AD of wheat straw, which was 51.33% higher than that of the blank control group and reached 333.14 mL CH4/g·TS. The peak concentration of volatile fatty acids (VFAs) in the negative experiment was 31.65% higher than that of the blank control group at the beginning of the experiment and 30.57% lower than that of the blank control group at the end of the experiment. The cyclic voltammetry curve showed that the NNPs improved the redox performance in the AD process, and the maximum value was reflected in the methane production. NNPs can increase the metabolic rate of VFAs and the content of active substances catalyzing biochemical reactions. The charge properties of Fe3O4 nanoparticles play an important role in improving the efficiency of AD, which is of great significance for nano-electron transfer mediators to improve the efficiency of AD. These results show that NNPs have the most significant effect on AD. The addition of this material is helpful in regulating the indices in the process of AD and can be applied to the AD of more substrates.