Potential of Crystalline and Amorphous Ferric Oxides for Biostimulation of Anaerobic Digestion

Iron oxides have been widely investigated to accelerate the conversion of organic wastes to methane. However, the potential mechanism involved with different types of iron oxides is a controversial topic. In this study, crystalline Fe2O3 and amorphous Fe (OH)3 were respectively supplemented to explore the effects of the crystal form of Fe­(III) on anaerobic digestion. The results showed that the addition of Fe2O3 and Fe­(OH)3 both significantly enhanced COD removal and biomethanation compared with the control. The reason was related to that the supplement of Fe­(OH)3 induced an efficient microbial dissimilatory iron reduction to enhance the decomposition of complex organics into simples. Consistently, 28.3% of Fe­(OH)3 dosed at the initial stage were reduced into Fe­(II), while no obvious iron reduction was observed with the Fe2O3 supplement. Interestingly, Fe2O3 significantly stimulated the secretion of protein and humic acid-like substances in EPS, leading to an electron-transfer capacity higher than that for Fe­(OH)3 and control reactors, which seemed to be an important reason for the improved anaerobic performance. The gene function prediction also showed a different degree of expression of functional genes involved in amino-acid, polysaccharides, and inorganic ion transport and metabolism in the presence of Fe2O3 and Fe­(OH)3. This study helped to give a more comprehensive insight for the mechanisms of ferric oxides on the improvement of anaerobic digestion.