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

Abstract

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 Fe 2 O 3 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 Fe 2 O 3 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 Fe 2 O 3 supplement. Interestingly, Fe 2 O 3 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 Fe 2 O 3 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.

Target-oriented recruitment of Clostridium to promote biohydrogen production by nano-ferrihydrite

“…The same scenario may have occurred in this study. In accordance with this result, supplementation with Fe (OH) 3 could enhance the decomposition of organic substrates [17,53].…”

The use of sludge as a micronutrient for the improvement of biogas production from seaweed: the integration of two sources of environmental concern to bring new opportunities

“…Direct interspecies electron transfer is an intrinsically faster mechanism for electron transfer than interspecies H 2 transfer, 44 and is promoted by semi‐conductive oxides, which might be the reason for the higher methane yield 45–47 . Direct interspecies electron transfer affects iron‐reducing bacteria (IRB) 48 .…”

“…It has been proved that EPS had more noticeable stimulatory effects on DIET processes after addition of Fe 2 O 3 52 . The degradation of organic matter increases because of enhancement in electron transfer rate from secondary fermenting bacteria to methanogens 46 . However, if Fe accumulation in the cells increases and exceeds tolerated levels, it may kill the cell 53…”

“…Direct interspecies electron transfer is an intrinsically faster mechanism for electron transfer than interspecies H 2 transfer, 44 and is promoted by semi-conductive oxides, which might be the reason for the higher methane yield. [45][46][47] Direct interspecies electron transfer affects iron-reducing bacteria (IRB). 48 Despite not having direct involvement in anaerobic transformation pathway and not being essential microorganisms in AD, IRBs can improve AD performance remarkably by using surplus H 2 and acetate.…”

“…52 The degradation of organic matter increases because of enhancement in electron transfer rate from secondary fermenting bacteria to methanogens. 46 However, if Fe accumulation in the cells increases and exceeds tolerated levels, it may kill the cell. 53 Baek et al 49 showed that co-digestion of waste activated sludge and iron oxyhydroxides increases methane production by accelerating the decomposition of complex substances.…”