Effects of different iron minerals on organics removal pathway and end-products during anaerobic digestion

The properties of different Fe(iii) minerals vary considerably, and the mechanisms of promoting anaerobic digestion by different Fe(iii) minerals are confusing, especially as regards organics removal pathway and end-products. This study investigated how different Fe(iii) minerals affected anaerobic digestion by separately adding amorphous ferrihydrite and crystalline magnetite into the anaerobic digestion of residual sludge. The results showed that the Fe(ii) content increased rapidly to 3098.3 mg L-1 on day 30 in the ferrihydrite-added group, much higher than that in the control group without adding Fe(iii) (600.3 mg L-1). More organics were removed in the ferrihydrite group compared with the control group. Fe(ii) content was nearly equal in the magnetite-added group and the control group, but more organics were removed in the former. Although ferrihydrite and magnetite both promoted organics removal, the cumulative CH4 production was 23.0% lower in the ferrihydrite-added group and 19.4% higher in the magnetite-added group, compared with the control group. Microbial analysis after the experiment also showed that iron-reducing bacteria were enriched in the ferrihydrite group. Geobacter and Methanosaeta were enriched in the magnetite group, which could promote direct interspecies electron transfer (DIET) during methanogenesis. These results indicated that dissimilatory iron reduction of ferrihydrite with strong oxidation capacity was thermodynamically favored over methanogenesis, leading to methanogenesis inhibition. Dissimilatory iron reduction of magnetite with weak oxidation ability was difficult thermodynamically, but magnetite could promote methanogenesis by strengthening DIET, due to its electrical conductivity.