Methionine-based insights into C-S-Fe-P transformations in anaerobic co-digestion of sludge containing iron–phosphorus compounds

Anaerobic co-digestion of sulfur-containing organic wastes with waste-activated sludge containing iron–phosphorus compounds (FePs) was recently suggested as an environment-friendly strategy to promote phosphate release, energy recovery, and hydrogen sulfide (H 2 S) control. Nevertheless, the mechanistic coupling between FePs speciation and the concurrent transformation of carbon, sulfur, iron, and phosphorus within this system remains to be fully elucidated. To address this knowledge gap, methionine, a typical hydrolysis product of sulfur-containing organics, and five FePs prevalent in sludge (ferric-phosphate tetrahydrate (FePO 4 ⋅4H 2 O), ferric-phosphate dihydrate (FePO 4 ⋅2H 2 O), vivianite (Fe 3 (PO 4 ) 2 ·8H 2 O), phosphate coprecipitated with Fe(III) (COP-P), and phosphate adsorption on hydrous ferric oxide (HFO-P)) were selected to elucidate C-S-Fe-P transformations in this study. The results showed that the H 2 S and methyl mercaptan productions decreased by >96% and >99%, respectively, while the methane production rate increased by 51.60–103.9% in the presence of FePs. The reaction between FePs and sulfide promoted the transformation of gaseous H 2 S and aqueous S 2− to solid sulfur species (elemental sulfur and iron sulfide precipitates), while simultaneously promoting the release of PO 4 3− from FePs. The formation of Fe(II) species derived from both abiotic sulfide-driven reduction and microbial-mediated iron reduction processes. The reduction rates of FePO 4 ⋅2H 2 O and FePO 4 ⋅4H 2 O were higher than those of COP-P and HFO-P, owing to their higher thermodynamic favorability. A negative correlation was observed between the Fe/P molar ratio and PO 4 3− release efficiency. FePO 4 (FePO 4 ⋅2H 2 O and FePO 4 ⋅4H 2 O) with the lowest Fe/P molar ratio achieved the highest P release efficiency (89.83–91.01%). Metagenomics analysis revealed that the SELENBP1 gene related to the degradation of methanethiol to sulfide was upregulated by 21.12–51.72% in the presence of FePs, and the genes involved in propionate metabolism, methylotrophic, and hydrogenotrophic methanogenesis were up-regulated concurrently. This study provides an in-depth understanding of C-S-Fe-P interactions and transformations during the anaerobic co-digestion of sulfur-containing organic wastes with FePs-containing sludge, helping to enhance methane production and the recovery of phosphorus and sulfur.