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 (H2S) 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 (FePO4⋅4H2O), ferric-phosphate dihydrate (FePO4⋅2H2O), vivianite (Fe3(PO4)2·8H2O), 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 H2S 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 H2S and aqueous S2- to solid sulfur species (elemental sulfur and iron sulfide precipitates), while simultaneously promoting the release of PO43- 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 FePO4⋅2H2O and FePO4⋅4H2O 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 PO43- release efficiency. FePO4 (FePO4⋅2H2O and FePO4⋅4H2O) 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.