Uncovering the Hindering Effect of Bonding Strength in Polygalacturonate–Metal Complexes on Methane Production from Waste Activated Sludge

The degradation of organic–metal complexes in waste activated sludge (WAS) offers a new avenue to recover CH4. However, the degradability of these complexes limited by the bonding strength has been historically overlooked. This work demonstrated that the bonding strength of polygalacturonate (PGA, a typical uronic acid in WAS)–metal complexes formed with various metal ions followed Fe3+ > Ca2+ > Mg2+ > Na+ by density functional theory calculations and other chemical analyses. Fe3+ with strong bonding was revealed as the key ion to reduce the activities of both polygalacturonate lyase (EC 4.2.2.2 and EC 4.2.2.9, >48.3%) and polygalacturonase (EC 3.2.1.15, >74.1%) secreted from the enriched polygalacturonate-degrading consortium (GDC), thereby notably reducing CH4 production (64.1–86.8%) and accumulating minor VFAs (0.85–0.97 g/L) from PGA. Interestingly, the activities of polygalacturonate lyase and polygalacturonase were not reduced notably by the weakly bound Mg2+, Ca2+, Fe2+, and iron compounds (Fe2O3, Fe(OH)3, and Fe3O4). Furthermore, the restriction of these weakly bound ions on the anaerobic digestion of PGA could be ignored, achieving CH4 production similar to that of the control group. Thus, this work revealed that the PGA–metal complex formed by weak bonding strength could be utilized when using the microbial method without any chemical pretreatments.