Contrasting sludge drying behavior induced by Fe, Al, and Ca-based conditioners correlates with differential...

Although traditional conditioners (e.g., ferric chloride, PAC, and lime) are widely applied to stabilize sludge, the roles of their metallic residues (Fe, Al, Ca) in drying dynamics remain unclear. Using integrated multispectral and FT-ICR MS analyses, the residual metals were found to drive biopolymer transformation via multifaceted physicochemical interactions (e.g., complexation and inter-chain bridging), altering hydrophilic-hydrophobic exposure and affecting sludge-water binding and evaporation. Specifically, Fe-based conditioners promoted evaporation, whereas Al- and Ca-based counterparts retarded subsequent sludge drying. In detail, Fe-based conditioning diminished the availability of hydration-related CO and OCO groups, enhanced hydrophobic-domain exposure and impaired the water-holding capacity of extracellular proteins, thereby promoting moisture release; in contrast, Al- and Ca-based conditioning preserved abundant oxygenous polar moieties, strengthened biopolymer-water affinity, and inhibited subsequent sludge drying. In addition, Fe species preferentially altered protein/lipid-like fractions in sludge biopolymer, whereas Al and Ca species enriched oxygenous groups, suggesting that conditioner-derived metals regulate drying by modulating biopolymer hydration and composition. These indicated that residual metals regulate biopolymer molecular structures during sludge drying, and Fe-based conditioners are preferable due to their diminished hydration related functional groups and enhanced protein hydrophobicity to facilitate water release. This study provides novel insights into biopolymer transformation after metal-containing conditioners stabilization, and offers a theoretical basis for developing more efficient and energy-saving sludge drying strategies.