Hydrogen bond driven deep eutectic solvent strategy for enhanced sludge dewatering via protein network remodeling

The high water-holding capacity of extracellular polymeric substances (EPS) is a key factor contributing to sludge dewatering challenges. Intra- and inter-protein hydrogen bond networks within EPS play a critical role in water retention. In this study, a deep eutectic solvent (DES) was synthesized from choline chloride and ferric chloride hexahydrate to enhance activated sludge dewatering. Spectroscopic analysis and theoretical calculations were conducted to investigate changes in EPS composition and structure and the mechanisms underlying DES-induced modifications of the secondary structure and hydrophilicity of extracellular proteins. Under optimal conditions (200 rpm, 0.41 mL/g DES, 60 °C, 70 s), the capillary suction time of the conditioned sludge significantly decreased from 71.0 s to 29.7 s (approximately 58 % reduction). DES-coupled thermal treatment disrupted the sludge floc structure, neutralized the particle surface charge, and significantly enhanced surface hydrophobicity. Consequently, the contact angle of the treated sludge increased from approximately10° to over 35°. DES weakened the binding capacity of EPS, causing proteins and polysaccharides to transition from a bound state into the liquid phase. The treatment disrupted hydrogen bonds between protein backbones, reduced α-helix content, and increased β-sheet content, thereby altering the protein secondary structure. These structural changes exposed hydrophobic residues, which further enhanced sludge dewatering performance. Density-functional theory confirmed that DES disrupted protein backbone hydrogen bonds via non-covalent interactions, leading to structural rearrangement. This hydrogen bond-regulating strategy provides a green, efficient approach for sludge dewatering, with theoretical support for sustainable resource utilization.