Efficient demulsification of emulsified oil wastewater by graft-functionalized β-cyclodextrin flocculants: The effect of charge density, hydrophobicity, and graft chain length.

Emulsified oil wastewater, characterized by diverse sources, low biodegradability, and high ecotoxicity, imposes a severe hazard on aquatic organisms and human health when released untreated. Flocculation stands out among current emulsified oil treatment methods for its simplicity, affordability, and role as a fundamental unit operation in water treatment processes. However, existing flocculants suffer from insufficient demulsification efficiency, ambiguous structure-property relationships, and unclear mechanisms.​ In this study, a series of cationic and hydrophobic β-cyclodextrin (β-CD)-based flocculants (β-CD-AAL) with varying charge density (CD), contact angle (CA), and graft chain length (CL) were synthesized via thermally initiated radical graft copolymerization by introducing quaternary ammonium groups, hydrophobic alkyl chains, and acrylamide onto the natural polymer β-CD. Systematic experiments were conducted to correlate the oil removal performance of β-CD-AAL flocculants with their CD, CA, and CL, revealing structure-property relationships underlying emulsified oil removal. Experimental data were fitted using a quadratic polynomial model, while density functional theory (DFT) calculations quantified the binding energies between the flocculant's characteristic structures and the contaminant, thereby revealing their relative influence on emulsified oil flocculation from both mathematical and molecular-level perspectives. The results indicated that CD exerted the greatest influence on flocculation performance, followed by CA, while CL had the least effect. Floc growth analysis demonstrated that CD governs initial demulsification through charge neutralization, while CA and CL control subsequent flocculation via hydrophobic association and adsorption bridging after charge neutralization saturation. This work furnishes technical guidance and a mechanistic underpinning for the rational design of high-efficiency flocculants and effective control of emulsified oil wastewater.