Emulsified oil removal from steel rolling oily wastewater by using magnetic chitosan-based flocculants: Flocculation performance, mechanism, and the effect of hydrophobic monomer ratio

Large amounts of wastewater that contained emulsified oil were generated in the steel rolling process of the steel industry. The weak interaction between flocculants and emulsified oil and the slow floc separation are the main bottleneck in oily wastewater treatment. To solve this problem, this study successfully prepared three magnetic chitosan-based flocculants (MCS) with different hydrophobic monomer contents to treat steel rolling oily wastewater (SROW). The oil content, turbidity, chemical oxygen demand (COD), and UV254 removal rates were investigated by using MCS and compared with commercial flocculants. Results showed that as the dosage was 6 mL/L, MCS-2 had an optimal effect on oil and turbidity removal with 95.79 % and 87.85 %, respectively. The best COD removal rate of 66.67 % was achieved at a dosage of 4 mL/L with MCS-3. GC–MS analysis showed that the removal efficiency of MCS on alkanes was significantly better than that of aromatic hydrocarbons. Density functional theory calculations demonstrated that the hydrophobic groups of MCS can combine alkanes and aromatic hydrocarbons through hydrophobic association. Furthermore, charge neutralization, hydrophobic association, and adsorption bridging contributed to emulsified oil removal synthetically. The increased content of hydrophobic monomers enhances the interaction between flocculants and emulsified oil, which was significant for SROW treatment.