Synergistic mechanism of alkali-organic separation enabled by iron ion magnetic resin for achieving recycling in high-alkalinity Gracilaria agar wastewater

The production of Gracilaria agar results in the production of highly alkaline wastewater with COD exceeding 5000 mg/L, which poses a significant challenge to wastewater treatment due to its complex chemical composition and high alkaline level. This study addresses this key issue by developing a novel iron-embedded polystyrene magnetic resin that overcomes the structural instability of traditional adsorption materials in a strongly alkaline environment, the dual goals of organic matter removal and alkali recovery were achieved simultaneously. The resin achieved a Chemical Oxygen Demand (COD) removal rate of 49.19% ± 0.34% with an alkali retention rate of up to 79.98%, and maintained performance over three reuse cycles (46.25% and 41.27% for secondary and tertiary applications). Redundancy analysis confirmed synergistic interactions between the resin's physical/chemical properties and separation efficiency. Iron (2.57–2.93 wt%) provided active sites for chemical adsorption, stabilizing the matrix at high alkalinity, while polystyrene scaffolds enabled selective organic compound capture. Thermogravimetric, infrared, and X-ray Photoelectron Spectroscopy (XPS) analyses verified the dual mechanism of alkali retention and organic removal, enhancing separation efficiency. The Fe 3 O 4 @Polystyrene magnetic separation device simplifies large-scale treatment by combining ion-driven adsorption with porous morphology, addressing high alkalinity complexity. This approach improves alkaline resource reutilization and offers sustainable solutions for industrial agar production.