Structure–performance relationship of agar-based hydrogels for multi-contaminant removal from oilfield wastewater

• Agar-based hydrogels were rapidly synthesized via microwave-assisted polymerization • Mixed oilfield effluent showed extreme salinity and complex organic–inorganic composition • AgCr 5% achieved 58.5% oil removal and 24% COD reduction from real MOE • High adsorption efficiencies (26–99%) were obtained for key dissolved metal ions • Multifunctional hydrogels enable pretreatment of complex oil and gas wastewaters This study develops agar-based hydrogels for treating complex high-salinity industrial wastewater (MOE) containing organic and inorganic pollutants. Hydrogels were synthesized via microwave-assisted free-radical polymerization, grafting agar with acrylamide at various loadings. The AgAM series varied acrylamide from 10-30 g, while the AgCr series had a fixed agar-to-acrylamide ratio of 1:20 (w/w) and was crosslinked with N, N'-methylenebisacrylamide. FTIR, TGA, and SEM confirmed successful grafting, improved thermal stability, and porous morphology. The AgCr5% sample removed 24-27% COD with an adsorption capacity of 628 mg/g, and 58.5% oil and grease at 10 g/L, with metal ion removal improving at 50 g/L. Kinetic studies showed oscillatory adsorption-desorption, supported by Weber-Morris diffusion analysis indicating multiple rate-limiting steps. Temperature impacts (25-55°C) were minimal, suggesting energy efficiency. The hydrogel performed through 7 regeneration cycles with performance decline. SEM revealed contaminant entrapment and decreasing pore size from 7.98 to 2.97 μm. FTIR and XPS confirmed interactions like hydrogen bonding, electrostatic attraction, and complexation via functional groups. These results indicate agar-based hydrogels are sustainable, multifunctional adsorbents for removing diverse contaminants from industrial wastewater, suitable for pretreatment before advanced polishing.