Magnetic biochar for removal of perfluorooctane sulphonate (PFOS): Interfacial interaction and adsorption mechanism

Perfluorooctane sulphonate (PFOS) as a long chain of persistent, bio-accumulative, and toxic emerging organic contaminant needs to be removed from contaminated water and soils. The development of cost-effective adsorbents is critical for remediation of PFOS. In this study, magnetic biochar (MBC) was synthesised by pre-modification of mineral-rich sugarcane bagasse (SB) with hematite nanoparticles (Fe2O3, ∼100 nm) for the adsorption of PFOS. Characterisation of MBC indicated the introduction of magnetic property and metals-based functional groups due to the incorporation of iron (hematite) nanoparticles. The oxygen-containing functional groups were retained by pyrolysis of sugarcane bagasse at low temperatures, which introduced hydrophilic properties of the MBC. The metal-based and hydrophilic functional groups could act as active sites for PFOS removal. Sorption isotherm results suggested maximum adsorption capacity of the MBC as 120.44 ± 12.37 mg/g for PFOS. Adsorption followed pseudo-second-order (PSO) kinetic model and Langmuir and Freundlich isotherm models, demonstrating monolayer and/or multilayer formation (depending on the solution concentration) of PFOS on the adsorbents through the physisorption and chemisorption of PFOS. The adsorption of PFOS decreased with increasing solution pH due to increase of electrostatic repulsion among negatively charged functional groups and anionic PFOS headgroup. Energy-dispersive X-ray spectroscopy (EDS) elemental mapping confirmed that PFOS (F) has strong co-distribution with oxygen, calcium, aluminium, potassium, and silicon, but limited co-distribution with carbon, indicating that electrostatic interaction and ion exchanges play a vital role in chemisorption.