Adsorption of Perfluorooctane sulfonate (PFOS) onto metal oxides modified biochar

Perfluorooctane sulfonate (PFOS) is a persistent organic pollutant (POP), which poses potential toxicity to biotic and abiotic ecosystem. Adsorption of PFOS from soil and water has emerged as promising remediation practice due to low cost, high removal performance, low environmental impact, and recalcitrant to induce secondary contamination. In this study, red mud modified sawdust (RMSDN600) and unmodified sawdust (SDN600) were prepared for sorption of PFOS from aqueous solution. The presence of magnetites (Fe3O4), ferrihydrites, and desilicated minerals are identified in the RMSDN600 using XRD (X-ray diffraction) and XANES (X-ray absorption near-edge structure). Sorption isotherm for RMSD600 and SDN600 showed close-fitting with Langmuir and Freundlich model demonstrated monolayer and multilayer sorption of PFOS over the active sites of the adsorbents. The potential formation of micelles and hemi-micelles can occur in interparticle porous biochars as the concentration of PFOS exceeds critical hemi-micelle concentration (4.57–45.7 mg/L). The kinetic study followed Pseudo-second-order model for both adsorbents, demonstrated both physisorption and chemisorption of PFOS. The results revealed the adsorption of PFOS was governed by both hydrophobic and electrostatic interaction, with hydrophobic interaction as the dominant sorption mechanism. The higher adsorption capacity for RMSDN600 (194.6 mg/g) was recorded than that for SDN600 (178.6 mg/g) at pH 3.1 due to the abundance of protonated metal-based functional groups, and more ordered graphitic carbon structure resulting from catalytic degradation and transformation of cellulose and hemicellulose. Aromatic structure can potentially enhance PFOS sorption by non-ionic interaction. In contrast, metal-based and other oxygen-containing functional groups of adsorbents enhance adsorption capacity through electrostatic interaction and ion exchange reactions. Lower solution pH and smaller particle size of the adsorbents could also enhance sorption of PFOS from aqueous phase.