Single and binary adsorption of perfluorooctanoic acid and perfluorooctane sulfonic acid on chitosan-carbon nanotubes hydrogel beads: Adsorption kinetics, isotherms, and thermodynamic parameters

The occurrence of perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) in bodies of water that receive contaminants has presented substantial health hazards for humans and aquatic organisms. Herein, the adsorption of PFOA (1.208E-05 mol/L to 1.208E-04 mol/L) and PFOS (9.997E-06 mol/L to 9.997E-05 mol/L) on chitosan-carbon nanotube (CCNT) hydrogel beads from aqueous solutions was studied. Findings on the adsorption kinetics studies suggest that the nonlinear pseudo-first-order kinetic model fits experimental data well at a contact duration of 48 h and an adsorbent load of 1.5 g/L for both PFOA and PFOS. Single adsorption isotherm data were accurately modeled by the nonlinear Freundlich model, with R² =0.991 for PFOA and R² =0.997 for PFOS, suggesting that the adsorption of PFOA and PFOS on the adsorbent was not restricted to the monolayer adsorption process. Binary adsorption isotherm data were well fitted by the extended-Langmuir isotherm model (R²=0.996 for PFOA and R²=0.995 for PFOS) and extended-Sips isotherm model (R²=0.996 for PFOA and R²=0.997 for PFOS). As such, it was inferred that the uptake of one adsorbate in the presence of the other resulted in antagonistic effects, which reduced the overall efficacy of the adsorbent. Thermodynamic studies explicitly indicated that the adsorption of PFOA and PFOS on CCNT was an endothermic process that can be characterized as a physicochemical adsorption process. The presence of sodium chloride as a competing ion synergized the uptake of adsorbates from an aqueous solution. It was concluded that hydrophobic interaction and electrostatic attraction were the predominant mechanisms in the sorption of PFOA and PFOS on CCNT hydrogel beads.