Adsorption mechanisms on perfluorooctanoic acid by FeCl₃ modified granular activated carbon in aqueous solutions

Perfluorooctanoic acid (PFOA) is an emerging organic pollutant that is persistent in the environmental, and has been detected in humans, and wildlife. Several technologies, such as activated carbon (AC) adsorption have been used to remove PFOA from water. In this study, Fe-impregnation with/without post-thermal treatment of AC was applied to improve the adsorption of PFOA. The adsorption mechanisms were evaluated using three kinetic models: pseudo-first-order model, pseudo-second-order model, and intra-particle diffusion models. Interpretation of experimental results with the kinetic models revealed that chemical interactions, such as electrostatic attraction or complexation were suggested as the adsorption mechanisms along with physical adsorption. Two isotherm models demonstrated that the modified ACs (171.0-189.9 mg g^-1) had increases in adsorption capacities than the unmodified AC (164.9 mg g^-1), which indicated that modification improved the maximum achievable surface concentrations and adsorption affinity to some extent. The evenly distributed iron content on the modified ACs was visualized using an energy dispersive X-ray spectroscopy. The Fe-impregnated AC showed a reduction in the specific surface area and total pore volume; however, post-thermal treatment largely recovered the pore structures. The isotherms normalized by the accessible surface area revealed the importance of the Fe-impregnated surfaces on PFOA adsorption. Comparable pH values of the point of zero charge and chemical compositions of the ACs implied that an increase in Fe-impregnated surface was crucial to improve PFOA adsorption. Thus, substantial enhancement of PFOA removal can be achieved by implementing a proper strategy for AC modification, especially using Fe-impregnation.