Enhanced adsorption and photocatalytic degradation of perfluorooctanoic acid in water using iron (hydr)oxides/carbon sphere composite

Abstract

Perfluorooctanoic acid (PFOA) has been widely detected in aquatic systems. Yet, cost-effective technologies for degrading PFAS have been lacking. We prepared and tested an adsorptive photocatalyst consisting of iron (hydr)oxides and carbon spheres (FeO/CS) through a one-step hydrothermal process. Characterization results revealed that the presence of carbon spheres affected the crystal formation of iron (hydr)oxides, and resulted in mutually modified mixed phases ferrihydrite and carbon spheres. FeO/CS was able to effectively adsorb and then degrade the pre-sorbed PFOA under simulated solar light. FeO/CS(1:1), prepared at an Fe:Glucose molar ratio of 1:1, showed the highest PFOA adsorption capacity and photoactivity. At a dosage of 1.0 g/L, FeO/CS(1:1) adsorbed nearly all 200 µg/L of PFOA within 1 h, and when the PFOA-laden FeO/CS(1:1) was subjected to simulated solar light at neutral pH, 95.2% of pre-concentrated PFOA was photodegraded and 57.2% defluorinated in 4 h. The efficient degradation also regenerated the material, allowing for repeated uses of the material without chemical regeneration. The much enhanced adsorption and photocatalytic activity of FeO/CS was attributed to: 1) CS facilitated formation of ferrihydrite, leading to adsorption of PFOA via binuclear and bidentate complexation, and 2) a hybrid ferrihydrite-CS structure, enabling multi-point, corporative adsorption of PFOA, and increasing direct electron extraction from PFOA under solar light irradiation. Moreover, OH radicals played an important role in PFOA degradation. Lastly, a photodegradation mechanism is proposed based on experimental findings and density functional theory calculations.