Efficiency and mechanisms of combined persulfate and nanofiltration for the removal of typical perfluorinated compounds

Abstract

Perfluorinated compounds (PFCs) are a class of emerging pollutants that are commonly detected in surface water and pose significant risks to both the environment and public health. This study investigates a combined treatment method for removing perfluorooctanoic acid (PFOA), a prevalent PFC found in micro-polluted surface water. The method integrates nanoscale zero-valent iron (nFe)-activated persulfate (PS) pre-oxidation with conventional water treatment processes—coagulation, sedimentation, and sand filtration—combined with nanofiltration (NF). This study primarily aims to evaluate the efficiency of this combined process for PFOA removal and to elucidate the mechanisms underlying PS oxidation and NF separation. The treatment sequence, comprising nFe/PS pre-oxidation, conventional treatment, and NF, was strategically designed considering the specific roles of each process in PFOA removal. In the initial stage, nFe-activated PS generates sulfate radicals (SO₄⁻·) and hydroxyl radicals (OH·), which oxidize and degrade PFOA. The subsequent conventional treatment removes the majority of degradation byproducts and suspended solids. Finally, NF retains both PFOA and its oxidation products, thereby ensuring high removal efficiency. Experimental results indicate that an optimal PS dosage of 0.2 mM and an nFe-to-PS molar ratio of 1 : 1 achieved the maximum efficiency for PFOA removal. Among the tested sequences, “nFe/PS pre-oxidation + conventional treatment + NF” achieved the highest removal rate, exceeding 99%. Furthermore, this sequence resulted in the lowest surface potential of the NF membrane, which enhanced electrostatic interactions between the membrane and PFOA. This reduction in surface potential, combined with the formation of C–O bonds between PFOA and the NF membrane, further enhanced PFOA adsorption onto the membrane surface. The combined process of nFe/PS pre-oxidation, conventional treatment, and nanofiltration effectively removes PFOA from micro-polluted surface water, thereby contributing to improved drinking water safety.