Unraveling adsorption behavior and mechanism of perfluorooctane sulfonate (PFOS) on aging aquatic sediments contaminated with engineered nano-TiO2.

Engineered nano-TiO2 (Enano-TiO2) have inevitably discharged into aquatic sediments that resulted from their widespread use. The physicochemical characteristics of sediments might be changed because of remarkable properties of Enano-TiO2 and affected by the aging of sediments, thereby altering the environmental behavior and bioavailability of other pollutants such as perfluorooctane sulfonate (PFOS) in sediments. Here, adsorption behavior and mechanism of PFOS on aging aquatic sediments spiked with Enano-TiO2 at a weight ratio of 5.0% were investigated. The results showed that Enano-TiO2 significantly altered zero points of charge (pHzpc) and pore surface properties of sediments, manifested as pHzpc, the total surface area (SBET), the micro-pore surface area (Smicro), and the external surface area (Sext) of sediment particles contaminated with Enano-TiO2 clearly increased, instead average pore size decreased. Rapid intra-particle diffusion processes were well fitted by the pseudo-second-order rate model with the sorption rate (K2) following the order single (5.764 mg/(g·h)) > binary systems (3.393 mg/(g·h)). Freundlich model best described the sorption isotherm data with the larger sorption capacity (KF) and sorption affinity (1/n) of sediments spiked with Enano-TiO2 than that of sediments only. Additionally, Enano-TiO2 changed the adsorption thermodynamics of PFOS on the sediments with the absolute value of ∆G0, ∆H0, and ∆S0 increased. Fourier transform infrared (FT-IR) spectroscopy suggested possible formation of a negative charge-assisted H-bond between PFOS and the functionalities on sediment surfaces, including O-H of carboxyl, alcohol, phenols, and chemisorbed H2O as well as carbonyl groups (C=O) of ketone groups. Furthermore, the multilayer sorption of PFOS on sediments contaminated with Enano-TiO2 is plausible because of bridging effect of Cu2+ and Pb2+.