Removal of perfluorooctanoic acid from water by adsorption on high surface area mesoporous materials

摘要

Removal of perfluorinated alkylated substances (PFAS) such as perfluorooctanoic acid (PFOA) from aqueous solution is an actual topic in light of their widespread diffusion and their persistence in the environment. The process of adsorption has been identified as an effective technique to eliminate PFAS in water, however the process efficiency strongly depends on the adsorbents employed (silica, alumina, activated carbon, layer doubled hydroxides). In this work three nanostructured mesoporous silica materials of similar pore diameter (～ 4 nm) featuring high surface area (～ 900 m~2/g) and high pore volume (0.7-1.0 mL/g) were evaluated in PFOA removal: calcined MCM-41 (MCM-41c), calcined hexagonal mesoporous silica (HMSc) and HMSe obtained after ethanol extraction of the amine templates from HMS. Sorption kinetics and isotherms were performed at PFOA concentrations from 10 μg/L to 10 mg/L. It appeared that HMSe showed much faster and higher adsorption capacity for PFOA than the other tested adsorbents (MCM-41c and HMSc) whatever the pH of the solution (5 ＜ pH ＜ 9). Thermogravimetric analysis of HMSe evidenced that the ethanol extraction of the templating amines was not complete (70 %) and HMSe possessed some remaining hexadecylamine (HDA) (0.08 mol amine per mol SiO_2) on the surface conferring some hydrophobic properties to the adsorbent and also some probable complex formation between anionic PFOA~- and protonated HDA. Indeed, the incomplete amine extraction is surely due to the presence of protonated HDA in strong electrostatic interactions with SiO~- avoiding their removal by simple ethanol extraction as for H-bonding amine with Si-OH groups. Considering both adsorption isotherms and adsorption kinetics, PFOA could be efficiently removed from contaminated water in a wide range of concentration by an environmental friendly adsorbent as HMSe.