Adsorption of Pyrene from Aqueous Solutions onto Sepiolite

Polycyclic aromatic hydrocarbons (PAHs) are a large class of organic compounds which are commonly mentioned and have been shown to be highly carcinogenic and to persist in the environment for many years. An inexpensive remediation method has yet to be found, so the current study was undertaken to test the use of sepiolite, a fibrous clay mineral, as a potentially inexpensive and effective solid-phase adsorbent for sequestering PAHs. Pyrene was chosen as a model PAH due to the specific volatility, miscibility, and relatively soluble properties of the compound. A sepiolite of Turkish origin was then investigated to explore its potential to adsorb hydrophobic organic compounds from aqueous solution. The microstructure and morphology of the sepiolite were characterized using elemental analysis, X-ray diffractometry (XRD), Fourier-transform infrared (FTIR) spectroscopy, field emission scanning electron microscopy (FE-SEM), and specific surface area from N2 adsorption isotherms. The pyrene adsorption isotherms were closely fitted to the Langmuir model and the coefficients of determination (RP2) were higher than 0.999. The results indicated that the high affinity of pyrene for sepiolite surfaces was dominated by the structural channels and the large number of Si-OH groups located on the basal surfaces. The intracrystalline interactions of pyrene with the sepiolite were, however, more favorable than pyrene interactions with sepiolite surface Si-OH groups, which can react directly with pyrene to form true covalent bonds (chemical interactions). Finally, the FE-SEM images initially revealed that, after sepiolite was loaded with adsorbed pyrene, a fairly straight and rigid arrangement of fibers occurred due to the aggregation of laths to form rods and the increased amounts of adsorbed pyrene.