Adsorption of perfluorooctanoic acid and perfluorooctanesulfonic acid to iron oxide surfaces as studied by flow-through ATR-FTIR spectroscopy
Xiaodong Gao A B and Jon Chorover A C
+ Author Affiliations
- Author Affiliations
A Department of Soil, Water and Environmental Science, University of Arizona, Tucson, AZ 85721, USA.
B Present address: Department of Earth Science, Rice University, Houston, TX 77251, USA. Email: xdgao@rice.edu
C Corresponding author. Email: chorover@cals.arizona.edu
Environmental Chemistry 9(2) 148-157 https://doi.org/10.1071/EN11119
Submitted: 29 September 2011 Accepted: 15 February 2012 Published:
Environmental context. Perfluoroalkyl compounds are organic contaminants that exhibit strong resistance to chemical- and microbial-degradation. As partitioning between solid and aqueous phases is expected to control the transport of perfluoroalkyl compounds, we studied the molecular mechanisms of their adsorption–desorption at a representative Fe oxide surface using in situ molecular spectroscopy. The results provide valuable information on the types of bonds formed, and enable a better understanding of the transport and fate of these organic contaminants in natural environments.
Abstract. The kinetics and mechanisms of perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) adsorption to nanoparticulate hematite (α-Fe2O3) from aqueous solutions were examined using in situ, flow-through attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy. Results indicate that both PFOA and PFOS molecules are retained at the hydrophilic hematite surface and the adsorption shows strong pH dependence. However, ATR-FTIR data reveal that PFOA and PFOS are bound to the iron oxide by different mechanisms. Specifically, in addition to electrostatic interactions, PFOA forms inner-sphere Fe–carboxylate complexes by ligand exchange, whereas the PFOS sulfonate group forms outer-sphere complexes and possibly hydrogen-bonds at the mineral surface. Both solution pH and surface loading affect adsorption kinetics. Faster adsorption was observed at low pH and high initial PFC concentrations. Sorption kinetics for both compounds can be described by a pseudo-second-order rate law at low pH (pH 3.0 and 4.5) and a pseudo-first-order rate law at high pH (pH 6.0). Sorption isotherm data for PFOA derived from spectroscopic results exhibit features characteristic of ionic surfactant adsorption to hydrophilic charged solid surfaces.
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