Feng Xiao - Effects of monovalent cations on the competitive adsorption of perfluoroalkyl acids by kaolinite: Experimental studies and modeling

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      Our hypothesis that longer-chained perfluoroalkyl acids (PFAAs)   outcompete shorter-chained PFAAs during adsorption was tested in   this study, wherein the adsorption interactions of six frequently   detected PFAAs with kaolinite clay were modeled and examined   experimentally using various suspension compositions. Competitive   adsorption of PFAAs on the kaolinite surface was observed for the   first time, and longer-chained PFAAs outcompeted those with a   shorter chain. The electrostatic repulsion between adsorbed PFAA   molecules is a primary inhibitory factor in PFAA adsorption. An   increase in aqueous sodium or hydrogen ion concentration weakened   electrostatic repulsions and changed the adsorption free energy.   Therefore, the adsorption of a shorter-chained PFAA with weaker   hydrophobicity could occur at high sodium or hydrogen ion   concentrations. The experimental and modeling data suggest that   the adsorption of shorter-chained PFAAs (≤4 perfluorinated   carbons) in freshwater with a typical ionic strength of   10–2.5 is not   thermodynamically favorable. Furthermore, by measuring the   electrokinetic potential of kaolinite suspension in the presence   of PFAAs, we found that the kaolinite surface became more   negatively charged because of the adsorption of PFAAs. This   observation indicates that the adsorbed PFAA molecules were   within the electrical double layer of the kaolinite surface and   that they contributed to the potential at the slipping plane. The   possible alignments of adsorbed PFAA molecules on the kaolinite   surface were then proposed.


      Address (URL): http://http://pubs.acs.org/doi/abs/10.1021/es202524y