Effects of monovalent cations on the competitive adsorption of perfluoroalkyl acids by kaolinite: experimental studies and modeling.
Effects of monovalent cations on the competitive adsorption of perfluoroalkyl acids by kaolinite: experimental studies and modeling.
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DOI：
10.1021/es202524y
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发表时间：
2011-10
影响因子：
11.4
通讯作者：
Feng-Xia Xiao;Xiangru Zhang;L. Penn;J. Gulliver;M. Simcik
中科院分区：
文献类型：
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作者：
Feng-Xia Xiao;Xiangru Zhang;L. Penn;J. Gulliver;M. Simcik
本研究验证了我们的假设，即长链全氟烷基酸（PFAAs）在吸附过程中优于短链全氟烷基酸（PFAAs），其中六种经常检测到的PFAAs与高岭石粘土的吸附相互作用进行了建模，并使用不同的悬浮液成分进行了实验检验。首次观察到PFAAs在高岭石表面的竞争性吸附，长链PFAAs优于短链PFAAs。吸附PFAA分子间的静电斥力是抑制PFAA吸附的主要因素。水溶液中钠离子或氢离子浓度的增加会减弱静电斥力，改变吸附自由能。因此，疏水性较弱的短链PFAA在高钠离子或高氢离子浓度下会发生吸附。实验和模拟数据表明，在典型离子强度为10（-2.5）的淡水中，短链PFAAs（≤4个全氟化碳）在热力学上是不利的。此外，通过测量PFAAs存在时高岭石悬浮液的电动势，我们发现由于PFAAs的吸附，高岭石表面的负电荷增加。这一观察结果表明，吸附的PFAA分子位于高岭石表面的双电层内，它们对滑动面上的电位有贡献。然后提出了PFAA分子在高岭石表面的可能排列。
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.