全氟化合物在阳离子表面活性剂改性碳纳米管上的吸附

Adsorption of perfluorinated compounds on cationic surfactant coated multiwalled carbon nanotubes

采用两种阳离子表面活性剂,四丁基溴化铵(TBA)及聚乙烯亚胺(PEI)对碳纳米管进行改性,并研究了全氟辛酸(PFOA)、全氟辛烷磺酸盐(PFOS)、全氟己烷磺酸盐(PFHxS)在改性碳纳米管上的吸附行为.研究结果显示,除PFOS在TBA修饰的碳纳米管(TBA-MWCNTs)上的吸附等温线呈线性外,全氟化合物在原碳纳米管(Pri-MWCNTs)、TBA-MWCNTs及PEI修饰的碳纳米管(PEI-MWCNTs)上的吸附均为非线性.吸附等温线可以用Freundlich及Langmuir模型拟合.与Pri-MWCNTs相比,PEI-MWCNTs的零电荷点(pHzpc)增加了6.2个单位,全氟化合物的吸附系数Kd值增加2—3倍;而TBA-MWCNTs的pHzpc增加了1.0个单位,PFOS在高平衡浓度下的Kd值增加了两倍.随着溶液pH值的降低,全氟化合物在Pri-MWCNTs和PEI-MWCNTs上的吸附量有所增加,而在TBA-MWCNTs上的吸附受pH值的影响不大.这说明疏水作用及静电作用是全氟化合物在Pri-MWCNTs和PEI-MWCNTs上吸附的主要机制,而疏水作用则为TBA-MWCNTs上吸附的主要机制.与单溶质体系相比,PFOS与PFOA同时存在时二者在PEI-MWCNTs上吸附的Kd值均有所减小,且PFOA的减小程度大于PFOS.PFOS在TBA-MWCNTs上吸附的Kd值在高浓度下几乎不受PFOA的影响.

In this study,two cationic surfactants,tetrabutylammonium bromide（TBA） and polyethyleneimine（PEI） were employed to modify the surface properties of multiwalled carbon nanotubes（MWCNTs）.Sorption isotherms of perfluorinated compounds（PFCs） on modified MWCNTs were studied.The sorption isotherms were nonlinear and fitted with Freundlich and Langmuir models,except that PFOS sorption on TBA modified MWCNTs was linear.Compared with pristine MWCNTs（Pri-MWCNTs）,modification by PEI increased the point of zero charge（PZC） of MWCNTs by 6.2 units and increased sorption coefficient（Kd） of all PFCs by 2—3 times.Modification by TBA increased the PZC by 1.0 unit and Kd of PFOS by 2 times at the high equilibrium concentration.Sorption on PEI modified MWCNTs（PEI-MWCNTs） increased with increasing perfluorocarbon chain length and decreasing solution pH.Sorption on TBA modified MWCNTs（TBA-MWCNTs） was not affected by solution pH significantly.These results suggest the importance of electrostatic and hydrophobic interactions on PEI-MWCNTs and the hydrophobic interaction on TBA-MWCNTs.Results of competitive sorption between PFOA and PFOS showed that Kd values on PEI-MWCNTs in the bi-solute system were lower than those in the single-solute system.PFOA had a higher reduction than PFOS.No significant difference between Kd values in bi-and single-solute systems was observed for PFOS sorption on TBA-MWCNTs,indicating no competition with PFOA.