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全氟辛烷磺酸(perfluorooctane sulfonate ,PFOS)是一种典型的全氟化合物,具有持久性、生物累积性和高毒性,2009年5月被增列入《关于持久性有机污染物的斯德哥尔摩公约》的名单中[1]。PFOS的大量生产和超稳定结构导致其在环境中持续累积,一些受污染场地的地下水中PFOS浓度可达到mg·L−1的水平[2-3]。研究发现接触PFOS可能对人体发育和生殖产生负面影响,甚至在低浓度情况下(10—15 ng·L−1)就可能存在人类健康风险[4]。由于PFOS在水中有较高的溶解度和稳定的化学结构,传统的生物处理工艺无法有效去除水中的PFOS[5-7]。吸附则是一种经济有效去除水中污染物的方法,PFOS在离子交换聚合物[8]、活性炭[9-10]、壳聚糖分子印迹聚合物[11]和树脂[12]等吸附剂上的吸附研究均有报道。
碳纳米管(carbon nanotubes,CNTs)是一种具有高稳定性和较强吸附性的纳米材料,由于其从废水中去除各种有机和无机污染物的突出能力而引起广泛关注[13-17]。近年来,许多研究集中在有机污染物在碳纳米管上的吸附[18-20]。Zhou等[21]研究发现,碳纳米管可以吸附高浓度的PFOS;Li等[22- 23]发现,由于受到了碳纳米管中氧含量的影响,电化学辅助增强了碳纳米管对PFOS的吸附。碳纳米管主要分为单壁碳纳米管(SWCNTs)和多壁碳纳米管(MWCNTs),SWCNTs是由单层石墨烯卷成柱状无缝管体而成,MWCNTs可看作由多层石墨体同轴套构而成。而双壁碳纳米管是最简单的多壁碳纳米管,将SWCNTs的形态和柔韧性与MWCNTs的物理化学阻力结合在一起,因此被认为是“碳纳米管”最好的模式[24-25]。Chen等[26]研究了3种碳纳米管对PFOS的吸附情况,其吸附能力依次为:DWCNTs>SWCNTs>MWCNTs。DWCNTs一般以纳米颗粒悬浮在水中,吸附后如何将其有效地从水环境中分离出来是值得研究的问题,而填充磁性纳米颗粒的吸附剂因具有磁性便于分离和收集,可以很好地解决这一问题[27]。
本研究采用化学沉淀法制备了磁性铁氧化物修饰的双壁碳纳米管复合材料,研究了磁性双壁碳纳米管(magnetic double-walled carbon nanotubes, m-DWCNTs )在不同pH和吸附剂投加量的情况下对低浓度PFOS的吸附影响,同时对m-DWCNTs进行表征以及吸附动力学和等温线模型的拟合,并初步探讨其吸附机理,为后续研究纳米材料处理水中的有机污染物提供了借鉴和参考。
磁性双壁碳纳米管(m-DWCNTs)对水中全氟辛烷磺酸(PFOS)的吸附
Adsorption of perfluorooctane sulfonate (PFOS) in water by magnetic double-walled carbon nanotubes (m-DWCNTs)
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摘要: 本文以双壁碳纳米管(double-walled carbon nanotubes ,DWCNTs)为基础材料,通过化学沉淀法制备了磁性双壁碳纳米管(magnetic double-walled carbon nanotubes ,m-DWCNTs),研究了pH和吸附剂投加量对全氟辛烷磺酸(perfluorooctane sulfonate ,PFOS)的吸附影响。采用扫描电镜(SEM)、傅里叶红外光谱(FTIR)、X射线衍射(XRD)和VSM对吸附前后的m-DWCNTs进行了观察,并进行了吸附动力学模型和等温线模型拟合。结果表明,磁化后的双壁碳纳米管包覆了铁的氧化物,出现了Fe3O4的特征衍射峰,具有超顺磁性,饱和磁化强度为66.07 emu·g-1。m-DWCNTs对PFOS的吸附更符合准二级动力学方程(R2>0.95)和Langmuir等温吸附模型(R2>0.95)。随着m-DWCNTs投加量的增加,吸附量逐渐降低,去除率逐渐升高。吸附量随着pH值的增加而逐渐降低,其主要原因有可能是m-DWCNTs对PFOS的吸附产生了静电作用。本研究结果为后续纳米材料处理水中的PFOS提供了借鉴和参考。Abstract: Magnetic double-walled carbon nanotubes (m-DWCNTs) are prepared by chemical precipitation method. The adsorption effects of perfluorooctane sulfonate (PFOS) under different pH and adsorbent dosages were studied, and m-DWCNTs were characterized by SEM, FTIR, XRD and VSM. The adsorption kinetic model and isotherm model were used for fitting. The results showed that the magnetized double-walled carbon nanotubes were coated with iron oxides, and had the characteristic diffraction peaks of Fe3O4. They were superparamagnetic and had a saturation magnetization of 66.07 emu·g-1. The adsorption of PFOS by m-DWCNTs was more in line with the pseudo-second-order kinetic equation (R2>0.95) and the Langmuir isotherm adsorption model (R2>0.95). With the increase of the dosage of m-DWCNTs, the adsorption capacity of PFOS gradually decreased, and the adsorption rate gradually increased. The adsorption capacity gradually decreases with the increase of pH value. The main reason may be the electrostatic effect of m-DWCNTs on the adsorption of PFOS. The results of this study provide a reference for the subsequent treatment of PFOS in water bodies with nanomaterial.
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Key words:
- perfluorooctane sulfonate(PFOS) /
- double-walled carbon nanotubes /
- magnetic /
- adsorption
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表 1 吸附动力学模型拟合参数
Table 1. Fitting parameters of adsorption kinetic model
模型
Model拟合参数
Fitting parameters数值
Value准一级
Pseudo-first-order kineticK1 0.0586 qe/(mg·g−1) 0.6538 R2 0.9547 准二级
Pseudo-second-order kineticK2/(g·mg−1·h) 0.1149 qe/(mg·g−1) 0.6521 R2 0.9814 扩散拟合
Diffusion fiiting第一阶段
The first stageKd/(g·g−1·h−1/2) 0.0861 C/(mg·g−1) 0.0254 R2 0.9834 第二阶段
The second stageKd/(g·g−1·h−1/2) 1.6056e−4 C/(mg·g−1) 0.6510 R2 0.7851 Elovich 拟合
Elovich fittinga/(g·mg−1·h−1) 1.3458 b/(g·mg−1) 3.6496 R2 0.9456 表 2 Langmuir和Freundlich模型拟合参数
Table 2. Parameters of Langmuir and Freundlich models
模型
Model拟合参数
Fitting parameters数值
ValueLangmuir模型
Langmuir fittingKL/(L·mg−1) 0.0077 qm/(mg·g-1) 1.5442 R2 0.9669 Freundlich模型
Freundlich fittingKF/(L·mg−1) 0.1322 n 2.8879 R2 0.8346 -
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