Environ Sci Pollut Res DOI 10.1007/s11356-014-3115-1
RESEARCH ARTICLE
Displacement and competitive sorption of organic pollutants on multiwalled carbon nanotubes Xiaofang Shen & Xilong Wang & Shu Tao & Baoshan Xing
Received: 27 June 2013 / Accepted: 28 May 2014 # Springer-Verlag Berlin Heidelberg 2014
Abstract Displacement of lindane presorbed on the pristine and OH-functionalized multiwalled carbon nanotubes (MWCNTs) by phenanthrene, naphthalene, and atrazine, and competition of these compounds with lindane on the aforementioned sorbents were investigated. Displacement of lindane presorbed on MWCNTs by atrazine, naphthalene, and phenanthrene, and competitive sorption effect of these chemicals with lindane on MWCNTs followed the same order: atrazine > naphthalene > phenanthrene. The lowest competition and displacement of lindane by phenanthrene were mainly because of the strong interactions between these two chemicals, whereas interaction of lindane with atrazine and naphthalene was quite low. The more pronounced displacement of lindane by atrazine than naphthalene and higher competitive sorption of lindane with atrazine than with naphthalene can be ascribed to the larger molecular volume of atrazine; thus, the steric hindrance effect is higher relative to naphthalene. This study is valuable for evaluating influence of the coexisting organic compounds on sorption of primary solute towards MWCNTs in the environment.
Keywords Multiwalled carbon nanotubes . Sorption . Displacement . Competition . Organic pollutant Responsible editor: Roland Kallenborn Electronic supplementary material The online version of this article (doi:10.1007/s11356-014-3115-1) contains supplementary material, which is available to authorized users. X. Shen : X. Wang (*) : S. Tao Laboratory for Earth Surface Processes, College of Urban and Environmental Sciences, Peking University, Beijing 100871, China e-mail: [email protected] B. Xing Stockbridge School of Agriculture, University of Massachusetts, Amherst, MA 01003, USA
Introduction Carbon nanotubes (CNTs) have been attracting increasing research attention because of their unique physiochemical properties and broad applications. Sheets of graphite in CNTs are arranged in concentric cylinders (Iijima, 1991). Different from fullerene and graphene which have zero- and twodimensional structures, CNTs have one-dimensional structure (Rao et al., 2009). According to the number of concentric cylinders, CNTs can be classified into two types: singlewalled carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes (MWCNTs). It was reported that the outer diameter of SWCNTs was about 1.4 nm, while that of MWCNTs was larger (Masciangioli and Zhang, 2003). With extremely hydrophobic surfaces, large surface area, and highly porous structure, CNTs were viewed as a promising adsorbent for removing hydrophobic organic compounds (HOCs) from water (Masciangioli and Zhang, 2003). It was reported that CNTs were toxic to organisms (Helland et al., 2007), and toxicity of pollutants would be enhanced after association with the ultrafine particles (Li et al., 2003). Furthermore, sorption of HOCs to CNTs made it more difficult to predict the environmental risks of both HOCs and CNTs (Ferguson et al., 2008). To date, much work has been done to examine sorption mechanisms of HOCs to CNTs in single-solute systems. It was documented that van der Waals, π-π, and electrostatic interactions as well as hydrogen bonds could influence HOC sorption by CNTs (Chen et al., 2008a; Gotovac et al., 2007; Lin and Xing, 2008). In addition, the functionalization treatment of CNTs with hydrophilic moieties changed their surface properties, thus facilitating their practical applications. Since the introduced polar functionalities enhanced their dispersibility and water cluster formation at the surfaces or tube ends of CNTs (Cho et al., 2008; Yu et al., 2011), interaction mechanisms between HOCs and functionalized CNTs differed from those with the pristine ones.
Environ Sci Pollut Res
Due to diversity in emissions of pollutants to the environment, the organic compounds may coexist in the environment in most cases. As a result, interactions between HOCs and CNTs would inevitably be affected by the coexisting solutes. Competition and displacement are two main processes that need to be considered while studying HOC sorption on CNTs in multisolute systems. So far, few studies have been done to examine the competitive sorption of the coexisting HOCs on CNTs. For instance, Yang et al. (2006, 2010) investigated competitive sorption of polycyclic aromatic hydrocarbons (PAHs) on MWCNTs and influence of the dissociation of ionizable organic compounds on their competitive sorption behaviors with a non-polar organic compound (i.e., naphthalene) on these sorbents. The authors found that interactions between the sorbed primary solute and the competitor affected their competition (Yang et al., 2006). It was also proposed that the chemicals introduced to the competitive sorption systems could change the dispersibility of CNTs and water solubility of the primary solute (Pan and Xing, 2008), thereby altering its sorption to CNTs (Wang et al., 2009). However, to date, little information is available on the influence of molecular structure and volume, as well as planarity of the coexisting compounds on their competitive sorption behaviors on MWCNTs. Moreover, to the best of our knowledge, almost no work focused on the underlying mechanisms controlling displacement of the sorbed HOCs by post-introduced chemicals from MWCNTs, and such mechanisms are largely unknown (Ju and Young, 2005). Based on the discussion above, the key objectives of this work were to (1) probe the mechanisms controlling competitive sorption behaviors of the coexisting organic chemicals on MWCNTs and (2) test if the aliphatic compound (e.g., lindane) sorbed to MWCNTs can be displaced by chemicals of dissimilar physicochemical properties (e.g., aromatics) and the associated mechanisms.
Materials and methods Sorbents and sorbates The pristine and OH-functionalized MWCNTs with outer diameter of 20–30 nm (purity >95 %; claimed by the manufacturer) were obtained from Chengdu Organic Chemicals Co., Ltd., Chinese Academy of Sciences. They were labeled as P30 and F30, respectively. The amorphous carbon and catalyst in the synthesized MWCNTs were removed with a mixture of HNO3 and H2SO4 by the manufacturer. The amorphous carbon content in the resulting MWCNTs was less than 5 % after the treatment as claimed by the producer, and it was not further determined in our lab. Very low ash content of P30 and F30 indicated their low remaining catalyst after the treatment (Table 1). Phenanthrene, naphthalene, atrazine, and lindane were selected as sorbates, and both the 14C-labeled and
non-labeled chemicals were purchased from Sigma-Aldrich Chemical Company, USA. Physicochemical properties of the selected sorbates are listed in Table 2. Characterization of sorbents Elemental composition, surface area, and porosity of P30 and F30 were reported in our previous study (Wang et al., 2010) and summarized in Table 1 as basic information for the research purposes of the present work. Contact angles between the sorbents and water were measured using a video contact angle system with sessile drop method (OCA20, Dataphysics, Germany) (Guo et al., 2012). Briefly, to obtain contact angle of a given sorbent, the sample was prepared by spreading P30 or F30 particles on a glass slide with the aid of double-faced adhesive tape to form a thin and uniform layer. After then, a droplet of 2 μL of background solution which contained 0.01 M CaCl 2 and 200 mg/L of NaN 3 was dripped onto the smooth surface of the sample layer, then the picture of the liquid droplet was captured with a built-in video camera. With the droplet image obtained, contact angle between the sorbent and water was calculated using the ellipse method (Rodríguez-Valverde et al., 2002), and each sample was run 10 times. Competitive sorption experiments As an aliphatic compound, lindane was selected as the primary solute, and three aromatic chemicals (phenanthrene, naphthalene, and atrazine) were used as competitors for competitive sorption experiments, with an aim to test if the aliphatic compound may compete for sorption sites on P30 and F30 on the basis that they had quite different molecular structure. A level of 2.44 mg/L for lindane was used for accurate determination of its equilibrium concentration. To perform competitive sorption experiment, the non-labeled stock solutions of phenanthrene, naphthalene, and lindane, as well as the 14C-labeled stock solution of lindane were prepared in methanol (Xihua Special Reagent Company, Tianjin, China). The non-labeled stock solution of atrazine was made in chloroform (Beijing Chemical Reagent Company, Beijing, China). Background solution (pH = 6.5) was prepared which contained 200 mg/L NaN3 (Fisher Scientific Co., USA) to inhibit bioactivity and 0.01 M CaCl2 (Chengnan Chemical Reagent Company, Quzhou, China) to maintain a constant ionic strength. Test solutions containing 14Clabeled and non-labeled lindane with a constant total concentration and non-labeled phenanthrene, naphthalene, or atrazine with varying concentrations were shaken for 1–1.5 h and then added to the screw cap vials with aluminum foil-Teflon liners which contained preweighed
Environ Sci Pollut Res Table 1 Physical properties of P30 and F30 Sorbents
Elemental composition (%) Bulk
P30 F30 a
Ash (%)
(O+N)/Ca
SAb (m2/g)
Surface
C
H
N
O
C
O
96.8 96.2
0.30 0.19
0.07 0.05
0.04 1.52
99.0 97.3
1.0 2.7
2.80 2.05
0.001 0.012
130.4 123.4
Pore volume (cm3/g) Vmicc
Vmes +Vmacd
0.074 0.070
1.169 1.060
Contact angle (°)
151.2±1.415e 146.1±1.086
Polarity index
b
Surface area
c
Micropore volume
d
A sum of meso- and macropore volumes
e
Standard deviation of contact angle
appropriate amount of solid P30 or F30 powder. The organic solvent content in the sorption systems was controlled below 0.1 % by volume to avoid cosolvent effect. It was proved in previous studies that such a low organic solvent content in the single- and bi-solute sorption systems may not affect sorption of organic chemicals to MWCNTs and other sorbents such as chars or soil organic matter (Wang and Xing, 2007; Wang et al., 2009; Guo et al., 2012). The sealed vials were placed on a rotary shaker (ZD-8801, Hualida Laboratory Equipment Co., Taicang, China) to mix for 5 days at room temperature because sorption equilibrium of all these chemicals was tested to have reached in 4 days in the single-solute systems (Wang et al., 2010). Solid-to-solution ratios for lindane in the single-solute systems were used here. After mixing, all vials were centrifuged for 30 min at 3,000 rpm with a centrifuge (LD5-10, Jingli Centrifuge Co., Ltd.,
Beijing, China), and then 2-mL supernatant was sampled and mixed with 4-mL cocktail (Fisher Scientific Co., USA) for radioactivity measurement with a liquid scintillation counter (LS 6500, Beckman Counter Co., USA). It was demonstrated that the dispersed P30 and F30 were completely removed after centrifugation as indicated by the fact that turbidity of the supernatant was comparable to that of the ultrapure water. Details of the sample preparation for turbidity analysis and the results are presented in the Supplementary Material (Table S1). Because of the negligible mass loss of lindane ( phenanthrene > atrazine, which was exactly reverse to the order of the molecular volumes of these chemicals (Table 2). Pelekani and Snoeyink (1999) examined competitive sorption of atrazine with dissolved natural organic matter (DNOM) on activated carbon fibers, where the authors also observed that DNOM with larger molecular size had stronger competition with atrazine. Besides, both phenanthrene and naphthalene were plannary molecules, while lindane and atrazine had non-plannary molecular structure. The non-planar compound
Environ Sci Pollut Res
(lindane) sorbed on MWCNTs was more strongly displaced by a chemical of similar planarity (atrazine) than other compounds with planar molecular structure (i.e., phenanthrene and naphthalene) (Fig. 1). A previous study also consistently showed that pyrene sorbed on SOMs was displaced by a compound of similar molecular structure and planarity (i.e., phenanthrene), although SOM and MWCNTs had quite different composition and structure (Wang et al., 2005). In the competitive sorption systems, natural organic matter exhibited greater competitive effect on sorption of a nonplanar compound (2-phenylphenol) relative to a planar one (phenanthrene) to carbonaceous materials including MWCNTs (Zhang et al., 2011). It was very possible that HOCs of similar planarity occupied the same sites on both SOMs and MWCNTs. It was documented that in the single-solute sorption systems, as the ionic strength was increased from 0.02 to 0.1 M, sorption of non-ionic aromatic compounds by single-walled carbon nanotubes was not significantly affected (Chen et al., 2008b). It could be that sorption of the non-ionic compounds (naphthalene, phenanthrene, lindane, and atrazine) used in the present work may not be substantially influenced by the ionic strength changes in a certain range. If this is true, displacement of presobed lindane on P30 and F30 by various displacers and the competitive sorption behaviors of lindane with the tested competitors on these two sorbents would not be considerably influenced by the variation of ionic strength. Difference in displacement of lindane by various displacers and competitive sorption behaviors of various competitors with lindane on P30 and F30 can be a result of their physicochemical property difference because the ionic strength in the sorption systems was constant. Even if the ionic strength variation may affect sorption of the competitors or displacers on a given sorbent in the single-solute systems, difference in displacement of lindane by various displacers and competitive sorption behavior of various competitors with lindane on P30 and F30 should have the same trend as that observed in the present study because the ionic strength for individual systems was identical. It was reported in a recent study that, as the solution pH was increased from 4 to 10, sorption of phenanthrene by MWCNTs and OH-functionalized MWCNTs (OHMWCNTs) was not affected (Zhang et al., 2010). Similarly, as the solution pH varied from 5.7 to 8.3, no significant effect on sorption of atrazine by single-walled carbon nanotubes was detected in a recent study (Brooks et al., 2012). Based upon these findings, it is most likely that surface chemistry of P30 and F30 may not change significantly if solution pH varies in a certain range mainly because of small amount of hydrophilic functional groups on these two sorbents. Hence, influence of solution pH changes on competitive sorption behaviors of lindane with the tested competitors on P30 and F30 and displacement of lindane presorbed on these sorbents by the displacers could be minimal.
Conclusions The aromatic compounds (i.e., phenanthrene, naphthalene, and atrazine) had competitive sorption effect on sorption of an aliphatic compound lindane on both pristine and OHfunctionalized MWCNTs. As a first report, it was observed in this work that although the aromatic chemicals phenanthrene, naphthalene, and atrazine had different chemical structures with the aliphatic compound lindane, both P30- and F30sorbed lindane can be displaced by these compounds. The coexisting compounds had dissimilar interaction intensity, which in turn had considerably different impact on their competition and displacement on the tested MWCNTs. Furthermore, in the case that sorption affinity of the coexisting solutes to sorbents was comparable and their interaction was weak, molecular volume of the coexisting compounds (e.g., naphthalene and atrazine) highly determined their competition intensity with the primary solute (e.g., lindane) on both P30 and F30 and displacement intensity of lindane by these chemicals from these two sorbents. These findings are critical for evaluating the environmental behaviors such as transport, fate, bioavailability, and potential health risks of the target compound in the bi-solute or even multiple-solute systems. Acknowledgments This work was in part supported by the National Natural Science Foundation of China (41271461, 41130754, 41328003, and 41390241), the National Key Basic Research Program of China (2014CB441101), and the Startup Fund for the Peking University 100Talent Program.
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Environ Sci Pollut Res Helland A, Wick P, Koehler A, Schmid K, Som C (2007) Reviewing the environmental and human health knowledge base of carbon nanotubes. Environ Health Perspect 115:1125–1131 Iijima S (1991) Helical microtubules of graphitic carbon. Nature 354:56–58 Jonker MTO, Koelmans AA (2002) Extraction of polycyclic aromatic hydrocarbons from soot and sediment: solvent evaluation and implications for sorption mechanism. Environ Sci Technol 36:4107–4113 Ju D, Young TM (2005) The influence of natural organic matter rigidity on the sorption, desorption, and competitive displacement rates of 1, 2-dichlorobenzene. Environ Sci Technol 39:7956–7963 Li N, Sioutas C, Cho A, Schmitz D, Misra C, Sempf J, Wang MY, Oberley T, Froines J, Nel A (2003) Ultrafine particulate pollutants induce oxidative stress and mitochondrial damage. Environ Health Perspect 111:455–460 Lin DH, Xing BS (2008) Adsorption of phenolic compounds by carbon nanotubes: role of aromaticity and substitution of hydroxyl groups. Environ Sci Technol 42:7254–7259 Lu YF, Pignatello JJ (2002) Demonstration of the “Conditioning effect” in soil organic matter in support of a pore deformation mechanism for sorption hysteresis. Environ Sci Technol 36:4553–4561 Masciangioli T, Zhang WX (2003) Environmental technologies at the nanoscale. Environ Sci Technol 37:102a–108a Pan B, Xing BS (2008) Adsorption mechanisms of organic chemicals on carbon nanotubes. Environ Sci Technol 42:9005–9013 Pelekani C, Snoeyink VL (1999) Competitive adsorption in natural water: role of activated carbon pore size. Water Res 33:1209–1219 Rao CNR, Sood AK, Subrahmanyam KS, Govindaraj A (2009) Graphene: the new two-dimensional nanomaterial. Angew Chem Int Ed 48:7752–7777 Rodríguez-Valverde MA, Cabrerizo-Vílchez MA, Rosales-López P, Páez-Dueñas A, Hidalgo-Álvarez R (2002) Contact angle measurements on two (wood and stone) non-ideal surfaces. Colloid Surf A 206:485–495
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RESEARCH ARTICLE
Displacement and competitive sorption of organic pollutants on multiwalled carbon nanotubes Xiaofang Shen & Xilong Wang & Shu Tao & Baoshan Xing
Received: 27 June 2013 / Accepted: 28 May 2014 # Springer-Verlag Berlin Heidelberg 2014
Abstract Displacement of lindane presorbed on the pristine and OH-functionalized multiwalled carbon nanotubes (MWCNTs) by phenanthrene, naphthalene, and atrazine, and competition of these compounds with lindane on the aforementioned sorbents were investigated. Displacement of lindane presorbed on MWCNTs by atrazine, naphthalene, and phenanthrene, and competitive sorption effect of these chemicals with lindane on MWCNTs followed the same order: atrazine > naphthalene > phenanthrene. The lowest competition and displacement of lindane by phenanthrene were mainly because of the strong interactions between these two chemicals, whereas interaction of lindane with atrazine and naphthalene was quite low. The more pronounced displacement of lindane by atrazine than naphthalene and higher competitive sorption of lindane with atrazine than with naphthalene can be ascribed to the larger molecular volume of atrazine; thus, the steric hindrance effect is higher relative to naphthalene. This study is valuable for evaluating influence of the coexisting organic compounds on sorption of primary solute towards MWCNTs in the environment.
Keywords Multiwalled carbon nanotubes . Sorption . Displacement . Competition . Organic pollutant Responsible editor: Roland Kallenborn Electronic supplementary material The online version of this article (doi:10.1007/s11356-014-3115-1) contains supplementary material, which is available to authorized users. X. Shen : X. Wang (*) : S. Tao Laboratory for Earth Surface Processes, College of Urban and Environmental Sciences, Peking University, Beijing 100871, China e-mail: [email protected] B. Xing Stockbridge School of Agriculture, University of Massachusetts, Amherst, MA 01003, USA
Introduction Carbon nanotubes (CNTs) have been attracting increasing research attention because of their unique physiochemical properties and broad applications. Sheets of graphite in CNTs are arranged in concentric cylinders (Iijima, 1991). Different from fullerene and graphene which have zero- and twodimensional structures, CNTs have one-dimensional structure (Rao et al., 2009). According to the number of concentric cylinders, CNTs can be classified into two types: singlewalled carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes (MWCNTs). It was reported that the outer diameter of SWCNTs was about 1.4 nm, while that of MWCNTs was larger (Masciangioli and Zhang, 2003). With extremely hydrophobic surfaces, large surface area, and highly porous structure, CNTs were viewed as a promising adsorbent for removing hydrophobic organic compounds (HOCs) from water (Masciangioli and Zhang, 2003). It was reported that CNTs were toxic to organisms (Helland et al., 2007), and toxicity of pollutants would be enhanced after association with the ultrafine particles (Li et al., 2003). Furthermore, sorption of HOCs to CNTs made it more difficult to predict the environmental risks of both HOCs and CNTs (Ferguson et al., 2008). To date, much work has been done to examine sorption mechanisms of HOCs to CNTs in single-solute systems. It was documented that van der Waals, π-π, and electrostatic interactions as well as hydrogen bonds could influence HOC sorption by CNTs (Chen et al., 2008a; Gotovac et al., 2007; Lin and Xing, 2008). In addition, the functionalization treatment of CNTs with hydrophilic moieties changed their surface properties, thus facilitating their practical applications. Since the introduced polar functionalities enhanced their dispersibility and water cluster formation at the surfaces or tube ends of CNTs (Cho et al., 2008; Yu et al., 2011), interaction mechanisms between HOCs and functionalized CNTs differed from those with the pristine ones.
Environ Sci Pollut Res
Due to diversity in emissions of pollutants to the environment, the organic compounds may coexist in the environment in most cases. As a result, interactions between HOCs and CNTs would inevitably be affected by the coexisting solutes. Competition and displacement are two main processes that need to be considered while studying HOC sorption on CNTs in multisolute systems. So far, few studies have been done to examine the competitive sorption of the coexisting HOCs on CNTs. For instance, Yang et al. (2006, 2010) investigated competitive sorption of polycyclic aromatic hydrocarbons (PAHs) on MWCNTs and influence of the dissociation of ionizable organic compounds on their competitive sorption behaviors with a non-polar organic compound (i.e., naphthalene) on these sorbents. The authors found that interactions between the sorbed primary solute and the competitor affected their competition (Yang et al., 2006). It was also proposed that the chemicals introduced to the competitive sorption systems could change the dispersibility of CNTs and water solubility of the primary solute (Pan and Xing, 2008), thereby altering its sorption to CNTs (Wang et al., 2009). However, to date, little information is available on the influence of molecular structure and volume, as well as planarity of the coexisting compounds on their competitive sorption behaviors on MWCNTs. Moreover, to the best of our knowledge, almost no work focused on the underlying mechanisms controlling displacement of the sorbed HOCs by post-introduced chemicals from MWCNTs, and such mechanisms are largely unknown (Ju and Young, 2005). Based on the discussion above, the key objectives of this work were to (1) probe the mechanisms controlling competitive sorption behaviors of the coexisting organic chemicals on MWCNTs and (2) test if the aliphatic compound (e.g., lindane) sorbed to MWCNTs can be displaced by chemicals of dissimilar physicochemical properties (e.g., aromatics) and the associated mechanisms.
Materials and methods Sorbents and sorbates The pristine and OH-functionalized MWCNTs with outer diameter of 20–30 nm (purity >95 %; claimed by the manufacturer) were obtained from Chengdu Organic Chemicals Co., Ltd., Chinese Academy of Sciences. They were labeled as P30 and F30, respectively. The amorphous carbon and catalyst in the synthesized MWCNTs were removed with a mixture of HNO3 and H2SO4 by the manufacturer. The amorphous carbon content in the resulting MWCNTs was less than 5 % after the treatment as claimed by the producer, and it was not further determined in our lab. Very low ash content of P30 and F30 indicated their low remaining catalyst after the treatment (Table 1). Phenanthrene, naphthalene, atrazine, and lindane were selected as sorbates, and both the 14C-labeled and
non-labeled chemicals were purchased from Sigma-Aldrich Chemical Company, USA. Physicochemical properties of the selected sorbates are listed in Table 2. Characterization of sorbents Elemental composition, surface area, and porosity of P30 and F30 were reported in our previous study (Wang et al., 2010) and summarized in Table 1 as basic information for the research purposes of the present work. Contact angles between the sorbents and water were measured using a video contact angle system with sessile drop method (OCA20, Dataphysics, Germany) (Guo et al., 2012). Briefly, to obtain contact angle of a given sorbent, the sample was prepared by spreading P30 or F30 particles on a glass slide with the aid of double-faced adhesive tape to form a thin and uniform layer. After then, a droplet of 2 μL of background solution which contained 0.01 M CaCl 2 and 200 mg/L of NaN 3 was dripped onto the smooth surface of the sample layer, then the picture of the liquid droplet was captured with a built-in video camera. With the droplet image obtained, contact angle between the sorbent and water was calculated using the ellipse method (Rodríguez-Valverde et al., 2002), and each sample was run 10 times. Competitive sorption experiments As an aliphatic compound, lindane was selected as the primary solute, and three aromatic chemicals (phenanthrene, naphthalene, and atrazine) were used as competitors for competitive sorption experiments, with an aim to test if the aliphatic compound may compete for sorption sites on P30 and F30 on the basis that they had quite different molecular structure. A level of 2.44 mg/L for lindane was used for accurate determination of its equilibrium concentration. To perform competitive sorption experiment, the non-labeled stock solutions of phenanthrene, naphthalene, and lindane, as well as the 14C-labeled stock solution of lindane were prepared in methanol (Xihua Special Reagent Company, Tianjin, China). The non-labeled stock solution of atrazine was made in chloroform (Beijing Chemical Reagent Company, Beijing, China). Background solution (pH = 6.5) was prepared which contained 200 mg/L NaN3 (Fisher Scientific Co., USA) to inhibit bioactivity and 0.01 M CaCl2 (Chengnan Chemical Reagent Company, Quzhou, China) to maintain a constant ionic strength. Test solutions containing 14Clabeled and non-labeled lindane with a constant total concentration and non-labeled phenanthrene, naphthalene, or atrazine with varying concentrations were shaken for 1–1.5 h and then added to the screw cap vials with aluminum foil-Teflon liners which contained preweighed
Environ Sci Pollut Res Table 1 Physical properties of P30 and F30 Sorbents
Elemental composition (%) Bulk
P30 F30 a
Ash (%)
(O+N)/Ca
SAb (m2/g)
Surface
C
H
N
O
C
O
96.8 96.2
0.30 0.19
0.07 0.05
0.04 1.52
99.0 97.3
1.0 2.7
2.80 2.05
0.001 0.012
130.4 123.4
Pore volume (cm3/g) Vmicc
Vmes +Vmacd
0.074 0.070
1.169 1.060
Contact angle (°)
151.2±1.415e 146.1±1.086
Polarity index
b
Surface area
c
Micropore volume
d
A sum of meso- and macropore volumes
e
Standard deviation of contact angle
appropriate amount of solid P30 or F30 powder. The organic solvent content in the sorption systems was controlled below 0.1 % by volume to avoid cosolvent effect. It was proved in previous studies that such a low organic solvent content in the single- and bi-solute sorption systems may not affect sorption of organic chemicals to MWCNTs and other sorbents such as chars or soil organic matter (Wang and Xing, 2007; Wang et al., 2009; Guo et al., 2012). The sealed vials were placed on a rotary shaker (ZD-8801, Hualida Laboratory Equipment Co., Taicang, China) to mix for 5 days at room temperature because sorption equilibrium of all these chemicals was tested to have reached in 4 days in the single-solute systems (Wang et al., 2010). Solid-to-solution ratios for lindane in the single-solute systems were used here. After mixing, all vials were centrifuged for 30 min at 3,000 rpm with a centrifuge (LD5-10, Jingli Centrifuge Co., Ltd.,
Beijing, China), and then 2-mL supernatant was sampled and mixed with 4-mL cocktail (Fisher Scientific Co., USA) for radioactivity measurement with a liquid scintillation counter (LS 6500, Beckman Counter Co., USA). It was demonstrated that the dispersed P30 and F30 were completely removed after centrifugation as indicated by the fact that turbidity of the supernatant was comparable to that of the ultrapure water. Details of the sample preparation for turbidity analysis and the results are presented in the Supplementary Material (Table S1). Because of the negligible mass loss of lindane ( phenanthrene > atrazine, which was exactly reverse to the order of the molecular volumes of these chemicals (Table 2). Pelekani and Snoeyink (1999) examined competitive sorption of atrazine with dissolved natural organic matter (DNOM) on activated carbon fibers, where the authors also observed that DNOM with larger molecular size had stronger competition with atrazine. Besides, both phenanthrene and naphthalene were plannary molecules, while lindane and atrazine had non-plannary molecular structure. The non-planar compound
Environ Sci Pollut Res
(lindane) sorbed on MWCNTs was more strongly displaced by a chemical of similar planarity (atrazine) than other compounds with planar molecular structure (i.e., phenanthrene and naphthalene) (Fig. 1). A previous study also consistently showed that pyrene sorbed on SOMs was displaced by a compound of similar molecular structure and planarity (i.e., phenanthrene), although SOM and MWCNTs had quite different composition and structure (Wang et al., 2005). In the competitive sorption systems, natural organic matter exhibited greater competitive effect on sorption of a nonplanar compound (2-phenylphenol) relative to a planar one (phenanthrene) to carbonaceous materials including MWCNTs (Zhang et al., 2011). It was very possible that HOCs of similar planarity occupied the same sites on both SOMs and MWCNTs. It was documented that in the single-solute sorption systems, as the ionic strength was increased from 0.02 to 0.1 M, sorption of non-ionic aromatic compounds by single-walled carbon nanotubes was not significantly affected (Chen et al., 2008b). It could be that sorption of the non-ionic compounds (naphthalene, phenanthrene, lindane, and atrazine) used in the present work may not be substantially influenced by the ionic strength changes in a certain range. If this is true, displacement of presobed lindane on P30 and F30 by various displacers and the competitive sorption behaviors of lindane with the tested competitors on these two sorbents would not be considerably influenced by the variation of ionic strength. Difference in displacement of lindane by various displacers and competitive sorption behaviors of various competitors with lindane on P30 and F30 can be a result of their physicochemical property difference because the ionic strength in the sorption systems was constant. Even if the ionic strength variation may affect sorption of the competitors or displacers on a given sorbent in the single-solute systems, difference in displacement of lindane by various displacers and competitive sorption behavior of various competitors with lindane on P30 and F30 should have the same trend as that observed in the present study because the ionic strength for individual systems was identical. It was reported in a recent study that, as the solution pH was increased from 4 to 10, sorption of phenanthrene by MWCNTs and OH-functionalized MWCNTs (OHMWCNTs) was not affected (Zhang et al., 2010). Similarly, as the solution pH varied from 5.7 to 8.3, no significant effect on sorption of atrazine by single-walled carbon nanotubes was detected in a recent study (Brooks et al., 2012). Based upon these findings, it is most likely that surface chemistry of P30 and F30 may not change significantly if solution pH varies in a certain range mainly because of small amount of hydrophilic functional groups on these two sorbents. Hence, influence of solution pH changes on competitive sorption behaviors of lindane with the tested competitors on P30 and F30 and displacement of lindane presorbed on these sorbents by the displacers could be minimal.
Conclusions The aromatic compounds (i.e., phenanthrene, naphthalene, and atrazine) had competitive sorption effect on sorption of an aliphatic compound lindane on both pristine and OHfunctionalized MWCNTs. As a first report, it was observed in this work that although the aromatic chemicals phenanthrene, naphthalene, and atrazine had different chemical structures with the aliphatic compound lindane, both P30- and F30sorbed lindane can be displaced by these compounds. The coexisting compounds had dissimilar interaction intensity, which in turn had considerably different impact on their competition and displacement on the tested MWCNTs. Furthermore, in the case that sorption affinity of the coexisting solutes to sorbents was comparable and their interaction was weak, molecular volume of the coexisting compounds (e.g., naphthalene and atrazine) highly determined their competition intensity with the primary solute (e.g., lindane) on both P30 and F30 and displacement intensity of lindane by these chemicals from these two sorbents. These findings are critical for evaluating the environmental behaviors such as transport, fate, bioavailability, and potential health risks of the target compound in the bi-solute or even multiple-solute systems. Acknowledgments This work was in part supported by the National Natural Science Foundation of China (41271461, 41130754, 41328003, and 41390241), the National Key Basic Research Program of China (2014CB441101), and the Startup Fund for the Peking University 100Talent Program.
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