Comparison of adsorption isotherms of single-ringed compounds between carbon nanomaterials and porous carbonaceous materials over six-order-of-magnitude concentration range
Introduction
Engineered carbon nanomaterials (CNMs, including carbon nanotubes/CNTs, fullerenes, and graphenes) are considered as promising candidates in many areas of environmental applications, in particular, as special adsorbents in water and wastewater treatment, or pre-concentrators for organic analysis [1], [2], [3], [4]. Owing to the large specific surface area and high surface hydrophobicity, CNMs exhibit strong adsorption affinity for a variety of hydrophobic organic contaminants in aqueous solution [5], [6], [7]. In addition, the sp2-bonding graphitic sheets of CNMs have very high electronic polarizability, and therefore are able to induce strong π–π stacking interaction with organic compounds bearing π-electrons [8], [9], [10], [11]. For example, Long and Yang attributed the much higher adsorption affinity of dioxin to CNTs than an activated carbon/AC to the strong π–π stacking interaction between the two benzene rings of dioxin and the graphitic surface of CNTs [9]. Similarly, Björk et al. predicted strong π–π stacking interaction of polyaromatic and antiaromatic hydrocarbons with graphene based on the ab initio van der Waals density functional calculation [11].
The large-scale, continuously increasing production of CNMs has lowered the cost of these materials substantially. This brings the possibilities of using CNMs as special adsorbents for the development of more effective nano-enabled environmental technologies that would otherwise be impossible. An understanding of the mechanisms controlling the differences in adsorption properties between CNMs and conventional porous carbonaceous materials (e.g., AC and carbon fiber)—in particular, the structure–activity correlations—will be of significant practical value for exploring the potential applications of CNMs as special adsorbents. It is necessary to note that even though conventional carbon adsorbents (e.g., AC) and CNMs share many similarities (e.g., large specific surface area and high hydrophobicity)—and thus similar adsorption mechanisms—the two classes of materials also differ significantly in certain characteristic physicochemical properties. In particular, the pore structures of AC and carbon fiber are pronouncedly different from those of CNMs. AC is dominated by irregularly shaped, rigid micropores (<2 nm in size), whereas CNMs are likely dominated by mesoporous interstices (2–50 nm in size) resulting from coagulation and aggregation of individual nanoparticles/nanotubes driven by π–π interactions. On one hand, AC is expected to show more prominent molecular sieving effect in adsorption than CNMs, particularly when the adsorbate molecules are bulky [12]. On the other hand, adsorption of small molecules to microporous AC and soot might be enhanced by the micropore-filling effect as the sizes of adsorbate molecules are similar to the pore sizes of the adsorbent [13], [14], [15]. Based on the analysis above, it is anticipated that some general rules should exist for the superiority of CNMs versus bulk carbonaceous materials as adsorbents for different organic molecules.
Thus far, only very limited studies [7], [12], [16], [17], [18], [19] have been conducted to compare the adsorption properties of organic contaminants between these two classes of carbonaceous adsorbents, and the reported results have been inconsistent, particularly for low-molecular-weight adsorbates (e.g., monoaromatic compounds) that may induce the micropore-filling effect [17], [18], [19]. Furthermore, in most of the literature studies adsorption isotherms of organic contaminants to carbonaceous adsorbents were collected within relatively narrow aqueous-phase concentration ranges (2–3 orders of magnitudes), and the concentrations examined were generally much higher than environmentally relevant concentrations, which are typically in the low-microgram per liter (μg/L) range or lower. Adsorption properties at low concentrations may not be extrapolated based on these observations because the relative dominance/contributions of different adsorption mechanisms can be quite different at different adsorbate concentrations (for instance, micropore-filling to the high-energy adsorption sites is often more dominant at low concentrations).
The main objective of this study was to understand the differences between CNMs and porous carbonaceous materials in adsorption of organic contaminants and the underlying mechanisms. This is of particular relevance and importance to many fields, such as water and wastewater treatment, nano-enabled environmental technologies and the associated environmental impacts. To answer this increasingly important question, we herein compared adsorption isotherms of a planar monoaromatic adsorbate (4-chloronitrobenzene) and a nonplanar single-ring aliphatic adsorbate (trans-1,2-dichlorocyclohexane) between three CNMs (single-walled CNTs/SWNT, multi-walled CNTs/MWNT, and graphene) and two commercial ACs over a concentration range of six orders of magnitude. A template-synthesized mesoporous carbon (CMK-3) and nonporous graphite were included as additional adsorbents for comparison to better understand the pore effects on adsorption. Adsorption experiments of two additional planar monoaromatic compounds (1,2-dichlorobenzene and 1,4-dichlorobenzene) to selected adsorbents were conducted to investigate the factors that may affect the possible micropore-filling effect associated with the microporous adsorbents. A Polanyi theory-based adsorption model was applied to further understand the adsorption mechanisms.
Section snippets
Adsorbents
Two CNT products, including a SWNT and a MWNT were purchased from Nanotech Port Co., Ltd. (Shengzhen, Guangdong Province, China). Based on the information provided by the manufacturer, the SWNT contained more than 90% (by volume) CNTs, and the content of SWNT with outer diameters less than 2 nm was above 50%; MWNT contained more than 97% CNTs and less than 3% impurities, and the outer diameters ranged from 10 to 20 nm. The length of the two CNTs ranged from 5 to 15 μm. Graphene was prepared from
Characterization of carbonaceous adsorbents
The pore size characterization and Brunauer–Emmett–Teller (BET) surface areas of the carbonaceous materials are summarized in Fig. 1 and SI Table S1, which were determined by N2 adsorption/desorption isotherms at 77 K (displayed in SI Fig. S2). According to the IUPAC [28], the N2 adsorption isotherms can be classified to type II for MWNT and graphite, type IV for SWNT, graphene and CMK-3, and type I for AC1 and AC2. The surfaces of all adsorbents were dominated by graphitized C (>95%) and the O
Conclusion
The present study shows that CNMs are likely superior adsorbents for planar aromatic compounds than bulk carbonaceous materials at environmentally relevant concentrations. Nonetheless, CNMs are not necessarily always the superior adsorbents. Nonplanar aliphatic compounds such as trans-1,2-dichlorocyclohexane had higher adsorption to conventional bulk carbonaceous materials. Findings of this study indicate that the effectiveness of carbonaceous adsorbents is highly adsorbate-specific and depends
Acknowledgement
This work was supported by the National Science Foundation of China (Grant 21237002), National Key Basic Research Program of China (Grant 2014CB441103), and China National Funds for Distinguished Young Scientists (21225729). We thank Dr. Yong Guo of Nanjing University for performing the DFT calculation.
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