Interaction of removal Ethidium Bromide with Carbon Nanotube: Equilibrium and Isotherm studies
© Moradi et al.; licensee BioMed Central Ltd. 2014
Received: 5 November 2013
Accepted: 10 November 2013
Published: 8 January 2014
Drinking water resources may be contaminated with Ethidium Bromide (EtBr) which is commonly used in molecular biology laboratories for DNA identification in electrophoresis. Carbon nanotubes are expected to play an important role in sensing, pollution treatment and separation techniques. In this study adsorption of Ethidium Bromide on single-walled carbon nanotubes (SWCNTs) and carboxylate group functionalized single-walled carbon nanotube (SWCNT-COOH) surfaces have been investigated by UV–vis spectrophotometer. The effect of contact time, initial concentration and temperature were investigated. The adsorbents exhibits high efficiency for EtBr adsorption and equilibrium can be achieved in 6 and 3 min for SWCNTs and SWCNT-COOH, respectively. The effect of temperature on adsorption of EtBr by toward adsorbents shows the process in this research has been endothermic. The results showed that the equilibrium data were well described by the Langmuir isotherm model, with a maximum adsorption capacity of 0.770 and 0.830 mg/g for SWCNTs and SWCNT-COOH, respectively. The adsorption of EtBr on SWCNT-COOH is more than SWCNTs surfaces. A comparison of kinetic models was evaluated for the pseudo first-order, pseudo second-order models. Pseudo second-order was found to agree well with the experimental data.
The contamination of drinking water with Non-radioactive materials such as Ethiduim bromide (EtBr) has become one of the most serious problems in water environment, especially in urban areas [1–3]. Ethidium bromide (3,8-diamino-6-phenyl-5-ethylphenanthridinium bromide, EtBr;), a powerful mutagenic , is an intercalating agent which resembles a DNA base pair. Due to its unique structure, it can easily intercalate into DNA strand. Therefore, it is commonly used as nucleic acid fluorescent tag in various techniques of the life science field. EtBr is a potent mutagen for which the Environmental Health & Safety advises a detoxification protocol , since earlier recommended oxidation by household bleach was recognized to lead to possible byproducts that could be more hazardous than the EtBr itself . Taking the adsorption of dyes and organic pollutant, the usability of various natural and synthetic adsorbents have been studied, such as banana peel , orange peel , calcined layered double hydroxides , hypercrosslinked polymeric adsorbent , pinecone derived activated carbon . Usually, the effectiveness of any adsorption process largely depends on the physicochemical properties of the adsorbent used. Since the discovery by Iijima , carbon nanotubes (CNTs) have attracted great attention in multidisciplinary areas due to their unique hollow tube structure and their many outstanding mechanical, electronic and optical properties . In particular, carbon nanotubes (both multi-walled (MWCNT) and single walled (SWCNT)) are promising materials for several applications such as high performance composites [13–15], components in water filters [16, 17], environmental sensors [18, 19], building blocks for electronic nanodevices , drug delivers , among others. CNTs are also considered to be extremely good adsorbents and successfully remove many kinds of organic and inorganic pollutants such as pentachlorophenol , benzene, toluene, ethylbenzene and p-xylene , o-xylene and p-xylene  and heavy metal ions such as U(VI) , Cr(VI) , Zn(II) , Cu(II) , Pb(II) , Hg(II)  and Cd(II) [31, 32] from water. Activation of CNTs plays an important role in enhancing maximum adsorption capacity. Activation causes modification in the morphology and functional groups surfaces and causes removal of amorphous carbon. The objective of this study was to investigate the possible use of single walled carbon nanotubes as an alternative adsorbent material for the removal of EtBr from drinking water. Adsorption isotherms and kinetics parameters were also calculated and discussed. The dynamic behavior of adsorption was investigated on the effect of initial EtBr concentration, contact times and temperature of the solution.
Material and methods
SWCNTs and SWCNT-COOH (Armchair (6,6), Young’s Modulus (0.94 T TPa), Tensile strength (GPa126.2 T), purity > 95%; diameter 1–2 nm; length, 5–30 nm; surface area, ~ 400 m2/ g; and manufacturing method purchased from NanoAmor Nanostructured & Amorphous Materials, Inc. (USA) and catalytic chemical vapor deposition (CVD)) were prepared from DC-PECVD (Model: SI-PE80) Toseye Hesgarsazan Asia Co. Doubly distilled water was used and all adsorbents were washed before using. A scanning electron microscope (SEM) (JEOL JSM-5600 Digital Scanning Electron Microscope) was used to characterize the SWCNT-COOH and SWCNTs for morphological information. Ethidium bromide (molecular Weight: 394.35; molecular Formula: C21H20BrN3) was supplied by Merck, Germany (maximum purity available, ≥95%). All solutions were prepared with deviations of less than ±0.1% from the desired concentrations.
where N is the number of data points. Each experiment was conducted in triplicate under identical conditions to confirm the results, and was found reproducible (experimental error within 3%).
where KF (1/mg) and n (dimensionless) are the Freundlich adsorption isotherm constants, being indicative of the extent of adsorption and the degree of nonlinearity between solution concentration and adsorption, respectively.
The adsorption data were analyzed according to Eq. (6). KT is the equilibrium binding constant (L/mg) corresponding to the maximum binding energy and constant b = RT/β (KJ/mol) is related to the heat of adsorption.
Therefore a minimization procedure is adopted to maximize the coefficient of determination, between the theoretical data for predicted from the linearized form of Redlich-Peterson isotherm equation and the experimental data.
Pseudo first-order kinetics
where qe (mg/ g) and q are the amount each of EtBr adsorbed on the adsorbents at equilibrium and at various times t and k1 is the rate constant of the pseudo first-order model for the adsorption (1/min) .
Pseudo second-order kinetics
where qe and q are defined as in the pseudo first-order model and k2 is the rate constant of the pseudo second-order model for adsorption (g/ mg min) .
Characterization of carbon nanotubes
Effect of contact time
Effect of EtBr initial concentration
In the present study, the adsorption experiments are performed to study the effect of EtBr initial concentration by varying it from 10 to 40 mg/L, while maintaining the SWCNT-COOH and SWCNTs amount 0.02 g/L.
Effect of temperature on the adsorption
Figure 2 shows the representative plots of adsorption amount of EtBr onto SWCNT-COOH and SWCNTs versus different temperature ranging from 298 to 328°K. In this section, concentration of EtBr was 30 mg/L and contacts time were 3 and 6 min for SWCNT-COOH and SWCNTs, respectively. It was found that the adsorption capacity of EtBr onto carbon nanotubes was found to increase with a rise in temperature.
Langmuir isotherm parameters and ARE parameter for EtBr by SWCNTs surface
Ce/qe = 1/KQm + Ce/Qm
1/qe = 1/Qm + 1/KQmCe
qe = Qm–qe/KCe
qe/Ce = KQm - Kqe
Langmuir isotherm parameters and ARE parameter for EtBr by SWCNT-COOH surface
Ce/qe = 1/KQm + Ce/Qm
1/qe = 1/Qm + 1/KQmCe
qe = Qm–qe/KCe
qe/Ce = KQm - Kqe
Freundlich, Temkin and Redlich-Peterson isotherm parameters for removal EtBr by SWCNT-COOH and SWCNTs surface
Comparison of the pseudo first- and second-order rate constants
(g/ mg min)
Effect of contact time
According to Figure 4, EtBr adsorption rate increased quickly with time and then reached equilibrium. SWCNT-COOH and SWCNTs surfaces were treated by EtBr solutions (30 mg/ L and T = 298 ± 1 K) in order to optimize contact time respect to EtBr. The amounts of adsorbed ion on the adsorbents were analyzed using UV-visible Spectrophotometer.
Effect of temperature
According to Figure 1; the adsorption capacity of EtBr onto carbon nanotubes was found to increase with a rise in temperature, suggesting the process in this research has been endothermic . An increase in the amount of equilibrium adsorption of each ion with the rise in temperature may be explained by fact that the adsorbent sites were more active at higher temperatures.
According to Figure 2 the constants of Langmuir models is obtained from fitting the adsorption equilibrium data and is listed in Tables 1 and 2. A comparison of the R2 and ARE values given in Tables 1 and 2 with 2 indicates that Langmuir isotherm better fits the experimental data than does the other isotherms. The validity of Langmuir isotherm suggests that adsorption is a monolayer process, and adsorption of all species requires equal activation energy. Moreover, K values for various adsorbents followed the order SWCNT-COOH > SWCNTs, suggesting that the affinity of the binding sites for each ion also followed this order. The RL parameter lies between 0.22 and 0.60 which proves that the adsorption process is favorable and SWCNTs and SWCNT-COOH are potential adsorbents for the removal of EtBr from drinking water.
The high correlations coefficient and high agreement that exist between the calculated and experimental qe values of the pseudo second-order kinetic model over the other model renders it best in adsorption of EtBr. This confirms that the sorption data for removal of EtBr on SWCNTs and SWCNT-COOH are well represented by the pseudo-second-order kinetics for the entire sorption period.
Repeated usage of CNT filters is one of the critical considerations in a treatment plant from economics standpoint. CNT filters are reusable as evidenced by Brady-Estevez et al. (2008) and Srivatsava et al. (2004). On the other hand CNT filters, due to their excellent mechanical properties prevent such deformational changes. Moreover, CNT filters support simple thermal regeneration techniques, whereas with polymeric membranes it is not possible [46, 47].
This investigation examined the equilibrium and the dynamic adsorption of EtBr on SWCNT-COOH and SWCNTs surfaces. The adsorption capacity was highest when 20 mg/ l SWCNT-COOH and SWCNTs were added. The results suggested that the adsorption of EtBr on SWCNT-COOH and SWCNTs surfaces increased with temperature. The adsorption of mentioned ions on SWCNT-COOH is more than SWCNTs surfaces. SWCNT-COOH and SWCNTs are after adsorption can be recycle and usable. In addition, SWCNT-COOH and SWCNTs are adsorbents have desorption capabilities.
This study was conducted in the Department of Chemistry, Shahr-e-Qods Branch and was financially supported by Islamic Azad University.
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