Enhanced adsorptive removal of Indigo carmine dye performance by functionalized carbon nanotubes based adsorbents from aqueous solution: equilibrium, kinetic, and DFT study
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The present work considers an adsorptive removal of Indigo carmine (IC) dye onto nanotube carbon (CNTs). The pure CNTs were prepared via chemical vapor deposition (CVD) method utilizing methane gas as a carbon source at 1000 °C in a quartz tube. The morphology and surface chemical structure of the adsorbents were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), nitrogen adsorption/desorption technique, and thermal gravity analysis (TGA). The parameters of the IC dye adsorption, such as initial concentration, contact time, pH, and mass-loaded adsorbent, were evaluated. The kinetic study confirmed that a pseudo-second-order model was best fitted to the adsorption data. The removal efficiency of adsorption onto pure and COOH-functionalized CNTs was 84% and 98.7% at 15 min, respectively. The equilibrium results were fitted well to the Langmuir isotherm model. The adsorption capacity of the CNT and COOH–CNT was 88.5 and 136 mg/g, respectively. The reusability of the adsorbents was studied, and after eight cycles, the efficiency decreased to 70%. Moreover, the density functional theory calculations confirmed that the functionalization of CNTs with COOH groups improves the adsorption properties of IC due to the formation of hydrogen-bonding interactions.
KeywordsIndigo carmine Adsorption Langmuir Carbon nanotube Regeneration
Dye affects human health directly, so the water treatment process is expanded with a significantly increasing trend. The existence of a double bond in the structure of IC dye causes it to be a permanent dye that is being utilized in the industry for coloring. There are salts and organic materials in the wastewater of the textile dye in a high amount. However, this dye harms the lives of humans, as it is carcinogenic, and leads to allergy [1, 2, 3]. Moreover, disturbances of IC on the environment are noticeable and hence pose many threats to the ecosystem. Various dye removal methods from wastewater are available. These include ultrafiltration , electro-chemical , and adsorption  techniques. Unfortunately, the utilization of the above methods has some crucial such as low removal efficiency, expensive cost, and catalytic reduction [7, 8, 9]. Among these dye removal techniques, adsorption is highlighted as an advanced, efficient, and economical method in low temperature and pressure conditions with a low-cost adsorbent. The adsorption process is the most conventional method. In this regard, vast numbers of low-cost adsorbents have been studied to remove such pollutants . Various materials have been concerned for adsorption of dyes for water treatment, such as biomass-derived adsorbents [10, 11]. Studies focused that activated carbons are suitable and promising adsorbent for organic water pollutants. The wastewater treatment with activated carbon is an effective method. Activated carbon is being extensively employed to remove unpleasant compounds, including different dyes, pigments, and other pollutants . Activated carbon is one of the considerable potentials in treating wastewater, as it possesses high surface area, high efficiency in removing dyes, and high adsorption capacity. Most of the pollutants which are present in wastewater could be removed by active carbon synthesized from different sources [12, 13].
Because of the large surface area, low cost, facile synthesis, and π-electrons on the surface, easy availability, and various functional groups on the surface of the CNTs, these carbonous honey ball structures have found many potential applications in different research fields like of water purification [14, 15]. One of the effective treatment methods of wastewaters is considered with carbon nanotubes for the removal of various dye from waste solution due to its large surface area, porous nature, high adsorption capacity, high purity, and easy availability [1, 14]. Lin and Xing  have evaluated the adsorption of aromatic compounds with the aid of carbon nanotubes. They confirmed that the adsorption of aromatic compounds by CNTs increases with increasing the number of aromatic rings and was greatly enhanced by –OH substitution. They also investigated the role of aromatic structure and –OH substitution in the polar aromatics–CNTs’ structure.
This research focuses on dye removal through adsorption by utilization of an adsorbent, where the equilibrium content of the adsorbed IC dye, the amount of used adsorbent, temperature, duration of the adsorption process, and pH of the solution are studied. In the evaluation of the adsorption parameters, the optimum values of the parameters are calculated by changing one parameter and keeping the other one constant. By the combination of experimental results and density functional theory simulations, we study the adsorbent capacity of the pure and COOH functionalized carbon nanotubes (CNTs), and also compare the kinetics, isotherm, and adsorption capacity of the adsorbents to select the appropriate adsorbent for IC dye removal.
Materials and methods
Synthesis of CNTs
The synthesis of CNTs as adsorbents was carried out by chemical vapor deposition (CVD) protocol . The catalytic reaction was utilized by Co–Mo/MgO catalyst in a quartz tube furnace reactor at 1000 °C for 30 min. The CVD reaction was performed using methane and hydrogen as a carbon source. The flow rate of the methane was 50 ml/min, and hydrogen passes through the tubular reactor with a 250 ml/min flow rate as the carrier gas. The furnace temperature was decreased and cooled to room temperature under argon atmosphere. After completion of the reaction, the black material was synthesized. The black product is mixed with HCl solution for about 16 h, due to refine the synthesized CNT and then wash with distillate water several times to maintain the pH in the neutral range.
Synthesis of CNT–COOH
To prepare the CNTs with carboxylic functionalized groups (COOH–CNT), sulfuric acid, and nitric acid was stirred with a 3:1 volumetric ratio, and then, CNTs were dispersed into above solution and sonicated for 3 hours. After that, the CNTs were washed with deionized water until the neutral solution pH, and then, the mixture was filtered and dried at 120 °C. The recited procedure is designed to synthesize the functionalized carbon nanotubes with covalent carboxylic groups (COOH–CNTs).
Characterization of the synthesized materials
The formation of the CNT samples was investigated by X-ray diffraction (XRD) patterns (Phillips PW 1840 X-ray diffractometer with CuKα X-ray radiation source). The morphology of the synthesized materials was determined by scanning electron microscopy (SEM) instrument utilization of Mira 3-XMU model with accelerating potential 7.0 kV. The surface bonding and surface chemical were examined by Fourier transform infrared spectra (FTIR) in the frequency range of 4000–400 cm−1 by KBr pellets. The surface area and porous characterization were analyzed with N2 adsorption/desorption at 77 K overnight by Micromeritics ASAP 2010 system using Brunauer–Emmett–Teller (BET) isotherm. To evaluate the stability of the synthesized adsorbents, Thermo gravimetric analysis (TGA) was implemented between 20 and 700 °C by SDT Q600, UK, with a heating rate of 10 °C/min under N2 atmosphere. The functionalization is proven by comparing the weight loss of the pristine CNT and COOH–CNT, respectively.
Results and discussion
Characterization of adsorbents
N2 adsorption isotherm data
Surface area (m2/g)
Average pore diameter (nm)
Pore volume (cm3/g)
The effect of adsorbent mass
The effect of pH
Kinetic study data of CNT and COOH–CNT
0.3 × 10−1
1.2 × 10−2
0.38 × 10−1
9.3 × 10−2
Isotherm study of the CNT and COOH–CNT for IC dye adsorption
2 × 10−2
6.5 × 10−1
3.76 × 10−4
4.3 × 10−1
7.8 × 10−1
5.21 × 10−4
Desorption and adsorbents regeneration experiments
To obtain a detailed understanding of the adsorption mechanism of IC over pure and COOH functionalized CNTs, high-level DFT calculations were performed on some appropriate model compounds. Taking the armchair (6, 6) CNT as the representative, the adsorption of IC molecule on both pure and COOH–functionalized CNTs was compared in detail. Figure s5 of Supporting Information shows the optimized structure of truncated (6, 6) pure and COOH–functionalized CNTs. The calculated average C–C bond distance in the pure CNT is about 1.43 Å, which is in excellent agreement with the previous theoretical reports [45, 46]. According to our results, the attachment of –COOH groups on the surface and edges of CNT are energetically favorable. The average adsorption energy of the COOH group on the pure CNT is about − 130 kJ/mol, which is good agreement with that obtained in the earlier theoretical studies . Note that this value is the highest (more negative) among all other possible configurations, most likely due to the less steric repulsion between the COOH groups. Meanwhile, the attachment of COOH pulls out the C atom of CNT a little from the tube wall, leading to a local rehybridization at the adsorption site. Consequently, a small radial distortion appears on the tube wall, as shown in Fig. s5. Besides, the electron density analysis of pure and COOH-modified CNTs indicates that the addition of COOH groups tends to induce some charge redistribution above the CNT surface. There is also a quite small charge transfer (≈ 0.08 e, according to the Mulliken charge density analysis) from each COOH group to the CNT, indicating that the CNT acts as an electron acceptor in COOH–CNT. Besides, the regions with the smallest electron density in COOH–CNT are associated with the hydrogen atom of COOH groups, which implies the potential of these sites to interact with electron-rich moieties.
In the present research, the deep adsorptive removal of IC dye from the liquid phase was carried out using the pure CNT and COOH–CNT as adsorbents. A parallel series of adsorption experiments were done to investigate and optimize the process parameters. We found that compared to the pure CNT, the COOH–CNT exhibits more considerable potential for IC dye removal from wastewater. The adsorption mechanism of the pristine CNT was obtained as the π–π interaction between the adsorbate and adsorbent. In addition to this mechanism, the adsorption of IC over COOH–CNT proceeds via the formation of hydrogen bonding interaction, which causes a fast adsorption process compared to the pristine CNT. The mass of the adsorbent loaded as an adsorption parameter was evaluated, and the optimum amount was 0.5 g. The equilibrium capacity of the CNT and COOH–CNT was 88.5 and 136 mg/g within 15 min at room temperature for 100 ppm initial concentration of IC dye. The Langmuir model was so suitable for the equilibrium data, which was indicated the monolayer adsorption process. The RL coefficient of the Langmuir was 0.53 for COOH–CNT, focused on the favorable adsorption process. The kinetic data were fitted the pseudo-second-order model well, which indicated the chemisorption process for IC dye adsorption removal. The rate-controlling step of the IC dye adsorption was substantiated in the pore diffusion and intraparticle step. Moreover, CNT-based adsorbents presented good reusability behavior. The DFT calculations revealed that the functionalization of CNT with COOH groups enhances the adsorption energy of the IC via the formation of hydrogen-bonding interactions. In conclusion, CNT was a promising alternative for IC dye removal from wastewater.
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