Dyes are one of the most hazardous chemical compound classes found in industrial effluents and need to be treated since their presence in water bodies reduces light penetration, precluding the photosynthesis of aqueous flora. In the present study, single-walled carbon nanotubes (SWCNTs) was used as an adsorbent for the successful removal of Reactive Red 120 (RR-120) textile dye from aqueous solutions. The effect of various operating parameters such as initial concentration of dye, contact time, adsorbent dosage and initial pH was investigated in order to find the optimum adsorption conditions. Equilibrium isotherms were used to identify the possible mechanism of the adsorption process. The optimum pH for removing of RR-120 dye from aqueous solutions was found to be 5 and for this condition maximum predicted adsorption capacity for RR-120 dye was obtained as 426.49 mg/g. Also, the equilibrium data were also fitted to the Langmuir, Freundlich and BET equilibrium isotherm models. It was found that the data fitted to BET (
Environmental pollution has recently become a severe problem worldwide [
Reactive dyes represent an important portion of the commercial synthetic dyes, mainly because of their excellent binding ability initiated by the formation of a covalent bond between their reactive groups and the surface groups of the textile and cellulose fibers. They are used extensively in textile industries, and their release in the ecosystem represents increasing environmental danger, because of their toxicity, mutagenicity, and nonbiodegradability [
Improper treatment and disposal of dye-contaminated wastewaters from textile, dyeing, printing, ink, and related industries have provoked serious environmental concerns all over the world [
The most efficient procedure for removal of synthetic dyes from industrial effluents is the adsorption procedure, because the dye species are transferred from the water effluent to a solid phase, diminishing the effluent volume to a minimum. Subsequently, the adsorbent can be regenerated or kept in a dry place without direct contact with the environment [
Carbon nanotubes (CNTs), ever since their discovery, have attracted extensive attention due to their unique physicochemical and electrical properties. CNTs include single-walled carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes (MWCNTs) depending on the number of layers comprising those [
In the present study, single-walled carbon nanotubes (SWCNTs) were chosen as sorbent for removal of RR-120 dye. The aim of this study is to investigate the adsorption of RR-120 dye on single-walled carbon nanotubes (SWCNTs) under various conditions. So, the influence of several operating parameters such as initial concentration, contact time, adsorbent dosage, and initial pH of solution was investigated. Equilibrium isotherms were used to identify the possible mechanism of the adsorption process. This information will be useful for designing and operating color removal systems based on different local water qualities.
The model textile reactive dye (Reactive Red 120) was purchased from Merck Company (Germany). The chemical structure and some properties of the Reactive Red-120 dye are presented in Table
Some characteristics of the investigated dye.
Characteristic | Reactive Red-120 (RR-120) |
| |
Molecular formula | C44H24C12N14Na6O20S6 |
Color index name | Reactive Red-120 |
Molecular weight | 1469,34 g/mol |
Water solubility | 70 (g L−1) |
|
515 |
Class | Diazo (–N=N– bond) |
The chemical structure of the Reactive Red-120.
SWCNTs (provided from Research Institute of Petroleum Industry (RIPI), Tehran, Iran) were selected as adsorbents to study the adsorption characteristics of RR-120 dye from aqueous solutions. On the basis of the information provided by the manufacturer, the SWCNTs were synthesized by catalytic chemical vapor deposition (CVD) method. The morphologies of SWCNTs were examined by using SEM (JEOL microscope, model JSM-6700F) (see Figure
SEM image of SWCNTs sample.
Because carbon nanotubes had the amorphous carbon and therefore the adsorption rate is very low, therefore, carbon nanotube should be purified. In order to functionalize SWCNTs, 0.3 g of the SWCNTs was dispersed in 25 mL of nitric acid (65 wt %) in a 100 mL round bottom flask equipped with a condenser and the dispersion was refluxed under magnetic stirring for 48 h. After that, the resulting dispersion was diluted in water and filtered. The resulting solid was washed up to neutral pH, and the sample was dried in vacuum at 40°C overnight.
Dye removal experiments with the synthesized SWCNTs were carried out as batch tests in 250 mL flasks under magnetic stirring. The experiments were conducted individually for each of the model dyes, but the same procedure was used for both, as detailed below. Each test consisted of preparing a 100 mL of dye solution with a desired initial concentration and pH by diluting the stock dye solutions with double distilled water, and transferring it into the beaker on the magnetic stirrer. The pH of the solution was adjusted using 0.1 NHCl or NaOH solutions by a pH meter model E520 (Metrohm Herisau, Switzerland). A known mass of SWCNTs powder (adsorbent dosage) was then added to the solution, and the obtained suspension was immediately stirred for a predefined time. After the mixing time elapsed, the suspension was allowed to settle and the supernatant was analyzed using a double beam UV/vis spectrophotometer (Shimadzu, Tokyo, Japan; Model 1601) at 515 nm for Reactive Red-120. Then the amount of dye (RR-120 dye) adsorbed,
To express the percent of dye removal, the following equation was used:
It is known that the solution pH can affect the surface charge of the adsorbent, the degree of ionization of the different pollutants, the dissociation of functional groups on the active sites of the adsorbent as well as the structure of the dye molecule [
At present study, the effect of pH on the RR-120 dye adsorption capacities of the SWCNTs was conducted at varying pH (pH 3–12) with 50 mg/L fixed initial dye concentrations and adsorbent dosage 0.01 g/L for 180 min. As can be seen from Figure
Effect of initial pH on the adsorption of RR-120 to SWCNTs. (
It is well known that the surface of CNTs contains some oxygen groups such as carboxylic groups (–COOH) and hydroxyl groups (–OH) after acid treatment [
The adsorbent concentration is an important parameter because this determines the capacity of the adsorbent (SWCNTs) for a given initial RR-120 dye concentration. In order to attain the optimal amount of SWCNTs for the adsorption of RR-120 dye, 0.01–0.05 g/L adsorbent (SWCNTs) was used for adsorption experiments at fixed initial pH (pH 5), initial dye concentration (50 mg/L), and temperature (23 ± 2°C) for 180 min.
As it can be seen from Figure
Effect of adsorbent dosage on RR-120 adsorption onto SWCNTs. (
Contact time is one of the most important parameters for practical application. Figure
Effect of contact time on RR-120 adsorption onto SWCNTs. (
The initial concentration provides an important driving force to overcome all mass transfer resistance of the dye between the aqueous and solid phases [
Remarkably, the adsorption capacity of RR-120 dye increases (Figure
Effect of initial dye concentrations on RR-120 adsorption onto SWCNTs. (Adsorbent dosage = 0.04 g/L, contact time = 180 min, pH = 5, temperature = 23 ± 2°C).
The initial dye concentrations provide an important driving force to overcome the mass transfer resistance of the dye between the aqueous phases and the solid phases, so increasing initial concentrations would enhance the adsorption capacity of dye. On the other hand, the dye adsorption process generally involves the first transport of dye molecules from bulk solution through liquid film to the exterior surface of adsorbent and then from the exterior surface to the pores of the adsorbent, which reflects that the adsorption of dye onto the adsorbent is relevant to the initial concentration. In general, the total number of available adsorption sites is fixed for a given adsorbent dose. It is reasonable to particulate that the larger ratio of active adsorption sites of the adsorbent is available at lower initial concentration. Therefore, the percentage removal of dye is greater at lower initial concentration but smaller at higher initial concentration.
Isotherms study can describe how an adsorbate interacts with adsorbent. The isotherm provides a relationship between the concentration of dye in solution and the amount of dye adsorbed on the solid phase when both phases are in equilibrium. At present study equilibrium adsorption data were analyzed by using the Langmuir, Freundlich and BET isotherm models (Figures
Langmuir isotherm for sorption of RR-120 onto SWCNTs (
Freundlich isotherm for sorption of RR-120 onto SWCNTs (
BET isotherm for sorption of RR-120 onto SWCNTs (
The adsorption isotherm models of Langmuir, Freundlich, and BET can be represented in the linear form as follows:
The
Isotherm parameters for adsorption of RR-120 onto SWCNTs at 23 ± 2°C.
Langmuir isotherm | Freundlich isotherm | BET isotherm | ||||||
---|---|---|---|---|---|---|---|---|
|
|
|
|
|
|
|
|
|
2500 | 0.031 | 0.9190 | 134.8 | 1.07 | 0.8819 | 384.6 | 0.00025 | 0.9897 |
From the present study, it could be concluded that the SWCNTs can be used effectively for the removal of the RR-120 dye from aqueous solutions. The percentage eliminated was found to depend on the amount of adsorbent, pH, the initial concentration of the dye, and the dye-adsorbent contact time. The SWCNTs were able to remove up to 89% of RR-120 dye from solutions whose initial concentration varied between 10 and 200 mg/L. The adsorption of RR-120 dye on SWCNTs has been described by the Langmuir, Freundlich, and BET isotherm models. Also the results of this study showed that the BET model is more suitable for describing the adsorption equilibrium of Reactive Red-120 dye onto SWCNTs.
This study was funded by the health research deputy of Zahedan University of Medical Sciences (Project no. 90-2377) and was conducted in the Chemical Laboratory of School of Public Health, Zahedan University of Medical Sciences. It is also worth to say that there is no direct relation or financial support by any commercial companies mentioned in the paper with this research.