Adsorption of Bismarck Brown R Dye Onto Multiwall Carbon Nanotubes

Dyes have long been used in different types of industries such as, dyeing, textiles, paper, plastics, leather and cosmetics [1]. Color stuff discharged from these industries pose hazards and has an environmental impact [2].The presences of dyes in water are causing problems, such as, reducing oxygen levels in water; interfering with penetration of sunlight into waters; retarding photosynthesis and interfering with gas solubility in water bodies [3].


Introduction
Dyes have long been used in different types of industries such as, dyeing, textiles, paper, plastics, leather and cosmetics [1]. Color stuff discharged from these industries pose hazards and has an environmental impact [2].The presences of dyes in water are causing problems, such as, reducing oxygen levels in water; interfering with penetration of sunlight into waters; retarding photosynthesis and interfering with gas solubility in water bodies [3].
Azo dyes are divided according to the presence of azo bonds (-N=N-) in the molecule; these include mono azo, diazo, triazoetc [4]. Azo dyes resist the effect of oxidation agents and light, thus they cannot be completely treated by conventional methods of anaerobic digestion [5]. It is necessary to find an effective method for the treatment of Bismarck Brown R. The degradation of Bismarck brown R dye in the presence of aqueous zinc oxide suspension has been reported before [6]. The adsorption technique proved to be an effective and attractive process for removing dyes from aqueous solutions in term of initial cost, ease of operation, insensitivity to toxic substance, high efficiency, easy recovery and simplicity of design [7,8]. Carbon nanotubes (CNTs) are relatively new adsorbents that can absorb organic pollutants from wastewater [9,10]. CNTs were discovered in 1991 as a minor byproduct of fullerene synthesis. CNTs are made up of concentric rolled graphene sheets. CNTs include single wall (SWCNTs) and multiwall (MWCNTs) depending on the number of sheets comprising them [11]. They have many applications because of their various properties such as high thermal and electrical conductivities, strength, mechanical and special adsorption properties [12].
In this work, commercial MWCNTs were used as an adsorbent to remove Bismarck Brown R from aqueous solution. The main objective of this research was to evaluate the adsorption ability of carbon nanotubes for the removal of Bismarck Brown R as a model compound for basic dyes. The effects of contact time, MWCNTs dosage, initial dye concentration, pH and temperature on adsorption capacity were studied. Kinetic and equilibrium models were used to fit experimental data and the adsorption thermodynamic parameters.
Adsorbate: Bismarck Brown R, having molecular formula C 21 H 24 N 8 .2HCl was chosen as the adsorbate. Bismarck Brown R was purchased from Sigma-Aldrich with water solubility as 11 g L -1 (25°C) and molecular weight as 461.39g mol -1 . The dye stock solution was prepared by dissolving Bismarck Brown R in distilled water to the concentration of 0.01M.The experimental solutions were obtained by diluting the dye stock solution in accurate proportions to required initial concentrations. The IUPAC name of the Bismarck Brown R is 4-[5-(2, 4-Diamino-5-methylphenyl) diazenyl-2-methylphenyl] diazenyl-6-methylbenzol-1, 3-diamin. The structure of Bismarck Brown R is shown in figure 1.

Adsorption equilibrium experiments
For equilibrium studies, solutions of 5×10 -5 M Bismarck Brown R, at the initial concentration, were treated with 25 mg of MWCNTs. The mixtures were agitated on shakers (Gemmy orbit, van 480 Gemmy Industrial Corp-Taiwan) continuously for 60 min, as the equilibrium time, at different temperature and pH. All adsorption experiments were done in dark to avoid illuminations effects [13]. After 60 min, the suspensions were filtered using a centrifuge and the filtrates were analyzed for residual Bismarck Brown R concentration by UVvisible spectrophotometer (PG instruments Ltd-Japan) at 459 nm. The amount of Bismarck Brown R uptake by CNTs in each flask was calculated using the mass balance equation: Where q e is the amount of Bismarck Brown R adsorbed by CNTs at equilibrium, C 0 and C e are the initial and final dye concentrations (M), respectively, V is the volume of solution (L), and W is the adsorbent weight (g).
The dye percent removal (%) was calculated using the following equation:

Adsorption kinetic experiments
For kinetic studies, solutions of 1, 3, 5, 7 and 10 ×10 -5 M Bismarck Brown R, as the initial concentration, were treated with 25 mg of MWCNTs at a constant temperature of 298.15K. The mixtures were then subjected to agitation using a shaker. In all cases, the working pH of solution was not controlled. Mixtures were taken from the shaker at appropriate time intervals (10,20,30,40, 50, 60min) and the remaining concentration of the Bismarck Brown R solution was determined.

Effect of contact time
The effect of contact time on the adsorption capacity of Bismarck Brown R onto MWCNTs is shown in figure 2, When the initial Bismarck Brown R concentration is increased from 1×10 -5 to 10×10 -5 M the amount of Bismarck Brown R adsorbed onto MWCNTs, at 60min contact time, pH value 5, 25mg adsorbent dose and the constant temperature 298.15 K, increased from 3.9 to 30.0 mg g -1 . The increase of loading capacity of CNTs with increasing initial Bismarck Brown R concentration may be due to higher interaction between Bismarck Brown R and adsorbent [14]. These results show that rapid increase in adsorbed amount of Bismarck Brown R is achieved during the first 10 minutes. Similar results were reported before for removal of hazardous contaminants from wastewater [15].

Effect of MWCNT dosage
To determine the effect of adsorbent dosage on the adsorption of BBR, a series of adsorption experiments were carried out with different adsorbent mass at initial dye concentration of 5×10 -5 M. Figure 3 shows the effect of adsorbent dose on the removal of BBR. The increasing of adsorbent dosage from 10to50 mg, the percentage of dye adsorbed increased from 48.61to 98.15% after 60min of adsorption time. This result was expected as that the number of available adsorption sites increases by increasing the adsorbent amount [16][17][18].

Effect of initial concentration of BBR dye
Different concentrations of BBR1, 3, 5, 7 and10×10 -5 M, were selected to study the effect of initial concentration of dye onto MWCNTs. The amounts of dye adsorbed at pH 5, adsorbent dosage 25mg and 298.15K are given in figure 4. With increasing initial concentration of BBR from 1 to 10×10 -5 M, the removal of dye molecules decreases from 97.87 to 75.15% after 60min of adsorption time. These results agree with adsorption of heavy metal ions on carbon nanotubes [19].

Effect of pH
The pH of the dye solution plays an important role in the whole adsorption process, particularly on the adsorption capacity. The solution pH can affect the surface charge of the adsorbent and the degree of the ionization of different pollutants [20]. The effect of pH on the BBR dye adsorption capacities of the MWCNTs was studied at varying pH (2-10) with 5×10 -5 M fixed initial dye concentration and adsorbent dosage 25mg for 60min. Figure 5 shows that the adsorption capacity of BBR dye increases with increasing the pH of solution from 2 to 5 and decrease slightly when solution pH is above 5.The maximum adsorption capacity of MWCNTs was 17.87 mg/g at pH 5. It is well known the MWCNTs surface contains carboxylic and hydroxyl groups after purification method by acid treatment. The change in solution pH will effect on the ionization of these functional groups [21,22].

Effect of temperature
To study the effects of temperature on the adsorption of dye by MWCNTs, the experiments were performed at temperatures from 278.15 to 298.15 K. Figure 6 shows the influence of temperature on the adsorption of dye on MWCNTs. As it was observed, the equilibrium adsorption capacity of BBR onto MWCNTs was found to increase with increasing temperature. This fact indicates that the mobility of dye molecules increased with the temperature, additionally the viscosity of dye solution reduces with rise in temperature and as a result, it increases the rate of diffusion of dye molecules. The results were in agreement with the effect of the solution pH, ionic strength, and temperature on adsorption behavior of reactive dyes on activated carbon [23].

Adsorption isotherm
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. The equilibrium of experimental data for adsorbed BBR on MWCNTs was studied by using the Langmuir, Freundlich and Temkin isotherm models. In this study, the best fit isotherm models to the experimental data were determined using the value of correlation coefficient R 2 [24].

Langmuir isotherm
The Langmuir adsorption isotherm assumes that adsorption takes place at specific homogeneous sites within the adsorbent and has found successful application in many sorption processes of monolayer adsorption [25]. Figure 7 shows the Langmuir isotherm for adsorption BBR on MWCNTs. The following equation is the Langmuir isotherm: Where q m is the maximum amount of BBR adsorbed per unit mass of MWCNTs and K L is the Langmuir constant related to rate of adsorption.
For the Langmuir equation the favorable nature of adsorption can be expressed in terms of dimensionless separation factor of equilibrium parameter (R L ) [26], which is defined by: where R L is the dimensionless equilibrium parameter and C 0 is the initial BBR concentration. The values of R L indicates the type of isotherm to be irreversible (R L = 0), favorable (0 < R L < 1), linear (R L =1) or unfavorable (R L > 1) [27]. The values of the dimensionless separation factor are given in table 1.

Freundlich isotherm
The Freundlich isotherm is an empirical equation employed to describe heterogeneous systems [28]. The Freundlich model [29] is based on the distribution of adsorbate between the adsorbent and aqueous phases at equilibrium. Figure 8 shows the Freundlich isotherm for adsorption BBR on MWCNTs. The basic Freundlich equation is: (5) where K F and n are Freundlich constants, which give a measure of adsorption capacity and adsorption intensity, respectively.

Temkin isotherm
This isotherm takes into accounts of indirect adsorbate-adsorbate interactions on adsorption isotherms and suggested that because of these interactions the heat of adsorption of all the molecules in the layer would decrease linearly with coverage [30]. Figure 9 shows the Temkin where K T and B 1 are the Temkin constants (K T is the equilibrium binding constant (L/g) and B 1 is related to the heat of adsorption).The values of the isotherm parameters are given in table 2.

Adsorption kinetics
Three kinetic models: Pseudo-first order, pseudo-second order and Intra particle diffusion kinetic models were used to fit experimental data to examine the adsorption kinetics. Equations 7,8 and 9 represent the linear forms of the pseudo-first order, pseudo-second order models and Intra particle diffusion kinetic model respectively ln (q t -q e ) = ln (q e )-k 1 t (7) The straight-line plots of ln (q e -q t ) versus t for the pseudo-first order reaction (Figure 10), t/q t versus t for the pseudo-second order reaction ( Figure 11) and q t versus t 1/2 for the Intra particle diffusion reaction ( Figure 12) for adsorption of BBR onto MWCNTs. The values of the kinetic parameters and correlation coefficient were calculated from these plots are given in table 3. The pseudo-second order model best represents this experimental data. Similar results have been observed before [31,32].
The negative values of ΔG° indicate that the adsorption process was a spontaneous process. The decrease in ΔG° with the increase of temperature indicates more efficient adsorption at higher temperature.
The positive values of ΔH° confirm that the sorption of BBR onto MWCNTs is endothermic in nature. Figure 13 shows the Van't Hoff plot for the adsorption of BBR on MWCNTs.

Conclusions
This study shows that MWCNT scan be used effectively for the removal of BBR from aqueous solution. 25 mg is the optimum dosage of MWCNTs to adsorb BBR. The adsorption capacity of the BBR on MWCNTs increased with the increasing of initial concentration of BBR. The equilibrium adsorption capacity of BBR increased with temperature. The optimum contact time and pH were 10 min and 5 respectively. The adsorption kinetics was fitted by a pseudo-second order kinetic model. The adsorption of BBR on MWCNTs has been described by the Langmuir, Freundlich and Temkin adsorption isotherm models. The equilibrium data were fitted with the Freundlich isotherm. R L values for BBR adsorption onto MWCNTs indicate favorable adsorption. Adsorption of BBR was found to be spontaneous at the temperatures under investigation. The positive value of ΔH° confirmed the sorption process was endothermic.