Adsorption Kinetic of Malachite Green Dye from Aqueous Solutions by Electrospun Nanofiber Mat

Introduction

Many industries throw their waste in large quantities such as dyes and organic or inorganic pollutants in water without any treatments. These pollutants have serious damage to the biotic ecosystem of all organisms^1-4. So these environmental pollutants can be treated in industrial waste water by using chemical, biologic and physical methods^5-10. Adsorption is one of the most effective methods of physical and economic treatments. In this technique the particles accumulate on the surface of the adsorbent material⁹.

Dyes are one of the types of organic pollutants, which are widely used in the manufacture of textiles, paper, plastics and printing as well as used as additives in the oil industry^11,12. Malachite green dye raises the concern about its use because it has toxic effects, so it must be treated before being given as a waste¹³.

Nanomaterials have been widely used for the removal of pollutants because they are effective adsorbent because of their large surface area and dispersion resistance¹⁴. Researchers used carbon nanotubes, nanoparticles and nanocomposites as adsorbent surfaces to remove pollutants from their solutions ^15-19. Nanofibers technology is one of the most important research topics in recent years in the field of the removal of pollutants. Polymer nanofibers contain a porous composition with a proportion of large surface to volume. As well as, they were used in electrical applications such as condensers, rechargeable batteries and sensor instruments^20,21. Continually, they have a great importance as they have been developed as highly efficient adsorbents because of the flexibility in surface modification, the very high ratio of surface area to volume and better mechanical properties compared to traditional adsorbents^22-24.

Nanofibers are prepared in a simple process called electrospinning technology, their diameters ranging from several micrometers (10 – 100) μm to sub-micron or nanometers (10^-1-10^-2) μm²⁵. Many of the polymers can be electrically spinning and their properties differ depending on the different properties of the solution, processing parameters and environmental conditions²⁶.

In the present study, 2-amino phenol-formaldehyde (2-A-Ph-F) resin was prepared and loaded on polystyrene (PS) as a matrices in order to produce electrospun nanofibers mat (2-A-Ph-F)/PS by electrospinning method. Kinetic adsorption of malachite green dye was studied by the resulting electrospun mat as a low cost adsorbent surface.

Materials and Methods

Instruments

Double beam FT-IR spectrophotometer-Shimadzu 1800, Double beam UV/VIS spectrophotometer- PG Instrument Ltd T80+, Electrospinning device- NE300, Scanning electron microscope- Angstrom Advanced AA8000, Shaking Water Bath-HYSC Ltd  SWB 25, Ultra sonic device-Wisd Lab Instrument WUC-A06H, Mechanical stirrer- DAIHAN Scientific WiseStir HS300 and vacuum drying oven -HYSC Ltd VO-27.

Materials

Malachite green dye (C₂₃H₂₆N₂Cl) Fig. 1, acetic acid and hydrochloric acid were supplied by BDH. Formaldehyde (40%), 2-aminophenol, N,N-dimethylformamide (DMF) and polystyrene (PS) with average Mw (200,000) were purchased from Sigma-Aldrich.

Figure 1: Chemical structure of malachite green dye. Click here to View figure

 

Preparation of The Resin  2-aminophenol-formaldehyde (2-A-Ph-F)

(2) mL of concentrated acetic acid was mixed with (5) mL of formaldehyde in a beaker has capacity (100) mL, then (16) g of 2-aminophenol added to the previous mixture with the continuous stirring. A few drops of concentrated hydrochloric acid added to the reaction mixture and after that the reaction container putted in a water bath at (80) °C with the continuously stirring and heating until a homogeneous brown sticky block was obtained.

Determination The Maximum Wavelength and Calibration Curve

The maximum wavelength λ_max which at the highest absorbance of the dye solution was determined by recording the absorption spectra by using the Uv-Vis spectrophotometer in the range (200-1000) nm by quartz cells with a pitch length of (1) cm, its value was (617) nm, Fig. 2. Also, the calibration curve, which represents the relation between absorbance and concentration, six concentrations (5, 10, 20, 30 ,40 and 50) ppm was prepared. The absorbance of these concentrations was measured at λ_max of the dye, and the standard curve was drawn between absorbance and concentration, Fig. 3.

Figure 2: Uv-Vis spectrum of malachite green dye. Click here to View figure

 

Figure 3: Calibration carve of malachite green dye  Click here to View figure

 

Preparation of The Standard Solution

The stock solution with a concentration of (500) ppm was prepared by dissolve (0.5) g of MG dye in (100) mL of distilled water. The standard solution (40) ppm of MG dye that studied its adsorption on the surface of the nanofiber mat (2-A-Ph-F)/PS was prepared by diluting the previous stock solution.

For the purpose of determining the time that required for the equilibrium between the surface adsorbent and the absorption material and studying the kinetic of this system, six round flasks have a volume (50) mL used, and (2) pieces of the electrospun nanofiber mat with a weight (0.004) g putted in every one of the round flasks then (20) mL of the dye solution with a concentration of (40) ppm added, after that all these systems putted in shaker water bath. The determination of solutions concentrations accrued at successive times. The adsorption kinetic of MG dye was studied at different temperatures (298, 303, 308, 313 and 318) K at a shaking speed (180) rpm for all temperatures.

Electrospinning Process Setting

(10) mL of polymer solution used consisting of mixing (9) mL of polystyrene with a concentration (20) %w/v and (1) mL of 2-aminophenol-formaldehyde resin with a concentration (30) %w/v by using DMF as a solvent for two polymer solutions, which mixed by a mechanical stirrer with a shaking speed (60) rpm.

The polymer mixture was transferred to a (10) mL plastic syringe that attached to a nozzle with an aperture (0.9) mm, which is the positive electrode of the electrical outfitter. The polymer mixture was pumped at a speed average (0.03) mL/min to a direction rotation cylinder at (50) rpm, that wrapped with an aluminum foil which is the negative electrode of the electrical outfitter. The voltage between the two electrodes was (32) kv while the distance between this electrodes was (15) cm.

The produced nanofibers mat was placed in the (50)°C at a vacuum oven for drying then it was cut by a mechanical cutter tool with diameter (10) mm, Fig. 4.

Figure 4: Picture of the produced electrospun nanofiber mat and the mechanical cutter tool. Click here to View figure

Results and Discussion

Characterization of The Prepared Resin (2-A-Ph-F)

Infrared FT-IR Spectroscopy

In the recorded IR spectrum of the prepared resin Figure (5), a broad beam at (3412) cm^-1 was observed due to the stretching vibration of the phenolic hydroxyl group O-H and N-H in the NH₂ group in the ortho position. The peaks at (2920) cm^-1 and (2926) cm^-1 were observed that attributed to stretching and bending vibration of alkaline group CH₂ respectively.

It is worth noting that the carbonyl group C=O disappeared, which is expected to appear at (1700) cm^-1 in the formaldehyde spectrum.

The emergence of peaks at (1637) cm^-1 and (1616) cm^-1 were also attributed to the stretching vibrations of C=C groups in the aromatic ring. While the vibration of the C=C group that connected to the mthylene bridge CH₂ appeared at (1521) cm^-1, so the bending vibration of the N-H group showed at (1508) cm^-1.

The appearance of the peak at (1436) cm^-1 is attributed to the intra level bending vibration of the O-H group. It is also observed that a peak at (1386) cm^-1 is likely to be due to the asymmetric stretching vibration of the phenolic group C-C-OH. The stretching vibrations of the single-bond C-O group at frequencies (1340) cm^-1 and (1053) cm^-1 in the CH₂OH group. The frequencies at (864) cm^-1 and (806) cm^-1 were attributed to the outside level bending vibrations of the C-H group for the para and ortho position respectively.

Figure 5: FT-IR spectrum of the preparation resin (2-A-Ph-F). Click here to View figure

 

Scanning Electron Microscopy SEM

The surface morphology of electrospun nanofiber mat (2-A-Ph-F)/PS was studied by the scanning electron microscope after the sample was coated with a layer of gold. As shown in Fig. 6, where a network of continuous fibers was observed in the form of smooth and the diameter average of the fiber (170±30) nm.

Figure 6: SEM picture of the electrospun nanofiber mat (2-A-Ph-F)/PS. Click here to View figure

 

The Adsorption Kinetic

The kinetic study was carried out at different temperatures to determine the effect of temperature on the kinetic functions.

The amount of material absorbed (q_t)mg.g^-1 at time (t) was calculated using the following formula:

Where  mgL^-1 is the primary concentration of the dye solution,) mgL^-1 is the concentration of the dye solution at time (t)  is the solution volume measured by L and  the weight of the surface is measured by g.

The amount of material absorbed (q_e)mg.g^-1 at equilibrium time was estimated according to the following equation:

Where (C_e) mgL^-1 is the concentration of the dye solution at equilibrium time.

The plot between the adsorbed amount of the MG dye (q_t)mg.g^-1 and time (t) min^-1 is shown in Fig. 7. The adsorption concentrations increase gradually with increasing time then approach a constant value at time (150) min for all studied temperatures, that means the adsorbed surface reaches to the saturation state at this time which considers as an equilibrium time.

Figure 7: Equilibrium time for adsorption of MG on the electrospun nanofiber mat (2-A-Ph-F)/PS at studied temperatures. Click here to View figure

 

In this study, the pseudo-first order equation was applied, Lagergren’s first order rate equation. The oldest and most famous equation to explain the rate of adsorption which depends on adsorption capacity, equation 3 shows the linear form of the first order Lagergren’s equation²⁷:

Where (q_e) mg.g^-1 is an amount of the adsorbed material at the equilibrium time, (q_t) mg.g^-1 is the adsorbed dye amount in time (t), (k₁) min^-1 is the rate constant of pseudo-first order, that calculated from the equation (3) when drawing In (q_e-q_t) versus time (t), while q_e was founded from the straight line equation intercept.

Fig. 8 shows plots of In (q_e-q_t) versus time (t) for all the studied temperatures. The linearity of the line and the large values of correlation coefficient R² show that the studied system follows the pseudo-first order equation.

Table 1 shows the adsorption kinetic coefficients. The theoretical quantities of the adsorbed material (q_e cal.) consistent with the experimental quantities of the adsorbed material (q_e exp.).

When the values of rate constants are extracted, generally we observe an increase in the values of these constants with increasing temperature, as a result to the increasing in the arriving rate to the equilibrium state of the studied adsorption system.

Figure 8: Plot of Ln (qe-qt) vis time for adsorption of MG on the electrospun nanofiber mat (2-A-Ph-F)/PS at studied temperatures. Click here to View figure

 

Table 1: The kinetic coefficients for adsorption of MG dye on the electrospun nanofiber mat (2-A-Ph-F)/PS.

T (K)	qeexp (mg.g-1)	qecal (mg.g-1)	k (min-1)	R2
298	14.585	18.150	0.017	0.994
303	28.846	27.182	0.018	0.980
308	32.362	32.774	0.019	0.983
313	157.433	171.591	0.021	0.989
318	107.339	185.892	0.022	0.998

 

In order to obtain activation energy Ea, It is determine the influence of the temperature on the reaction rate, ln k versus  was plotted according to the following equation²⁸:

Where k is the rate constant, A is the frequency coefficient, Eα is the activation energy, R  is the gas constant and T is the temperature in Kelvin.

Generally, from the value of activation energy Ea we can recognize if there is a physical or chemical adsorption occur in the studied system. In the physical adsorption, the equilibrium is speedily and easily reversible so that the activation energy has usually a small magnitude less than (4.2) kJmol^-1, while the activation energy in the chemical adsorption is larger than this value, that involves bonding forces larger than in physical adsorption.

Fig. 9 represents the plot of Arrhenius’s equation. The value of activation energy Ea was extracted from the slope of equation 4, which represents the minimum energy that required for a molecules of MG dye to adsorb on the electrospun nanofibers mat (2-A-Ph-F)/PS. It has been value (10.542) kJ.mol^-1, that means the adsorption involves activated chemical adsorption in addition to physical adsorption since the rate increase with increasing temperature.

Figure 9: Arrhenius plot for adsorption of MG on the electrospun nanofiber mat (2-A-Ph-F)/PS at 298 K. Click here to View figure

 

In order to take a look at the mechanism and the rate controlling steps that affecting on the adsorption kinetic, the experimental results kinetic were applied to Weber’s intraparticle diffusion²⁹ to explain the diffusion  mechanism, according to the following equation(5), the kinetic results were analyzed with the intraparticle diffusion model:

Where C is the intercept, k_id is intraparticle diffusion rate constant (mg/gh^1/2), which can be estimated from the slope of the straight line drawn that represents q_t against t^1/2. While, the straight line intercept represents an influence of the secondary layer thickness. Fig. 10 shows the plots of Weber’s equation for all studied temperatures. So, the values of calculated intraparticle diffusion coefficients were abstracted in Table 2.

If the slope of q_t versus t^1/2 is linear and passes through the original points, the intraparticle diffusion is the limiting controlling step for the specified rate. While if the straight line does not pass through the original points this indicates that intraparticle diffusion is not only a rate controlling step²⁹.

The first step (surface) shows that adsorption occurs on the outer surface and ends at time (60) min, then begins the step of intraparticle diffusion control until time (120) min, and finally the equilibrium step that begins at time (150) min. From the values of correlation coefficient, it can be observed that the adsorption of the MG dye on the surface of electrospun nanofibers mat (2-A-Ph-F)/PS involves more than one process, and the intraparticle diffusion process is not just a rate controlling step³⁰. The straight line fails to pass through the original points largely at temperatures (313) K and (318) K, as a result to the difference in the rate of molecules transfer in the initial and final stages of adsorption that causes from the increasing in temperatures. Continuously, the correlation coefficient values of intraparticle diffusion system were smaller than that in pseudo first order system, this confirms the adsorption process may be followed by intraparticle diffusion process. So, the large value of intercept reflects the increase of the surface contribution in the sorption at the rate controlling step.

Figure 10: Plot of qt versus t1/2 for adsorption of MG on the electrospun nanofiber mat (2-A-Ph-F)/PS at studied temperatures. Click here to View figure

 

Table 2: Intraparticle diffusion rate constants and the influence of the secondary layer for adsorption of MG dye on the electrospun nanofiber mat (2-A-Ph-F)/PS.

Temperature (K)	kid(mg/gt1/2)	The influence of the secondary layer
298	1.096	3.781
303	2.036	0.414
308	2.312	2.742
313	10.32	37.07
318	7.202	92.77

Conclusion

In this study, the kinetic adsorption of a basic dye Malachite green (MG) from aqueous solution at different temperatures was investigated, by using electrospun nanofiber mat (2-A-Ph-F)/PS as an adsorbent surface. The results obtained from two kinetic models, the adsorption kinetic described by the Lagergren’s pseudo-first order rate equation , while the intraparticle diffusion analyzed by Weber’s model. The results obtained there is more than one process in studied system, an adsorption process followed by intraparticle diffusion process.

Acknowledgement

We are grateful to Department of Chemistry and The Central Service Laboratory in College of Education for Pure Sciences/ Ibn Al-Haitham/ University of Baghdad for providing this work.

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