This work reports the adsorption of crystal violet (CV) dye onto magnetic zeolite (MZ) nanoparticles, synthesized by direct fusion of fly ash (FA) and magnetite particles. The synthesised MZ showed high capacity for CV dye adsorption, removing 95% of the dye at an equilibrium adsorption time of 10 min and 25℃. The effects of adsorbent dosage, dye concentration, and pH, on adsorption were evaluated. Adsorption data were best described by the Langmuir adsorption isotherm (R 2 = 0.9986), while the adsorption kinetics was best fitted by the pseudo-second-order kinetic model (R 2 = 0.9999). Application of the MZs synthesised from inexpensive resources such as FA could ensure the sustainability and cost effectiveness of treating industrial effluent containing basic dyes, especially effluent from the textile industries.
Textile industry has been listed as one of the six key industrial sectors for priority prevention and control of chemical pollutants. In spite of this, some of the chemicals that have been outlawed are still in use in the textile industries, particularly in many developing countries. An estimated 2000 different chemicals are used in the textile industry [
Dyes are generally classified, based on their precursors, as either natural (derived from plants and animals) or synthetic (derived from organic and inorganic compounds). Synthetic dyes are relatively inexpensive, and as such, are widely used in the textile industry [
Crystal violet (CV) dye (
Several techniques have been used for the removal of dyes from industrial effluents, including biological (microbial decolorization, biodegradation and bioremediation), chemical (coagulation and flocculation), and physical (electrolysis, reverse osmosis, membrane-filtration, and adsorption) [
Although activated carbon is predominantly used in adsorption studies [
Though some researchers have reported the adsorption of dye onto natural adsorbents [
Zeolite was synthesized from FA―collected from Arnot Eskom power station, Mpumalanga, South Africa. Sodium hydroxide, hydrochloric acid, and anhydrous sodium aluminate were obtained from Sigma Aldrich, while the CV dye and magnetite (Fe3O4) were obtained from Merck. All reagents were of analytical grade (99%).
The raw FA samples were first screened through a 212 µm sieve to eliminate the larger particles. Mixture of sodium hydroxide, FA, and the magnetite particles, in a predetermined ratio of 1:1.5:y (by weight), respectively, was milled and fused in an oven at a temperature of 550˚C for 1/2 h [
Adsorption experiments were carried out batch wise, at 25˚C, using synthetic samples of CV prepared in distilled water. A stock solution of the CV containing 500 mg/L was used for the adsorption isotherm and kinetic studies. The equilibrium adsorption of dye was performed by shaking 0. 2 g of adsorbent in 50 mL of dye solution, in an incubator shaker at 200 rpm for 6 h, after which, the mixture was centrifuged at 10,000 rpm and 4˚C, for 10 min. The supernatant was gently removed and the concentration of CV in the supernatant was determined using a UV-VIS spectrophotometer (Shimadzu Corp., Japan), by measuring the absorbance at a wavelength of 590 nm. The experiment was repeated with varying adsorbent dosage (0.1 - 0.5 g/50mL of CV solution), concentration of the CV (100 - 1000 mg/L), and the pH (2 - 10), in order to determine the effects of different adsorbent dosage, dye concentration, and pH on CV removal from the simulated waste water. All experimental runs were performed at 25˚C in triplicate. The MZ with the highest adsorption capacity was used to assess suitable adsorption isotherms and kinetics.
Adsorption capacity was calculated from the data obtained from the adsorption studies by a mass-balance relationship (Equation (1)),
where, qe is the amount of dye adsorbed at equilibrium (mg/g), V is the volume of the solution (mL), m is the mass of the adsorbent (g), co and ce are the initial and equilibrium concentrations of the dye (mg/L), respectively.
Equilibrium adsorption data are commonly described with isotherms. Classical adsorption isotherms of Langmuir and Freundlich [
By taking the reciprocal of Equation (2), its linearized form is given in Equation (3) as:
where qe is the maximum amount of the CV dye adsorbed per mass of absorbent (mg/g), ce is the equilibrium concentration (mg/L), and KL is a Langmuir constant (L/mg) related to the affinity of the binding sites, with QL being the limiting adsorption capacity (mg/g), when the MZ surface is saturated with the dye. QL and KL were determined from the linear plot of ce/qe versus ce.
The Freundlich adsorption isotherm, which is often based on a heterogeneous surface adsorption, is given by Equation (4):
with the linearized form being presented in Equation (5):
where qe is the amount of dye adsorbed at equilibrium (mg/g), ce is the equilibrium concentration (mg/L), while KF and n are the Freundlich model constants, whose values were obtained from the plot of Lnqe against Lnce.
Kinetic studies were performed to investigate the effects of contact time on the quantity of dye adsorbed, at a fixed initial dye concentration (500 mg/L), by adding 50 mL of the dye solution to 0.2 g of MZ. The mixture was shaken in an incubator shaker at 200 rpm and 25˚C, while samples were taken periodically. The concentration of the adsorbed dye was determined at 590 nm wavelength, as described previously. The quantity of CV adsorbed (qt) at time t was determined by Equation (6):
where, qt is the amount of dye adsorbed at time t (mg/g), V is the volume of the solution (mL), m is the mass of the adsorbent (g), co and ct are the concentrations of the dye at initial (t = 0) and at time t, respectively. The rate constants were calculated by using the pseudo first-order, pseudo-second-order, and the Elovich equations [
The pseudo-first-order expression is given in Equation (7) as:
where qt is the amount of adsorbed dye (mg/g) on the adsorbent at time t, and k1 (min−1) is the rate constant of the pseudo-first-order adsorption. From the intercept of a plot of log(qe − qt) versus t, qe and k1 were determined.
The pseudo-second-order kinetic model is expressed in Equation (8) as:
where k2 (g∙mg−1∙min−1) is the rate constant of pseudo-second-order kinetic. k2 and qe were determined from the gradient and intercept of the plot t/qt versus t.
The initial adsorption rate (ho) is expressed in Equation (9) as:
The rate of adsorption of dye on the MZ surface decreases with time, without desorption of the products, due to increased adsorbent surface coverage. One of the most relevant models used for describing such phenomenon is the Elovich equation [
where a and b are the Elovich coefficients, which can be determined from the plot of qt against lnt.
The MZ was synthesized in a batch system, by direct fusion of FA, sodium hydroxide and magnetite nanoparticles, in a ratio of 1:1.5:y, respectively. Where y represents 0.1, 0.2, 0.3, 0.5, 0.75, while, the MZ produced at these ratios were represented as MZ1, MZ2, MZ3, MZ4, and MZ5, respectively. Another zeolite (Z) sample was synthesized from the FA, without the addition of magnetite particles, in order to assess the effects of magnetite on the affinity of zeolites adsorbent for CV dye adsorption. From the preliminary adsorption studies, MZ1 gave the optimum adsorption of CV dye; hence, it was characterized and used for further studies.
The surface morphology of the samples was examined using SEM, and the corresponding micrographs obtained, at 5000× magnification, are shown in
Chemical Element | FA | Z | MZ |
---|---|---|---|
C | 34.40 | 22.55 | 20.68 |
O | 46.63 | 46.38 | 46.19 |
Na | - | 9.97 | 23.83 |
Al | 4.44 | 8.78 | 3.19 |
Si | 19.04 | 10.44 | 3.09 |
Ca | 2.25 | 1.29 | 0.22 |
Fe | - | 0.59 | 2.80 |
According to International Zeolite Association (IZA) and the International Mineralogical Association (IMA), zeolites with a Si/Al ratio of 1 - 1.5, in their framework, is classified as zeolite X [
Typical N2 adsorption/desorption isotherms for the synthesized zeolite (Z) and MZare shown in
In order to further understand the adsorption capacity of the synthesized zeolites, a t-plot was generated using equation: t = [13.99/(0.034 − log(P/PO))]0.5, proposed by Harkins-Jura, to determine the micropore volume (Vmic) and mesopore volume (Vmes) as well as the external surface area (
Langmuir surface areas were determined for the zeolite materials by measuring the quantity of N2 adsorbed at different relative pressures.
Adsorption experiments were conducted to compare the adsorption capacity of FA, Z and the MZs produced at the different ratios of the magnetite particles. The results presented in
The effects of adsorbent dosage and pH of CV dye solution, in addition to different dye concentrations, on adsorption, were evaluated. It was observed that adsorption was nearly constant, even at low adsorbent dosage (
would have been completely covered such that further increase in the adsorbate concentration has no effects on the adsorption.
The adsorption isotherms of CV dye were explained by the classical Langmuir and Freundlich models [
Isotherm | Kinetics | ||||||||
---|---|---|---|---|---|---|---|---|---|
Langmuir | Freundlich | Pseudo-first-order | Pseudo-second-order | Elovich | |||||
QL KL R2 | 0.9711 0.033209986 | KF n R2 | 4.894 × 10−41 0.03468 0.9507 | qe (mg/g) k1 (min−1) R2 | 84.605 0.3518 0.9402 | qe (mg/g) k2 (g/mg/min) ho (mg/g/min) R2 | 117.647 0.018 249.135 0.9999 | a b R2 | 2.18 × 104 7.414 0.7027 |
gave a better fit of the experimental results.
The Langmuir model assumes monolayer CV dye adsorption onto homogeneous MZ surface with finite number of identical sites. The Langmuir constant (KL) and the limiting adsorption capacity (QL) when the MZ surface is fully covered with the CV dye are 0.0332 L/mg and 0.9711 mg/g, respectively.
The kinetics of CV dye adsorption onto MZ was investigated, and experimental results were modelled using pseudo-first-order, pseudo-second order, and Elovich equations. Kinetic study is invaluable in adsorption processes, as it helps in predicting the rate at which adsorbates are removed from the effluents being treated. The rate of adsorption can be affected by a couple of factors such as adsorbate-adsorbent affinity ratio; adsorbate concentration and adsorbent dosage―which were also investigated in this study; thermodynamics; etc. CV dye adsorption increased exponentially to equilibrium adsorption point in 10 mins (
The pseudo-second-order kinetic model depends on the assumption that chemisorption is the rate-limiting step for the adsorption. In chemisorption, the CV dye ions attach to the MZ surface by forming a chemical bond and thus tend to find sites that maximize their coordination number with the surface [
117.647 mg/g and the rate constant of pseudo-second-order adsorption (k2) was 0.018 g/mg/min.
The synthesised MZ in this study demonstrated a high capacity for CV dye adsorption, removing 95% of the dye at an equilibrium adsorption time of 10 mins, with an equilibrium concentration of 500 mg/L. Varying adsorbent dosage did not show any significant effects on the adsorption of CV dye. However, the dye adsorption was significantly lowered at pH below 4, while equilibrium adsorption was maintained at pH values between 6 and 10. The adsorption isotherm data were best explained by the Langmuir adsorption isotherm (R2 = 0.9986), while the adsorption kinetics was best fitted by the pseudo-second-order kinetic model (R2 = 0.9999). The application of MZs synthesised from inexpensive resources such as FA could ensure the sustainability and cost effectiveness of treating industrial effluents, containing basic dyes, especially effluent from textile industries.
Authors acknowledge the Cape Peninsula University of Technology for providing fund for this research.
Olusola S.Amodu,Tunde V.Ojumu,Seteno K.Ntwampe,Olushola S.Ayanda, (2015) Rapid Adsorption of Crystal Violet onto Magnetic Zeolite Synthesized from Fly Ash and Magnetite Nanoparticles. Journal of Encapsulation and Adsorption Sciences,05,191-203. doi: 10.4236/jeas.2015.54016