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Original Article
DOI: 10.1016/j.jmrt.2018.06.002
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Effective adsorptive removal of azo dyes over spherical ZnO nanoparticles
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Muhammad Nadeem Zafara,
Corresponding author
, Qamar Dara, Faisal Nawazb, Muhammad Naveed Zafarc, Munawar Iqbald,e, Muhammad Faizan Nazara
a Chemistry Department, University of Gujrat, Gujrat, Pakistan
b Basic Sciences & Humanities Department, University of Engineering and Technology Lahore (Faisalabad Campus), Lahore, Pakistan
c Chemistry Department, Quaid-e-Azam University Islamabad, Islamabad, Pakistan
d Chemistry Department, The University of Lahore, Lahore, Pakistan
e Chemistry Department, Qurtuba University of Science and Information Technology, Peshawar, Pakistan
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Table 1. Adsorption isotherms, kinetics and thermodynamics parameters for adsorption of MO and AM dyes by ZnO-NPs.
Table 2. Comparison of adsorption capacities of AM and MO onto various adsorbents.
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Abstract

To minimize the detrimental effects of contaminated water, technology-based smart treatment processes are prerequisite for sustainable supply of drinking water. Nano-sized metal oxides are the best choice futuristic adsorbents for the removal of water toxins as such materials are associated with the characteristics of simplicity, versatility, efficacy and high surface reactivity. In this study, we describe a nanostructured ZnO adsorbent, which displays remarkable efficiency toward the removal of widely used azo dyes, methyl orange (MO) and amaranth (AM), from aqueous systems. The ZnO nanoparticles (ZnO-NPs) were prepared by simple co-precipitation method, and the structural morphology of the as-prepared NPs was revealed by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform IR (FTIR) and Brunauer–Emmett–Teller (BET). After complementary characterization, as-prepared ZnO-NPs were further used as adsorbent for the removal of toxic azo dyes (MO and AM) from water. The results revealed that an amount of 0.3g ZnO-NPs showed maximum removal efficiency of each dye (40ppm) at pH 6. It was further confirmed that the adsorption of both dyes on ZnO-NPs strongly followed the Langmuir model whereas the kinetics studies revealed that each adsorption process was pseudo second order. Moreover, the findings suggested that R-SO3– groups were active sites and the electrostatic attraction between the dyes (MO, AM) and ZnO-NPs+ may be the prime adsorption mechanism of designated removal systems.

Keywords:
ZnO nanoparticles
Adsorption
Wastewater
Anionic dyes
Regeneration
(0.08MB).
Full Text
1Introduction

From last few years, many developing countries are facing serious challenges in providing sustainable supply of drinking water for their population, as their natural water resources are heavily contaminated from industrial/agricultural effluents and man-made pollution threats [1]. Large amount of clean water is either consumed in chemical processes and/or products involving dyes (e.g. textile, leather, paints, paper and pulp manufacturing, cloth dyeing, leather treatment, and printing). Depending on chromophores, there are about 20–30 groups of dyes, among which azo, phthalocyanine, triarylmethane and anthraquinone are the largest contributor of dyes [2]. Azo (around 70%) composes the largest class of dyes. Sodium 4-[(4-dimethylamino)phenyldiazenyl]benzenesulfonat or methyl orange (MO) and Trisodium (4E)-3-oxo-4-[(4-sulfonato-1-naphthyl)hydrazono]naphthalene-2,7-disulfonate or amaranth (AM) are two of the most popular azo dyes. MO dye is used as the coloring agent, disinfector in dyestuffs, rubbers, pharmaceuticals, pesticides, and varnishes. Similarly, the AM dye is widely used as coloring agent for textiles, papers, phenol-formaldehyde resins, woods and leathers. Though, the toxicity, gene mutation, allergic, mutagenicity, and carcinogenic activities of MO and AM have been experimentally proven, nonetheless these toxic dyes are still illicitly used in some parts of the world, due to their low cost and high efficacy [3].

Since dyes are recalcitrant, stable, colorant, and even possibly toxic and carcinogenic, releasing these pollutants into the environment will lead to serious consequences on the health of human beings, microbes, animals and plants. Dyes are distributed throughout every environmental matrices and great advances have been made in the elimination of these dyes from the environment. However, the fast and sensitive adsorption and detection of dyes in water are still attracting much attention since water remediation is a global challenge [4,5].

A thorough review of the literature suggest that the adsorption is one of the most studied techniques for the effective removal of dyes primarily due to its ease, cost effectiveness and great efficiency [6]. Numerous types of both natural and synthetic adsorbents have been assessed for the removal of dyes and other pollutants from wastewater [7–14]. Though, these materials are good in removing pollutants but normally have some disadvantages, such as poor mechanical strengths, difficulty in separating these powdery natural adsorbents (except by high-speed centrifugation from the treated effluent) and nonresistance against acid solutions [15,16].

Thus, various researchers around the world have engaged their efforts to improve adsorption processes and develop cost effective novel alternative adsorbents with high adsorptive power. In this respect, great attention has lately been focused to benefit from the process involving nanoparticles. At present, nanomaterials are receiving significant attention for their use as adsorbents as these have been used for the remediation of environmental pollutants and dyes removal [17]. A number of recent studies have reported the successful use of nanoparticles for different pollutants treatment and remediation [17–21]. Nano-sized metal oxides are proved to be effective materials as adsorbents due to their high surface reactivity, adsorption capacity and destructive sorbent [22–24] compared to their commercial analogs [25], and the ease of their synthesis from abundant natural minerals [26].

The present investigation reports a simple method for removal of MO and AM by ZnO nanoparticles (ZnO-NPs). This oxide is extra specific in elimination of anionic organic compounds because of its high point of zero charge (pHzpc) of ZnO=9.3–10.5 besides its favorable electrostatic attraction to various anionic particles [27]. To our knowledge, there is scarcity of information about the removal of AM and MO using the ZnO-NPs. There are reports on the application of ZnO nanomaterials in photocatalytic degradation and antibacterial activities [28–30]. The objectives of this study were to (i) synthesize ZnO-NPs by simple alkaline precipitation method and characterize the ZnO-NPs; (ii) explore the adsorption performance of AM and MO onto the ZnO-NPs; and (iii) investigate kinetics and isotherms of adsorption of AM and MO onto the ZnO-NPs using the hypothetical models.

2Experimental2.1Materials

Sodium hydroxide (NaOH), Hydrochloric acid (HCl), Zinc acetate, Triethanolamine and ethanol were purchased from Sigma (Sigma–Aldrich, Taufkirchen, Germany). Methyl orange (MO) and amaranth (AM) were also purchased from Sigma (Sigma–Aldrich, Taufkirchen, Germany) and their structure and properties are given in Table S1. These dyes were involved as primary pollutants model.

2.2Preparation of ZnO nanoparticles

The ZnO-NPs were synthesized by simple alkaline precipitation method. In a typical synthesis, appropriate amount of zinc acetate was dissolved in 50mL distilled ethanol in a glass vial to produce 0.001M solution. Next, 100mL solution of 0.003M of triethanolamine and 100mL solution of 0.002M of sodium hydroxide in distilled ethanol were prepared. Both solutions were mixed well and added drop wise to zinc acetate solution. After that, this mixture was heated and stirred (80°C, 350 RPM) using a hot plate for 2h and was later allowed to cool naturally till room temperature. The white precipitates of ZnO-NPs formed this way were separated by filtration. These precipitates were washed twice with ethanol and dried by using an electric oven at 100°C for 1h. In the alkaline precipitation of ZnO-NPs, the ethanol rich phase serves as best dispersing agent and the best solvent. After the ZnO-NPs adsorb surface active agent triethanolamine in the particles surface, a diffuse layer of ethanol-rich liquid around the particles is created. Since, the zinc cation is soluble in water, its growth was controlled by the diffusion of Zn2+ ions in the ethanol-rich layer.

The precipitation of ZnO-NPs can be either from Zn(OH)2 or Zn(OH)42− depending on the reaction parameters such as temperature, pH and synthetic methods [31]. The possible reactions for the precipitation of ZnO-NPs in presence of triethanolamine and NaOH can be explained by the following equations.

Zn(CH3COO)2·2H2O + triethanolamine → Zn[triethanolamine] + 2CH3COO
Zn[triethanolamine] → Zn2+ + triethanolamine
NaOH → Na+ + OH
Zn2+ + 2OH → Zn(OH)2
Zn(OH)2 → ZnO + H2O

2.3Batch adsorption studies

MO and AM adsorption studies were carried out in batch form to investigate the effect of different factors such as pH, adsorbent dose, agitation speed, temperature, initial concentration of dyes and contact time while studying the adsorption isotherm and kinetics of the reactions. Dyes solutions with known concentrations were prepared and required amount of ZnO-NPs nanoparticles was added followed by shaking on orbital shaker (Wisd, WiseShake SHO-2D, Seoul, South Korea) for predefined time. The time necessary to achieve equilibrium conditions was estimated by initial kinetic measurements. Then the solutions were filtered and residual concentration of MO and AM in the filtrate was determined by monitoring the absorbance at 460nm and 520nm, respectively using UV-Vis spectrophotometer. The effect of pH was also investigated by controlling the pH of the dyes solutions using HCl and NaOH solutions. The adsorption kinetics was determined by analyzing the adsorption capacity of dyes solution at different contact times. To study adsorption isotherms, MO and AM solutions of various concentrations were agitated with the adsorbent until the equilibrium was attained. The percentage removal of dyes by ZnO-NPs was calculated using Eq. (1).

where Co is initial dye concentration and Ce is the concentration of dye at equilibrium.

2.4Characterization

The X-ray diffraction (XRD) analysis was recorded on a PANalytical X’ Pert PRO 3040/60 X-Ray diffractometer (Almelo, The Netherland) containing x-rays source of Cu Kα at 45kV and 40mA. The morphology was detected with a JSM-5910 scanning electron microscope, SEM (JEOL, Tokyo, Japan). The Fourier transform infrared (FTIR) spectra were recorded on Nicolet 6700 FTIR spectrometric analyzer using KBr pellets. A pH meter (inoLab pH 720, WTW, Weilheim, Germany) was employed for the pH measurements. The nitrogen adsorption isotherm was recorded with a Belsorp-mini II (BEL Japan). The spectrophotometric measurements were carried out with a UV-Vis spectrophotometer (UV 4000, MRI, Germany).

3Results and discussions3.1Characterization of ZnO-NPs

XRD analysis was carried out to explore the crystalline size and structure of the ZnO-NPs (Fig. 1A). XRD data collected for the ZnO powder showed that the sphere-like annealed products were identified by XRD as predominately ZnO. All the Bragg peaks of 2θ values centered at (ca.) 32.1, 34.7, 36.5, 47.8 and 56.9 correspond to the (hkl) planes (100), (002), (101), (102) and (110), respectively of ZnO and are consistent with those of the ZnO (JCPDS file no. 79-0208). In addition, ZnO-NPs obtained in this work simply matched with the single phase of ZnO. The surface area of ZnO-NPs was much higher, indicating the superior adsorption capacity of nanoparticles toward dyes removal. No other secondary or amorphous phase and no diffraction peaks for impurities were present, which indicated the high purity of the sample.

Fig. 1.
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(A) X-ray diffraction (XRD) patterns, (B) FTIR spectra of ZnO-NPs and SEM images of ZnO-NPs (C) before adsorption, (D) after AM adsorption and (E) after MO adsorption.

ZnO-NPs were further investigated by FTIR spectroscopy. The sample for IR analysis was prepared by mixing KBr with ZnO powder and then pressing into a pellet. The IR spectrum of ZnO-NPs was presented in Fig. 1B, which revealed a medium peak at 3500–3200cm−1 arising from O—H groups coordinated with zinc ions or could be due to the water on the nanoparticles surface and the peak at 1516cm−1 corresponded to O—H bending vibrations. Weak bands were observed at 1384cm−1 due to Zn—O bond vibrations and the bands at 1050–700cm−1 appeared to be due to Zn—O bonds.

The surface and textural morphology of ZnO-NPs before and after adsorption of MO and AM by SEM images was illustrated in Fig. 1. The SEM image of the ZnO-NPs before adsorption (Fig. 1C) exhibited the formation of aggregates in the form of small spherical grains. The sizes of the aggregate were in the range of 75–150nm. The SEM images of ZnO-NPs loaded with MO and AM were shown in Fig. 1D and 1E, which indicated the formation of smaller particles agglomerated and interconnected with each other, ranging in size from 120–250nm.

The behavior and adsorption capacity of ZnO-NPs could be predicted from the porosity and surface area information. The nitrogen desorption/adsorption isotherm for ZnO-NPs was presented in Fig. S1. The specific BET surface area as determined by Brunauer–Emmett–Teller (BET) method was found to be 49.36m2g−1. The average pore diameter and pore volume calculated using the Barrett–Joyner–Halenda (BJH) method were found to be 27.44nm and 0.211cm3g−1 respectively.

3.2Dyes adsorption analysis

MO and AM are examples of azo group and were selected as model dyes to explore the adsorption capacity of ZnO-NPs.

3.2.1Effect of pH

The influence of pH on removal of MO (40ppm) and AM (40ppm) dyes by 0.1g ZnO-NPs at room temperature was investigated from pH 2 to 11, while keeping the other parameters constant. The results (Fig. 2A) showed that the removal of AM increases up to 95% at pH 5 and remain constant up to pH 7 and the removal of MO also increased up to pH 6 when the pH was increased from 2 to 11. For further study, the pH value 6 was used as optimum pH for both dyes. Since the maximum removal was found at pH 6 and 7 for MO and AM, respectively, the electrostatic attraction between the anionic dyes molecules and positively charged ZnO-NPs (pH PZC=9.8) might be the dominant adsorption mechanism (Scheme 1). The pH less than PZC of the ZnO-NPs made them positively charged which develop enhanced electrostatic attraction between dye and ZnO surface. Similar observation has been reported in past for different adsorbents [15,32–34].

Fig. 2.
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Effect of (A) pH, (B) adsorbent dose, (C) stirring speed and (D) temperature on adsorption of dyes by ZnO-NPs after equilibrium time of 50min [AM (red line), MO (blue line)].

Scheme 1.
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Effect of pH on the electrostatic interaction responsible for adsorption of dyes onto ZnO-NPs.

At low acidic pH, protons can attach with any of the nitrogen in a NN double bond, and so MO and AM become protonated and thus the generated repulsive forces between protonated AM and MO and the positively charged ZnO-NPs results in a decline in the removal of dyes. In addition it could also be due to decreased adsorption sites, which resulted from the part dissolution of ZnO-NPs in acid solutions [35,36]. Since the PZC of ZnO-NPs is 9.8 and pKa values for MO and AM are 3.46 and 10.5 respectively, so that could be the reason why that there is sharp increase in percentage removal observed up to pH 6 for MO and a little increase in percentage removal observed up to pH 5–7 for AM when pH increased from 2 to 11. At higher pH, a reduction in dye removal is also observed. At higher basic pH, less positively charged sites and more negatively charged sites were developed on the ZnO-NPs surface, which does not support the adsorption of anionic dye due to electrostatic repulsion. At the same time the formation of OH ions and competition between OH ions and anionic dyes for occupying the adsorption sites on nanoparticles also disfavor the adsorption of anionic dyes [37].

3.2.2Effect of ZnO-NPs amount

The adsorbent amount is also one of the most important factors that present the capacity of the adsorbent for a given initial amount of the adsorbate. To investigate the correlation of MO and AM removal on ZnO-NPs amount, various amounts (0.1–0.7g) of ZnO-NPs at pH 6 and room temperature were added into 100mL of 40ppm solutions of MO and AM. The results were shown in Fig. 2B, from which it can be seen that the removal percentage of MO and AM increased on increasing the ZnO-NPs amount. As the ZnO-NPs amount increased from 0.1g to 0.3g, the removal efficiency of MO and AM ions increased significantly from 68.5% to 82.5% and 76.7% to 97.4%, respectively. However the higher amount of ZnO-NPs results in lower removal capacity. The increase in percentage removal at low adsorbent amount might be due to better dispersion of ZnO-NPs in an aqueous solution. All of the active sites on the adsorbent surface are entirely uncovered, which could accelerate the approachability of dyes molecules to a large number of the adsorbent active sites. On the other hand, at higher adsorbent amounts, the accessibility of adsorbent active sites with higher energy decreases and a larger fraction of the active sites with lower energy become occupied, leading to a decrease in the adsorption capacity [15]. Therefore, a 0.3g ZnO-NPs amount was chosen as the optimal dosage for further study.

3.2.3Effect of shaking speed

The effect of shaking speed on removal of MO (40ppm) and AM (40ppm) dyes by 0.3g ZnO-NPs at pH 6 was investigated by varying shaking speed from 75 to 225rpm. The results (Fig. 2C) indicated that the percentage removal of MO increased from 57.5 to 70.3% as the shaking speed increased from 75 to 175rpm, whereas the percentage removal of AM increased up to 94.2% at 125rpm when shaking speed increased from 75 to 125rpm. At higher shaking speed a little decrease in percentage removal of both dyes was observed. The possible explanation of this increase could be that as the shaking speed increased the collision between dyes and adsorbent surface was also increased which results in fast reaction between positively charged ZnO-NPs and anionic dyes. A little decrease in percentage removal of both dyes with increasing shaking speed might be due to desorption of dyes molecules from the surface of ZnO-NPs at higher rpm.

3.2.4Effect of temperature

The equilibrium percentage removal of MO and AM onto ZnO-NPs were studies by varying the temperature from 35 to 55°C. The experiments were performed by adding 0.3g of ZnO-NPs into 100mL of 40ppm solutions of MO and AM at pH 6 (Fig. 2D). The obtained results showed that the increase in the temperature of the solutions of MO and AM from 35 to 55°C leads to an increase in the adsorption percentage removal of ZnO-NPs, which shows that the adsorption process is endothermic and chemical in nature. The possible explanation of this increase in percentage removal of MO and AM onto ZnO-NPs could be due to the availability of more active sites and activation of the adsorbent surface at higher temperatures. It could also be due to the increased diffusion and mobility of MO and AM dye ions from the bulk solution toward the ZnO-NPs surface, thereby increasing the number of dyes molecules acquiring sufficient energy to undergo chemical reaction with ZnO-NPs at higher temperature. The same trend was also reported for the adsorption of MO onto Mn@Si/Al adsorbent [15] and adsorption of AM onto Fe3O4/MgO nanoparticles [32].

3.3Adsorption isotherms

The effect of initial dye concentration on dyes removal was discussed in section S1.1 (Supplementary data). Adsorption isotherms are mathematical model that are used to describe the distribution of adsorbate molecules between adsorbent and liquid based on homogeneity/heterogeneity of adsorbent, interaction between molecules of adsorbate and coverage type. The equilibrium data of MO and AM onto ZnO-NPs was analyzed using the most common Langmuir, Freundlich and Temkin models to find out the suitable model that may be used for design and optimization of adsorption processes. The resulting plots for AM and MO were shown in Fig. 3 (A–C and D–F), respectively. Table 1 summarizes the constants and coefficients of Langmuir, Freundlich and Temkin models.

Fig. 3.
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Adsorption isotherms for the adsorption of dyes on ZnO-NPs (A) Langmuir isotherm model for AM, (B) Freundlich isotherm model for AM, (C) Temkin isotherm model for AM, (D) Langmuir isotherm model for MO, (E) Freundlich isotherm model for MO and (F) Temkin isotherm model for MO.

Table 1.

Adsorption isotherms, kinetics and thermodynamics parameters for adsorption of MO and AM dyes by ZnO-NPs.

Isotherms/Models  ParametersAMMO
Pseudo first order  k1 (1/min)  0.138      0.066   
  qe (mg/g)  57,942.7      186.1   
  R2  0.186      0.277   
Experimental adsorption capacity  qe (mg/g)  38.75      25.82   
Pseudo second order  k2 (g/mg.min)  0.024      0.048   
  qe (mg/g)  39.20      26.04   
  R2  0.999      0.999   
Intraparticle diffusion  ki (mg/g min1/2)  5.456      4.763   
  C  105.4      98.56   
  R2  0.745      0.876   
  Temperature →35°C  45°C  55°C  35°C  45°C  55°C 
Langmuir  RL0.045  0.025  0.021  0.260  0.200  0.095 
  b (L/g)0.306  0.501  1.421  0.070  0.058  0.064 
  qmax (mg/g)74.02  75.87  66.89  48.31  62.27  65.23 
  R20.997  0.983  0.998  0.994  0.997  0.994 
Freundlich  Kf (mg/g)17.11  24.30  35.67  4.646  4.751  5.396 
  n1.812  2.094  3.775  1.686  1.545  1.560 
  R20.989  0.997  0.977  0.976  0.986  0.993 
Temkin  AT (L/mg)3.532  9.197  269.2  0.689  0.650  0.736 
  bT (J/mol)167.2  198.3  376.7  238.1  205.0  203.5 
  R20.978  0.880  0.749  0.993  0.976  0.975 
Experimental adsorption capacity  qe (mg/g)58.20  63.10  65.06  34.30  40.61  42.71 
  CO (mg/L) →50  60  70  50  60  70 
Thermodynamic  ΔH (kJ/mol)5.670  9.500  11.75  3.430  5.350  8.450 
  ΔS (J/mol.K)28.45  35.55  41.55  23.67  29.55  32.66 
  ΔG° (kJ/mol)  35°C  −1.233  −4.150  −7.345  −2.455  −4.170  −6.765 
    45°C  −2.542  −5.770  −8.124  −3.220  −4.455  −7.345 
    55°C  −2.985  −6.520  −8.753  −3.988  −5.785  −7.886 

The Langmuir isotherm states that during adsorption process the monolayer is formed when there is no interaction between adsorbate molecules. Freundlich isotherm is mostly applied to heterogeneous solid catalyst and is a mathematical relation used to describe the multilayer adsorption. Temkin isotherm is used to describe the phenomenon of adsorption of heterogeneous systems and this isotherm supposes that the adsorption heat (a function of temperature) decreases linearly with coverage due to adsorbent-adsorbate interactions and the adsorption binding energies are distributed uniformly.

The linear form of Langmuir isotherm assuming monolayer adsorption is expressed by Eq. (2)[38], the linear form of Freundlich isotherm can be expressed by Eq. (3)[39] and the linear form of Temkin isotherm can be expressed by Eq. (4)[40]:

where Ce is equilibrium concentration of dye, qmax (mg/g) is the maximum adsorption capacity of the adsorbent corresponding to monolayer formation. The b parameter is a coefficient related to the energy of adsorption. Values of qmax and b are determined from the linear regression plot of (Ce/qe) versus Ce. The Kf and n are constants from the Freundlich equation. Kf is capacity of adsorbent for adsorbate and n is intensity of adsorption. The values of Kf and n can be derived from linear regression plot between log Ce and log qe. The B in the Eq. (4) is constant which is equal to RT/bT. AT (L/mg) and bT (J/mol) are the Temkin constants. bT is related to heat of adsorption and AT is the equilibrium binding constant corresponding to the maximum binding energy. R (8.314J/mol K) is the universal gas constant and T (K) is the absolute temperature (308, 318 and 328K). The values of AT and bT are calculated from the linear regression plot of qe versus ln Ce.

The values of b in the range of 0 to 1, indicates that the ZnO-NPs are favorable for adsorption of the two dyes. The Langmuir model shows that maximum theoretical adsorption capacities are similar to the experimentally obtained values for MO and AM dyes. From the Freundlich model, 1/n values indicate the type of isotherm to be irreversible (1/n=0), favorable (1/n lower than 1) and unfavorable (1/n higher than 1). The value of n should be less than 10 and higher than unity for favorable adsorption conditions. The values of Freundlich constant n, were more than 1 for both dyes, indicate that the adsorption process is favorable (Table 1). From the Temkin isotherm model, the higher value of bT demonstrates a strong interaction force between adsorbates and absorbents. The R2 values of Freundlich and Temkin models were lower than that obtained from Langmuir isotherm and also Langmuir model shows that the maximum theoretical adsorption capacity for MO and AM are close to the experimentally obtained values, which suggests that adsorption of MO and AM dyes is accompanied predominantly by monolayer formation.

The important feature of Langmuir model can be described by dimensionless constant separation factor RL given by:

where Co (mg/L) is the initial concentration of adsorbate. The value of RL for MO and AM are found to be in between 0 and 1 indicating that the adsorption is favorable (0<RL<1) under the optimized conditions. In addition, the RL values for AM are low than MO, which indicates that the interaction of AM dye molecules with ZnO-NPs might be relatively strong and that might be the reason of high percentage removal of AM onto ZnO-NPs [41].

The monolayer adsorption capacity of AM and MO obtained from Langmuir isotherm was 75.9 and 65.2mgg−1, respectively. By making comparison with the adsorption capacities of other reported adsorbents (Table 2), the adsorption capacity of ZnO-NPs was found comparable or better than many reported adsorbents in literature [21,32,42–50].

Table 2.

Comparison of adsorption capacities of AM and MO onto various adsorbents.

AdsorbentAdsorption capacity (mgg−1)Reference
AM  MO 
Ni-CLDH    860  [21] 
Crosslinking chitosan microsphere    48.0  [42] 
Calcined (MgNiAl–C) hydrotalcites    375  [43] 
Nanorod-like γ-Al2O3    84.0  [44] 
ZnO-SnO2    91.0  [45] 
[(CS/REC)10:1]-20%CNTs foam    50.8  [46] 
MoSe2 microspheres    36.9  [47] 
Fe3O4/MgO nanoparticles  38.1    [32] 
MgAlCO3  120    [48] 
Fe3O4@mZrO2/rGO  78.6    [49] 
Alumina reinforced polystyrene  20.2    [50] 
ZnO-NPs  75.9  65.2  This study 
3.4Adsorption kinetics

The effect of contact time on dyes removal was discussed in section S1.2 (Supplementary data). In order to define the mechanism of AM and MO adsorption onto ZnO-NPs, experimental kinetic data were fitted to the pseudo first and second order models as well as intra-particle diffusion model. The first kinetic model predicts that the rate of change of solute uptake has direct relation to difference in saturation concentration and the amount of solid uptake with time, which is normally valid over the early stage of an adsorption process. The pseudo-second kinetic model predicts that the rate-limiting step is chemisorption and valid over the whole range of adsorption. From the linear form of these two models, equations for pseudo first and second order can be represented by Eqs. (6) and (7), respectively [51,52]:

where k1 and k2 are the adsorption rate constants of first and second order kinetic models, in min–1 and g (mg min)–1, respectively, qe and qt, in mg g–1, are the equilibrium adsorption uptake (at time t=∞) and the adsorption uptake (at time t), respectively. The pseudo-first-order parameters k1 and qe were calculated from the plots of log (qeqt) versus t (Fig. S4 A and B) and the values of k2 and qe can be calculated from the slope and intercept of plots of t/qt versus t (Fig. S4 C and D). Table 1 summarizes the constants and coefficients of pseudo first and second order models.

The value of the correlation factor R2 for MO and AM are 0.277 and 0.186 and also the values of theoretical qe significantly disagree from the experimental value recommending that pseudo-first-order model does not elucidate the kinetics of adsorption. However the pseudo-second-order model displayed a good fit with experimental data for both dyes (R2>0.99). The value of qe,cal also seemed to agree well with the experimentally observed value of qe,exp. These findings suggest that the adsorption kinetics obeyed the pseudo-second-order model. This dependability in the experimental data with the pseudo-second-order kinetic model suggests that the rate limiting step for the adsorption of AM and MO ions on the ZnO-NPs is chemical adsorption (Scheme 1). Additionally, the low value of pseudo-second-order rate constants (k2=0.048 for MO and 0.024 for AM) can be ascribed to the poorer competition for the adsorption surface sites at lesser concentration.

Considering that the pseudo-first-order and pseudo-second-order kinetic models cannot recognize the diffusion mechanism, the kinetic data were further evaluated by Weber's intraparticle diffusion model, which can be defined as [53]:

where ki is the intra-particle diffusion rate constant in mg g–1min–1/2 and C is the intercept. The value of C tells the thickness of the boundary layer. The higher C means the greater effect of the boundary layer. As shown in Fig. 4, the plot of qt against t1/2 for MO and AM presented multi-linearity, demonstrating that adsorption process of AM and MO onto ZnO-NPs was affected by more than one process. The first linear section is the external surface adsorption or instantaneous diffusion stage, during which a large amount of MO (about more than 65%) and AM (about more than 90%) was rapidly adsorbed by the exterior surface of the adsorbent. The second linear portion was a slow adsorption phase related to intra-particle diffusion of dye molecules within the pores of adsorbent. Furthermore, none of the lines pass through the origin, which revealed that adsorption process, was not controlled only by intra-particle diffusion but involved other mechanism pathway, like chemical adsorption etc. [54,55].

Fig. 4.
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Intra-particle diffusion model for adsorption of AM and MO dyes by ZnO-NPs.

3.5Thermodynamic studies

In order to investigate the effects of temperature on adsorption of two dyes onto ZnO-NPs, the adsorption experiments were carried out at (35, 45, and 55)°C with varying initial concentration of dyes. Fig. 5 showed that the adsorption capacities for the MO (5A) and AM (5B) increased with the increase of the initial concentration of dyes solutions and became higher as the temperature increased for the different dye initial concentrations from 35 to 55°C. The results indicated that the adsorption of MO and AM on ZnO-NPs is endothermic in nature and implying a chemical adsorption process [41,56].

Fig. 5.
(1.35MB).

Effect of temperature in different initial (A) MO, (B) AM dyes concentrations and (C) regeneration of ZnO-NPs for AM and MO dyes by ethanol, 1% NaOH/ethanol (v/v), and 6% NaOH/ethanol (v/v), respectively (pH 6; contact time, 120min; initial concentration, 40mgL−1).

The thermodynamic parameters reveal the spontaneous nature and feasibility of the adsorption process. Thermodynamic parameters such as, free energy change (ΔG°), enthalpy (ΔH) and entropy (ΔS) were estimated to endorse the nature of adsorption of MO and AM onto ZnO-NPs. Thermodynamic parameters were calculated from the following equations.

where Kd is the thermodynamic equilibrium constant. The values of ΔH and ΔS were calculated from the slope and intercept by plotting ln Kd versus 1/T using Eq. (11) and the values of ΔG° were calculated according to Eq. (10). To confirm the accuracy of our experiment, thermodynamic studies were completed in different initial dye concentrations (50, 60 and 70ppm). Table 1 summarizes the values of ΔG°, ΔH and ΔS. The values of ΔG° are negative for both dyes and the negative values increases with increasing temperature in different initial dyes concentrations, suggesting that the adsorption is spontaneous and the adsorption is more favorable at higher temperatures. The positive values of ΔH and ΔS in the temperature range from 35 to 55°C show that the adsorption of MO and AM onto ZnO-NPs is endothermic and dominated by chemisorption. They also confirm that the adsorption process takes place randomly at the solid solution interface [15,37].

3.6Desorption studies

Based on the analysis of the adsorption kinetics, isotherm and thermodynamics, it can be considered that the adsorption of AM and MO dyes was due to the chemical action between anionic dyes and ZnO-NPs based adsorbents. The negative charged groups on the anionic dyes and positive charge groups at the ZnO-NPs generated electrostatic interactions. Scheme 2 represents the suggested adsorption and desorption mechanism between the ZnO-NPs and anionic dyes.

Scheme 2.
(0.12MB).

Schematic representation for the possible interactions between the ZnO-NPs and AM and MO dye ions.

From a practical point of view, removal efficiency, stability and reproducibility are significant properties of an advance and good adsorbent. Organic solvents could be good candidate for the recovery of dyes so in present study, dye loaded ZnO-NPs was regenerated using pure ethanol and also using low concentration of alkali in ethanol solution. Fig. 5C shows that the AM and MO loaded adsorbents could not be effectively generated using pure ethanol. But with increasing NaOH concentration from 1 to 6% (v/v), the regeneration efficiency raised to almost 90% at first 2 cycles and retained above 78% after the fourth cycle for AM. For MO the regeneration efficiency raised to 75% in first cycle and remained above 60% after fourth cycle. These results indicate the stability and reusability of ZnO-NPs in applied applications and makes ZnO-NPs suitable for high-efficiency and low cost water pollution treatment. For the practical application, the potential of the ZnO-NPs adsorbents for dye removal will further be studied in continuous flow fixed-bed column in our future work.

4Summary and conclusions

In conclusion, zinc oxide (ZnO) nanoparticles were synthesized by co-precipitation method and were characterized by XRD, FTIR, SEM and BET techniques with successful application as adsorbents for the removal of MO and AM dyes from aqueous solution. The maximum removal of MO by ZnO-NPs was observed at pH 6, shaking speed 175rpm, adsorbent dose 0.3g and at temperature 35°C, whereas maximum removal of AM by ZnO nanoparticles was observed at pH 6, shaking speed 125rpm, adsorbent dose 0.3g and at temperature 35°C. The adsorption studies confirmed that the adsorption process of both dyes on ZnO-NPs strongly followed the Langmuir model. The kinetics studies revealed that both adsorption processes were pseudo-second-order. The thermodynamic studies provided evidence for the endothermic and spontaneous nature of the adsorption process. The as-prepared nanoparticles posses high surface area and exhibit improved adsorption ability with good recyclability for AM and MO removal. Finally the ZnO-NPs could be used for removal of anionic dyes under convenient conditions and regarded as a kind of cost-effective adsorbent in future water treatment.

Associated content

Supporting information

Properties and structure of MO and AM dyes, BET isotherm plot, Effect of initial dye concentration on adsorption of AM and MO dyes by ZnO-NPs, Effect of time on adsorption of AM and MO dyes by ZnO-NPs and Adsorption kinetics for the adsorption of dyes on ZnO-NPs (Table S1, Figs. S1–S4, Sections S1.1 and S1.2).

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

M.N. Zafar acknowledges financial support by Higher Education Commission of Pakistan (NRPU Project. 6515/Punjab/NRPU/R&D/HEC/2016). M.F. Nazar also extends his thanks to Higher Education Commission of Pakistan for providing financial support through NRPU Project. 20-4557/NRPU/R&D/HEC/14/481. The authors express their gratitude to the Department of Chemistry, University of Gujrat, Pakistan, for the provision of lab facility.

Appendix A
Supplementary data

The following are the supplementary data to this article.

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