Removal of methylene blue on zinc oxide nanoparticles nonlinear and linear adsorption isotherms and

2.1. Materials

Methylene Blue (MB), a cationic dye with a molar mass of 319.85g/mol, molecular formula: C16H18N3CLS, pKa:3.5 and wavelength of maximum absorption (λmax):668nm was procured from Sigma-Aldrich Chemical Company (USA). Zinc oxide nanoparticles (ZnO-NPs) with a diameter of less than 20nm, purity of 99%, pKa of 7.5 and specific surface area of 90m2/g were also purchased from Sigma-Aldrich Chemical Company (USA).

2.2. Structural characterization of ZnO NPs

X-ray diffraction (XRD) was used to define the structural features, diffractive arrangement and interplanar spacing of the ZnO-NPs (adsorbent) using an X-ray diffract meter with Philips PNA-analytical diffract meter. The morphology of ZnO-NPs was determined via scanning electron micro¬scopy (SEM). Fourier-transform infrared spectroscopy (FT-IR) of the adsorbent was done on a JASCO 640 plus machine (4000-400 cm-1) at room temperature to determine the functional groups present in the adsorbent and taking part in the MB adsorption process.

2.3. Batch adsorption technique

All adsorption experiments were carried out at room temperature. The study was performed on a batch mode using the one-factor-at-a-time (OFAT) method. To carry out the experiments, a 290

stock solution of MB at a concentration of 1000 mg/L was prepared. The different concentrations of MB were prepared by dilution in deionized water (applying the relation: C 1V1=C2V2). The effects of ZnO-NPs dosage (0.1-0.4 g/L), contact time (15-120 min), pH (3-11) and initial MB concentration (20-80 mg/L) on MB removal were investigated. At first, the effect of the pH (3, 5, 7, 8 and 11) on the process was studied at a contact time of 60 min by adding 0.1 g/L of ZnO-NPs to each beaker with 100 ml of the MB solution with an initial MB concentration of 60 mg/L. The pH of the solution was adjusted using 0.1N HCl or 0.1N NaOH solutions. The beaker with its contents was placed on a shaker and rotated at a speed of 180 rpm. After a specific time of contact, the samples were filtered using the Whatman filter paper (size: 40µm). The residual MB concentration of the filtrate was measured to determine the adsorption capacity and removal efficiency. The optimum dosage of the adsorbent was determined at a contact time of 60 min, MB initial concentration of 60 mg/L at the optimum pH obtained. Also, the effect of initial MB concentration was studied by varying the contact time at the optimum pH and optimum ZnO-NPs dosage. The residual concentrations were measured using a UV-visible spectrophotometer (MODEL: LUV-100A). The amount of MB adsorbed on ZnO-NPs (qe) was evaluated using Eq. (1):

Also, the removal efficiency (%R) was calculated based on Eq. (2) [23-26]:

Where C0 (mg/L) is the initial concentration of MB and Ce (mg/L) is the equilibrium liquid phase concentration of MB, Cf (mg/L) is the final concentration, V is the volume of the solution (L) and M is the amount of ZnO-NPs used (g).

3.1. XRD, SEM and FTIR analysis on ZnO-NPs

The XRD result (Figure 1a) indicates that the ZnO-NPs possess a crystalline structure. This crystalline structure increases the adsorption, where the adsorbate is adsorbed on the upper layer of the crystalline structure of the adsorbent surface by means of physisorption [27]. The XRD image also showed that the maximum peak is around 2θ=36 0 with very high intensity. The surface morphology of ZnO-NPs is shown in Figure 1b. The ZnO particles look lamellar-like in arrangement. The specific surface area is one of the parameters determining the adsorption capability of an adsorbent [28]. Figure 1b also shows that the NPs are highly porous in nature which indicates the presence of high surface area which will lead to a higher level of contact with the adsorbate, MB [28]. A plot of the infrared transmittance (%) against the wavelength (cm-1) can be seen in Figure 1c. The FTIR analysis on the ZnO-NPs indicates the existence of C–O stretch of alcohols, carboxylic acids, esters, ethers (1097.08 cm-1) and C–H rock of alkanes (1384.50 cm-1). The peaks 1638.09 and 1617.65 cm-1 show the presence of C–C stretch (in-ring) of aromatics. Peak 2925.73 cm-1 indicates the existence of C–H stretch alkanes, which are strong, broad and multi-banded. The presence of O–H stretch, H–bonded of alcohols, phenols (3415.78 cm-1), which are also strong and broad bands can be observed. The O–H bands are very important sites for adsorption [29]. The hydroxyl group effect is more felt due to the hydrogen bonding with other hydroxyl bonds since they do not exist in isolation establishing a stable structure [28, 29].

cm-1 Name Description 2 Sample 074 By Administrator Date ‫تبسلا‬, ‫ ربمسيد‬23 2017

Figure 1. (a) XRD, (b) SEM image, and (c) FTIR spectra of ZnO-NPs 292

3.2. Effect of pH

The influence of pH (3-11) on adsorption of MB using ZnO-NPs was studied at a time of 60 min, initial MB concentration of 60 mg/L, and ZnO-NPs dosage of 0.1 g/L. Figure 2 shows that the percentage removal of MB increased from 92.16 to 93.3 % as the pH was increased from 3 to 7; the amount of MB adsorbed also increased from 27.65 to 28 mg/g. The solution pH is one of the parameters affecting the adsorbent level and ionization of pollutants. The influence of pH depends on the point zero charge (pHzpc) of the catalyst and acidity constant (pKa). MB has a pKa value of 3.5, and pHzpc value for ZnO-NPs is 7.5 [25, 30]. At pH higher than these values, MB and ZnO-NPs had a negative charge. Therefore, the force between them will be repulsive, and they will have no tendency to react. Thus, MB removal declined at alkaline pH values. At pH values below 7.5, ZnO-NPs surface had a positive charge and thus possesses a greater adsorption force, causing greater MB removal [31].

pH Figure 2. Effect of pH on removal efficiency of MB (Time: 60 min, ZnO-NPs dosage: 0.1 g/L, initial MB concentration: 60 mg/L).

3.3. Effect of adsorbent dosage

The effect of ZnO-NPs dosage on MB removal was studied by varying the dosage from 0.1 to 0.4 g/L at an initial MB concentration of 60 mg/L and a pH of 7. Figure 3 shows that the removal efficiency was enhanced as the dosage was increased from 0.1 to 0.2 g/L. Maximum removal efficiency occurred at a dosage of 0.2 g/L (97.5 %). The amount of MB adsorbed on ZnO-NPs (qm) at the dosage of 0.2 g/L was 21 mg/g. This may be owed to the increase in the number of active sites available for MB adsorption [32]. The removal efficiency was decreased as the adsorbent dosage was increased above 0.2 g/L, which may be due to the fact that as the amount of adsorbent is increased, the total surface area available for the adsorption of MB diminishes as a result of overlapping or aggregation of adsorption sites [33, 34].

Figure 3. Effect of adsorbent dosage on removal efficiency of MB (Time: 60 min, pH: 7, initial MB concentration: 60 mg/L).

3.4. Effect of initial MB concentration

Figure 4 shows the effect of initial MB concentration (20–80 mg/L) on the removal of MB on ZnO-NPs. The adsorption capacity increased and the removal efficiency decreased with increasing MB concentration. The dye was adsorbed on the outer surface of the adsorbent; after complete adsorption on the outer surface, the dye entered into the inner surface through the pores [35, 36]. The adsorption efficiency decreased as the initial concentration of MB increased from 40 to 80 mg/L because of the decrease in the active sites [25]. The reduction in the efficiency by increasing the concentration of pollutants can be expressed by the constant of other parameters effective against the increase of pollutant concentration, and the filling of active nanoparticles sites by pollutants in the process. Other researchers also confirmed that as the initial concentration of the pollutant was increased, the removal efficiency was decreased [37].

Initial MB concentration,mg/L Figure 4. Effect of initial MB concentration on removal efficiency of MB (Time: 60 min, pH: 7, dosage: 0.2g/L). 294

3.5. Effect of contact time

The effect of contact time on the MB adsorption process was investigated by varying the contact time from 30 to 120 min using ZnO-NPs dosage of 0.2 g/L and MB concentration of 40 mg/L at pH of 7. Figure 5 shows the effect of contact time on the removal of MB on ZnO-NPs. The MB adsorption was increased with increasing the time of contact from 30 to 60 min and reached equilibrium at 60 min. This was due to the increase in the period of contact between the adsorbent and the adsorbate [32]. But as the time of treatment was increased beyond 60 min, the percentage of MB adsorbed decreased. After 60 min, desorption took place [38].

Time(min) Figure 5. Effect of contact time on removal efficiency of MB (ZnO-NPs dosage: 0.2 g/L; pH: 7, initial MB concentration: 40 mg/L).

3.6. Adsorption isotherms and kinetics

The Langmuir and Freundlich isotherms are the two well-known adsorption isotherm models. These isotherms were used in this study to determine the relationship between the amount of dye adsorbed and its equilibrium concentration in solution. For the isotherm study, 0.2 g/L ZnO-NPs were added to the MB solution with a concentration of 40 mg/L at pH 7 and stirred for 60 min. Adsorption kinetic models are used to examine the rate of adsorption process and the potential rate-controlling step. To determine the kinetic order and the time required to reach adsorption equilibrium, the adsorption kinetic study of MB on ZnO-NPs biomass was carried out at room temperature. The adsorption kinetics experiment was carried out using ZnO-NPs dosage of 0.2 g/L, MB concentration of 40 mg/L, pH 7 at different contact times (15, 20, 30 60, 90, and 120 min). The different isotherm and kinetics equations (linear and nonlinear equations) are presented in Table 1 [12, 39-41]. Where qe (mg/g) and qt (mg/g) are the amounts of MB adsorbed at equilibrium and at time t, respectively; qm (mg/g) is the maximum or monolayer adsorption capacity, and Ce is the equilibrium concentration of MB in the solution (mg/L); k1 (/min) is the pseudo-first-order rate constant of adsorption, K2 (g mg/min) is the pseudo-second-order rate constant, and KL (L/mg) is the Langmuir isotherm constant related to the affinity of the binding sites and energy of adsorption; KF (mg/g)(L/mg) and n are the constants incorporating the factors affecting the adsorption capacity and intensity of adsorption, respectively. 295

Table 1. The equation of isotherms and kinetics. Isotherm and kinetic type Freundlich Langmuir

Main equation (nonlinear) 1/𝑛 𝑞𝑒 = 𝐾𝐹 𝐶𝑒 𝑞𝑚 𝐾𝐿 𝐶𝑒 𝑞𝑒 = 1 + 𝐾𝐿 𝐶𝑒

Linear form 𝐿𝑜𝑔𝑞𝑒 = 1⁄𝑛 𝐿𝑜𝑔𝐶𝑒 + 𝐿𝑜𝑔𝐾𝐹 𝐶𝑒⁄ 𝐶𝑒 1 𝑞𝑒 = ⁄𝑞𝑚 𝐾𝐿 + ⁄𝑞𝑚 𝐿𝑜𝑔(𝑞𝑒 − 𝑞𝑡 ) = 𝐿𝑜𝑔𝑞𝑒 −

In order to further validate the adsorption mathematical models (nonlinear isotherm and kinetic models) with the experimental results, a number of error functions were used, which are available in the literature. The use of only the R2 for the validation of nonlinear isotherm and kinetic data analysis is not sufficient enough, because the experimental results may have high R 2 value. Therefore, it is necessary to diagnose the result of regression for residue analysis. In this study, five types of statistical functions were used in this study. The different error functions equations used to validate the nonlinear isotherm and kinetic models are presented in Table 2 [40, 42-44]. Where N is the number of performed experiments, P is the number of parameters of the fitted model, qc is the value that is calculated from the model fit, qe is calculated from test elements and p is the number of test element and R2 is the coefficient of determination. Table 2. The equations of error functions. Isotherm and kinetic type Standard deviation (SD)

The coefficient of determination𝑅2 2 (R2) ∑𝑁 ̅𝑜𝑏𝑠𝑒𝑟𝑣𝑒𝑑 ) − ∑𝑁 𝑖=1(𝑞𝑖,𝑜𝑏𝑠𝑒𝑟𝑣𝑒𝑑 − 𝑞 𝑖=1(𝑞𝑖,𝑜𝑏𝑠𝑒𝑟𝑣𝑒𝑑 − 𝑞𝑖,𝑐𝑎𝑙𝑐𝑢𝑙𝑎𝑡𝑒𝑑 ) = [ ] 2 ∑𝑁 ̅𝑜𝑏𝑠𝑒𝑟𝑣𝑒𝑑 ) 𝐼=1(𝑞𝑖,𝑜𝑏𝑠𝑒𝑟𝑣𝑒𝑑 − 𝑞 𝑁−1 Adjusted R-squared (𝑹𝟐𝒂𝒅𝒋 ) 2 𝑅𝑎𝑑𝑗 = 1 − (1 − 𝑅2 ) ( ) 𝑁−𝑃−1 𝑁 𝟐 2 Reduced Chi-squared (𝑿𝒓𝒆𝒅 ) (𝑞𝑖,𝑜𝑏𝑠𝑒𝑟𝑣𝑒𝑑 − 𝑞𝑖,𝑐𝑎𝑙𝑐𝑢𝑙𝑎𝑡𝑒𝑑 ) 2 𝑋𝑟𝑒𝑑 = ∑ 𝑁−𝑃 Residual sum of squares (RSS)

The linear adsorption isotherm and kinetics plots are shown in Figures 6 and 7. The estimated linear isotherm and kinetics parameters and their respective correlation coefficients, R2 are presented in Table 3. The MB adsorption experimental data was found to conform more to the pseudo-second-order kinetic model (with R2 = 0.9974) than the pseudo-first-order model (R2 = 296

0.7029), which suggests a chemisorption process [8, 45]. It is found that the Freundlich model (R2 = 0.9384) is more suitable to satisfactorily describe the studied sorption phenomenon than the Langmuir (R2 = 0.9368) model using the linear forms of the isotherm equations. The estimated nonlinear isotherm and kinetics parameters and their respective correlation coefficients, R2 are presented in Table 4. A lower value of RMSE shows a better fit. The higher 2 2 values of R2 and lower values of RMSE, SD, 𝑅𝑎𝑑𝑗 , and 𝑋𝑟𝑒𝑑 imply that the pseudo-second-order kinetic model best explained the studied adsorption process. The estimated nonlinear isotherm parameters, fixed correlation coefficient (R2), RMSE and the other error analysis functions are presented in Table 4. Considering the error range for each model, it was deduced that the Langmuir and Freundlich models were adequate for describing the adsorption process. But the MB adsorption equilibrium data conformed more to the Langmuir isotherm compared to the Freundlich isotherm with regards to its correlation coefficient, R2 (Table 4). Langmuir isotherm indicates the surface homogeneity of the adsorbent [46]. In fact, by comparing the values of the error functions, the Freundlich was found to be the more suitable model for the description of the 2 2 adsorption of MB on ZnO-NPs with the lowest values of RMSE, RSS, SD, 𝑅𝑎𝑑𝑗 , and 𝑋𝑟𝑒𝑑 . The Freundlich adsorption model assumes a heterogeneous surface [46]. Generally, the results (shown in Tables 3 and 4) indicated that the nonlinear method of regression analysis provided better results based on fitting the experimental data with the isotherm and kinetic models studied. Also, n (the intensity of adsorption) was determined as 9.62, which lies within the range of 1 to 10 suggesting that the adsorption of MB on ZnO-NPs is favorable [47]. The monolayer adsorption capacity (qm) for ZnO-NPs with MB was 12.78 mg/g. Table 3. Parameters of linear isotherm and kinetics for MB adsorption on ZnO-NPs. Freundlich

Figure 6. Linear (a) Freundlich, (b) Langmuir model plots of MB adsorption on ZnO-NPs.

Table 4. Parameters of nonlinear isotherm and kinetics for MB adsorption on ZnO-NPs. Freundlich

SD 0.081 R2 0.988 2 0.934 𝑅𝑎𝑑𝑗 RMSE 0.47 Pseudo-second-order

Figure 7. Linear (a) pseudo-first-order, (b) pseudo-second-order plots of MB adsorption on ZnONPs. The comparison of ZnO NPs with other adsorbents for methylene blue removal The removal efficiencies (%R) of methylene blue (MB) using various adsorbents are listed in Table 5. Table 5 reveals that ZnO-NPs is a capable material that can be used for the removal of MB from water containing MB when compared with the other adsorbents. Table 5 also proves that methylene blue, a cationic dye can be removed from effluents using the adsorption technique via different adsorbent materials.

Table 5. Comparison of ZnO-NPs with other adsorbents for the removal of MB. Adsorbent material Maximum removal efficiency 100 % Sulfuric acid– treated orange peel

Luffa adsorbent modified with sodium dodecyl sulfate anionic surfactant

pH: 8 Adsorbent dose: 0.4 g/L Contact time: 45 min MB concentration: 50 mg/L Temperature: 30 0C Volume of solution: 100 mL pH: 4.0 adsorbent dosage: 1.5g/L Contact time: 300 min initial dye concentration: 15 mg/L Temperature: 45 0C Agitation speed: 150 rpm Volume of solution: 250 mL pH: 6 Adsorbent dose: 1 g Contact time: 60 min MB concentration: 50 mg/L Temperature: 25 0C Agitation speed: 115 rpm Volume of solution: 100 cc pH: 11 AC-CS dose: 1.4 g/L Contact time: 50 min MB concentration: 10 mg/L Temperature: 25 ± 2 °C Volume of solution: 100 mL pH: 2.03 nanoparticles dose: 1.91 g/L Contact time: 15.16 min MB concentration: 100.65 mg/L Agitation speed: 180 rpm Temperature: 25 ± 2 °C Volume of solution treated: 100 mL pH: 10 nanoparticles dose: 0.2 g/L Contact time: 120 min MB concentration: 30 mg/L Volume of solution treated: 50 cc pH: 8 Adsorbent dose: 1 g/L Contact time: 60 min MB concentration: 10 mg/L Volume of solution treated: 100 mL pH: 5.6 OPAC dosage: 0.16 g/L Contact time: 100 min MB concentration: 50 mg/L Temperature: 333 K Agitation speed: 110 stroke/min Volume of solution treated: 200 mL

pH: 7 This study ZnO-NPs dose: 0.2 g/L Contact time: 60 min MB concentration: 40 mg/L Temperature: 25 ± 2 °C Agitation speed: 180 rpm Volume of solution treated: 100 mL