2.1 Catalyst preparation

The exchange technique was used to prepare CuO/NaA according to the following protocol: 1 g of the NaA synthesized zeolite was added to 200 mL of Cu (NO₃)₂·3H₂O (0.1 M) (Assay, 98%) and shaken for 8 h in a polyethylene bottle at room temperature. The NaA zeolite was synthesized by hydrothermal process, as reported in our previous work [18]. The solid product was filtered and washed with doubly distilled water until Cu²⁺ becomes undetectable in solution. Then, the sample was dried at room temperature and referred to as CuO/NaA zeolite. Finally, CuO/NaA was calcined at 450 °C for 4 h.

2.2 Characterization

The NaA, CuO and CuO/NaA samples were characterized by X-ray diffraction (Philips PW 1800, using Cu Kα radiation), over the 2θ range between 5 and 35°. The surface morphology was observed with a scanning electronic microscope (SEM, Philips XL 30) equipped with an energy dispersive spectrometry (EDS) unit for chemical analysis.

2.3 Batch adsorption

Batch adsorption experiments of MO were carried out in a double-walled Pyrex reactor of capacity 200 cm³, whose temperature was regulated by a thermostated bath. The MO solutions (50–200 mg/L) were prepared by dilution from a stock solution (1000 mg/L) of MO (Merck, 99.5%). The effect of pH on the adsorption of MO was examined by mixing 0.2 g of CuO/NaA with 100 mL of a MO (50 mg/L) solution whose pH ranged from 3.0 to 11.0; the pH was adjusted with NaOH (0.1 N) or HCl (0.1 N). The adsorption kinetics and isotherms of the dye solutions (50–250 mg/L) were performed at different temperatures (25, 30, 40, and 55 °C). The final MO concentration was determined using a UV–Vis spectrophotometer (Optizen 2120 UV–Visible, λ_max = 465 nm). The percentage of MO removal was calculated using the relationship:

2.4 Photoactivity experiments

The photocatalytic degradation is undertaken with a volume of 100 mL (MO: 100 mg/L, pH ∼ 7) and 200 mg of CuO/NaA in the same reactor as above. The light source is a tungsten lamp (200 W, Philips) emitting over the range from 400 to 750 nm and located 14 cm above the reactor, thus providing a light flux of 10.3 mW·cm⁻².

3.1 Characterization

Fig. 1 shows the XRD patterns of three samples. Fig. 1a corresponds to the sodalite NaA structure with strong peaks in the 2θ range from 5 to 35°, which indicates that the zeolite was well prepared, while Fig. 1b corresponds to the CuO structure. The two phases are in good agreement with those already reported in the literature [13,18]. As expected, the XRD pattern (Fig. 1c) shows the peaks of the zeolite NaA, which coexist with those of CuO.

Scanning electronic micrographs (Fig. 2a) show that the crystallites of an NaA zeolite form fine cubic particles with an average size of 2–5 μm. Fig. 2b shows that CuO particles crystallize as needles, with an average size of about 0.1–0.5 μm, and form spherical aggregates. It seems that the preparation method of this oxide has a positive influence on crystal morphology. Fig. 2c shows that CuO particles are well dispersed in the structure surface of the NaA zeolite. The chemical analysis by EDS confirmed the presence of copper on the surface of the NaA zeolite.

3.2 Adsorption

3.2.1 Effect of pH

The pH of the solution is a controlling parameter that strongly affects the adsorption of dyes on the CuO/NaA surface. The influence of pH on MO adsorption was investigated over the range from 3 to 11 and the results are displayed in Fig. 3, which shows a maximum removal of MO (99%) at pH ∼ 7. At lower pH_s values, the adsorbants have a net positive charge. Hence, the minimum MO removal obtained at low pH may be due to the electrostatic attractions between negatively charged functional groups located on MO and the positively charged adsorbent surface. Hydrogen ion also acts as a bridging ligand between the adsorbent and dye molecules [20]. As pH increases, the number of hydroxide ions increases and competes with anionic ion MO on the adsorption. A similar type of behavior was reported for the adsorption of the dye onto different adsorbents [20]. Therefore, a pH value ∼ 7 was selected for further studies.

3.2.2 Effect of contact time

The effect of the contact time on the removal percentage of MO was investigated for a concentration of 50 mg/L (Fig. 4). The uptake of MO by CuO/NaA was rapid at the beginning, due to a larger surface area of adsorbent, and gradually decreases with time until it reaches saturation. The plots reveal that the maximum removal is achieved after 240 min of shaking. After adsorption, MO uptake is controlled by the rate of dye transported from the exterior to the internal pores of the adsorbent.

3.2.3 Effect of the catalyst dose

The adsorbent dose is an important parameter since it determines the uptake capacity of an adsorbent for a given initial MO concentration under the operating conditions. The influence of the dose on MO adsorption (Fig. 5) increases with increasing the dose of CuO/NaA, and the adsorption is almost constant at doses higher than 2 g/L. This may be due to the increased availability of surface active sites resulting from the increased dose and conglomeration of the adsorbent.

3.2.4 Effect of initial concentration of MO

The effect of MO concentration over the range (50–250 mg/L) was investigated with a dose of 2 g/L, pH ∼ 7, a temperature of 25 °C and a duration of 6 h. The results show that the percentage removal of MO was found to decrease with increasing the initial MO concentration, achieving a maximum removal of 97% at 50 mg/L and of 60% for 250 mg/L (Fig. 6). This strong adsorption for low initial MO concentrations might be explained by the active sites available on the adsorbent. Similar results have been reported in the literature [21–23].

3.2.5 Effect of temperature

The adsorption of MO onto CuO/NaA was studied for a concentration of 50 mg/L by varying the temperature from 25 to 55 °C (Fig. 7). The percentage of MO removal increased with increasing the temperature from 25 to 55 °C; it can be seen that higher temperatures were favorable for adsorption. The equilibrium adsorption capacity was affected by the temperature and the amount of MO adsorbed increased from 67.55 to 94% when the temperature was raised from 25 to 55 °C. It has been well documented that the temperature has two major effects on the adsorption process. An increase in temperature is known to increase the diffusion rate of the adsorbate molecules across the external boundary layer and in the internal pores of the adsorbent particles as a result of the decreased viscosity of the solution.

3.3 Adsorption isotherms

In order to describe the removal mechanism of MO from water onto CuO/NaA, three isotherm models (Freundlich, Langmuir and Temkin isotherm models) were applied. The Langmuir, Freundlich and Temkin isotherm models were applied to establish the relationship between the amount of MO adsorbed by CuO/NaA and their equilibrium concentration in aqueous solution. The experimental data are fitted by the Langmuir model [24]:

The adsorption equilibrium data were also applied to the Freundlich model in logarithmic form [25]:

The Temkin isotherm contains a factor that explicitly takes into account adsorbent–adsorbate interactions. By ignoring the extremely low and large concentrations, the model assumes that the heat of adsorption (function of temperature) of all molecules in the layer would decrease linearly rather than logarithmically with coverage. The derivation is characterized by a uniform distribution of binding energies (up to some maximum binding energy) that was evidenced by plotting q_e against ln C_e [Eq. (6)]:

where A_T (L/g) and B (J/mol) are the Temkin constants related to the binding equilibrium isotherm and to the heat of adsorption, respectively. The constants were determined from the slope and the intercept.

The adsorption of MO onto CuO/NaA was well fitted with the Langmuir, Freundlich and Temkin models because of the high correlation coefficients (R²). The Langmuir model (R² = 0.995) was more applicable than those of Freundlich (R² = 0.979) and Temkin (R² = 0.955). The Langmuir adsorption maximum, q_m, is quite high (79.49 mg/g), with a constant b of 0.108 L/mg. The Freundlich coefficient (n = 3.28) was smaller than 10, indicating that the adsorption of MO on CuO/NaA under the studied conditions was favored. By contrast, Fig. 8 shows experimental data that do not arrange linearly. It seems that the data are well fitted by using the models of Langmuir and Temkin.

3.4 Adsorption kinetics

The adsorption kinetics is important because it controls the efficiency of the process and the equilibrium time. It also describes the rate of adsorbate uptake on CuO/NaA. In order to identify the potential rate-controlling steps involved in the adsorption process, two kinetic models were used to fit the experimental data, namely the pseudo-first-order and pseudo-second-order models.

3.4.2 Pseudo-second-order model

The second-order kinetics may be tested on the basis of the following equation [29]:

$$\frac{\text{d}q}{\text{d}t}=k_2{\left(q_{\text{e}}-q_{\text{t}}\right)}^2$$

(9)

where k₂ is the pseudo-second-order adsorption rate coefficient (g/mg/min). For the boundary conditions (t = 0 to t and q_t = 0 to q_t), Eq. (9) becomes:

$$\frac{t}{q_t}=\frac{1}{(k_2{q_{\text{e}}}^2)}+\frac{1}{q_{\text{e}}}t$$

(10)

The plots of t/q_t versus time (t) for different concentrations (C_o) and various temperatures fit well with the experimental data and provide the rate constants k₂ and q_e. Plots of Lagergren first-order and pseudo-second-order kinetic models are shown in Figs. 9 and 10 for different MO concentrations and temperatures (Table 2). The R² coefficients of the pseudo-second-order kinetics were higher than 0.994 for both solutions. Moreover, the calculated q_e value agrees well with the experimental one, indicating that the model fits better the adsorption data.

Table 2

Pseudo-first-order and pseudo-second-order models constants for the adsorption of MO on CuO/NaA under different initial concentrations and different temperatures.

	Pseudo-first-order	Pseudo-second-order
	qecal (mg/g)	k1 × 102 (min−1)	R2	qecal (mg/g)	k2 × 104 (g·mg−1·min−1)	R2
MO (mg/L)
50	31.74	1.48	0.941	31.84	3.22	0.994
100	44.53	1.19	0.990	54.05	2.21	0.997
150	55.49	1.51	0.899	76.92	1.57	0.996
200	75.07	1.40	0.992	86.20	1.62	0.995
250	66.63	1.14	0.975	90.09	1.69	0.997
T (K)
298	31.76	1.08	0.9458	72.67	5.18	0.9989
308	61.72	1.07	0.9988	98.91	1.94	0.9965
318	63.25	1.28	0.9006	101.01	2.81	0.9961
328	48.58	1.03	0.9879	102.46	3.19	0.9962

The rate constants k₂ at different temperatures (Table 2) were used to estimate the activation energy of MO adsorption onto CuO/NaA. The relationship among the rate constant k₂, temperature (T) and activation energy (E_a) follows the Arrhenius equation [30]:

$$k_2=A_{\text{o}}\exp (-E_{\text{a}}\text{/}RT)$$

(11)

the linearization gives:

$$\text{ln}{k}_2=-E_{\text{a}}\text{/}R\left(\text{1/}T\right)+\text{ln}{A}_{\text{o}}$$

(12)

where R (8.314 J/mol·K) is the universal gas constant and A_o is a pre-exponential factor. The slope of plot ln k₂ versus 1/T (Fig. 11) gives an activation energy E_a (20.98 kJ/mol) that indicates physical adsorption [18,30].

3.5 Thermodynamic study

The thermodynamic parameters were evaluated to confirm the nature of the adsorption and the inherent energetic changes involved during MO adsorption. Standard enthalpy (ΔH°), free energy (ΔG°) and entropy change (ΔS°) were calculated to determine the thermodynamic feasibility and the spontaneous nature of the process. Therefore, the values of ΔH° and ΔS° were obtained from the slope and intercept of the ln K_d versus 1/T curve according to Eq. (12) (Fig. 12) [18]:

As can be seen in Table 3, the ΔG° values are negative for all temperatures, indicating that MO adsorbed spontaneously onto CuO/NaA and that the system does not gain energy from an external source. The positive value of ΔH° further confirms the endothermic nature of MO adsorption, while positive entropy (ΔS°) indicates the increased randomness with MO adsorption, probably because the number of desorbed water molecules is larger than that of adsorbed MO molecules.

3.6 Photocatalysis

It is worthwhile to outline that MO (with a concentration of 100 mg/L) was partially adsorbed (88%). So, the remaining concentration was subjected to visible illumination. The kinetic was studied by withdrawing aliquots at regular time intervals. Fig. 13 clearly shows that MO photodegradation obeys a first-order kinetic with a rate constant of 7.47 ×10⁻³ min⁻¹ and a removal percentage of 76%.

From Table 4, comparing the maximum adsorption capacities q_m (mg/g) of MO on different adsorbents, shows that the CuO supported on NaA zeolite performs considerably better than the other ones.