## 3 Results and Discussion
### Effect of Solution pH  
The pHpzc value of 4 AZW is 10.5 (Figure 6). Then, at pH < pHpzc, the surface of the adsorbent is predominantly positive; while when the solution pH > pHpzc, the adsorbent surface is predominantly negative [44]. The effect of the initial solution's pH (in the range of 3 to 12) on the removal of each dye, at initial concentrations of MB and MG of 11.5 mg/L and 45 mg/L, respectively, at 25 \({}^{\circ}\)C, is shown in Figure 7.  
Generally, for a cationic dye, at a lower pH, the percentage of dye removal will decrease, but it will increase at higher pH values [45]. Although methylene blue has no determinable pKa between pH 0 and 14, it is in the form MB\({}^{+}\) in this aqueous pH range, but several species can be formed according to the redox status of the solution [46]; for malachite green, however, pKa = 6.90 [47]. Then, for MB or MG cations, the protonated form is the predominant species in solutions at pH < pKa, and above this pH value, the dye becomes ever more de-protonated.  
Figure 5: Effect of NaCl concentration on the adsorption percentage of MB and MG (C\({}_{0}\)(MB) = 8 mg/L, C\({}_{0}\)(MG) = 41 mg/L, S/L = 1 g/L, contact time 70 min, T = 25 \(\pm\) 1 \({}^{\circ}\)C and unadjusted pH).  
Figure 6: The plot of \(\Delta\)pH vs. initial pH for the determination of 4 AZW’s point of zero charge.  
The highest adsorption capacities were obtained in the pH range of 5 to 10 for MB with a maximum adsorption efficiency of 99% at pH = 8.5. For MG, a relatively high adsorption capacity can be kept at pH = 5 to 11 with maximum adsorption of 99.5% at pH = 8 (Figure 7). Similar observations have also been reported by other researchers [48; 49; 50; 51; 52; 53].  
However, a definitive explanation of this point cannot be established on the basis of pH values only, because pH can be influenced, separately or together, by the surface charge of the adsorbent, the degree of ionization of the adsorbate, and the extent of dissociation of functional groups at the adsorbent active sites [44; 54]. Lower sorption of dyes obtained at very acidic pH values can be explained through the behavior of the zeolite in acidic and basic mediums, as reported elsewhere [55].  
First, zeolite in an acidic medium can easily exchange its M\({}^{+}\) metal cation with protons according to the following reaction:  
\[[\text{Z-OM}]_{\text{n}}+\text{H}^{+}{}_{(\text{aq})}\leftrightarrows[\text{ Z-OH}]_{\text{n}}+\text{M}^{+}{}_{(\text{aq})} \tag{4}\]  
where Z is the Silicon or Aluminum atom at the solid surface and within the zeolite structure. This ion-exchange reaction leads to an increase in the solution pH as observed in Figure 7. If the initial H\({}^{+}\) concentration is high, proton adsorption at the neutral surface of the zeolite will take place:  
\[[\text{Z-OH}]_{\text{n}}+\text{H}^{+}{}_{(\text{aq})}\leftrightarrows[\text{ ZOH}_{\text{2}}{}^{+}]_{\text{n}} \tag{5}\]  
The protonation of the zeolite surface (Equation (5)) diminishes the possible dye adsorption due to electrostatic repulsive forces that may occur between the zeolite surface and the cationic dye. On the other hand, in an alkaline medium, hydroxyl ions can deprotonate the surface as follows:  
\[[\text{Z-OH}]_{\text{n}}+\text{OH}{}_{(\text{aq})}\leftrightarrows[\text{ZO} ^{\prime}]_{\text{n}}+\text{H}_{\text{2}}\text{O} \tag{6}\]  
Under such conditions, the attraction forces between the negatively charged zeolite surface and the positively charged MB and MG species favor the adsorption. Similar results have been obtained with other inorganic pollutants such as heavy metals in wastewater by using zeolite nanoparticles impregnated in polysulfone membranes [14].  
Figure 7: Effect of pH on the removal of MB and MG by 4 AZW from aqueous solution. (C\({}^{0}\) (MB) = 11.5 mg/L, C\({}^{0}\) (MG) = 45 mg/L, S/L = 1 g/L, contact time 70 min, T = 25 \(\pm\) 1 \({}^{\circ}\)C).