# Ni/Zn Layered Double Hydroxide (LDH) Micro/Nanosystems and Their Azorubine Adsorption Performance
## 4 Results and Discussion
### Adsorption Process from Aqueous Solutions
#### 4.2.2 Contact Time, pH, Stirring Speed and Temperature Effect on the Adsorption Process  
The adsorption as a function of contact time was conducted at room temperature (25 \({}^{\circ}\)C) using 30 mg L\({}^{-1}\) of Ni/Zn LDH and is reported in Figure 5a. The azo dye uptake was rather fast initially (first 25 min), then increased slowly up until 60 min, when the equilibrium was reached with a removal ratio of 94.3%. At the beginning, there are phenomena attributed to the existence of a large number of vacant active sites on the adsorbent surface. However, the remaining vacant surface sites are less available for adsorption with time probably due to the repulsive forces that occur between the Az molecules and the Ni/Zn LDH surfaces [46]. Otherwise, the decrease in the Az adsorption kinetics is due to the presence of carbonates from LDH structure; the dye species are in competition with carbonate anions which originate from strong interactions between the CO\({}_{2}\) molecules and the strongest basic sites in the LDH structure and, consequently, it takes more time to reach the adsorption equilibrium [47]. The pH of the dye solution is also an important parameter which controls the rate of synthetic azo dye solution degradation. In order to study this pH effect, the degradation experiments were carried out using 30 mg of Ni/Zn hydroxide in a solution of 50 ppm Az (Figure 5b) by varying the pH in the range 2-12 with addition of (0.1 M) either HCl or NaOH. The adsorption process affected by the milieu pH is possibly due to the interactions between the dye anions in the solution and the adsorbent surface charge contents [48; 49]. Therefore, we noted firstly that the removal efficiency of Az increases from 82.52% to 96.02% when the pH is varied from 2 to 7, and the maximum was achieved at neutral pH. Then, the removal rate decreases in basic environment from 33.96% to 27.54% in the pH range from 10 to 12. This phenomenon can be attributed to the electrostatic attraction  
Figure 5: Contact time (**a**), pH (**b**) and stirring speed (**c**) effect on the Carmoisine (Az) removal using Ni/Zn LDH adsorbent (initial dye concentration = 50 mg L\({}^{-1}\) in 100 mL; sorbent mass = 30 mg; T = 25 \({}^{\circ}\)C; initial pH). (**d**) Kinetics of Az uptake by nano adsorbents at various temperature (initial dye concentration = 50 mg L\({}^{-1}\) in 100 mL; sorbent mass = 30 mg; stirring speed = 300 rpm; initial pH). between the positively charged adsorbent surface layer and the anionic dye at pH less than the point of zero chare (PZC), which is equal to 7.53 for our Ni/Zn LDH adsorbent. Therefore, at lower pH (acidic region), the surface of Ni/Zn LDH adsorbent with more positively charged (pH < pHPZC) leads to increase the percentage of dye removal due to strengthening the attractive forces among the positive charge of the surface of the LDHs and negative charge of the dye [50]. On the other hand, the surface of LDHs become more negatively charged along with the increase in solution pH (pH > pHPZC), leading to a decline inthe nanohybrid adsorption ability. This decrease phenomenon is related to strong repulsion forces between the dye and negative surface of the LDHs containing inorganic anions. Consequently, for pH values under the pHPZC, there can be two probable mechanisms to propose for Az removed using this kind of LDH; the first mechanism could concern the anionic exchange of carbonate anions in the interlayer with dye anions Az-SO3, while the second is the fixation by adsorption with the positively charged surface H2O+ of Ni/Zn LDH and the Az anions [23]. Therefore, these solutions were used directly without any pH adjustment in the rest of the work. According to the curves in Figure 5c, it can be seen that the capacity for retention of the dye increases slightly with increasing stirring speed, which acts favorably on the contact probability between substrate and support. Figure 5d presents Az removal as a function of time at three different temperatures (15, 25 and 45 \({}^{\circ}\)C) on Ni/Zn LDH adsorbent.