## 3 Results and Discussions
### Photocatalysis for Congo Red Dye Removal  
3.2.1. Dark Adsorption Studies  
Adsorption capacity is an important characteristic of the solid photocatalysts because the generation of the oxidant species and the oxidation process of organic dye compounds takes place at the surface of the solid. On the one hand, a good adsorption capacity ensures the presence of the organic dye compound on the catalyst surface, where the oxidation process takes place [24]. The studies that have been carried out regarding the synergistic effect of adsorption and photocatalysis on the removal of dyes from the water have shown that the materials that have high adsorption capacities have higher removal efficiencies [24], but these performances depend especially on the affinity of the adsorbent material towards the organic compound. On the other hand, the use of adsorbent materials with high adsorption capacity does not economically justify the use of these materials as photocatalysis. In addition, a very high adsorption capacity prevents the radiation generated by the lamp from reaching the surface of the catalyst to initiate the process of generating electron-hole couples that give rise to oxidizing radicals. Studying the synergistic effect of adsorption and photocatalysis developed by carbon nitride-based composite nanosheets, it was concluded that a high adsorption efficiency is followed by a low photocatalysis efficiency [25].  
The dark adsorption studies give also information about the time needed to achieve the equilibrium and about the extent of adsorption. The variation in the CR concentration across time when conducting the adsorption studies in the dark is shown in Figure 5. A sharp decrease in the CR concentration is observed in the first 5 min after the contact between the dye solution and the solid materials, then the concentration decreases more slowly, the equilibrium being reached after 30 min of contact.  
Figure 4: UV-VIS spectra of the synthetized samples. Inlet: Kubelka–Munk transformed function from diffuse reflectance spectra.  
The adsorption efficiency developed by the two materials is: 34.4% for EthylAmAcid and 12.5% for BenzylAmAcid. Under these experimental conditions, the equilibrium adsorption capacities reached the values of 8.62 mg/g for EthylAmAcid and 1.95 mg/g for BenzylAmAcid, respectively.  
The main application of polymers and composite polymers in water treatment, and especially in the tertiary treatment process of dye removal, is adsorption. A varied class of polymers and polymeric composites was synthesized, characterized, and tested for the study of their adsorbent performances. For example, a polyaniline-zinc titanate composite has proven an adsorption efficiency of over 95% in 15 min, when removing CR with an initial concentration of 50 mg/L [26]. A polystyrene/CuO/calcined layered double hydroxide composite has developed a maximum adsorption capacity of 485 mg/g, with an efficiency of over 95% in removing CR dye from water [27]. Comparing the adsorption efficiencies developed by other polymeric materials with those of the two studied materials, it can be concluded that their use as adsorbents is not economically feasible.