**11. Conclusion**

*Green Chemistry Applications*

RH and MB, respectively.

The kinetics k rate indicates higher values for binary systems with MO component. When the MO concentration reaches a constant value, the reaction depends on the photocatalyst mass. The preparation of ternary mixtures with RH or MB and different azo species as orange G (OG), methyl red (MR), and Eriochrome Black T (EBT) clarifies the reaction mechanism dependency. The synergistic effect after azo compound addition is confirmed, and the time decreases about 23 and 13 min for

The photodecomposition acceleration effect is positively proportional to the azo dye concentration and no longer changes after reaching a specific equilibrium value. The comparison with the k values indicates higher rates for EBT > OG > MO > MR; the sequence is in agreement with the polarity of the four azo dye compounds. The azo compounds in the experiments were acid orange 7 (AO7), Congo red (CR), and amido black 10B (AB10B). The results were the same obtained for the other azo

The possible decomposition mechanism includes the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) as a type of molecular frontier orbitals. Roughly, the HOMO level is for organic semiconductors, the equivalent of the valence band for the inorganic semiconductors, and the LUMO is the equivalent of the semiconductors' conduction band. The energy difference between them is called a HOMO-LUMO gap. The energy gap between the two frontier orbitals can be used to predict the strength and stability of the transi-

The simulated changes of the azo molecule methylene orange (MO) in molecular energy structure in the photocatalyst Ag2O surface indicate the LUMO composed by the atomic orbital contributions of benzene, nitrogen, and the nitrogen double bond, and the HOMO is a result of the paired electron orbits of negatively charged

After the MO adsorption in Ag2O surface, the electrons on the surface of the Ag2O can transfer to the molecules' LUMO and participate in feedback coordination at the bond. The C▬N bond links the benzene ring to the azo group, and it becomes longer on adsorption. Such coordination effects weaken the π bonding conjugated over the whole molecular skeleton and start to be attacked by photogenerated electrons or radicals, with the presence of active fragmented intermediates. The resultant intermediates caused subsequent acceleration of azo bond cleavage also

The infrared results of the MO adsorbed in Ag2O indicate a shift for N〓N bond

, and for C▬N bonds from 817 and 749 cm<sup>−</sup><sup>1</sup>

the sulfonic group acting as an electron donor and Ag2O as an electron acceptor, weakening the conjugated π bonding and activating the N〓N and C▬N bonds confirming the adsorption of MO by Ag2O. The MO peaks disappear including the sulfonic group and the azo bond after 2 min of irradiation, suggesting to be the first degraded group. The suspension started to be colorless, and the peaks assigned to be C〓C bond start to appear after 5 min of exposition, an indication of the benzene rings broken. There is no observation of MO chemical structure after 10 min of light irradiation; it is an indication of the complete dye decomposition. The azo bond or C▬N bond connected with the benzene rings broken first and produces active intermediates which accelerate the degradation of non-azo organics followed

Generally, the C▬N bond linked to the benzene ring and the azo group of the MO is the first target for the free radicals produced by the photocatalyst. The possible intermediates are aminobenzenesulfonates, aromatic amines, phenolic

, for -SO3<sup>−</sup> was observed from 1314 and 1121 cm<sup>−</sup><sup>1</sup>

to 1320

.

to 819 and 751 cm<sup>−</sup><sup>1</sup>

. They are the confirmation of

compounds confirming the synergistic oxidation effect.

tion metal complexes and also their colors in solution.

for non-azo organic cleavages due to their oxidative activity.

Another shift for Ag2O was from 650 to 705 cm<sup>−</sup><sup>1</sup>

oxygen atoms in the sulfonic group.

from 1392 to 1396 cm<sup>−</sup><sup>1</sup>

by the benzene ring broken.

and 1120 cm<sup>−</sup><sup>1</sup>

**82**

Environmental sustainability demands the advance in water treatment and the use of lighting natural resources. Brazil has one of the most stable and intense solar irradiation in the word. It has to be used not only for energy generation purposes but also and mostly for water treatment, water quality polishment, and furthermore water disinfection. The chapter performs a comparison of different green technologies for water treatment as natural solar irradiation. The photocatalytic hydroxide radicals are the photodecomposition potent oxidants and react fast and unselectively with surrounding chemical species via radical addition, hydrogen abstraction, or e<sup>−</sup> transfer mechanisms. The transformation by-products of pharmaceuticals and EDC compounds (TBPs) with higher photodecomposition effectiveness ends up in complete mineralization with the production of CO2, H2O, and inorganic salts. The heterogeneous photocatalysis shows a strong dependence of the operating temperature, and the kinetics is usually dependent on the first step of the adsorption and the equilibrium modeled by Langmuir isotherms and Langmuir-Hinshelwood model. The first pseudo-order usually appears at the beginning of the reaction, just in the initial steps, and as the reaction proceeds, the intermediates' production could interfere with the radiation incidence. There is a competition of the adsorption sites of the catalyzer surface between the pollutant and others adsorbed species; the pollutants start concentration is a limiting reactant step with mass transfer limitations in lower concentrations. The semiconductor TiO2 photocatalytic process has shown great potential as a low cost, environment-friendly treatment technology in degrading a wide range of pollutants with the formation of reactive oxygen species upon excitation of a semiconductor particle with light energy greater than the respective bandgap energy of the photocatalyst. The photocatalyst TiO2 has superior characteristics over others with wide bandgap energy which requires the UV light which is 3–5% of natural solar light. The application of a variety of strategies improved the photocatalytic efficiencies from photocatalysts as dispersed solids to second-generation photocatalysts (chemically doped and physically modified dispersed solids) achieving better spectral sensitivity and photoactivity. Many studies indicate the scavengers' presence reduces the photodecomposition effect in water suspension. The ions HCO3 <sup>−</sup>/CO3 <sup>2</sup><sup>−</sup>, SO4 <sup>2</sup><sup>−</sup>, Cl<sup>−</sup> and NO3 <sup>−</sup> showed inhibitory effects

toward the hydroxyl radicals generated by AOPs; the natural organic matter (NOM) presence showed a synergistic effect increasing the E2 degradation, such degradation produces other radicals. The application of the visible light photodecomposition in a dye mixture of methylene blue (MB), methyl orange (MO), and rhodamine (RH) indicates the MO as the more stable azo compound than the other organic pollutants due to the aromatic groups attached at the end of the azo bond. Despite this fact when the light-driven photodecomposition uses the Ag2O as a catalyzer, it was the fastest and easiest decomposed compound. Published results indicate the visible light photodecomposition with Ag2O with the elimination of 90.2% of MO, 96.5% of RH, and 99.5% of MB using 4, 50, and 20 min, respectively. The photodecomposition acceleration synergistic effect is positively proportional to the azo dye concentration and no longer changes after reaching a specific equilibrium value. The comparison with the k values indicates higher rates for EBT > OG > MO > MR; the sequence is in agreement with the polarity of the four azo dye compounds. The azo compounds in the experiments were acid orange 7 (AO7), Congo red (CR), and amido black 10B (AB10B). The results were the same obtained for the other azo compounds confirming the synergistic oxidation effect. The possible decomposition mechanism includes the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) as a type of molecular frontier orbitals. The description of the acceleration of the photodegradation process with azo dyes' presence in a mixed dye solution is a synergy between the azo structure and Ag2O with the generation of aniline, sulfanilic acid, and phenol compounds which also accelerates the degradation of the non-azo compounds. The synergetic effect is beneficial for the Ag2O photodecomposition applicability to treat the ordinary real wastewater with a complex dye mixture.
