**2. Photodecomposition: kinetics and process parameters**

The heterogeneous photocatalysis shows a strong dependence on the operating temperature. 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. As the reaction proceeds, the intermediate production could interfere with the radiation incidence. The observed decomposition rates as the pollutant start to decompose and begin the competition for the adsorption sites between the pollutants and the adsorbed species. The initial pollutant concentration starts to be the limiting reactant with mass transfer limitations with no kinetic control for lower concentrations.

The increase of the catalyst mass promotes the photodegradation rate and the active catalyst sites. If the system is a slurry, the reaction rate reaches a maximum or optimal value and after that declines. The higher suspended particle concentration enhances the light scattering, the particle agglomeration, with light opacity enhancement, and after a certain point, the photodecomposition efficiency decreases. There is an equilibrium between the available surface site and the suspended particles control.

The oxygen plays a vital role in the photodecomposition reactions; dissolved O2 reacts with the photogenerated electrons leading to O2 <sup>−</sup> radicals and preventing the recombination of the generated e<sup>−</sup> and the h+ pairs. The comparison between the kinetics of the air-saturated solutions and the pure oxygen solution often results in smaller rates for the first.

The pH values influence the catalyst aggregations and its surface charges, with the valence band (VB) and conduction band (CB) position. At low pH values, the CB holes are more effective in comparison with VB. The change in the pH values allows the surface charge modification mostly when the amphoteric groups are present [5].

The adsorption equilibrium after decomposition depends on the pollutant pH speciation and the reactive species. Considering the low pH values, the h<sup>+</sup> can be the more oxidizing species, the amount of ˙OH increases under alkaline conditions when the OH<sup>−</sup> ions are available to react with h+ , and the ˙OH become the primary oxidant (Eqs. (1) and (2)). This effect increases the ˙OH availability at higher pH values in spite of the negatively charged catalyst surface and also the pollutant repelling action at such pH values; the ˙OH radicals' attack can explain the pH medium behavior.

$$\text{(M - OH)}\text{surface} + \text{H}^+ <= > \text{(M - OH}\_2^{\cdot \cdot}\text{)}\text{surface} \tag{1}$$

$$\text{(M - OH)}\text{surface} + \text{OH}^- <= > \text{(M - O}^-\text{)}\text{ surface} + \text{H}\_2\text{O}\tag{2}$$

The presence of some anions like chloride, nitrate, and sulfate reduces the photocatalytic performance as a result of the competition for the adsorption sites to scavenge the ˙OH radicals. Natural organic matter and humic acids are scavengers of the reactive species and usually show negative photodegradation influence. Nevertheless, the presence of the carbonate and bicarbonate increases photodecomposition efficiency.

The temperature has a limited effect in the photodecomposition efficiency until 80 Cº; after that there is the tendency to reduce the photocatalytic efficiency as a result of lower oxygen solubility in water. Different temperatures can also promote intermediaries and by-product formation.
