**2. Materials and methods**

#### **2.1. TiO2 synthesis**

**1. Introduction**

282 Titanium Dioxide - Material for a Sustainable Environment

of TiO<sup>2</sup>

ers (Mg(OH)<sup>2</sup>

formula [M2+

immobilizing TiO<sup>2</sup>

tion of the TiO<sup>2</sup>

difficulty with the use of TiO<sup>2</sup>

1−xM3+ x (OH)<sup>2</sup> ] x (An−)

used: the first one is to obtain the TiO<sup>2</sup>

photocatalytic activity at the end of the test.

leads to the formation of an electron-hole pair (e−

a support material or coadsorbent to immobilize TiO<sup>2</sup>

the subject of various investigations [9, 10].

Synthetic organic composites like phenol are widely used in a great variety of industries including paper, wood, paint, and fertilizers [1]. Wastewater from such industrial processes contains this kind of composite and poses a threat to aquatic life and the environment. It is therefore important to remove or degrade such composites before discharging the wastewater into the environment. Among the technologies used for the degradation of organic pollutants in aqueous media are advanced oxidation processes (AOPs), specifically heterogeneous photocatalysis, which promotes the degradation of several pollutants by broadband semiconductor excitation [2–5]. Photon adsorption by the semiconductor with higher bandgap energy

valence band are powerful oxidants, while the conduction band electrons are good reducers. The formation of other highly oxidant species (mainly ●OH radicals) can also occur; these redox-type reactions occur when the electron-hole recombination is minimized [6]. The use

chemical corrosion, low toxicity, and efficiency [7]. The anatase crystalline phase of TiO<sup>2</sup>

more effective than the rutile phase for the photodegradation of several contaminants [5]; however, photoactivity has been found to increase in mixed anatase-rutile phases [8]. One

Layered double hydroxides (LDHs) are synthetic composites belonging to the anionic clay family, having a hexagonal or octahedral crystalline structure. They consist of layers of positively charged metal cations, where the surface of the layers is occupied by hydroxyl groups, anions, and water molecules. LDHs are the result of isomorphic variations of brucite-type lay-

the predominant anion [11]. Hydrotalcite is an LDH-type layered material, with the chemical

and Al3+) and An− is the intercalated anion. LDH and its calcined products are porous materials with large surface area, have the capacity to adsorb pollutants, and have proven suitable for

the LDH anionic clays to study the influence of the preparation method on the photocatalytic capacity of those composites in a phenol solution. Two groups of synthesis methods were

to previously optimized procedures [14, 15] in relation to its photoactivity evaluated with the degradation of phenol. The synthesized composites in this work were also tested for phenol photodegradation in aqueous solution. These materials also showed advantages in their reusability and were able to be used in four photocatalytic cycles with a minimum loss in the

particles for the photodegradation of organic pollutants [12, 13].


charge residue which is offset by the presence of intercalated anions, carbonate (CO<sup>3</sup>

x/n•mH<sup>2</sup>

The aim of this work is to synthesize composites derived from the TiO<sup>2</sup>

) when Mg2+ cations are substituted by Al3+ cations, thereby generating a positive

BC-h<sup>+</sup>

is its separation and recovery for possible reuse; the addition of

O, where M2+ and M3+ are di- and trivalent cations (Mg2+

photocatalyst, and the second one is in the prepara-

as photocatalyst has caused great interest due to its high activity, resistance to mild

BV). The photogenerated holes in the

particles to facilitate recycling has been

is

2−) being

photocatalyst and

The photocatalysts were synthesized using three methods. (i) First, following the sol–gel procedure [16], 5.25 mL of titanium isopropoxide (TTIP) Ti[OCH(CH<sup>3</sup> )2 ]4 were added to 97% (Sigma-Aldrich) in 47 mL of ethanol (Civeq); the mixture was agitated for 3 hours after which 12.25 mL of deionized water were added. Agitation continued at 78°C for 20 hours. The solid was washed with deionized water by centrifugation and dried at 80°C during 1 hour. The resulting powder was ground and calcined (TiO<sup>2</sup> I). (ii) In the second method, 90 mL of 1-butanol C<sup>4</sup> H10O (Sigma-Aldrich) were mixed with 120 mL of deionized water; both reagents were heated at 70°C in a water bath with continuous agitation and reflux system. Subsequently, 45 mL of titanium butoxide (TOBT) C16H36O<sup>4</sup> Ti 97% (Sigma-Aldrich) were added by dripping. The mixture was aged during 24 hours at constant agitation and temperature. The resulting solid was recovered and washed by centrifugation using ethanol, dried during 24 hours at 100°C, and finally calcined (TiO<sup>2</sup> B). (iii) The third method of obtaining TiO<sup>2</sup> was direct calcination of TTIP by placing 10 mL in mechanical agitation in air for 30 minutes, before being calcined (TiO<sup>2</sup> T). In all cases, calcination took place at 550°C for 3.5 hours.
