**4. Results and discussion**

### **4.1. Photo-oxidation experiments (reactor PR1)**

**Figure 6** shows the degradation kinetics of OTC photocatalysis (scenarios S1–S4) and photolysis (scenarios S5 and S6) performed in two different aqueous matrices (distilled and tap water), always with an initial concentration of 20 mg/L and exposed to solar radiation (a free and renewable energy source) during 210 minutes (experimental).

For OTC degradation using solar radiation exposure, the maximum average value of 88% was reached for the scenarios S1 and S3 (different water matrix), which correspond to the highest TiO<sup>2</sup> concentration.

The constituents present in the tap water, namely the iron, showed to have a significant effect on the OTC degradation efficiency, with special emphasis in photolysis experiments (almost quintupled), while in photocatalysis, this increase was only about 20%, under similar conditions of accumulated UV energy. Indeed, auxiliary control testing of tap water quality parameters detected the presence of iron concentrations in the range of 0.08–0.1 mg/L.

In order to assess a potential efficiency increase in OTC removal, due to an alternative UV radiation source (although with energy costs), those two photo-oxidative processes were also performed for the same aqueous matrices and OTC initial concentration but using the described UV lamp reactor with an exposure time of 60 minutes (scenarios S7–S12). The obtained results are depicted in **Figure 7**.

Photocatalytic Treatment Techniques using Titanium Dioxide Nanoparticles for Antibiotic... http://dx.doi.org/10.5772/intechopen.69140 137

**Figure 6.** OTC photo-oxidation efficiency with solar radiation.

Each of the lettuce seed root growth inhibition test was performed with 20 seeds in a Petri dish, containing a filter paper embedded in 2 mL of each sample dilution (100, 75, 50, and 25%). Root lengths were measured after 72 hours of incubation (**Figure 5**), and the average

The samples used consisted of the oxytetracycline before and after photocatalytic treatment and, as negative control, distilled water. The tests were always carried out in triplicate.

**Figure 6** shows the degradation kinetics of OTC photocatalysis (scenarios S1–S4) and photolysis (scenarios S5 and S6) performed in two different aqueous matrices (distilled and tap water), always with an initial concentration of 20 mg/L and exposed to solar radiation (a free

For OTC degradation using solar radiation exposure, the maximum average value of 88% was reached for the scenarios S1 and S3 (different water matrix), which correspond to the highest

The constituents present in the tap water, namely the iron, showed to have a significant effect on the OTC degradation efficiency, with special emphasis in photolysis experiments (almost quintupled), while in photocatalysis, this increase was only about 20%, under similar conditions of accumulated UV energy. Indeed, auxiliary control testing of tap water quality param-

In order to assess a potential efficiency increase in OTC removal, due to an alternative UV radiation source (although with energy costs), those two photo-oxidative processes were also performed for the same aqueous matrices and OTC initial concentration but using the described UV lamp reactor with an exposure time of 60 minutes (scenarios S7–S12). The

eters detected the presence of iron concentrations in the range of 0.08–0.1 mg/L.

lethal concentration (LC50) was calculated as stated by Dutkka [42].

**4. Results and discussion**

136 Application of Titanium Dioxide

TiO<sup>2</sup>

concentration.

**4.1. Photo-oxidation experiments (reactor PR1)**

obtained results are depicted in **Figure 7**.

and renewable energy source) during 210 minutes (experimental).

**Figure 5.** Preparation and final result of the acute toxicity bioassay using *L. sativa*.

**Figure 7.** OTC photo-oxidation efficiency with UV lamp reactor.

For OTC degradation using UV reactor exposure, the same behavior was observed. The maximum efficiency (near 96%) was reached for the scenarios S7 and S9 (different water matrix), which correspond to the highest TiO<sup>2</sup> concentration. As depicted in **Figure 7**, the use of those two different aqueous matrices had a negligible effect on final OTC removal efficiency.

In this case (UV lamp reactor), the efficiency gains on OTC removal, related to the catalyst action, are much less significant than in the case of the solar radiation tests. Due to this finding the benefit of the use of photocatalysis would not be sufficiently attractive given the costs inherent to the necessary removal process of suspended TiO<sup>2</sup> nanoparticles.

**Table 3** summarizes the major experimental results obtained for OTC removal using suspended TiO<sup>2</sup> , namely the maximum average efficiencies, some photo-oxidation kinetic parameters, and the coefficient of determination (*R*<sup>2</sup> ) observed in the adjustment of the *Langmuir-Hinshelwood* model to the experimental data set obtained for each assay.

The obtained *R*<sup>2</sup> values (**Table 3**) allow to conclude that the *Langmuir-Hinshelwood* model adapts adequately to the kinetic behavior observed in the OTC photo-oxidation for any of those experimental scenarios tested and analyzed in this study.

For both water matrices solutions and in the scenarios using 50 mg/L of TiO<sup>2</sup> , OTC removal efficiencies may achieve values higher than 88% if the accumulated solar energy quantity is higher than 113 kJ/L.

Comparing the results obtained using these two different UV radiation sources, the photocatalysis using TiO<sup>2</sup> with solar radiation seems to be a sustainable alternative for antibiotic removal in WTPs due to its minor energy costs and high efficiency removal, even requiring more exposure/retention time and achieving lower efficiencies, when compared with the ones observed in UV reactor tests.

### **4.2. Photocatalytic filtration experiments (reactor PR2)**

The results of OTC removal efficiency by photocatalytic filtration performed in the reactor PR2 are depicted in **Figure 8**, considering the experimental scenarios F1–F5, which were defined aiming to assess the influence of different filtration fluxes, OTC initial concentration, and the OTC solution aeration in the feed tank.

The results showed that slower flux resulted in better OTC removal efficiency at the beginning of the experiment, due to longer retention times in the filter (curves F1, F3 and F4). Aeration is important for the oxidation reaction in photocatalytic processes. This process requires dissolved oxygen to act as an oxidant and to slow down the electron-hole recombination reaction. The curve F5 for the experiment with aeration shows the highest value for the initial photo-degradation rate.

In **Figure 8**, it can be seen that the experiments F1, F4, and F5 with 4 and 12 L/h had higher initial degradation rates, and these tests removed more than 96% of OCT by 270 minutes of solar irradiation time. The highest OTC removal efficiency obtained for photocatalytic filtration,


**Table 3.** Results synthesis of OTC photo-oxidation experiments in reactor PR1.

**Figure 8.** OTC removal efficiency using photocatalytic filtration with TiO<sup>2</sup> (PR2).

using a quartz porous medium coated with TiO<sup>2</sup> , was 98% achieved for scenarios F4 and F5, which correspond to the higher flow rates tested (without and with filter aeration).

**Table 4** summarizes the major experimental results of OTC removal experiments using photocatalytic filtration with a porous medium coated with TiO<sup>2</sup> , namely the maximum average efficiencies, some photo-oxidation kinetic parameters, and the coefficient of determination (*R*<sup>2</sup> ) observed in the adjustment of the *Langmuir-Hinshelwood* model to the experimental data sets.

The results presented were obtained on different days with variations in the amount of accumulated energy from solar radiation received on the surface of the porous medium.

The calculated *R*<sup>2</sup> values (**Table 4**) allow to verify that the *Langmuir-Hinshelwood* model also adapts adequately to the kinetic behavior observed in the OTC photocatalytic filtration performed in this study for any of the analyzed experimental scenarios.

The effect of the flow rate variation on OTC adsorption was assessed using the reactor PR2 in darkness conditions and filtration with two different porous media (quartz without and with TiO<sup>2</sup> functionalization). The results of the OTC adsorption tests are depicted in **Figure 9**,


**Table 4.** Results synthesis of OTC photo-oxidation experiments in reactor PR2.

**Parameter S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12**

(ppm) 19.25 20.59 18.69 20.83 15.39 20.69 19.57 20.62 12.13 18.13 20.69 21.04

**Table 3** summarizes the major experimental results obtained for OTC removal using sus-

adapts adequately to the kinetic behavior observed in the OTC photo-oxidation for any of

efficiencies may achieve values higher than 88% if the accumulated solar energy quantity is

Comparing the results obtained using these two different UV radiation sources, the photo-

removal in WTPs due to its minor energy costs and high efficiency removal, even requiring more exposure/retention time and achieving lower efficiencies, when compared with the ones

The results of OTC removal efficiency by photocatalytic filtration performed in the reactor PR2 are depicted in **Figure 8**, considering the experimental scenarios F1–F5, which were defined aiming to assess the influence of different filtration fluxes, OTC initial concentration,

The results showed that slower flux resulted in better OTC removal efficiency at the beginning of the experiment, due to longer retention times in the filter (curves F1, F3 and F4). Aeration is important for the oxidation reaction in photocatalytic processes. This process requires dissolved oxygen to act as an oxidant and to slow down the electron-hole recombination reaction. The curve F5 for the experiment with aeration shows the highest value for the initial photo-degradation rate. In **Figure 8**, it can be seen that the experiments F1, F4, and F5 with 4 and 12 L/h had higher initial degradation rates, and these tests removed more than 96% of OCT by 270 minutes of solar irradiation time. The highest OTC removal efficiency obtained for photocatalytic filtration,

*Langmuir-Hinshelwood* model to the experimental data set obtained for each assay.

For both water matrices solutions and in the scenarios using 50 mg/L of TiO<sup>2</sup>

parameters, and the coefficient of determination (*R*<sup>2</sup>

**4.2. Photocatalytic filtration experiments (reactor PR2)**

and the OTC solution aeration in the feed tank.

those experimental scenarios tested and analyzed in this study.

, namely the maximum average efficiencies, some photo-oxidation kinetic

values (**Table 3**) allow to conclude that the *Langmuir-Hinshelwood* model

with solar radiation seems to be a sustainable alternative for antibiotic

) observed in the adjustment of the

, OTC removal

 (ppm) 2.48 4.80 2.48 7.60 13.67 10.06 0.86 2.16 0.48 1.43 4.02 3.22 *K*aap (minutes−1) 0.013 0.009 0.011 0.006 0.001 0.004 0.061 0.047 0.078 0.052 0.030 0.040 *R*<sup>2</sup> 0.844 0.909 0.974 0.987 0.931 0.982 0.899 0.866 0.816 0.807 0.992 0.321

 (ppm∙minutes−1) 0.25 0.19 0.21 0.12 0.01 0.08 1.19 0.97 0.95 0.94 0.62 0.84 OTC removal (%) 87 77 87 64 11 51 96 90 96 92 81 85

**Table 3.** Results synthesis of OTC photo-oxidation experiments in reactor PR1.

*C*0

pended TiO<sup>2</sup>

138 Application of Titanium Dioxide

The obtained *R*<sup>2</sup>

higher than 113 kJ/L.

catalysis using TiO<sup>2</sup>

observed in UV reactor tests.

*Cf*

*r*0

**Figure 9.** Results of the OTC adsorption tests and final look (color changes) of the porous medium.

as well as the final look (color changes) of the porous medium in the following four distinct situations:


In darkness and after 120 minutes, the quartz (without TiO<sup>2</sup> ) has a negligible OTC adsorption, but in the column filter with the coated quartz, the adsorption is function of the feed flow rate. With a flow rate of 6 L/h, the equilibrium concentration was reached within 90 minutes, and for 4L/h, the equilibrium concentration was only reached after 120 minutes.

Moreover, it was also observed a high regeneration ability by the photocatalytic porous medium, which can completely recover its oxidative properties after a simple solar radiation exposure of about 4 hours [15]. **Figure 10** presents the time evolution of saturation and regeneration processes observed in this photocatalytic filter.

As reported on item 3.3, the toxicity of the oxytetracycline both before and after the photocatalytic degradation (performed in each reactor – PR1 and PR2) was evaluated by using *L. sativa* seeds germination as a bioindicator.

The results of these toxicity tests toward lettuce seed growth showed a toxicity decrease after the photocatalytic OTC degradation, enabling the adoption of this emerging water treatment technique as an apparently safe alternative for the antibiotics removal challenge.

Photocatalytic Treatment Techniques using Titanium Dioxide Nanoparticles for Antibiotic... http://dx.doi.org/10.5772/intechopen.69140 141

**Figure 10.** Saturation and regeneration processes evolution observed in the photocatalytic filter (PR2).
