**3.1. Photo-oxidation experiments using suspended TiO2**

alkoxy radical (RO•)—involving chemical (e.g., O<sup>3</sup>

groups: with suspended nanoparticles (e.g., TiO<sup>2</sup>

), or photocatalytic (TiO<sup>2</sup>

130 Application of Titanium Dioxide

stainless steel, zeolites).

water supply systems.

organic micropollutants in water [37].

**3. Experimental methodology**

photocatalysis was performed using suspended TiO<sup>2</sup>

a granular porous medium coated by immobilized TiO<sup>2</sup>

H2 O2 , O<sup>3</sup> /H<sup>2</sup> O2

tor photocatalytic process has shown a great potential as a low-cost, environmental friendly, and sustainable treatment technology to align with the "zero" waste scheme in the water/ wastewater industry. The ability of this advanced oxidation technology has been widely demonstrated to remove persistent organic compounds and microorganisms in water [35] and

Recent research works were mainly focused on AOPs assisted by solar radiation (a clean and renewable energy source), such as heterogeneous photocatalysis, in order to develop more sustainable and *low-cost* processes. The photocatalytic reactors can be divided into two main

wastewater) and with immobilized nanoparticles on a carrier material (e.g., glass, quartz,

When the catalyst is in suspension, the active surface is greater. However, its particles have to be removed from the treated water after the detoxification, and the manipulation of pow-

extensive and relatively costly installation technology is necessary, including pumps. Very promising techniques for solving problems concerning separation of the photocatalyst as well as products and by-products of photo-degradation from the reaction mixture are the use of photocatalytic membrane reactors (PMRs) and the introduction of a magnetic into the nanocomposite [36]. However, the energy costs evolved in membrane processes can compromise the economic sustainability of the water treatment utilities, namely in medium and small

A solution for avoiding the contamination with the photocatalytic nanoparticles is their immobilization on the surface of specified materials by use of suitable coating techniques, as a wet chemical process. Quartz has been found to be the best support for titanium dioxide, because it is the most neutral and stable one at high temperatures. As a consequence, it has

During this research work, a set of experiments under different test scenarios were performed in order to assess the antibiotic removal efficiency and to characterize its photo-oxidation kinetics, using two different lab-scale photoreactors. In the first one (PR1), the heterogeneous

antibiotic from water. In the second (PR2) one, a photocatalytic filtration was performed using

In these experiments, the antibiotic used to prepare all synthetic solutions was the oxytetracycline hydrochloride (MW = 496.89, CAS# 2058-46-0), supplied by Sigma-Aldrich with

some hazardous inorganic micropollutants (e.g., arsenic, heavy metals, uranium).

dered semiconductors are difficult. To ensure complete rejection of TiO<sup>2</sup>

been chosen as the ideal support for new experiments with TiO<sup>2</sup>

/UV, ZnO/UV) oxidation processes. In recent years, semiconduc-

), photochemical (UV/O<sup>3</sup>

, ZnO) in the reaction mixture (water and

nanoparticles, an

in the photodegradation of

nanoparticles as catalyst to remove the

nanoparticles.

, UV/

In reactor PR1, photo-oxidation experiments were performed, with and without suspended TiO<sup>2</sup> nanoparticles, using two different UV radiation sources: solar radiation and UV lamp reactor (**Figure 2**).

For the OTC photo-degradation under solar radiation, bottles of colorless polyester with a capacity of 1.5 L were used as reactor. These water bottles were placed vertically, being shaken manually every 10 minutes to prevent the deposition of TiO<sup>2</sup> at the bottom. The sun exposure time was 210 minutes for all photodegradation tests.

The UV reactor (*Heraeus Noblelight, System 2*) used in photodegradation assays consists of an UV immersion lamp TQ 150, an immersion tube, a cooling tube, and a reactor vessel. The UV

**Figure 2.** UV radiation sources used in OTC degradation experiments: solar (polyester bottles); UV reactor *Heraeus Noblelight*.

immersion lamp is a medium-pressure mercury vapor lamp with a broad emission spectrum in the UV range above 190 nm and lamp output of 150 W. The reactor vessel has a capacity of 0.8 L and three openings (one central and two sideways). In the central opening, the UV lamp tube is inserted, and only a side opening is used to carry out the extraction of the samples during the tests. The container is placed on a magnetic stirrer that was in operation throughout the test. The UV lamp exposure time was 60 minutes for all tests.

Equation (1) allows the calculation of the amount of accumulated UV energy (Q450-950n) received on any surface in the same position with regard to the sun, per unit of volume of water inside the reactor, in the time interval Δ*t*.

the reactor, in the time interval  $\Delta t$ .

$$Q\_{450-950\_{\text{s}}} = Q\_{450-950\_{\text{s}}} + \Delta t\_{\text{s}} \times \overline{450-950} \times \frac{A\_{\text{r}}}{V\_{\text{i}}} ; \Delta t\_{\text{s}} = t\_{\text{n}} \cdot t\_{\text{n}1} \tag{1}$$

Where *tn* is the experimental time of each sample (s); *Vt* is the total reactor volume (L); *Ar* is the exposed surface area (m2 ) of the reactor; and ¯¯<sup>450</sup>–<sup>950</sup> is the average solar radiation (W/m2) measured during the period ∆*t n* (s).

Photolytic and photocatalytic experiments were carried out under static hydraulic conditions using 20 mg/L of OTC, as initial pollutant concentration, in all tests. For photocatalysis, the chosen initial suspended catalyst concentrations were 50 and 25 mg/L of TiO<sup>2</sup> , in order to assess the effect of doubling the value of this parameter on OTC removal efficiency.

In order to assess the photocatalysis ability as post-treatment unit in WTPs for antibiotic removal, OTC solutions were prepared using two different water matrices (distilled and tap water) in order to assess the potential influence of other water supply constituents on OTC removal efficiency. The pH values measured in all experiments ranged between 4.3–4.9, for distilled water, and 6.6–7.3, for tap water.

To evaluate the influence of radiation in OTC degradation, at any given irradiation time interval, the dispersion was sampled (5 mL), filtered through a Millipore filter (pore size of 0.22 μm) to separate the TiO<sup>2</sup> particles, and the absorption was monitored to obtain OTC concentration.

**Table 1** summarizes the different assay conditions (scenarios) under which the OTC photodegradation tests, using suspended TiO<sup>2</sup> , were performed (reactor PR1).

Most of the studies carried out on heterogeneous photocatalysis with TiO<sup>2</sup> have shown that the kinetics underlying the photo-oxidation of emerging pollutants can be represented by Eq. (2), according to the *Langmuir-Hinshelwood* model [38, 39].

$$r\_o = -\frac{d\mathbf{C}}{dt} = \frac{k \times K \times \mathbf{C}\_0}{1 + K \times \mathbf{C}\_0} \tag{2}$$

Where *r0* is the initial rate of photo-oxidation (ppm minutes−1); *C0* is the initial pollutant concentration (ppm); *k* is the reaction rate constant (ppm minutes−1); and *K* is the pollutant adsorption coefficient (L/mg) measured during the period ∆*tn* (s).

Photocatalytic Treatment Techniques using Titanium Dioxide Nanoparticles for Antibiotic... http://dx.doi.org/10.5772/intechopen.69140 133


**Table 1.** Scenario analysis for OTC photo-oxidation in reactor PR1.

immersion lamp is a medium-pressure mercury vapor lamp with a broad emission spectrum in the UV range above 190 nm and lamp output of 150 W. The reactor vessel has a capacity of 0.8 L and three openings (one central and two sideways). In the central opening, the UV lamp tube is inserted, and only a side opening is used to carry out the extraction of the samples during the tests. The container is placed on a magnetic stirrer that was in operation throughout

Equation (1) allows the calculation of the amount of accumulated UV energy (Q450-950n) received on any surface in the same position with regard to the sun, per unit of volume of water inside

> *n* ×¯¯ 450–950 × \_\_

Photolytic and photocatalytic experiments were carried out under static hydraulic conditions using 20 mg/L of OTC, as initial pollutant concentration, in all tests. For photocatalysis, the

In order to assess the photocatalysis ability as post-treatment unit in WTPs for antibiotic removal, OTC solutions were prepared using two different water matrices (distilled and tap water) in order to assess the potential influence of other water supply constituents on OTC removal efficiency. The pH values measured in all experiments ranged between 4.3–4.9, for

To evaluate the influence of radiation in OTC degradation, at any given irradiation time interval, the dispersion was sampled (5 mL), filtered through a Millipore filter (pore size of

**Table 1** summarizes the different assay conditions (scenarios) under which the OTC photo-

kinetics underlying the photo-oxidation of emerging pollutants can be represented by Eq. (2),

concentration (ppm); *k* is the reaction rate constant (ppm minutes−1); and *K* is the pollutant

*dt* <sup>=</sup> *<sup>k</sup>* <sup>×</sup> *<sup>K</sup>* <sup>×</sup> *<sup>C</sup>* \_\_\_\_\_\_\_\_0 1 + *K* × *C*<sup>0</sup>

Most of the studies carried out on heterogeneous photocatalysis with TiO<sup>2</sup>

is the initial rate of photo-oxidation (ppm minutes−1); *C0*

particles, and the absorption was monitored to obtain OTC

(s).

, were performed (reactor PR1).

*Ar Vt* ; ∆*t <sup>n</sup>* = *t <sup>n</sup>* - *t*

*<sup>n</sup>*-1 (1)

is

, in order to

have shown that the

is the initial pollutant

(2)

is the total reactor volume (L); *Ar*

<sup>450</sup>–<sup>950</sup> is the average solar radiation (W/m2)

the test. The UV lamp exposure time was 60 minutes for all tests.

= *Q*<sup>450</sup>–950*<sup>n</sup>*-1

is the experimental time of each sample (s); *Vt*

*n* (s). + ∆*t*

) of the reactor; and ¯¯

chosen initial suspended catalyst concentrations were 50 and 25 mg/L of TiO<sup>2</sup>

assess the effect of doubling the value of this parameter on OTC removal efficiency.

the reactor, in the time interval Δ*t*.

*Q*<sup>450</sup>–950*<sup>n</sup>*

132 Application of Titanium Dioxide

the exposed surface area (m2

measured during the period ∆*t*

distilled water, and 6.6–7.3, for tap water.

degradation tests, using suspended TiO<sup>2</sup>

according to the *Langmuir-Hinshelwood* model [38, 39].

adsorption coefficient (L/mg) measured during the period ∆*tn*

*<sup>r</sup>*<sup>0</sup> <sup>=</sup> <sup>−</sup>\_\_\_ *dC*

0.22 μm) to separate the TiO<sup>2</sup>

concentration.

Where *r0*

Where *tn*

Considering that "*K*×*C*<sup>0</sup> " product can be a value quite low for photo-oxidation processes, which can be described by a pseudo-first order decay kinetics [35], the final pollutant concentration (*Ct* ) is given by Eq. (3).

$$C\_t = C\_0 \times e^{-\mathcal{K}\_{\alpha\gamma}t} \tag{3}$$

Where *K*aap is the apparent velocity reaction constant (minutes−1).

So, the initial rate of photo-oxidation can be obtained by Eq. (4) when the pollutants present vestigial concentrations.

$$r\_0 = K\_{app} \times \mathbb{C}\_0 \tag{4}$$

#### **3.2. Photocatalytic filtration experiments using immobilized TiO<sup>2</sup>**

The lab-scale reactive filter applied on photocatalytic oxidation of OTC consists of two borosilicate glass cylinder (DURAN®) with 750 mm length, 70 mm external diameter, and 62 mm inner diameter. The filtration columns, with this quartz porous medium coated with TiO2, are assembly as showed in **Figure 3**, and the OTC solution was feed to the columns by a peristaltic pump (Watson-Marlow 503U).

The porous bed consists of a quartz extracted from a quarry located in Ponte da Barca (Portugal), which was characterized by X-ray diffraction (XRD) (**Figure 4**).

The quartz was crushed and sieved in order to reduce its grains size till the desired granulometry, as well as, to facilitate the removal of the usual impurities. After sieving out, a grain

**Figure 3.** Filtration columns with a quartz porous media for OTC photo-oxidation.

**Figure 4.** Characterization of a quartz sample by X-ray diffraction.

size distribution between 2.36 and 4.75 mm was dipped coated with TiO<sup>2</sup> , also from Degussa (Aeroxide®), using the method described by Jeong et al. [40].

Prior to the start of the photocatalytic filtration tests, a study was carried out to optimize the hydraulic operation of the filter (e.g., flow rates ranges, head losses, hydraulic retention times) in order to select the most suitable flow rates: for photocatalysis, experiments were defined 4, 6 and 12 L/h; for adsorption, tests were defined 2, 4 and 6 L/h.

The selected range of flow rates for photocatalysis allows to simulate filtration (loading) rates similar to those occurring in WTP rapid and high rate filters (real scale hydraulic conditions) and also leads to OTC contact times with the TiO<sup>2</sup> that can provide an efficient photodegradation.

The hydraulic tests were performed both in open and closed (looped) circuit. An open circuit operation (without filtered water recycling) allows to maintain the initial OTC concentration constant and thus to evaluate the maximum capacity of retaining pollutant mass corresponding to the occurrence of porous medium saturation. A closed circuit operation allows to perform the number of loops (cycles) necessary to obtain the desired OTC contact time with the porous medium coated with TiO<sup>2</sup> nanoparticles.

The photocatalytic filtration tests of OTC solutions were performed in looped circuit during 270 minutes, considering different flow rates, initial OTC concentration (20 and 40 ppm), and aeration conditions. Final OTC concentrations were obtained by absorbance measurement using an UV-VIS spectrophotometer (Shimadzu UV—1800) at 354 nm wavelength. The effect of the aeration on the photo-degradation efficiency of OTC feed solution was also evaluated.

**Table 2** summarizes the different test conditions (scenarios) under which the photocatalytic filtration was performed (reactor PR2)

Adsorption test was carried out under similar hydraulic conditions and the same duration of photodegradation tests, passing the OTC solution through the filter, first with quartz and after with quartz coated with TiO<sup>2</sup> , in darkness to avoid any photodegradation contribute on final OTC removal.

#### **3.3. Acute toxicity test**

size distribution between 2.36 and 4.75 mm was dipped coated with TiO<sup>2</sup>

Prior to the start of the photocatalytic filtration tests, a study was carried out to optimize the hydraulic operation of the filter (e.g., flow rates ranges, head losses, hydraulic retention times)

(Aeroxide®), using the method described by Jeong et al. [40].

**Figure 3.** Filtration columns with a quartz porous media for OTC photo-oxidation.

134 Application of Titanium Dioxide

**Figure 4.** Characterization of a quartz sample by X-ray diffraction.

, also from Degussa

In order to assess the toxicity of OTC and oxidation by-products, it was used a simple toxicity test, not normalized but standardized by the international organization WaterTox Network [41]. In this toxicity assay, lettuce seeds (*Lactuca sativa*) are used.


**Table 2.** Scenario analysis for OTC photocatalysis in reactor PR2.

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

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