**2.3 Experimental procedure**

Initially, a model solution of 50 mg/L concentration was prepared for each dye. This concentration is within the range of concentrations regularly found in the effluents of the textile industry [5]. Subsequently, concentrations of 100 and 150 mg/L were used to study the effect of the concentration on the kinetics of the reaction, and solutions of 5 g/L, to carry out a preliminary analysis by FTIR.

The ozonation experiments were carried out on 400 mL of each model solution in the 500 mL reactor, with 2 mg/L of initial ozone concentration and a gaseous mixture flow (O3/O2) of 500 mL/min, at room temperature (20–25°C). The maximum ozonation time was 120 min. Samples were taken every 10 min and, in some cases, every 5 min.

Preparation cycles of the model solution and ozonation were made for 10 min, up to five times, with 50 mg/L solutions under the described conditions. That is, the sample solution of dye was prepared at 50 mg/L, and the ozone treatment was applied for 10 min; after this, the same sample was reconstituted with a concentration of 50 mg/L of textile dye, and it was treated again with ozone for 10 min. This cycle of contamination-discoloration was repeated up to five times with each textile dye, so that at the end there was a series of five samples of 400 mL, for each dye. The first sample had been subjected to a single cycle, the second sample to two cycles, the third sample to three, and so on until completing the five cycles. The purpose of this part of the work was to represent the recirculation of water within the process.

These samples were used for dyeing tests of textile material with reactive dyes and direct dyes. For the dyeing with reactive dyes, Reactive Black 5 (RB5), Reactive Yellow 84 (RY84), and Reactive Red (RR141) were used, while for dyes with direct dyes, a trichromy composed of Direct Yellow 50 (DY50), Direct Blue 80 (DB80), and Direct Red 23 (DR23) was selected. The dyes were carried out in a closed machine with infrared heating (Mathis). The DY50 corresponds to group A of the SDC classification of direct dyes (self-regulating); DB80, group B (controllable by means of salt); and DR23, group C (controllable by means of salt and temperature). The color difference was measured with respect to control samples, dyed with distilled water by reflectance spectrophotocolorimetry.

A second series of samples was prepared in the manner described in the previous paragraph, adding from the first cycle the equivalent to 50 mg/L of sodium ion. This was carried out in order to verify their presence throughout the ozonation cycles, using atomic absorption spectroscopy.

## **2.4 Analytic methods**

### *2.4.1 pH and conductivity measurement*

The variation of the pH during the ozonation was measured with a Jenway pH meter model 3310 equipment. Likewise, the variation of conductivity was

measured, in order to indirectly determine the formation of ionic species. The latter was used as a criterion to verify the separation of the groups capable of forming ions, located in the molecules of the dyes, during the ozonation process.
