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according to theearlier results reported by Muruganandhan and Swaminathan(2007) for eliminating such compounds. For homogeneoustests, as different authors reported that the optimum pH valuefor the photo-Fenton reaction is 2.8 mainly due the predom-inance of iron species more photoactive (FeOH2+)insolution,as well as it avoids ferric hydroxide precipitation, the pH wasadjusted to 2.8 (Machulek Júnior et al. 2011; Pignatello et al.2006). After the pH was adjusted, another sample was taken15 min later to confirm these values. For heterogeneous pho-tocatalytic tests, titanium dioxide was added (Degussa, P25,80 % anatase and 20 % rutile, 200 mg L−1)andthemixturerecirculated for 15 min and another sample was taken forcharacterization. Afterward, irradiation of the solution wasstarted, and samples were taken at different time intervals toevaluate the degradation process. For TiO2/H2O2/UV, prior tothe start of irradiation, 500 mg L−1of hydrogen peroxide wasadded. For the photo-Fenton tests, iron salt (60 mg Fe2+L−1)was added after the pH adjustment, the mixture was wellhomogenized for 15 min, and another sample was taken for characterization. Afterwards, hydrogen peroxide wasadded (500 mg L−1) and irradiation of the solution wasstarted. Samples were taken at different time intervals toevaluate the degradation process. Regarding the photoly-sis and H2O2/UV experiments, reactions occurred at pH4.5, without addition of any reagent in the case of pho-tolysis and with the addition of hydrogen peroxide for theH2O2/UV system. Whenever hydrogen peroxide wasused, its concentration was maintained between 100 and500 mg L−1during the entire run through the addition ofsmall amounts of hydrogen peroxide to compensate theconsumed ones.Lab-scale photoreactor experimental procedureAlthough the best process was chosen among the AOPs tobe applied in a pilot plant scale, such process should beimproved in order to obtain the best efficiency with a lesscost associated by the reactant addition, temperature vari-ations, and low energy consumption. Nonetheless, varia-tions on the natural solar irradiance during the experimentshave justified the use of an artificial UV solar (SUNTESTreactor) for assessment of the best reactor operating pa-rameter values. Thus, a lab-scale photoreactor with asunlight simulator was used under controlled operationalconditions to assess the influence of operating parameters(iron concentration, pH, temperature, and irradiance) onprocess efficiency. Thus, the glass recirculation vessel ofthe lab-scale prototype was filled with 1.0 L of textilewastewater (Vi=270 mL; Vi/Vt=0.27; ti=0.43 min; tdark=1.16 min; ACPC=0.023 m2), which was pumped to theCPC unit and homogenized by recirculation in the closedsystem during 15 min in the dark. The pH was adjustedto the desired value (2.4, 2.8, 3.2, 3.6, or 4.5) usingsulfuric acid, and another sample was taken 15 min laterto confirm this value. The temperature set point of therefrigerated thermostatic bath was controlled to maintainthe intended wastewater temperature (10, 20, 30, 40, or50 °C).
The SUNTEST was turned on and the radiationintensity was defined as 250, 500, or 750 W m−2, whichis equivalent to 22, 44 or 68 WUVm−2, respectively,measured in the wavelength range from 280 to 400 nm.Ferrous sulfate was added according to the desired con-centration of 20, 40, 60, 80, or 100 mg Fe2+L−1.After15 min, another sample was taken for iron concentrationcontrol and the first dose of hydrogen peroxide wasadded. Samples were taken at pre-defined times to evalu-ate the degradation process. The concentration of hydrogenperoxide was maintained in excess, between 100 and500 mg L−1during the entire reaction, by adding amountsto compensate the ones that were consumed, as deter-mined by analyses throughout the experiments.Results and discussionTextile wastewater characterizationThe wastewater was collected at the equalization tank withoutany form of treatment from a textile company located at thenorth of Portugal. The wastewater results mainly from thedyeing of cotton fibers using several classes of dyes, such asReactive, Direct, Dispersive, Acids, and Cuba. Table 3 pre-sents the main chemical–physical characteristics of the textilewastewater. The wastewater presents a beige color, with amaximum absorbance peak at 641 nm. In addition, the efflu-ent presents an alkaline pH, low total dissolved nitrogen, and amoderate organic load expressed in COD and DOC values,exceeding the discharge limits into receiving waters imposedby Portuguese legislation (Decree Law no. 236/98). The highconductivity of the wastewater is related mainly to its highconcentrations of chloride, sulfate, and sodium ions. Thepresence of these toxic and recalcitrant compounds, normallyresulting in a textile wastewater with low biodegradability,requires alternative processes such as AOPs, as a suitabletreatment before discharge into water bodies. Lotito et al.(2012) and Soares et al. (2014) reported similar characteristicsfor textile wastewaters from dyeing, printing, and scouringprocesses of cotton.Table 3 Textile wastewater characteristicsParameters Units ValuespH Sorensen scale 8.1Temperature °C 30.9Conductivity mS cm−113.6COD mg O2 L−1496Total dissolved carbon mg C L−1264Inorganic carbon mg C L−1135DOC mg C L−1129Absorbance at 641 nm – 0.089Chloride g Cl−L−11.1Sulfate g SO42−L−10.4Total dissolved nitrogen mg N L−121.0Nitrate mg N‐NO3−L−1<0.2Nitrite mg N‐NO2−L−14.1Ammonia mg N‐NH4+L−18.8Phosphate mg P‐PO43−L−16.4Sodium g Na+L−11.2Potassium mg K+L−130.1Magnesium mg Mg2+L−18.7Calcium mg Ca2+L−112.0Total suspended solids mg TSS L−1101Volatile suspended solids mg VSS L−178 Solar-driven AOPsPreliminary tests at a pilot scale were performed under naturalsolar conditions, in order to compare the efficiency of differentAOPs on the treatment of the textile wastewater.