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The miner-alization profiles for all solar-driven AOPs tested can be seenin Fig. 2. All operational conditions can be seen in Table 4.During the acidification procedure (pH=2.8 for photo-Fenton reaction and pH=4.5 for the others systems), a DOCreduction of 5 and 19 % (see Fig. 2) for the heterogeneousphotocatalytic and photo-Fenton tests, respectively, was ob-served. This can be associated with the precipitation of someorganic compounds, observed as sludge formation during theacidification process.Among all the processes evaluated, the solar photo-Fentonwas the most efficient, achieving a complete decolorizationand74%mineralizationafter10.5kJUVL−1consuming79.5 mM of H2O2. Although TiO2/H2O2/UV process wasthe most efficient among the heterogeneous systems, themineralization efficiency was less than the achieved byphoto-Fenton reaction (50–74 %, respectively). In addition,the accumulated UV energy increased by a factor of 4, justi-fying the choice of photo-Fenton process to the work conti-nuity. The initial part of the mineralization curves follows apseudo-first-order reaction kinetic. The photo-Fenton reaction(k=0.046 L kJUV−1) presents a pseudo-first-order kinetic con-stant almost three times higher than for the TiO2/H2O2/UVsystem (k=0.015LkJUV−1) (see Table 4). In addition, hydrogenperoxide consumption profiles show a linear correlation withthe accumulated UVenergy per unit of volume of wastewaterduring the reaction period(kH2O2=0.186,1.2,and 7.3 mM H2O2 kJUV−1for H2O2/UV,TiO2/H2O2/UV, and Fe2+/H2O2/UV systems, respectively).Although all the studied processes contributed to a completedecolorization, with the exception of photolysis, the photo-Fenton process showed to be the better one, being necessaryonly 0.5 kJUVL−1consuming 9 mM H2O2.
Optimization of the photo-Fenton variables in the lab-scalephotoreactorThe photo-Fenton reaction showed the highest efficiencyamong all the AOPs studied; thus, the influence of the mainreaction variables, such as iron concentration, pH, tempera-ture, and UV irradiance, was evaluated under controlled con-ditions in a lab-scale photoreactor using a sunlight simulator.Table 4 presents the photo-Fenton reaction conditions used inall sets of experiments.Influence of iron concentrationThe presence of other light-absorbing species in textile waste-waters reduces the number of photons absorbed by the ironcomplexes, being necessary higher iron concentrations (above1 mM) to achieve the desired reaction rates (Malato et al.2004). Thus, aiming at cost reduction and reaction rate max-imization, the optimum iron concentration must be optimizedfor each particular application.The textile wastewater mineralization profiles for five dif-ferent initial iron concentrations (20, 40, 60, 80, and 100 mgFe2+L−1) at a constant temperature of approximately 30 °C(similar to the real cotton-textile wastewater conditions), pH2.8, and UVirradiance of 44WUV m−2are presented in Fig. 3.Immediately after adjusting the pH to 2.8, a similar DOCabatement was observed for all experiments, due to the pre-cipitation of some organic compounds at low pH. Althoughhigher iron concentration enhanced the reaction rates in termsof mineralization, and hydrogen peroxide consumption, foriron concentration values higher than 60 mg L−1, similarprofiles were obtained. Analyzing the tested amounts of ironfor optimum photo-Fenton process, removals of around 75and 79 % of DOC were achieved for textile wastewatertreatments using 60 and 100 mg L−1of iron, respectively.An increase of 40 mg L−1of iron has improved the photo-Fenton performance at about 3–4 %, however, taking intoaccount that higher amounts of iron added in the solutioncan increase the process costs; 60 mg Fe2+L−1can be consid-ered suitable for the treatment of this textile wastewater. Inaddition, regarding the discharge limit imposed byPortuguese legislation (<2 mg L−1), total iron concentrationon treated effluent must be monitored and removed by otherprocesses before its final discharges on water bodies. Afterapproximately 4 kJUVL−1, a very small mineralization rateand a negligible H2O2 consumption were observed, indicat-ing the formation of stable low molecular weight carboxylicacids, such as acetic and formic acids, and its complexeswith ferric ions, which present a low photoactivity and arevery resistant to the attack of HO•, being necessary highenergy doses for their complete mineralization (Soares et al.2014; Vilar et al. 2011).Fig. 2 Mineralization of textile wastewater; comparison between thedifferent AOPs evaluated