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Colorimetric determination of totaliron concentration was done with 1,10-phenantroline accord-ing to ISO 6332 (1988). UV–Vis spectra between 200 and700 nm, particularly absorbance at 450 nm (vanadate meth-od), 510 nm(phenanthroline method), and 641 nm(maximumabsorbance of the wastewater in the visible region), wereobtained using a UNICAM Helios spectrophotometer. Dueto the textile wastewater absorption at the selected wave-lengths, a blank of the wastewater, diluted as for the colori-metric analyses, was always prepared and the absorbancemeasured at the same wavelength was used as a correctionfactor. Sulfate, chloride, nitrate, phosphates, nitrite, ammoni-um, sodium, potassium, calcium, and magnesium were mea-sured by ion chromatography (Dionex ICS-2100 and DionexDX-120 for anions and cations, respectively), using a DionexIonpac (columns: AS9-HC/CS12A 4 mm×250 mm; suppres-sor: ASRS®300/CSRS®300 4 mm, respectively, for anionsand cations). The program for anion/cation determinationcomprised a 12-min run with 30 mM NaOH/20 mMmethanesulfonic acid at a flow rate of 1.5/1.0 mL min−1.Total suspended solids (TSS) and volatile suspended solids(VSS) were determined according to standard methods(Clesceri et al. 2005).Solar CPC pilot plantThe solar-driven AOP experiments were carried out in a CPCpilot plant installed on the roof of the Chemical EngineeringDepartment of the Faculty of Engineering, University of Porto(FEUP), Portugal. The solar collector consists of a CPC unit(0.91 m2) of four borosilicate (Duran) tubes (Schott-Durantype 3.3, Germany, cutoff 280 nm, internal diameter 46.4 mm,length 1,500 mm, and width 1.8 mm) connected in series bypolypropylene junctions, with their CPC mirrors in anodizedaluminum, supported by an aluminumstructure oriented to thesouth and tilted 41° (local latitude). The pilot plant also hastwo recirculation tanks (10 and 20 L), two recirculation pumps(20 L min−1), two flow ratemeters, five polypropylene valves,and an electric board for process control. The pilot plant canbe operated in two ways, using the total area of the CPC(0.91 m2) or using 0.455 m2of the CPC’s surface area indi-vidually, offering the option of performing two different ex-periments at the same time and under the same solar radiationconditions. A schematic representation of the pilot plant canbe consulted in Pereira et al. (2011).The intensity of solar UVradiation is measured by a globalUV radiometer (ACADUS 85-PLS) mounted on the pilotplant with the same inclination, which provides data in termsof incident irradiance in watt per square meter (WUV m−2).The accumulated UV energy (QUV,n kJ L−1)receivedonanysurface in the same position with regard to the sun per unit ofvolume of water inside the reactor in the time interval Δt canbe calculated using the following equation:QUV;n ¼ QUV;n−1 þ ΔtnUVG;nAr1; 000 Vt; Δtn ¼ tn−tn−1ð1Þwhere tn is the time corresponding to n water sample (s), Vt thetotal reactor volume (L), Ar the illuminated collector surfacearea (m2), and UV G;n the average solar UV radiation (Wm−2)measured during the period Δtn (s).Lab-scale photoreactorOptimization of photo-Fenton reaction variables was carriedout in a lab-scale photoreactor with a sunlight simulator.
Figure 1 presents a scheme of the photocatalytic systemwhichcomprised (i) a solar radiation simulator (ATLAS, modelSUNTEST XLS+) with 1,100 cm2of exposition area, a1,700-W air-cooled xenon arc lamp, a daylight filter, andquartz filter with IR coating; (ii) a CPC with 0.023 m2ofilluminated area with anodized aluminum reflectors and bo-rosilicate tubing (Schott-Duran type 3.3, Germany, cutoff280 nm, internal diameter 46.4 mm, length 160 mm, andthickness 1.8 mm); (iii) one glass vessel (1.5 L capacity) witha cooling jacket coupled to a refrigerated thermostatic bath(Lab Companion, model RW-0525G) to ensure a constanttemperature during the experiment; (iv) a magnetic stirrer(Velp Scientifica, model ARE) to ensure complete homogeni-zation of the solution inside the glass vessel; (v) one peristalticpump (Ismatec, model Ecoline VC-380 II, with a flow rate of0.63 L min−1) to promote water recirculation between theCPC and the glass vessel; (vi) and pH and temperature meter(VWR sympHony SB90M5). All the systems were connectedusing Teflon tubing. The intensity of the UV radiation was measured by a broadband UV radiometer (Kipp & ZonenB.V., model CUV5), placed on the interior of the sunlightsimulator at the same level as the photoreactor center. Theradiometer was plugged into a handheld display unit (Kipp& Zonen B.V., model Meteon) to record the incidentirradiance (WUV m−2).Solar pilot plant experimental procedureA set of AOPs (UV, TiO2/UV, H2O2/UV, TiO2/H2O2/UV, andFe2+/H2O2/UV-vis) was applied to the treatment of a realtextile wastewater using a solar CPC pilot plant in naturalsolar conditions, in order to choose the best AOP among allthat were evaluated. In this stage, the optimal operationalparameters values based on the literature reported by differentauthors were used, being 200 mg TiO2 L−1and 60 mg Fe2+L−1for the heterogeneous photocatalytic (TiO2/UVand TiO2/H2O2/UV-vis) and homogeneous (Fe2+/H2O2/UV) systems,respectively. Based on accurate modeling of radiation fieldin a CPC solar photoreactor by a six-flux absorption scatteringmodel (SFM), different authors reported that the value used(200 mg TiO2 L−1) is the optimum catalyst concentration ableto absorb 100 % of the solar UV photons in a similarphotoreactor to that used in this work (Colina-Márquez et al.2010;Malatoetal. 2004; Vilar et al. 2009). According toSoares et al. (2014), the value used (60 mg Fe2+L−1)isthesuitable catalyst concentration to absorb the maximum solarUV photons in a similar effluent and photoreactor to that usedin this work. Beside these conditions on the improvement ofthe AOP performance, an increase on the mineralization rate,a low reaction time, and a low-energy consumption wereincluded in the efficiency analysis of the set of AOPs,allowing selecting the most efficient AOP. Thus, a volumeof 15 L of textile wastewater was added to the recirculationtank of the CPC unit (illuminated volume (Vi)=5.1 L; Vi/Vt=0.34; illuminated time (ti)=0.25 min; dark time (tdark)=0.50 min; ACPC=0.455 m2) and homogenized by turbulentrecirculation during 15 min in darkness (a first control samplewas taken for further characterization). High alkalinity andsalinity generally present on the textile wastewaters can beresponsible for the low photoactivity of TiO2, due to theformation of a double layer of salt (Guillard et al. 2003)andinteraction of carbonates and bicarbonates at the TiO2 surface.In addition, carbonates and bicarbonates can work as HO•scavengers, leading to the formation of less reactive species(Kormann et al. 1991). In addition, pH was adjusted to 4.5 inall the heterogeneous photocatalytic tests,