Treatment of textile wastewaters by solar-driven advanced oxidation processes

Available online at www.sciencedirect.com

Solar Energy 85 (2011) 1927–1934 www.elsevier.com/locate/solener

Treatment of textile wastewaters by solar-driven advanced oxidation processes Vı´tor J.P. Vilar ⇑, Lı´via X. Pinho, Ariana M.A. Pintor, Rui A.R. Boaventura LSRE-Laboratory of Separation and Reaction Engineering, Departamento de Engenharia Quı´mica, Faculdade de Engenharia da Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal Received 12 April 2011; accepted 30 April 2011 Available online 25 May 2011 Communicated by: Associate Editor Gion Calzaferri

Abstract Heterogeneous (TiO2/UV, TiO2/H2O2/UV) and homogenous (H2O2/UV, Fe2+/H2O2/UV) solar advanced oxidation processes (AOPs) are proposed for the treatment of recalcitrant textile wastewater at pilot-plant scale with compound parabolic collectors (CPCs). The textile wastewater presents a lilac colour, with a maximum absorbance peak at 516 nm, high pH (pH = 11), moderate organic content (DOC = 382 mg C L1, COD = 1020 mg O2 L1) and high conductivity (13.6 mS cm1), associated with a high concentration of chloride (4.7 g Cl L1). The DOC abatement is similar for the H2O2/UV and TiO2/UV processes, corresponding only to 30% and 36% mineralization after 190 kJUV L1. The addition of H2O2 to TiO2/UV system increased the initial degradation rate more than seven times, leading to 90% mineralization after exposure to 100 kJUV L1. All the processes using H2O2 contributed to an effective decolourisation, but the most efficient process for decolourisation and mineralization was the solar-photo-Fenton with an optimum catalyst concentration of 100 mg Fe2+ L1, leading to 98% decolourisation and 89% mineralization after 7.2 and 49.1 kJUV L1, respectively. According to the Zahn–Wellens test, the energy dose necessary to achieve a biodegradable effluent after the solar-photo-Fenton process with 100 mg Fe2+ L1 is 12 kJUV L1. Ó 2011 Elsevier Ltd. All rights reserved. Keywords: AOPs; Solar radiation; Textile wastewater; Biodegradability; Pilot plant; Compound parabolic collectors

1. Introduction The textile industry is one of the largest consumers of water in the world, and consequently, one of the largest producers of wastewaters. The wastewater in textile industry is generated in several stages of textile manufacturing and processing, such as sizing of fibres, scouring, desizing, bleaching, washing, mercerization, dyeing and finishing (Rodriguez et al., 2002).The main characteristics of these wastewaters include a strong colour associated with residual dyes (leading to moderate levels of organic matter), high concentrations of salts (like sodium chlo⇑ Corresponding author. Tel.: +351 918257824; fax: +351 225081674.

E-mail address: [email protected] (V.J.P. Vilar). 0038-092X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2011.04.033

ride), detergents and soaps, oil and grease, sulphide and sodas (Rodriguez et al., 2002). It is estimated that 1– 20% of the world’s dyes production is lost during the dyeing process, due to low-fixation on the fabrics (Muruganandham and Swaminathan, 2006a; Prieto et al., 2005). Dyes have a complex aromatic and polymeric structure, constituting high solubility in water, and at the same time are often toxic and non-biodegradable, becoming extremely dangerous for ecosystems. Some problems which may arise due to dye pollution include the absorbing of sunlight by dyes, affecting the photosynthesis of aquatic plants, and the possibility of production of aromatic amines by the degradation of azo dyes, which are considered highly carcinogenic (Bali et al., 2004; Torrades et al., 2004).

1928

V.J.P. Vilar et al. / Solar Energy 85 (2011) 1927–1934

Due to the low biodegradable nature of textile wastewaters, conventional biological processes are commonly ineffective for their treatment (Torrades et al., 2004), being necessary to find out alternative technologies for the efficient removal of dye containing wastewaters. Some of the common methods for this purpose include: adsorption on activated carbon (Ahmad and Hameed, 2010; Khaled et al., 2009; Sirianuntapiboon et al., 2007), coagulation/ flocculation followed by sedimentation or dissolved air flotation (Khouni et al., 2011; Riera-Torres et al., 2010; Szygula et al., 2009) and reverse osmosis (So´jka-Ledakowicz et al., 1998; Suksaroj et al., 2005). However, these methods are not completely effective as they simply transfer the contaminant from water to a solid phase or sludge (Torrades et al., 2004; Pekakis et al., 2006). Advanced Oxidation Processes (AOPs) are chemical oxidation processes based on the use of the hydroxyl radicals (OH), which are one of the most powerful oxidizing species. Their attack is not very selective, which is a useful attribute for use in the treatment of textile wastewaters, as they are able to degrade different recalcitrant compounds effectively, including dyes. Most of the AOPs use UV radiation combined with a semiconductor such as titanium dioxide or powerful oxidants such as hydrogen peroxide or ozone for the generation of hydroxyl radicals. The most recent development in AOPs is the use of solar radiation, as UV photon source, decreasing the artificial energy requirements for the application of these methods (Malato et al., 2007). A variety of AOPs have been proposed for treatment of textile wastewaters or degradation of dyes: UV/H2O2 (Bali et al., 2004; Muruganandham and Swaminathan, 2004, 2006b, 2007); TiO2/UV (heterogeneous photocatalysis) (Muruganandham and Swaminathan, 2006a,b, 2007; Prieto et al., 2005; Pekakis et al., 2006); H2O2/Fe2+ (Fenton’s reagent) (Pe´rez et al., 2002); UV/H2O2/Fe2+ (photo-Fenton process) (Rodriguez et al., 2002; Bali et al., 2004; Torrades et al., 2004; Muruganandham and Swaminathan, 2006b, 2007; Pe´rez et al., 2002). Although there is currently some information on the degradation of dyes using chemical oxidation processes, most of this information is limited to the removal of specific dyes from pure dye solutions or simulated wastewaters. The information on the kinetics of a solar photocatalytic treatment for real textile wastewaters is quite scarce. Rodriguez et al. (2002), Pekakis et al. (2006) and Pe´rez et al. (2002) studied the treatment of real textile wastewaters using photocatalytic processes, but none of them used solar light as UV radiation source. There is a lack of published studies on the use of solar photocatalytic treatment at pilot plant scale which limits the application of this technology on industrial scale. Therefore, the main goal of this work is to evaluate the degradation efficiency of a real textile wastewater by different AOPs, UV/H2O2, UV/TiO2, UV/TiO2/H2O2, Fenton and solar-photo-Fenton, using solar light, at pilot plant scale.

2. Experimental methodology 2.1. Solar CPC pilot plant All the experiments were carried out in a 100-L CPC pilot plant placed at the roof of the Chemical Engineering Department, Faculty of Engineering, University of Porto (FEUP), Portugal. The pilot plant consists of a CPC illuminated area of 4.16 m2, two storage tanks of 55 and 100 L, two recirculation pumps (20 L min1) (ARGAL, model TMB), two flowmeters (Stu¨be, model DFM 165-350), polypropylene valves (FIP) and connecting tubing, being operated in batch mode and, a biological oxidation system (Rocha et al., 2011). The solar collectors are made up of four CPC units (1.04 m2) with five borosilicate tubes each (Schott-Duran type 3.3, Germany, cut-off at 280 nm, internal diameter 46.4 mm, length 1500 mm and thickness 1.8 mm) connected by plastic junctions and tilted 41° (local latitude). The plant can be operated in two ways: using the CPCs total area of 4.16 m2 or using 2.08 m2 of CPC area individually, which makes it possible to perform two different experiments at the same time using the identical solar radiation conditions. The intensity of solar UV radiation is measured by a global UV radiometer (ACADUS 85-PLS) placed at the top of the pilot plant at the same angle. The instantaneous UV radiation data (W m2) is collected and used to calculate the amount of accumulated UV energy (QUV,n kJ L1) per unit of volume of water inside the reactor, in the time interval Dt (Malato et al., 2002): QUV;n ¼ QUV;n1 þ Dtn UVG;n

Ar ; Vt

Dtn ¼ tn  tn1

ð1Þ

where tn is the time corresponding to n-water sample, Vt is the total reactor volume, Ar is the illuminated collector surface area and UVG;n is the average solar ultraviolet radiation measured during the period Dtn. 2.2. Analytical determinations The quantification of H2O2 concentration during the experiments was performed by the metavanadate method, based on the reaction of H2O2 with amonium metavanadate in acidic medium, which results in the formation of a red–orange colour peroxovanadium cation, with maximum absorbance at 450 nm (Nogueira et al., 2005). Iron concentration was determined by colourimetry with 1,10phenantroline according to ISO 6332. pH and temperature were monitored using a pH meter HANNA HI8424. Sulphate, chloride, nitrate, phosphate, nitrite and ammonium were measured by ion chromatography (Dionex DX-120), using a Dionex Ionpac (columns: AS9-HC/CS12A 4 mm  250 mm; suppressors: ASRSÒ300/CSRSÒ300 4 mm, respectively for anions and cations). The programme for anions/cations determination comprises a 20/ 10 min run with 9 mM Na2CO3/20 mM methanesulfonic acid at a flow rate of 1.0 mL min1. COD was determined

V.J.P. Vilar et al. / Solar Energy 85 (2011) 1927–1934

by MerckÒSpectroquant kits (Ref. 1.14541.0001). Total suspended solids (TSS) were determined by gravimetry, according to Standard Methods (Clesceri et al., 2005). Dissolved organic carbon (DOC) was measured in a TC–TOC–TN analyzer (Shimadzu, model TOC-VCSN) calibrated with standard solutions of potassium phthalate. Total nitrogen was measured in the same TC–TOC–TN analyzer coupled with a TNM-1 unit (Shimadzu, model TOC-VCSN) by thermal decomposition and NO detection by chemiluminescence method after calibration with standard solutions of potassium nitrate. UV–Vis spectrum between 200–700 nm, absorbance at 450 nm (vanadate method), 510 nm (phenantroline method), 516 nm (maximum absorbance of the wastewater in the visible region) and 254 nm (aromatic content) were obtained using a UNICAM Helios a spectrophotometer. Before analysis, all samples were centrifuged in a HIMAC CT 6E centrifuge at a speed of 4000 rpm for 5 min. 2.3. Biodegradability assays Before biodegradability tests and other analysis involving chemical oxidation, excess H2O2 present in samples was removed using a small volume of 0.1 g L1 catalase solution (2500 U mg1 bovine liver) after adjusting the sample pH to approximately 7. Biochemical oxygen demand (BOD5) was carried out according to Standard Methods using an OXITOPÒ system (Clesceri et al., 2005). A 28 days biodegradability test (Zahn–Wellens test) was performed according to the EC protocol, Directive 88/ 303/EEC (OECD, 1992). A volume of 240 mL of the pretreated samples, obtained at different reaction times, without hydrogen peroxide, was added to an open glass vessel, magnetically stirred and kept in the dark inside a thermostatic refrigerator at 25 °C. Activated sludge from the municipal wastewater treatment plant of Freixo (Porto, Portugal), previously centrifuged, and mineral nutrients (KH2PO4, K2HPO4, Na2HPO4, NH4Cl, CaCl2, MgSO4 and FeCl3) were added to the samples. The control and blank experiments were prepared using glucose and distiled water, respectively. The percentage of biodegradation (Dt) was determined by the following equation (EPA, 1996):   Ct  CB Dt ¼ 1   100 ð2Þ C A  C BA where CA and CBA are the DOC (mg L1) in the mixture and in the blank, measured 3 h. after the beginning of the experiment, Ct and CB are the DOC (mg L1) in the mixture and in the blank, measured at the sampling time t. The samples are considered biodegradable when Dt is higher than 70%. 2.4. Experimental procedure A volume of 35 L of textile wastewater was added to the recirculation tank of the CPC units (2.08 m2) and homogenized by turbulent recirculation during 15 min in dark (a

1929

sample was taken for characterization). The solution pH was adjusted with H2SO4 (96%, Pronalab) to 2.6–2.9 for Fenton and photo-Fenton experiments and to 4.5–4.6 for UV/TiO2, UV/H2O2, and UV/TiO2/H2O2, and a second sample was taken. For Fenton and photo-Fenton experiments, iron salt (20, 80, 100 or 120 mg Fe2+ L1) was added as FeSO4.7H2O (Panreac) and after homogenization for 15 min., a third sample was taken. The reaction started with the addition of the first dose of hydrogen peroxide (50% w/v, Quimite´cnica), and in the case of UV/H2O2 and photo-Fenton reaction, the CPCs were uncovered. In the kinetic studies, hydrogen peroxide concentration was maintained in excess, between 200 and 500 mg L1, by the addition of small amounts of H2O2 as consumed, and samples were taken at pre-defined times, to evaluate the degradation process. For the TiO2 photocatalytic tests, after collecting the first sample for wastewater characterization, TiO2 (Degussa, P25, 80% anatase and 20% rutile, 200 mg L1) was added and the mixture recirculated for next 15 min. Just before uncovering the CPC units to start the photocatalytic process, another sample was collected to quantify the DOC and COD removal by adsorption on the TiO2 surface. In the case of the TiO2/H2O2/UV experiment, hydrogen peroxide (50% w/v, Quimite´cnica) was added to the mixture of TiO2 and wastewater, before uncovering the CPCs. Samples were also taken at regular time intervals to evaluate the progress of the photocatalytic oxidation. 3. Results and discussion 3.1. Textile wastewater characterization A brief characterization of the raw textile wastewater used in this study is shown in Table 1. The wastewater was collected before undergoing to any treatment by the industrial company (Vizela, Portugal). The textile wastewater presents a lilac colour, resulting from the mixture of difTable 1 Textile wastewater characterization. Parameter

Value

pH Temperature (°C) Conductivity (mS cm1) TSS (mg L1) DOC (mg L1) COD (mg O2 L1) BOD5 (mg O2 L1) BOD5/COD Chloride (mg Cl L1) Total nitrogen (mg N L1) 1 Ammonia (mg N–NHþ 4 L ) 1 Nitrite (mg N–NO L ) 2 1 Nitrate (mg N–NO 3 L ) 1 Phosphate (mg P–PO3 4 L ) 2 1 Sulphate (mg SO4 L )

10.8 31.2 13.6 <3 382 1020 110 0.11 4578 32.4 0.8 <0.2 7.5 <0.2 265

V.J.P. Vilar et al. / Solar Energy 85 (2011) 1927–1934

11

350

10

300

9

250

8

200

7

150

6

100

5

1.2 1.0

pH

0.8 0.6 0.4

50 0

0

50

0.2

4

RAD-ON

100

150

200

250

Abs/Abs0 at 516 nm

400

3 300

0.0

QUV (kJ/L) Fig. 1. Decolourisation and mineralization of the textile wastewater by TiO2 solar photocatalysis: DOC degradation curve, Abs/Abs0 at 516 nm and pH evolution.

400

DOC

350

Abs/Abs0 516 nm H2O2 cons.

200 175

300

150 250 125 200 100 150

75

100

50 RAD-ON

50

1.4

225

1.2

25 0

0 0

1.0 0.8 0.6 0.4

Abs/Abs0 at 516 nm

A first approach to the treatment of textile wastewaters was done by heterogeneous photocatalysis with TiO2 after acidification at pH 4.5 (0.34 mL H2SO4 L1 wastewater, H2SO4, 96%). The high alkalinity and salinity are generally responsible for the low photoactivity of TiO2 surface, presumably due to the formation of a double layer of salt at the TiO2 surface (Guillard et al., 2003), interaction of carbonates and bicarbonates with TiO2 surface, and on the other hand to the modification of the catalyst surface, since for pH < pzc and pH > pzc (pzc-point of zero charge) the predominant species are respectively, –TiOH2+ and –TiO (Kormann et al., 1991), therefore, in this case pH was corrected to 4.5. Muruganandham and Swaminathan (2007) also reported that adsorption of dyes onto the titanium dioxide surface is more significant at acidic pH. The catalyst concentration was 200 mg L1, as it has been considered the optimum concentration for the type of photoreactors used in this study (Malato Rodrı´guez et al., 2004). Fig. 1 shows 68% removal of absorbance at 516 nm (associated with colour removal), but only 30% mineralization after 293.7 kJUV L1. The mineralization curve follows first-order kinetics with a kinetic constant, k, of 8.8  104 1 L kJ1 UV and an initial degradation rate, ro, of 0.34 mg kJUV . Despite wastewater acidification, decolourisation and mineralization rates were very low, presumably due to the high concentration of chloride, responsible for hydroxyl radicals scavenging and formation of less reactive inor ganic radicals ðCl ; Cl 2 and SO4 Þ (Guillard et al., 2003; Neta et al., 1988; Martell and Smith, 1977) and direct absorption of photons by dyes rather than by the catalyst, thus reducing the photocatalytic efficiency (Muruganandham and Swaminathan, 2006a). Therefore, an improvement was attempted by adding H2O2 to TiO2, preventing the electron–hole recombination by accepting a photogenerated electron from the conduction band and producing additional OH radicals. A clear improvement on mineralization (k = 3.3  102 L kJ1 UV , ro = 12.2 mg kJ1 ) and decolourisation was observed UV (Fig. 2) by the addition of H2O2, yielding values of 90%

DOC Abs/Abs0 516 nm pH

H2O2 consumed (mM)

3.2. Solar heterogeneous photocatalytic treatment

1.4

12

450

DOC (mg/L)

ferent dyes, a moderate organic load (COD = 1020 mg O2 L1 and DOC = 382.4 mg L1), a high alkaline pH and high conductivity, associated with a high concentration of chloride ions (nearly 4.6 g L1). Sulphate, phosphate, ammonium, nitrite and nitrate were present in moderate/low concentrations, not exceeding the discharge limits imposed by Portuguese legislation. UV–Vis absorbance spectrum of the textile wastewaters, between the wavelengths of 200 and 700 nm, reveal a high absorbance in the UV region, due to the aromatic compounds (at 256 nm, absorbance is 2.87) and a maximum absorbance peak in the visible region at 516 nm (0.33) associated with the lilac colour of the solution. The low ratio of BOD5/COD (0.1) reveals the low biodegradability of the textile wastewater.

DOC (mg/L)

1930

0.2 0.0

10 20 30 40 50 60 70 80 90 100

QUV (kJ/L) Fig. 2. Decolourisation and mineralization of the textile wastewater by solar UV/TiO2/H2O2: DOC degradation curve, H2O2 consumption and Abs/Abs0 at 516 nm.

and 94%, respectively, after 99.4 kJUV L1 and consuming 208.1 mM of H2O2 (kH2O2 = 2.3 mmol kJ1 UV ). It was found that 21.3 mM of H2O2 was necessary for achieving 74% decolourisation, suggesting that the addition of hydrogen peroxide is a major factor for the effective removal of the effluent’s colour. 3.3. Solar homogeneous photocatalytic treatment In order to know the influence of H2O2 in the textile wastewater treatment, a new experiment with only H2O2 and sunlight was performed. This was already done by other authors with good results for decolourisation, although not much for mineralization (Muruganandham and Swaminathan, 2004). It was observed that the use of sunlight and hydrogen peroxide leads to much better results in terms of decolourisation (93%) than using

V.J.P. Vilar et al. / Solar Energy 85 (2011) 1927–1934

250

20

200

15

DOC H2O2 cons. Abs/Abs0 516 nm

150 RAD-ON

100 50 0 0

50

100

150

10

0.8

0.6

0.4

5

0.2

0 200

0.0

a

150

0.6 0.4

100

0.2

50

0.0

0 0 10 20 30 40 50 60 70 80 90 100 110

QUV (kJUV/L)

(b) 50

90

40

70

Fig. 3. Decolourisation and mineralization of the textile wastewater by solar UV/H2O2: DOC degradation curve, H2O2 consumption and Abs/ Abs0 at 516 nm.

80

60

T ( C)

TiO2/UV (Fig. 3), although similar mineralization was obtained (36%). However, TiO2/H2O2/UV system presents an initial reaction rate more than 10 times higher when compared with the UV/H2O2 system leading to a faster decolourisation and mineralization. The degradation of dyes in textile wastewaters by UV/ H2O2 occurs by the generation of hydroxyl radicals through the photolysis of hydrogen peroxide under solar irradiation (Muruganandham and Swaminathan, 2007). It is worth to mention that this process might not have been operated at optimum pH, as two previous studies. Muruganandham and Swaminathan (2004, 2007) pointed out an optimum pH of 3 for mineralization and decolourisation of dyes by UV/H2O2. However, in this study, pH was maintained in the range 4.5–5.0 for comparison with UV/ TiO2 and UV/TiO2/H2O2. This suggests that hydrogen peroxide is an effective agent for decolourisation, being able to easily degrade the dyes’ structure into intermediates but not into further degradation of these intermediate products into CO2 and other more oxidised products. Finally, the combination of iron with H2O2 and solar radiation (solar-photo-Fenton) was tested. Fig. 4a illustrates the mineralization of the textile wastewater by photo-Fenton reaction at four different initial iron concentrations (20, 80, 100 and 120 mg Fe2+ L1). Similar DOC abatement (20–35%) was observed after pH adjustment to pH 2.8 due to the destruction of most oxidized compounds by the acid attack, which is a powerful oxidant, since it was not observed the formation of foam and sludge (possible precipitation of some compounds at low pH). Afterwards, it can be observed an induction period, characterized by low mineralization and hydrogen peroxide consumption (which is more visible in the experiment with the lowest iron dose), although decolourisation is very rapid suggesting a fast breaking down of the dyes’ molecules. The higher the iron concentration, the smaller is the induction period, being almost eliminated for the two highest iron concentra-

250

200

0.8

QUV (kJ/L)

-1

H2O2 consumed (mM)

25 300

H2O2 consumed (mM)

350

a

1.0

2+

a - addition of 40 mg Fe L

RAD-ON

30

50 40

20

30 10

20 RAD-ON

Dissolved Iron (mg/L)

30

(a) 1.0

DOC/DOC0

400

DOC (mg/L)

1.2

35

Abs/Abs0 at 516 nm

450

1931

10 2+

0

a

a - addition of 40 mg Fe L

-1

0 0 10 20 30 40 50 60 70 80 90 100 110

QUV (kJUV/L) Fig. 4. Mineralization of the textile wastewater by solar-photo-Fenton: comparison between different iron concentrations, 20 mg Fe2+ L1 (h, j), 80 mg Fe2+ L1 ( , ), 100 mg Fe2+ L1 ( , ) and 120 mg Fe2+ L1 ( , ) – open symbols: H2O2 consumed and dissolved iron concentration; solid symbols: DOC/DOC0 and temperature.

tions tested. This suggests that after the first oxidation of iron in the presence of hydrogen peroxide, as in Eq. (3), the limiting step of the reaction is the availability of radiation for the regeneration of ferrous ions (reduction of Fe3+ to Fe2+ according to Eq. (4)) (Malato et al., 2009), as absorption of photons by iron complexes is more difficult in strongly coloured solutions. This explains the very slow reaction rate observed in a dark Fenton reaction performed with 100 mg Fe2+ L1, achieving only 67% mineralization in 4 days and consuming 41.3 mM H2O2, although complete decolourization was accomplished. Fe2þ þ H2 O2 ! Fe3þ þ OH þ OH ½FeðOHÞ

2þ

þ hm ! Fe

2þ

þ OH



ð3Þ ð4Þ

The second part of mineralization curve follows firstorder kinetics (k = 0.031, 0.042, 0.042, 0.042 L kJ1 UV , r0 = 10.7, 13.0, 11.3, 11.3 mg kJ1 for initial iron doses of UV 20, 80, 100 and 120 mg L1, respectively). The H2O2 consumption profile shows a linear correlation with the UV energy accumulated per unit of volume of wastewater during the second reaction period (kH2O2 = 2.3, 2.7, 4.2, 3.8 mmol H2O2 kJ1 UV , respectively for initial iron doses of 20, 80, 100 and 120 mg L1).

1932

V.J.P. Vilar et al. / Solar Energy 85 (2011) 1927–1934

Reaction rates were found to increase with catalyst concentration, which is in agreement with previous reports by Malato et al. (2009). Yet this increase stabilizes for higher concentrations of iron, as dark zones might be created inside the photoreactor, making absorption of photons by Fe3+ and its related complexes difficult. In another study, Malato Rodrı´guez et al. (2004) reported a range between 0.2 and 0.5 mM as the optimum concentration of iron, for solar-photo-Fenton with CPCs with an internal diameter tube of 46.4 mm. However, in coloured effluents, catalyst concentrations above 1 mM might be necessary as dyes compete with iron for light absorption. Fig. 4b shows a decrease of iron concentration between 58% and 66% relatively to the initial iron dose added, principally in the initial part of the photo-Fenton reaction, which can be possible explained by complexation of iron with intermediate products present in solution after the initial breaking down of the dyes’ structure, such as lowmolecular-weight carboxylic acids. For example, in the first experiment, it was necessary to add different doses of iron during the reaction, since the iron concentration was lower than 10 mg Fe2+ L1. Temperature usually rises from morning start-up (17– 24 °C) to an almost constant value for several hours until 14:00 h (48 °C was the maximum temperature achieved) and decreases again during the afternoon, depending on the sunlight intensity. It must be emphasized that experiments were performed in consecutive days (Fig. 4b). According to the induction time and reaction rate, no significant difference could be observed between the experiments at the two highest iron concentrations. Considering that a lower iron concentration reduces the reactant costs, the catalyst concentration of 100 mg Fe2+ L1 was selected as the optimum Fe2+ concentration for further studies. In these conditions, 98% decolourisation (in terms of absorbance at 516 nm) after 3.7 kJUV L1 (14 mM of H2O2) and 89% mineralization after 49.1 kJUV L1 were achieved with a consumption of 203.2 mM of H2O2.

A better visualization can be seen from Fig. 5 revealing that the solar photo-Fenton reaction is much more efficient than UV/H2O2 and UV/TiO2/H2O2 systems. Although the kinetic constants of the photo-Fenton reaction and UV/ TiO2/H2O2 system are very similar, the amount of energy to achieve 80% mineralization is two times higher for the UV/TiO2/H2O2 system, mainly due to the acidification process until pH 2.8, which leads to more than 30% mineralization of the wastewater. Nevertheless, the consumption of H2O2 is 1.3 times higher for the photo-Fenton process, when compared with the UV/TiO2/H2O2 system, to achieve the same mineralization. 3.4. Biodegradability assays A Zahn–Wellens test was carried out to evaluate the biodegradability at different stages of the most efficient previously studied AOP (solar-photo-Fenton reaction with 100 mg Fe2+ L1), aiming at determining the optimum phototreatment time for coupling with a biological process. To collect pre-treated samples for this test, the previous solar-photo-Fenton experiment was reproduced under the same operating conditions, except the H2O2 dose. In this run, a small amount of H2O2 was added to the photoreactor, and after its total consumption, a sample was taken for bioassays and another amount of H2O2 was added. This process was important not only to prevent any reaction under dark conditions after sample collection but also to prevent the inhibition of microorganisms in bioassays due to the presence of residual H2O2. COD, chloride, sulphate, nitrate and total nitrogen analyses were also performed for those samples, in order to have a more comprehensive assessment of each step of the phototreatment. Fig. 6 shows 83% COD removal (from 1020 to 170 mg O2 L1), a very fast abatement of the absorbance at 516 nm at the beginning of the treatment and an increase of Carbon Oxidation State (COS) (Amat et al., 2007; Arques et al., 2007) from an initial value of 0.0 to a maximum of +3.3, which corresponds to a strong oxidation of the organic matter.

250

1,0

50

0,2

0,0 0

50

100

150

0 200

QUV (kJUV/L)

S

2

3

COD DOC COS Abs/Abs0 516 nm

800 S

4

600

2

5

-1

400 S

S

S

7

0 40

0.6 0.4

-2

6

200

20

0.8

1 0

S

0

Fig. 5. Mineralization of the textile wastewater: comparison between, ), UV/TiO2/H2O2 ( , ) and photo-Fenton with UV/H2O2 ( , 100 mg Fe2+ L1 (h, j) – open symbols: H2O2 consumption; solid symbols: DOC/DOC0.

1.0

3 S

60

80

8

0.2

Abs/Abs0 at 516 nm

100

0,4

4

RAD-ON

1

COS

DOC/DOC0

150

0,6

S

1000

DOC, COD (mg/L)

200

0,8

H2O2 consumed (mM)

RAD-ON

-3 -4 100

0.0

H2O2 consumed (mM) Fig. 6. Solar-photo-Fenton treatment of the textile wastewater: COD, DOC, COS and Abs/Abs0 at 516 nm.

100

100

90

90

80

80

70

70

60

60

50

50

40

40

30

30

20

20

10

10

Dt (%)

Dt (%)

V.J.P. Vilar et al. / Solar Energy 85 (2011) 1927–1934

0

0 0

4

8

12

16

20

24

1933

after the acidification step with sulphuric acid and the addition of iron sulphate (100 mg Fe2+ L1) and remained almost constant during the photo-treatment, which is in accordance with the discharge limits of Portuguese legislation. In order to achieve the discharge limits in terms of pH and iron concentration, the effluent must be neutralized to pH  7, using lime or sodium hydroxide. 4. Conclusions

28

Time (days) Fig. 7. Zahn–Wellens test for selected samples during the solar-photoFenton treatment of the textile wastewater: I – S1, DOC = 382.4 mg L1; / – S2, DOC = 325.0 mg L1; } – S3, DOC = 306.9 mg L1; r – S4, DOC = 248.1 mg L1; – S5, DOC = 197.2 mg L1; O – S6, 1 DOC = 105.0 mg L ; H – S7, DOC = 61.3 mg L1; s – S8, DOC = 50.6 mg L1; j – reference (DOC = 349.1 mg L1).

Fig. 7 presents the results obtained in the Zahn–Wellens test, showing that the first three samples (non-treated, after pH adjustment and after iron addition) present a moderate biodegradation of 50, 43 and 49%, respectively, below the 70% biodegradability threshold defined in Zahn–Wellens test methodology (Sarria et al., 2002). The results indicate that the acidification process destroys the most oxidized compounds with a higher biodegradability and the Fenton reaction promotes a slight increase in the biodegradability. However, as expected, the biodegradability of the textile wastewater was enhanced during the photo-Fenton treatment, up to a value higher than 70% degradation after 28 days for samples 5 (71%), 6 (80%), 7 (83%) and 8 (81%) (according to Fig. 6). Based on these results, sample 5 can be considered as corresponding to the optimum phototreatment time to reach a biodegradable effluent. Therefore, the optimum energy dose to reach a biodegradable effluent is 12 kJUV L1, consuming 52 mM of hydrogen peroxide (added in excess), as calculated from the kinetic studies, and leading to 55% mineralization (DOCfinal = 197 mg C L1) and 96% decolourisation. Other parameters were analysed to verify the compliance of the effluent at the optimum phototreatment time with the discharge limits defined by Portuguese legislation. Total nitrogen and nitrates remain constant throughout the treatment, at around 32.5 mg N L1 and 8 mg N–NO 3 L 1 , respectively. Ammonia, however, increases by a factor 1 higher than 5, from 0.8 to 4.2 mg N–NHþ 4 L , associated to the oxidation of the organic nitrogen. At the optimum phototreatment time, a concentration of 31.9 mg N L1 1 was obtained (7.9 mg N–NO and 3.9 mg N–NHþ 3 L 4 1 L ). Coupling the photo-oxidation with a biological treatment can simultaneously eliminate the residual biodegradable carbon and total nitrogen. Chloride concentration does not change throughout the treatment, remaining in the range of 4.5–4.7 g L1. Sulphate concentration increases from 265 to 1043 mg L1

Solar AOPs are a valid technology to degrade these kinds of refractory compounds until complete mineralization or transforming them in more biodegradable compounds. The combination of catalysts and hydrogen peroxide as an oxidant (UV/TiO2/H2O2 and photo-Fenton) proved to be more efficient than TiO2 or H2O2 alone with solar irradiation. Solar-photo-Fenton was the most efficient of all solar AOPs studied, for an optimum catalyst concentation of 100 mg Fe2+ L1, enhancing the biodegradability of the leachate and making possible its combination with a biological oxidation process. Acknowledgements Financial support for this work was in part provided by LSRE financing by FEDER/POCI/2010. V. Vilar’s acknowledges Cieˆncia 2008 Program. A. Pintor and L. Pinho acknowledge their Ph.D. scholarships by FCT (SFRH/ BD/70142/2010) and CNPq (200544/2010-1), respectively. References Ahmad, A.A., Hameed, B.H., 2010. Effect of preparation conditions of activated carbon from bamboo waste for real textile wastewater. J. Hazard. Mater. 173, 487–493. Amat, A.M., Arques, A., Galindo, F., Miranda, M.A., Santos-Juanes, L., Vercher, R.F., Vicente, R., 2007. Acridine yellow as solar photocatalyst for enhancing biodegradability and eliminating ferulic acid as model pollutant. Appl. Catal., B 73, 220–226. Arques, A., Amat, A.M., Garcı´a-Ripoll, A., Vicente, R., 2007. Detoxification and/or increase of the biodegradability of aqueous solutions of dimethoate by means of solar photocatalysis. J. Hazard. Mater. 146, 447–452. Bali, U., C ¸ atalkaya, E., Sengu¨l, F., 2004. Photodegradation of Reactive Black 5, Direct Red 28 and Direct Yellow 12 using UV, UV/H2O2 and UV/H2O2/Fe2þ : a comparative study. J. Hazard. Mater. 114, 159–166. Clesceri, L.S., Greenberg, A.E., Eaton, A.D. (Eds.), 2005. Standard Methods for Examination of Water & Wastewater, 21st ed. American Public Health Association (APHA), American Water Works Association (AWWA) & Water Environment Federation (WEF). EPA, 1996. US Environmental Protection Agency, Prevention Pesticides and Toxic Substances (7101). Fates, Transport and Transformation Test Guidelines OPPTS 835.3200 Zahn–wellens/EMPA Test, EPA 712-C-96-084, Washington, DC, 1996. Guillard, C., Lachheb, H., Houas, A., Ksibi, M., Elaloui, E., Herrmann, J.-M., 2003. Influence of chemical structure of dyes, of pH and of inorganic salts on their photocatalytic degradation by TiO2 comparison of the efficiency of powder and supported TiO2. J. Photochem. Photobiol., A 158, 27–36.

1934

V.J.P. Vilar et al. / Solar Energy 85 (2011) 1927–1934

Khaled, A., Nemr, A.E., El-Sikaily, A., Abdelwahab, O., 2009. Removal of Direct N Blue-106 from artificial textile dye effluent using activated carbon from orange peel: adsorption isotherm and kinetic studies. J. Hazard. Mater. 165, 100–110. Khouni, I., Marrot, B., Moulin, P., Ben Amar, R., 2011. Decolourization of the reconstituted textile effluent by different process treatments: enzymatic catalysis, coagulation/flocculation and nanofiltration processes. Desalination 268, 27–37. Kormann, C., Bahnemann, D.W., Hoffmann, M.R., 1991. Photolysis of chloroform and other organic molecules in aqueous TiO2 suspensions. Environ. Sci. Technol. 25, 494–500. Malato, S., Blanco, J., Vidal, A., Richter, C., 2002. Photocatalysis with solar energy at a pilot-plant scale an overview. Appl. Catal., B 37, 1– 15. Malato, S., Blanco, J., Alarco´n, D.C., Maldonado, M.I., Ferna´ndezIba´n˜ez, P., Gernjak, W., 2007. Photocatalytic decontamination and disinfection of water with solar collectors. Catal. Today 122, 137–149. Malato, S., Ferna´ndez-Iba´n˜ez, P., Maldonado, M.I., Blanco, J., Gernjak, W., 2009. Decontamination and disinfection of water by solar photocatalysis: recent overview and trends. Catal. Today 147, 1–59. Malato Rodrı´guez, S., Blanco Ga´lvez, J., Maldonado Rubio, M.I., Ferna´ndez Iba´n˜ez, P., Alarco´n Padilla, D., Collares Pereira, M., Farinha Mendes, J., Correia de Oliveira, J., 2004. Engineering of solar photocatalytic collectors. Sol. Energy 77, 513–524. Martell, A.E., Smith, R.M., 1977. Critical stability constants. Plenum Press, New York. Muruganandham, M., Swaminathan, M., 2004. Photochemical oxidation of reactive azo dye with UV-H2O2 process. Dyes Pigm. 62, 269–275. Muruganandham, M., Swaminathan, M., 2006a. Photocatalytic decolourisation and degradation of Reactive Orange 4 by TiO2-UV process. Dyes Pigm. 68, 133–142. Muruganandham, M., Swaminathan, M., 2006b. Advanced oxidative decolourisation of Reactive Yellow 14 azo dye by UV/TiO2, UV/ H2O2, UV/H2O2/Fe2þ processes–a comparative study. Sep. Purif. Technol. 48, 297–303. Muruganandham, M., Swaminathan, M., 2007. Solar driven decolourisation of Reactive Yellow 14 by advanced oxidation processes in heterogeneous and homogeneous media. Dyes Pigm. 72, 137–143. Neta, P., Huie, R.E., Ross, A.B., 1988. Rate constants for reactions for inorganic radicals in aqueous solution. J. Phys. Chem. Ref. Data 17, 1027–1247. Nogueira, R.F.P., Oliveira, M.C., Paterlini, W.C., 2005. Simple and fast spectrophotometric determination of H2O2 in photo-Fenton reactions using metavanadate. Talanta 66, 86–91.

OECD, 1992. Test No. 302B: Inherent Biodegradability: Zahn–Wellens/ EMPA Test in: OECD Guidelines for the Testing of Chemicals. OECD Publishing, pp. 8. Pekakis, P.A., Xekoukoulotakis, N.P., Mantzavinos, D., 2006. Treatment of textile dyehouse wastewater by TiO2 photocatalysis. Water Res. 40, 1276–1286. Pe´rez, M., Torrades, F., Dome`nech, X., Peral, J., 2002. Fenton and photoFenton oxidation of textile effluents. Water Res. 36, 2703–2710. Prieto, O., Fermoso, J., Nun˜ez, Y., del Valle, J.L., Irusta, R., 2005. Decolouration of textile dyes in wastewaters by photocatalysis with TiO2. Sol. Energy 79, 376–383. Riera-Torres, M., Gutie´rrez-Bouza´n, C., Crespi, M., 2010. Combination of coagulation-flocculation and nanofiltration techniques for dye removal and water reuse in textile effluents. Desalination 252, 53–59. Rocha, E.M.R., Vilar, V.J.P., Fonseca, A., Saraiva, I., Boaventura, R.A.R., 2011. Landfill leachate treatment by solar-driven AOPs. Sol. Energy 85, 46–56. Rodriguez, M., Sarria, V., Esplugas, S., Pulgarin, C., 2002. Photo-Fenton treatment of a biorecalcitrant wastewater generated in textile activities: biodegradability of the photo-treated solution. J. Photochem. Photobiol., A 151, 129–135. Sarria, V., Parra, S., Adler, N., Pe´ringer, P., Benitez, N., Pulgarin, C., 2002. Recent developments in the coupling of photoassisted and aerobic biological processes for the treatment of biorecalcitrant compounds. Catal. Today 76, 301–315. Sirianuntapiboon, S., Sadahiro, O., Salee, P., 2007. Some properties of a granular activated carbon-sequencing batch reactor (GAC-SBR) system for treatment of textile wastewater containing direct dyes. J. Environ. Manage. 85, 162–170. So´jka-Ledakowicz, J., Koprowski, T., Machnowski, W., Knudsen, H.H., 1998. Membrane filtration of textile dyehouse wastewater for technological water reuse. Desalination 119, 1–9. Suksaroj, C., He´ran, M., Alle`gre, C., Persin, F., 2005. Treatment of textile plant effluent by nanofiltration and/or reverse osmosis for water reuse. Desalination 178, 333–341. Szygula, A., Guibal, E., Palacı´n, M.A., Ruiz, M., Sastre, A.M., 2009. Removal of an anionic dye (Acid Blue 92) by coagulation-flocculation using chitosan. J. Environ. Manage. 90, 2979–2986. Torrades, F., Garcı´a-Montan˜o, J., Antonio Garcı´a-Hortal, J., Dome`nech, X., Peral, J., 2004. Decolorization and mineralization of commercial reactive dyes under solar light assisted photo-Fenton conditions. Sol. Energy 77, 573–581.