Application of Fe-doped TiO2 specimens for the solar photocatalytic degradation of humic acid

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Application of Fe-doped TiO2 specimens for the solar photocatalytic degradation of humic acid Nazmiye Cemre Birben a , Ceyda Senem Uyguner-Demirel a,∗ , Sibel Sen Kavurmaci a , Yelda Yalc¸ın Gürkan b , Nazli Turkten c , Zekiye Cinar c , Miray Bekbolet a a

Institute of Environmental Sciences, Bogazici University, Bebek, Istanbul 34342, Turkey Department of Chemistry, Namik Kemal University, 59030, Tekirdag, Turkey c Department of Chemistry, Yildiz Technical University, 34220, Istanbul, Turkey b

a r t i c l e

i n f o

Article history: Received 29 February 2016 Received in revised form 7 June 2016 Accepted 9 June 2016 Available online xxx Keywords: EEM Fe-doped TiO2 Humic acid Molecular size distribution Solar photocatalytic degradation

a b s t r a c t This study aimed to assess solar photocatalytic activity of Fe-doped TiO2 specimens for the degradation of natural organic matter (NOM) represented by a model humic acid (HA). Fe-doped TiO2 specimens (P-25 and UV-100) were prepared by a wet impregnation method. Humic acid (molecular size < 100 kDa) was characterized by UV–vis and fluorescence spectroscopic parameters and dissolved organic carbon (DOC) content both prior to and following solar photocatalysis. Molecular size fractionation data revealed a shift to lower molecular size fractions i.e.: 30 kDa, 10 kDa and 3 kDa being more pronounced in case of Fe-doped UV-100 in comparison to Fe-doped P-25. Excitation-emission matrix (EEM) fluorescence features of indicated humic molecular size fractions did not illustrate any significant difference related to the formation of new fluorophoric regions but displayed the shift from humic-like fluorophores to fulvic-like fluorophores even in 3 kDa fraction indicating that humic acid followed a photocatalyst type independent pathway via solar photocatalysis. Slower DOC removal rates were attained for Fe-doped TiO2 specimens with respect to bare specimens indicating the importance of substrate properties rather than the inefficiency of visible light activation by metal doping of photocatalyst. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Photocatalytic oxidation has been successfully applied as an alternative treatment process for the destruction of pollutants in aqueous systems. Its major advantage is its effectiveness for the complete mineralization of the target compound as achieved by the non-selective oxidation mechanism of hydroxyl radicals. TiO2 has been proven to be the most powerful photocatalyst so far [1,2]. Photocatalytic reactions are performed under ultraviolet (UV) light due to the band gap energy of TiO2 (Ebg = 3.0 or 3.2 eV in rutile or anatase phase), which is a significant drawback in the practical application of TiO2 limiting the possibility of utilization of solar light (5% of solar spectrum). The major disadvantage of UV photocatalysis could be overcome through chemical and morphological modifications of the photocatalyst to enable the light absorption in the visible region. Over the past years, most of the research has been devoted to the synthesis and characterization of second gen-

∗ Corresponding author. E-mail addresses: [email protected], [email protected] (C.S. Uyguner-Demirel).

eration photocatalysts [3–5]. Among many catalyst improvement techniques, doping has been shown to be one of the most promising options. Most of the recent studies were devoted to the application of non-metal doping (e.g. C, N, S, F or co-doping) and metal deposition (e.g. Fe, Cr, V) [6–8]. Metal ion doping improves trapping of the photo excited conduction band electrons at the surface while minimizing charge carrier recombination and leads to an increased rate in the formation of OH radicals [9]. For this purpose, in recent years, the technique of metal ion doping into TiO2 has been widely studied [10–13]. Among a variety of transition metal ion dopants investigated, Fe3+ has been found to be a good candidate due to its similar radius (0.69 Å) to that of Ti4+ (0.75 Å) [14]. Due to the energy level of Fe2+ /Fe3+ which is close to that of Ti3+ /Ti4+ , the separation of photogenerated electron-hole pair is favored ending up with improved quantum efficiency as given in Eqs. (1)–(5) [15–17]. Fe3+ + e− → Fe2+

(1)

Fe2+ + O2(ads) → Fe3+ + O2 −

(2)

Fe2+ + Ti4+ → Fe3+ + Ti3+

(3)

http://dx.doi.org/10.1016/j.cattod.2016.06.020 0920-5861/© 2016 Elsevier B.V. All rights reserved.

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Fe3+ + hvb + → Fe4+

(4)

Fe2+ + hvb + → Fe3+

(5)

Fe-doped TiO2 specimens could be prepared by using various preparation methods. Almost all of the prepared Fe-doped TiO2 specimens expressed band gap energy lowering in the range of 2.45–3.11 eV with a red shift in wavelength region of incident light leading to successful utilization of solar energy [18]. Formation of extrinsic energy levels originating from 3d electrons of Fe3+ is another reason for visible light activity [14,19]. The use of both UV (␭ = 365 nm) and vis light (␭ > 380 nm) sources were reported in photocatalytic degradation experiments. Generally, activity testing studies of Fe-doped TiO2 specimens were performed using dyes as model coumpounds (Supplementary information, Table S1). Moreover, aromatic compounds as phenol and chlorinated phenol derivatives, 4-nitrophenol, benzene, and nitrobenzene, aliphatic compounds as dichloromethane, 1,2-dichloroethane, formic acid and acetic acid were also used as substrates. More specifically, 17␤-estradiol was investigated under visible light using a solar simulator [20]. However, all of these model compounds are in the molecular weight range of 46 g mol−1 to 272 g mol−1 . Considering the molecular size of humic acids being mainly < 0.45 ␮m and displaying molecular weight distribution profiles ranging from 500 Da to 100 kDa, the organic compounds provided in the Supplementary information, Table S1 could not be regarded as representative models of humic acids. Literature survey reveals very few studies on the application of Fe-doped TiO2 specimens for the degradation of neither NOM nor humic acids [21,22]. In a recent paper by Yuan and colleagues the removal of HA by Al and Fe co-doped TiO2 nanotubes under UV light and preapplied O3 was investigated whereas, removal of HA was not clearly explained in the absence of O3 [21]. Following this study, the photocatalytic degradation of HA (10 mg L−1 ) by Fe-TiO2 supported on spherical activated carbon (SAC) in a fluidized bed photoreactor was reported where humic acid characterization was based on per cent removal of chemical oxygen demand (COD) [22]. Considering that HA is a complex aromatic macromolecule difficult to degrade, chemical oxidation via COD would not be successfully accomplished and erroneously measured. It should be indicated that low range COD measurements are highly non reproducible especially in the case of HA displaying very complex aromatic macromolecules. In that respect, there is still a definite lack of information on the degradation of HA using doped TiO2 . Bekbolet and colleagues directed their attention to the application of photocatalysis in natural waters mainly to the removal of the major components as natural organic matter [23–26]. For that purpose, TiO2 photocatalytic removal of NOM, namely HA and fulvic acids have been studied using a continuously stirred, bench scale, UV (␭max = 360 nm) initiated system [24]. The retardation and competition effects on the TiO2 /UV system in the presence of different types of humic and fulvic acids, TiO2 brands, various metal ions, common cations, anions, oxyanions (OCl− , ClO2 − , ClO3 − ), oxidizing species (H2 O2 ) and inhibitory effects due to alkalinity have also been revealed to understand the limitations of the photocatalytic system [25]. Moreover, further interest was shown in the molecular size distribution of HAs and their degradation profiles by photocatalytic oxidation [26]. Due to the complexity and diverse molecular size distribution of NOM (humic acids), molecular size fractionation profiles both prior to and following photocatalysis were also investigated. Referring to the above mentioned visible light active photocatalyst applications, this study aimed to fulfill the gap lacking in literature on solar photocatalytic removal of NOM by Fe-doped photocatalysts. For that purpose the following points were considered.

i Fe-doped TiO2 application on NOM removal using model compounds (e.g. HA) and utilizing solar radiation. ii To eluciate the effect of different types of TiO2 , i.e. P-25 and UV100. iii To understand compositional variations/changes in humic matrix during degradation by molecular size fractionation. iv To elucidate the humic matrix by spectroscopic analysis, UVvis, fluorescence and further by advanced techniques, i.e. three dimensional fluorescence spectroscopy in the form of excitation–emission matrix (EEM). v To compare photocatalytic removal efficiencies of HA in the presence of bare and doped TiO2 .

With this respect, photocatalytic performances of Fe-doped Evonik P-25 and Hombikat UV-100 TiO2 were investigated under simulated solar light for the degradation of HA as a representative of complex organic matrix. As the major molecular size fraction of humic acid, 100 kDa fraction was subjected to solar photocatalytic oxidation process [26]. Molecular size fractionation and degradation profiles were comparatively evaluated in terms of UV–vis and fluorescence parameters. To understand the structural and functional properties of humic substances and to determine the relationships during and after photocatalytic degradation, EEM contour plots were evaluated.

2. Materials and methods 2.1. Materials Commercial humic acid was supplied from Aldrich (humic acid sodium salt). A stock HA solution (50 mg L−1 ) was prepared in deionized water and prefiltered through 0.45 ␮m cellulose acetate membrane filters. Subsequent ultrafiltration (Amicon 8050 stirred cell unit) was applied to prepare humic molecular size fraction of 100 kDa to be used as substrate for the photocatalytic degradation reactions. Titanium dioxide Evonik P-25 (80% anatase and 20% rutile) and Hombikat UV-100 (100% anatase) were used as photocatalysts for the preparation of Fe-doped specimens.

2.2. Preparation and characterization of Fe-doped TiO2 specimens Fe-doped TiO2 specimens (P-25 and UV-100) were prepared by an incipient wet impregnation method using Fe(NO3 )3 ·9H2 O (Merck) as the Fe3+ source. For the preparation of 0.25% Fe-doped TiO2 , 8 g TiO2 and an appropriate amount of Fe(NO3 )3 ·9H2 O were mixed with definite volumes of doubly distilled water and stirred for 1 h. Then, the prepared photocatalysts were washed with water and centrifugally separated three times, heat-treated at 378 K for 24 h to eliminate water, calcined at 623 K for 3 h, ground and sieved [14]. This dose was selected according to results reported therein for 4-nitrophenol degradation. Surface morphologies were assessed by Scanning Electron Microscope (SEM) in combination with Energy Dispersive X-ray analysis (ESEM-FEG/EDX Philips XL30) operating at 20 kV using sample powders supported on carbon tape. The nitrogen adsorption/desorption isotherms were obtained at liquid nitrogen temperature 77 K (Quantochrome Nova 2200e automated gas adsorption system). All the samples were vacuumdegassed at 105 ◦ C overnight prior to measurements. Specific surface area was determined by using multi-point BrunauerEmmett-Teller (BET) analysis. pH dependent zeta potential change was determined by Nano/Zetasizer (ZS90, Malvern Instruments Ltd.) for the calculation of isoelectric point (pHiep ).

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Table 1 Properties of bare and Fe-doped TiO2 specimens. TiO2 specimens

Ebg , eVa

␭, nma

pHzpc

Crystallite size, nma

BET m2 g−1

Bare P-25 Bare UV-100 Fe-doped P-25 Fe-doped UV-100

3.01 3.20 2.55 2.43

411 387 486 509

6.25 6.30 4.80 5.10

22.3 16.6 16.4 17.1

∼55 >250 33 67

a

[14].

2.3. Experimental procedure and methodology Solar photocatalytic degradation experiments were carried out in an Atlas-Suntest CPS+ solar simulator. The light source was an air cooled Xenon lamp (wavelength range, ␭ = 300 to 800 nm and light intensity, I = 250 W m−2 ). The solar simulator was equipped with quartz and UV special glass filters to simulate typical outdoor sunlight irradiation time period (0–120 min with 10 min intervals). 100 kDa humic fraction was photocatalytically treated using 0.25 mg mL−1 doped and bare TiO2 specimens with respect to irradiation time. pH was followed throughout the photocatalytic experimental time periods. Preliminary photocatalytic experiments were carried out according to the below given sequence; i. humic acid was subjected to irradiation in the absence of photocatalyst for time periods of 0–120 min for the assesment of photolytic degradation, ii. initial adsorption of HA onto bare and doped TiO2 specimens were determined by preparing the slurry composed of HA and TiO2 followed by immediate filtration through 0.45 ␮m membrane filters, iii. photocatalytic experiments were performed by using bare TiO2 specimens. Moreover, doped TiO2 specimens in deionized water were also exposed to irradiation to check the stability and release of Fe species. Under all conditions following treatment, immediate filtration through 0.45 ␮m membrane filters was applied for the removal of TiO2 from the solution matrix. Furthermore, photocatalytically treated humic acid samples (60 min) were subsequently fractionated into nominal molecular size fractions of 30 kDa, 10 kDa and 3 kDa. Dissolved organic carbon (DOC) content of the samples was determined by a Total Organic Carbon Analyzer (Shimadzu TOCVWP). Humic acid was analyzed by UV–vis (Perkin Elmer Lambda 35) and fluorescence spectroscopy (Perkin Elmer LS 55). Specified UV–vis parameters were described in terms of selected absorbance values measured at 436 nm (Color436 ), 365 nm (UV365 ), 280 nm (UV280 ) and 254 nm (UV254 ). In order to acquire EEM fluorescence spectra, excitation wavelengths were incrementally increased from 200 to 500 nm at 10 nm steps; for each excitation wavelength, the emission at longer wavelengths was detected at 0.5 nm steps. Excitation and emission slit widths were set to 10 nm and photomultiplier tube voltage was 900 V. Three-dimensional contour plots were created by plotting fluorescence intensity as a function of emission (x-axis) and excitation (y-axis) wavelengths. Matlab R2013a was used to process EEM data. Fe concentration was measured by ICP-OES (Perkin Elmer Optima 2100DV).

3. Results and discussion 3.1. Characteristics of bare and Fe-doped TiO2 Properties of bare and Fe-doped TiO2 specimens were presented in Table 1. Fe doping caused a reduction in Ebg of both bare P25 (Ebg = 3.01 eV) and UV-100 TiO2 (Ebg = 3.20 eV) specimens to 2.55 eV and 2.43 eV, respectively. Narrowing in band-gap values was explained by the interaction between valence band electrons and localized d electrons of Fe3+ , but the main reason for visible

light activity is the formation of additional electronic states originating from 3d electrons of Fe3+ in the band gap [14]. Absorption wavelength values shifted from UV range to visible range as a result of Fe doping indicating possibility of solar light activity. As shown by the scheme given in reference [27], the excitation of 3d electrons from ferric ions to the conduction band of TiO2 is the origin of the red shift [14,27]. Properties of bare and Fe-doped TiO2 specimens were presented in Table 1. Using the DRS spectra of the Fe-doped samples presented by Yalcin et al. [14], the band gap energies of the doped photocatalyst samples were calculated through the use of the Kubelka-Munk formula; F(R) =

(1 − R)2 2R

(6)

where R is the reflectance read from the spectrum. Using the Tauc equation by plotting [F(R).h␯]n vs h␯, where h␯ is the photon energy and n = 1/2, the band gap energies were deduced from the intersection of the Tauc’s linear portion extrapolation with the photon energy axis. The results indicated that Fe doping caused a reduction in Ebg of both bare P-25 (Ebg = 3.01 eV) and UV-100 TiO2 (Ebg = 3.20 eV) specimens to 2.55 eV and 2.43 eV, respectively. Narrowing in band gap values were explained by the interaction between valence band electrons and localized d electrons of ferric ion. The tailing and the diffused character of the DRS spectrum indicated the presence of midgap levels originating from 3d electrons of Fe3+ in the band gap. Fe3+ doping of P-25 ended up with smaller crystallite size than bare TiO2 as 16.4 nm and 22.3 nm, respectively [14]. On the other hand, UV-100 specimen displayed almost similar size 16.6 nm for bare UV-100 and 17.1 nm for doped UV-100. A significant reduction in BET surface area was observed for both doped P-25 and UV-100 being more pronounced for Fe-doped UV-100 compared to bare ones which could be attributed to a possible agglomeration of Fedoped TiO2 specimens. pHiep conditions displayed a shift to acidic region being more pronounced for P-25 (pHiep = 4.80) than UV-100 (pHiep = 5.10). The pH change would directly affect the negatively and positively charged site distribution profile on the surface of both of the TiO2 specimens. The resultant effect would possibly lead to diverse electrostatic interactions between the oppositely charged surface sites and pH dependent deprotonated humic functional groups displaying carboxylic (pKa = 3–5) and for phenolic groups (pKa = 7–9) [28]. Surface morphologies of bare and Fe-doped TiO2 specimens were evaluated by SEM images with EDX analysis as presented in Fig. 1. EDX analysis designated the presence of Fe in accordance with doping. Surface morphologies were distinctly different from each other, all for P-25, UV-100 and their respective Fe-doped TiO2 specimens [14]. 3.2. Kinetic considerations The specified UV–vis parameters and DOC content of 100 kDa HA were determined as: Color436 = 0.107; UV365 = 0.227; UV280 = 0.547; UV254 = 0.640 and DOC = 7.294 mg L−1 . It was previously reported that upon photocatalytic treatment, degradation profiles of humic

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Fig. 1. SEM images of bare and Fe-doped TiO2 specimens. Table 2 Pseudo first-order kinetic model parameters. k × 10−2 , min−1

R, m−1 min−1

t1/2 , min

Color436 Bare P-25 Bare UV-100 Fe-doped P-25 Fe-doped UV-100

5.20 3.45 1.49 1.55

0.551 0.366 0.159 0.166

13.3 20.1 46.5 44.7

UV365 Bare P-25 Bare UV-100 Fe-doped P-25 Fe-doped UV-100

5.90 3.88 1.34 1.43

1.316 0.865 0.304 0.325

11.7 17.9 51.7 48.5

UV280 Bare P-25 Bare UV-100 Fe-doped P-25 Fe-doped UV-100

6.10 3.96 1.23 1.29

3.306 2.146 0.673 0.706

11.4 17.5 56.3 53.7

UV254 Bare P-25 Bare UV-100 Fe-doped P-25 Fe-doped UV-100

5.90 3.89 1.14 1.22

3.758 2.478 0.730 0.781

11.7 17.8 60.8 56.8

DOC Bare P-25 Bare UV-100 Fe-doped P-25 Fe-doped UV-100

k × 10−2 , min−1 2.64 2.58 0.605 0.844

R, mg L−1 min−1 0.188 0.183 0.044 0.062

t1/2 , min 26.3 26.9 114.5 82.1

parameters as a function of irradiation time successfully follow an exponential decay [29]. Therefore, data were modeled by pseudo first-order reaction kinetics (R2 > 0.80) revealing the following rate constants (k), reaction rates (R) and half-life (t1/2 ) values (Table 2). As presented previously, UVA light initiated (␭max = 360 nm and I = 5.6 mW cm−2 ) photocatalytic removal trend of UV–vis parameters of humic matter displayed a decreasing order of Color436 > UV365 > UV280 > UV254 [24]. A similar trend could not be assessed for Fe-doped TiO2 specimens under solar light simulated conditions. Moreover, almost all of the UV–vis parameters expressed comparable pseudo-first order rate constants

(k, min−1 ). Based on information in Table 2, the average rate constants (k, min−1 ) for the specified UV–vis parameters were acquired as 5.78 × 10−2 ± 0.39 × 10−2 for bare P-25 and 3.80 × 10−2 ± 0.23 × 10−2 for bare UV-100. Almost similar mineralization rates in terms of DOC removal (R, mg L−1 min−1 ) were determined as R = 0.188 and R = 0.183 for bare P-25 and UV-100, respectively. However, in the presence of Fe-doped P-25 and UV100, slower humic acid mineralization rates were assessed as R = 0.044 mg L−1 min−1 and R = 0.062 mg L−1 min−1 , respectively. Under similar experimental conditions, the use of Fe-doped TiO2 for the photocatalytic removal of a simple model compound, 4nitrophenol, enhanced the degradation rate compared to undoped TiO2 [14]. However, in the presence of a complex macromolecule i.e. HA contrary results were achieved. The use of doped TiO2 specimens resulted in retardation of the photocatalytic removal of the specified UV–vis parameters in comparison to bare ones. The differences between the rate constants of bare specimens could not be observed in the presence of Fe-doped specimens. The reason could be attributed to the possible interactions occuring between Fe3+ and HA molecular size fractions through surface complexation mechanism prevailing in the reaction medium. As determined by DOC measurements, initial adsorption of HA onto bare TiO2 P-25 was 33% whereas it was 19% for UV-100 contrary to available surface area of P-25 (55 m2 g−1 ) and UV-100 (250 m2 g−1 ). Fe doping significantly altered the initial adsorptive interactions as 45.6% for doped P-25 (33 m2 g−1 ) and 52.2% for doped UV-100 (67 m2 g−1 ). Considering extensive surface coverage (>45%) of humic fractions on doped TiO2 , light absorption capacity could be diminished possibly leading to a retardation effect. Contrary to our findings, removal efficiencies for 10 mg L−1 HA were reported to increase when Fe3+ ions were doped into the TiO2 nanotubes and subjected to O3 prior to degradation [21]. On the other hand, Baek et al. [20] reported low photocatalytic activity for the removal of HA using Fe-TiO2 /SAC under UVC light (100–280 nm). Changing Fe contents (0.4–0.8 wt%) and the light source to UVA (315–400 nm) resulted in higher photocatalytic activity. However, no detailed information was provided about the adsorption properties of HA under the defined experimental conditions. Considering the high specific

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surface area of Fe-TiO2 /SAC in the range of 483–584 m2 g−1 , direct comparison of results would not be possible. Therefore, taking into account various experimental conditions such as HA type, photocatalyst loading, light source and intensity, photoreactor type, doping method and dopant concentration, differences in removal rates could be expected. Related to these findings, comparative assessment of the removal efficiencies of HA under controlled solar photocatalytic experimental conditions using Fe-doped TiO2 specimens would be leading to erroneous conclusions. Due to the strong multi-site complexation capacity of iron with humic fractions, leaching and stability of iron species were checked by performing similar photocatalytic experiments in deionized water. The control reactions under irradiation periods up to 60 min in deionized water revealed that in the sole presence of doped P-25, released iron species concentration was 0.013 mg L−1 . In the presence of HA, almost five-fold increase in leaching of iron species was attained as 0.070 mg L−1 . On the other hand, in control reactions of doped UV-100, leaching of iron species was 0.073 mg L−1 however, after 60 min of irradiation period, significant iron leaching (0.116 mg L−1 ) was attained in the presence of HA. The reason could be attributed to the strong chelation effect of humic sub-fractions leading to release of iron to the aqueous medium. Formation of Fe (III)-HA complex in neutral aqueous medium under solar irradiation leads to several reactions (Eqs. (7)–(15)) as follows [19,30]: HA + h␯ → HA∗

(7)

HA ∗ + O2 → Products + O2 O2

•−

•−

/HO2 •

(8)

/HO2 → H2 O2

(9)

Fe(III) + HA → Fe(III)-HA Fe(III)-HA + h␯ → HA

•+

(10)

+ Fe(II)

(11)

Fe(III)-HA + h+ → Fe(IV)-HA → HA∗+ + Fe(III) Fe(II) + O2 → Fe(III) + O2 Fe(II) + H2 O2 → Fe(III) +

Fig. 2. A. Specified UV–vis parameters, DOC and FI; B. specific UV–vis parameters and SFIsyn of humic acid and its molecular size fractions treated by both of the Fedoped TiO2 specimens.

•−

/HO2 •

• OH

+ OH

(12) (13)

−

HA + OH → Photocatalyticdegradationproducts

(14) (15)

The role of these photo initiated reactions on the removal efficiencies could possibly be attributed to the release of iron being more pronounced for doped UV-100 in comparison to doped P25. It should also be mentioned that the released amount of iron in aqueous medium is far below the naturally present concentrations. Therefore, the major cause of the slower rates could be due to the outer sphere/inner sphere surface complexation rather than the reactions taking place in aqueous medium. Furthermore, photo sensitization via visible light absorption leading to the formation of reactive oxygen species could also initiate self-degradation of HA. However, no direct photolysis of HA was observed under the studied experimental conditions (≤5%). The faster removal rates attained by bare TiO2 specimens in comparison to Fe-doped specimens could be related to the complex substrate properties of humic acid and its interactions with the oxide surface rather than the inefficiency achieved in the presence of a visible light active photocatalyst. 3.3. Spectroscopic evaluation of solar photocatalytic degradation Upon irradiation period of 60 min at which approximately 50% UV254 removal was attained, the samples were subjected to molecular size fractionation in a decreasing sequence of 30 kDa, 10 kDa and 3 kDa. For each fraction, specified UV–vis parameters, DOC, specific UV–vis absorbance, (SCoA and SUVA254 as DOC normalized

Color436 or UV254 , m−1 mg−1 L) and specific fluorescence intensity (SFIsyn as DOC normalized fluorescence intensity measured at ␭exc = 350 nm and ␭emis = 450 nm) were determined. Moreover, a fluorescence-derived index, (FI) defined as the ratio of the emission intensity at ␭emis 450 nm to that at ␭emis 500 nm following excitation at ␭exc 370 nm was also presented in Fig. 2A and B [31]. Specified UV–vis parameters and DOC contents expressed a decreasing profile with respect to decreasing molecular size contrary to the increasing trend of FI irrespective of the type of TiO2 (Fig. 2 A). An inverse relationship was elucidated between SFI and aromaticity of humic matter (SUVA254 ) as well as SCoA (Fig. 2B). A contrary trend in specific UV–vis parameters was observed for the lowest molecular size fraction (3 kDa) of humic acid treated by both of the Fe-doped TiO2 specimens. EEM contour plots were addressed as a useful tool for further assessment of humic properties [32]. Depending on the excitationemission wavelength regions, five regions were ascribed: i. Region I: Aromatic Proteins I (␭exc 220-250 and ␭emis 280-332), ii. Region II: Aromatic Proteins II (␭exc 220-250 and ␭emis 332-380), iii. Region III: Fulvic-like (␭exc 220-250 and ␭emis 380-580), iv. Region IV: Microbial byproducts (␭exc 250-470 and ␭emis 280-380), and v. Region V: Humic-like (␭exc 250-470 and ␭emis 380-580). Regional speciation of the EEM fluorescence spectral features of initial HA (Fig. 3AA–AD) and HA after solar photocatalytic oxidation conditions using Fedoped P-25 (Fig. 3BA–BD) and Fe-doped UV-100 (Fig. 3CA–CD) were illustrated. Fig. 3AA–AD displayed EEM contour plots of untreated humic acid and respective molecular size fractions with a decreasing order in intensities of both humic-like (Region V) and fulvic-like regions (Region III). Upon photocatalytic treatment, EEM contour plots displayed the successive removal of both humic and fulviclike fluorophores for all of the molecular size fractions of HA (Fig. 3BA–BD and CA–CD). Almost insignificant presence of Regions I, II and IV were evident for all of the HA samples. Emergence of

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Fig. 3. EEM contour-plots of A. humic acid untreated and its molecular size fractions (AA: 100 kDa; AB:30 kDa fraction, AC:10 kDa fraction and AD:3 kDa fraction); B. humic acid treated using Fe-doped P-25 and its molecular size fractions (BA: 100 kDa; BB:30 kDa fraction, BC:10 kDa fraction and BD:3 kDa fraction); C. humic acid treated using Fe-doped UV-100 and its molecular size fractions (CA: 100 kDa; CB:30 kDa fraction, CC:10 kDa fraction and CD:3 kDa fraction).

fulvic-like (Region III) and disappearance of humic-like (Region V) fluorophores could be noted for treated HA in accordance with the shift to lower molecular size fractions. However, these regions were still present for 30 kDa and 10 kDa fractions. The absence of Region V and the presence of fulvic-like fluorophores (Region III) were evident in the lowest fraction <3 kDa which expressed the highest FI (Fig. 2A). Due to the photocatalytic removal of DOC, 3 kDa fraction underwent reactions acting simultaneously in various pathways such as consecutive mineralization and/or transformation from higher molecular size fractions. It should be emphasized that no significant difference could be observed in the presence of different types of Fe-doped TiO2 . 4. Conclusion Fe-doped TiO2 specimens were evaluated in terms of their solar photocatalytic activity for the degradation of 100 kDa molecular size fraction of HA. Degradation efficiency was presented by pseudo first-order kinetic parameters, and evaluated by both UV–vis and fluorescence spectral parameters more specifically by EEM contour plots. A retardation effect on the photocatalytic removal of the specified UV–vis parameters and DOC were observed for doped TiO2 specimens compared to bare TiO2 specimens which could be attributed to the surface complexation reactions rather than the reactions taking place in aqueous medium. The faster removal rates attained by using bare TiO2 specimens could be regarded as substrate specific rather than being related to the inefficiency of the visible light activated catalytic performance. EEM fluorescence

features showed no significant role of specimen type on the transformation from humic-like to fulvic-like macromolecules via solar photocatalytic degradation process. Successful use of visible light activated photocatalysts strongly depends on the activity testing using model compounds and stability tests (Suppl. A). As observed in this study, leaching of iron in solution poses a significant drawback in the presence of strong ligands i.e. humic acids. Since in natural waters, NOM is the major source of complex organics and various inorganic and organic species constitute the water matrix, the efficiency of Fe-doped specimens should be carefully assessed. Therefore, covering all these drawbacks, applications in drinking water treatment require detailed investigation. Acknowledgement The financial support by Research Fund of Bogazici University (Projects No: 6729 and 6750) is gratefully acknowledged. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cattod.2016.06. 020. References [1] A.L. Linsebigler, G. Lu, J.T. Yates Jr., Chem. Rev. 95 (3) (1995) 735–758.

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Please cite this article in press as: N.C. Birben, et al., Application of Fe-doped TiO2 specimens for the solar photocatalytic degradation of humic acid, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.06.020