TiO2 systems revealed by fluorescence EEM-PARAFAC

Water Research 87 (2015) 119e126

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Insight into photocatalytic degradation of dissolved organic matter in UVA/TiO2 systems revealed by fluorescence EEM-PARAFAC Diep Dinh Phong a, b, Jin Hur a, * a b

Department of Environment and Energy, Sejong University, Seoul, 143-747, South Korea Vietnam Academy of Science and Technology, Hanoi, Vietnam

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 April 2015 Received in revised form 21 August 2015 Accepted 10 September 2015 Available online 14 September 2015

Photocatalytic degradation of dissolved organic matter (DOM) using TiO2 as a catalyst and UVA as a light source was examined under various experimental settings with different TiO2 doses, solution pH, and the light intensities. The changes in UV absorbance and fluorescence with the irradiation time followed a pseudo-first order model much better than those of dissolved organic carbon. In general, the degradation rates were increased by higher TiO2 doses and light intensities. However, the exact photocatalytic responses of DOM to the irradiation were affected by many other factors such as aggregation of TiO2, light scattering, hydroxyl radicals produced, and DOM sorption on TiO2. Fluorescence excitation-emission matrix (EEM) coupled with parallel factor analysis (PARAFAC) revealed that the DOM changes in fluorescence could be described by the combinations of four dissimilar components including one proteinlike, two humic-like, and one terrestrial humic-like components, each of which followed well the pseudo-first order model. The photocatalytic degradation rates were higher for protein-like versus humic-like component, whereas the opposite order was displayed for the degradation rates in the absence of TiO2, suggesting different dominant mechanisms operating between the systems with and without TiO2. Our results based on EEM-PARAFAC provided new insights into the underlying mechanisms associated with the photocatalytic degradation of DOM as well as the potential environmental impact of the treated water. This study demonstrated a successful application of EEM-PARAFAC for photocatalytic systems via directly comparing the kinetic rates of the individual DOM components with different compositions. © 2015 Elsevier Ltd. All rights reserved.

Keywords: EEM-PARAFAC Photocatalytic degradation Titanium dioxide Hydroxyl radicals Adsorption

1. Introduction Dissolved organic matter (DOM) is a heterogeneous mixture of organic compounds with different molecular sizes and functional groups, ubiquitous in natural waters (Thurman, 1985). DOM has been reported to be the precursor of disinfection by-products (Drikas et al., 2003) and membrane foulants (Jermann et al., 2007). A portion of DOM remaining and/or transformed after drinking water treatment can promote the re-growth of bacteria and the formation of biofilm, facilitating corrosion in distribution systems (Volk and Lechevallier, 2002). Because of the major concerns proposed, a great deal of effort has been made to remove DOM in water treatments based on adsorption, coagulation, membrane filtration, ion-exchange,

* Corresponding author. E-mail address: [email protected] (J. Hur). http://dx.doi.org/10.1016/j.watres.2015.09.019 0043-1354/© 2015 Elsevier Ltd. All rights reserved.

advanced oxidation process, and biofiltration (Ivancev-Tumbas, 2014). Among the treatments studied and operated, UV/TiO2, a heterogeneous photocatalytic process, has proven to be a promising technique in eliminating DOM from natural water owing to its superior advantages of high removal efficiency, recyclable and chemically stable catalyst, being chemical-free, and its lack of sludge production (Fu et al., 2006; Chong et al., 2010). In recent many studies of DOM photodegradation, dissolved organic carbon (DOC) and/or ultraviolet/visible spectra have been used as proxies for evaluating the removal efficiency (Liu et al., 2010; Mori et al., 2013; Rajca and Bodzek, 2013). However, these parameters are inadequate in fully understanding the behaviors of the highly heterogeneous DOM. Fluorescence spectroscopy is a non-destructive, reliable, and highly sensitive optical technique for fast probing of DOM in natural and engineered systems (Henderson et al., 2009; Fellman et al., 2010). There were many reports using fluorescence spectroscopy to understand photo-oxidation of DOM under either artificial or natural light (Del Vecchio and Blough,

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2002; Mostofa et al., 2007; Hur et al., 2011). For example, Hur et al. (2011) applied synchronous fluorescence spectroscopy for noncatalytic photodegradation of DOM under UVA irradiation and found that fulvic- and humic-like fluorescence were initially degraded to a greater extent than protein-like fluorescence. In contrast, only limited studies are available to explore the applicability of fluorescence spectroscopy into photocatalytic degradation of DOM. For example, Cho and Choi (2002) reported the reduction in the synchronous fluorescence spectra of DOM at an excitation wavelength of 350 nm in TiO2 mixed solutions under visible light irradiation. More recent studies employed fluorescence peaks from excitation-emission matrices (EEMs), which comprise fluorescence signals at several thousand pairs of excitation/emission wavelengths (Ex/Em), as a monitoring tool for photocatalytic oxidation of DOM based on UV/TiO2 systems (Valencia et al., 2013a, 2014; Sen Kavurmaci and Bekbolet, 2014). However, these studies evaluated the degradation trend of DOM simply by peaking fluorescence peak intensities, neglecting the potential bias arising from the spectral overlaps from a complex mixture of different fluorescence components. Parallel factor analysis (PARAFAC), a statistical tool to decompose a data set of EEMs into different individual components representing the unique fluorescence features, has been widely applied in tracking the behaviors of different DOM components in natural/engineered environments (Ishii and Boyer, 2012; Murphy et al., 2008; Yang et al., 2015). The intensities and the relative abundance of the separate PARAFAC components, which reflect different DOM sources and structures, have been used as the surrogates for water quality, the treatability of DOM, and the treatment performance (Yang et al., 2015) in various water treatment processes. Despite its successful application in water treatment systems, no effort has been made to date on extending the applicability of the EEM-PARAFAC into the photocatalytic oxidation process of DOM. Utilizing EEM-PARAFAC for photocatalytic degradation of DOM may provide valuable and quantitative information on the individual degradation behaviors of different DOM components with their own environmental significance, avoiding the problem from overlapped peak intensities in EEM. For example, Protein-like and humic-like fluorescence components have been often used for the quantitative estimation of the bioavailability of treated water and the formation of disinfection by-products, respectively (Yang et al., 2015). In this work, we investigated the photocatalytic degradation of leaf litter-derived organic matter (LLOM), a representative terrestrial DOM source in drinking water supplies, using TiO2 as a photocatalyst and UVA as a light source. Pony lake fulvic acid (PLFA) and Elliott soil humic acid (ESHA) were also used as reference DOM sources. The objectives of this study were (1) to compare the changes in DOC, UV absorbance at 254 nm (UV254), and fluorescence of DOM during UVA irradiation under different experimental setups with varying TiO2 doses, pH, and light intensities, and (2) to track the individual behaviors of different fluorescent components of DOM based on EEM-PARAFAC in order to obtain further insight into catalytic photodegradation in UV/TiO2 systems. 2. Materials and methods 2.1. Reagents A type of TiO2 (AEROXIDE® P25) was purchased from SigmaeAldrich and was used as a catalyst for this study without further purification. The nanoparticle has a BET surface area of 50 ± 15 m2/g and an average size of 21 nm (supplied from the manufacturer). The point zero of charge (PZC) of the particle was reported at 6.2e6.9 (Dutta et al., 2004). The catalyst was mixed with distilled, deionized

water (DDW) to prepare a TiO2 slurry system before appropriate amounts of DOM stock solutions (~100 mg C/L) were added to achieve the desired HS concentrations. LLOM was prepared following the method described in Hur et al. (2011) with certain modifications. Briefly, fallen leaves were collected from the Han River basin in Korea and were air-dried, shredded, and mixed with DDW. LLOM was the water soluble extract after the solids were removed. PLFA and ESHA were obtained from the International Humic Substance Society (IHSS) and were dissolved in DDW. All DOM solutions were filtered through a pre-cleaned 0.45 mm pore size membrane filter (cellulose acetate, Advantec) prior to use. The starting DOM solutions were prepared with a concentration of 100 mg C/L. 2.2. Photocatalytic experiments Photocatalytic degradation of the DOM solutions was evaluated in a stirred cylindrical quartz reactor with a working volume of 1 L and an inner diameter of 95 mm. The reactor was placed in the middle of a closed cabinet equipped with twelve 8W-UVA lamps (Sankyo Denki, F8T5BL) inside as a light source, the wavelength of which was centered at 352 nm. The light intensity was measured at the center of the reactor with a UVA/B light meter (model 85009, Sper Scientific). In order to evaluate the effect of light intensities on DOM degradation, different numbers of the lamps (i.e., 6, 9, and 12 lamps) were alternatively turned on with all surrounding the reactor, which provide the intensities of 3680, 4670, and 6540 mW/ cm2, respectively. Otherwise, all the experiments were conducted under the irradiation of 12 lamps, or 6540 mW/cm2. The reactor temperature was maintained at 25 ± 3
C by circulating cooling air with fans. Batch experiments were also conducted with different TiO2 doses at a fixed DOM concentration (~10 mg C/ L). Before the irradiation, the DOM solutions were adjusted to desired pH values using 0.1 N HCl or NaOH solutions and stirred in the dark for 1 h to ensure the establishment of adsorptiondesorption equilibria between DOM and TiO2. An aliquot (20 mL) of the DOM solution was taken at appropriate time intervals during the 4-h irradiation (i.e., every 15 min for the first 90 min and every 30 min afterward). The adsorption of DOM onto TiO2 reached the equilibrium in dark condition in 10 min (DOC as an indicator), thus the adsorption time was set at 60 min for all the experiments. The samples were immediately filtered via a pre-washed 0.45 mm membrane filter to remove the catalyst, and the pH was re-adjusted to 7.0 before further analyses (Yang and Hur, 2014). 2.3. Analytical methods DOC concentrations were determined using a TOC analyzer (Shimadzu V-CPH) with the relative precision of <3% as determined based on repeated measurements. UVevisible spectra from 200 to 700 nm were recorded for the samples using a UVevisible spectrophotometer (Evolution 60, Thermo Scientific) with a 1-cm quartz cuvette. Fluorescence EEMs were measured with a luminescence spectrometer (LS-55, PerkineElmer) by scanning the emission spectra from 280 to 550 nm at 0.5 nm wavelength increments and stepping through the excitation wavelengths from 250 to 500 nm at 5 nm intervals. Excitation and emission slits were adjusted to 10 nm and 5 nm, respectively, and the scanning speed was set at 1200 nm min1. To limit second order Raleigh scattering, a 290 nm cutoff filter was used for all the measurements. The samples with the UV254 value exceeding 0.05 cm1 were diluted to avoid innerfilter correction (Yang and Hur, 2014). The background fluorescence EEM from a blank solution (DDW) was also taken into account. Fluorescence intensity was normalized using quinine sulfate

D.D. Phong, J. Hur / Water Research 87 (2015) 119e126

equivalent units (QSU), in which 1 QSU corresponds to the maximum fluorescence intensity for 1 mg L1 of quinine in 0.1 N H2SO4 at Ex/Em of 350/450 nm. Relative precisions of <2% were routinely obtained based on replicated UVevisible and fluorescence measurements. 2.4. PARAFAC modeling PARAFAC modeling was performed using MATLAB 7.1 (MathWorks, Natick, MA, USA) with the DOMFluor Toolbox (http://www. models.life.ku.dk). The details of the modeling are well described elsewhere (Stedmon and Bro, 2008; Kowalczuk et al., 2009). Splithalf analysis was used to validate the identified components and their number. The maximum fluorescence intensities (Fmax) of the individual components were used to represent their relative concentrations. The EEM data for the PARAFAC modeling were obtained based on 143 EEM data from 11 separate experiments. 3. Results and discussion 3.1. Changes in DOC during photodegradation Table 1 summarizes the major results on the responses of DOM to UVA irradiation. In the absence of TiO2, LLOM showed negligible changes in DOC at pH 7 after 4 h-irradiation (Exp. #1). This was consistent with previous similar studies, in which no change or minor removal of DOC was reported under the irradiation of UVA light (Liu et al., 2008; Portjanskaja et al., 2009) or solar simulated light (Valencia et al., 2013a). In our experiments using TiO2 as a photocatalyst, the total removal of DOC reached 73e90% depending on the experimental conditions and the DOM sources. A simple first order kinetic model has been widely used to describe the photodegradation of DOM in the previous literature (Uyguner and Bekbolet, 2010; Hur et al., 2011; Mori et al., 2013). For this study, all the observed trends in the DOC changes did not exactly follow the first-order kinetic model. For example, the experiments with the TiO2 doses from 0.1 g/L to 0.5 g/L for LLOM (i.e., Exp. #2, 3, and 4) and those using PLFA and ESHA fitted (i.e., Exp. #10 and 11) the model well (R2 ¼ 0.86e0.99), while the others did not (R2 ¼ 0.52e0.78). The low degree of the model fitting may be

121

attributed to the high release of DOC from the TiO2 surface (i.e., photodesorption), which corresponded to 11.7%e35.9% of the adsorbed DOC amounts (Table 1). Photodesorption was explained by the decomposition of high molecular adsorbed fractions into smaller compounds upon irradiation and the subsequent release from the TiO2 surface due to reduced binding affinity (Shahid et al., 2014). The effect may also account for the lag phase and/or the low reduction in DOC during the initial irradiation period (Fig. 1; Supplementary Information, Figs. S1eS3). DOM were not completely mineralized in DOC even after the 4 h-irradiation, indicating that a portion of DOM might be resistant and/or transformed into refractory products (Cho and Choi, 2002). 3.1.1. Effects of TiO2 loading Effects of TiO2 loading were examined through the experiments # 2, 3, 4, and 5 at pH 7 with 12 UV lamps (Table 1). As expected, a stepwise increase in the dose from 0.1 to 0.7 g/L enhanced the DOC adsorption by 19.3%e39.1%, presumably resulting from the increased surface area of TiO2. The total removals and the apparent degradation rates of DOC were accordingly increased with a higher TiO2 dose up to 0.5 g/L. For the maximum dose used in this study (i.e., 0.7 g/L), however, the total removal slightly decreased and the rate constant became much lower than that of 0.5 g/L. The decrease at the high TiO2 loading is probably due to light scattering (i.e., light attenuation) resulting from the increased turbidity and the subsequent reduction in the UVA light penetration and the photocatalyst efficiency (Fu et al., 2006; Chong et al., 2010). The measured turbidity in our study was 400, 1316, 2175, 2880 NTU for the TiO2 doses increasing from 0.1 g/L to 0.7 g/L. It was previously reported that the optimum loading of photocatalyst varied depending on the experimental setups and the DOM sources. The suggested optimum loadings of TiO2 were 0.4 g/L (Rajca and Bodzek, 2013), 0.5 g/L (Fu et al., 2006), or 0.6 g/L (Valencia et al., 2013a). 3.1.2. Effects of solution pH The effects of pH on the photodegradation of LLOM were evaluated at pH 4, 7, and 10 in the experiments with 0.5 g/L of TiO2 (i.e., the optimum loading) and 12 UV lamps (Table 1). The adsorption of DOC increased on the order of pH 4 > pH 7 > pH 10. The result can be explained by the PZC of TiO2 and the charge characteristic of

Table 1 Extent of adsorptiona, photodegradation, photodesorptionb, and total removal in percentages of HS and the first order degradation rate constants for different UVA irradiation systems on the basis of DOC and UV254. No. Experimental conditions DOC

UV254

TiO2 dose pH Number of Adsorption Photo(g/L) lamps (%) degradation (%) Leaf litter 1 0.0 2 0.1 3 0.3 4 0.5 5 0.7 6 0.5 7 0.5 8 0.5 9 0.5 Pony Lake 10 0.5 Elliott soil 11 0.5 a

Photodesorption (%)

Total removal (%)

Photo-degradation rate (min1)

Adsorption Photo(%) degradation (%)

Total removal (%)

Photo-degradation rate (min1)

12 12 12 12 12 12 12 6 9

e 19.3 27.8 34.5 39.1 41.3 19.2 34.0 33.8

e 53.2 53.7 54.5 48.8 48.5 43.8 38.3 44.7

e 6.5 5.5 2.1 13.5 11.7 31.2 35.9 18.4

e 72.5 81.5 89.1 87.9 89.9 63.0 72.3 78.4

e 0.0042 0.0044 0.0059 0.0042 0.0037 0.0024 0.0019 0.0028

0.0002 0.0006 0.0006 0.0009 0.0007 0.0005 0.0006 0.0006

16.9 24.4 28.9 29.5 39.4 15.3 28.9 25.3

3.0 79.1 73.2 69.7 68.6 59.4 79.3 68.4 71.5

e 95.9 97.6 98.6 98.1 98.7 94.7 97.4 96.8

e 0.0107 0.0157 0.0192 0.0155 0.0115 0.0088 0.0086 0.0100

7 12

24.6

63.6

3.0

88.3

0.0078 ± 0.0006

12.5

85.5

98.0

0.0243 ± 0.0006

7 12

34.6

46.1

0.4

80.7

0.0053 ± 0.0002

12.6

84.2

96.7

0.0130 ± 0.0002

7 7 7 7 7 4 10 7 7

± ± ± ± ± ± ± ±

± ± ± ± ± ± ± ±

0.0013 0.0019 0.0021 0.0015 0.0007 0.0013 0.0015 0.0012

Adsorption percentage was calculated based on the following equation: Adsorption(%) ¼ 100 - [(C0 - Cads)/C0] where C0 is the DOC at t ¼ 0, Cads is the DOC at t ¼ 60 min (i.e., adsorption for 60 min-contact with TiO2). b Photodesorption percentage was calculated based on the following equation: Desorption (%) ¼ 100 - [(Cdes - Cads)/(C0 - Cads)] where Cdes is the highest DOC value after irradiation (Shahid et al., 2014).

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(a)

1.2

UV on

Normalized values

Normalized values

1.2

UV on

1.0

1.0 0.8 0.6 0.4

0

60 120 Time (min)

180

240

0.8 0.6 0.4

0.0 -60

(c)

UV on

1.0

0.2

0.2 0.0 -60

(b)

Normalized values

1.2

0.8 0.6 0.4 0.2

0

60 120 Time (min)

180

240

0.0 -60

0

60 120 Time (min)

180

240

Fig. 1. Changes in normalized DOC, UV254, and PARAFAC components of (a) leaf litter organic matter, (b) Pony lake fulvic acid, and (c) Elliott humic acid under the irradiation systems with 0.5 g/L TiO2 at pH 7 and 12 UV lamps.

LLOM. In other words, TiO2 surface becomes net positively charged at a solution pH lower than the PZC (i.e. 6.2e6.9), facilitating the adsorption of negatively charged matter such as DOM through electrostatic attraction (Yang et al., 2009; Erhayem and Sohn, 2014). It is interesting that the greatest adsorption of LLOM at pH 4 did not lead to the highest removal of DOC as compared to other pH conditions. The overall removal of DOC at pH 4 and pH 7 was similar (89.9% and 89.1%) and higher than that at pH 10 (63.0%) (Table 1). Furthermore, the degradation rates of DOC decreased on the order of pH 7 > pH 4 > pH 10. The inconsistency between the adsorbed amount of DOM and the overall removal efficiency may be attributed to the surface modification of TiO2 upon DOM adsorption, aggregation of TiO2, and the limiting step of the DOM oxidation by OH in the aqueous phase. The previous literature demonstrated that the interactions between DOM and TiO2 could result in the modification of the photoactive properties of the TiO2 surface (Yang et al., 2009; Li and Sun, 2011). The aggregation of TiO2 particles driven by decreasing pH may lead to lower mass transport rates and thus a lower surface area for light (Palmer et al., 2002). This explanation is partially supported by the lowest turbidity values observed at pH 4 for this study (turbidity was 1848 NTU, 2175 NTU, and 2262 NTU at pH 4, pH 7 and pH 10, respectively) and also by a study of Li and Sun (2011) who reported an increased particle size of TiO2 at a lower pH in the presence of DOM. The aggregated TiO2 would further reduce the active surface area. In addition, an acidic condition (i.e., less OH) may be less favorable for OH to form via the hole oxidation of OH, lowering the efficiency of hydroxyl radical attack on DOM and finally reducing the photocatalytic oxidation rates (Franch et al., 2002; Liu et al., 2008). Meanwhile, the highest amount of the released DOC was observed at pH 10, at which the electrostatic attraction between TiO2 and DOM is weak and the adsorbed DOM are subsequently likely to be easily detached by the UV light. 3.1.3. Effects of light intensity and DOM sources Removal of DOC and the degradation rates both increased with a higher light intensity (i.e., more number of the lamps on) (Table 1; Supplementary Information, Fig. S3). With greater light intensity, the photodesorption was less pronounced with the degrees corresponding to 35.9%, 18.4%, and 6.5% of the total adsorbed DOC at 6, 9, and 12 lamps, respectively. It appears that the enhanced removal with stronger light intensity offsets the photodesorption effect. Adsorption of DOM onto TiO2 was greater on the order of LLOM ~ ESHA > PLFA. The photodegradation rate was the highest for PLFA followed by LLOM and ESHA, whereas the order of total DOC removal was inconsistent with the degradation rates, being higher on the order of LLOM > PLFA > ESHA. Our results suggest that the dominant mechanisms responsible for photocatalytic

degradation may be dependent on the sources of DOM (i.e, chemical composition of DOM) (Uyguner and Bekbolet, 2005; Remoundaki et al., 2009; Rajca and Bodzek, 2013). 3.2. Changes in UV254 during photodegradation UV254 has been widely accepted as a surrogate for probing the degradation rates of DOM and thus is used as an alternative parameter for DOC (Uyguner and Bekbolet, 2005; Sitnichenko et al., 2011; Rajca and Bodzek, 2013). The photodgradation rates of UV254 values fit the pseudo first order kinetics well (R2 ¼ 0.83e1.00) and the total removal was fairly high, ranging from 94.7 to 98.6%. The UV254 values were highly correlated with DOC (R2 ¼ 0.86e0.96 for LLOM, 0.93 for PLFA, and 0.97 for ESHA). The adsorption and the photodegradation of UV254 for the three DOM sources showed the same trend as those of DOC in their responses to varying light intensity and pH (Table 1; Supplementary Information, Figs. S2 and S3). The change in the extent of the DOM adsorption with TiO2 dose was also the same for UV254 and DOC, showing greater adsorption with higher amounts of photocatalyst. However, the order of the degradation rates with TiO2 dose (i.e., 0.5 g/L > 0.3 g/ L ~ 0.7 g/L > 0.1 g/L) was not the same as that of DOC, probably due to different degrees of light scattering effects on the UV-absorbing components (i.e., aromatic compounds). The rapid reduction of UV254 values over the irradiation time (Fig. 1) suggests that the chromophores in DOM, which mostly consist of high molecular sized aromatic rings, might be rapidly broken down into smaller sized non-aromatic structures (Sanly et al., 2007). The photodegradation rates and the total removal on the basis of UV254 were consistently higher than those of DOC (p < 0.001), suggesting that aromatic moieties in DOM were preferentially removed and/or some chromophores within DOM may be partially transformed into non-UV-absorbing compounds by the photochemical reaction (Uyguner and Bekbolet, 2005; Sanly et al., 2007; Hur et al., 2011; He, 2013). It is notable that many photoproducts of DOM constitute low molecular weight organic acids, alcohols, aldehydes, and inorganic carbon (Pullin et al., 2004). 3.3. PARAFAC components and the behaviors of the components during photodegradation 3.3.1. Fluorescence components identified by PARAFAC Four fluorescence components (C1, C2, C3, and C4) were decomposed by the PARAFAC modeling on the EEM dataset of all 143 samples from eleven experiments (Fig. 2). The peak locations of the four components were compared with those previously reported in Table S1 (Supplementary Information). C1 presented a fluorescence peak at Ex/Em of 280/358 nm,

D.D. Phong, J. Hur / Water Research 87 (2015) 119e126

123

Fig. 2. PARAFAC model output showing (a) the four fluorescent components and (b) the corresponding excitation/emission loadings (b). Excitation/emission loadings consist of two independent halves of the dataset (red and blue dotted lines) and the complete dataset (black lines). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

indicating the presence of a tryptophan-like (peak T) or a proteinlike component. This component is often found in the aquatic systems subject to anthropogenic input and/or to high primary productivity (Yamashita et al., 2008; Hong et al., 2012). Alternatively, it was assigned to a microbial product of DOM with biological sources (Fellman et al., 2011; Kothawala et al., 2012). The latter appears to be more reasonable here for the component's source because peak T was prominent in the EEM of LLOM (Supplementary Information, Fig. S5). C2 exhibited a primary and a secondary maxima at 250/418 nm (Ex/Em) and 335/418 nm (Ex/Em), respectively. Similar components were previously assigned to humic-like fluorescence (Murphy et al., 2008) or fulvic acid-like component (Santin et al., 2009; Pifer and Fairey, 2014). The component is likely to be associated with microbial origins because two peaks (i.e., peak A and peak C) were dominant in the EEM of PLFA (i.e., a microbially-derived fulvic acid) for this study (Supplementary Information, Fig. S5). C2 also resembles a humic-like component of DOM released from the degradation of phytoplankton (Zhang et al., 2011). C3, another humic-like component, had its maximum at 310/ 428 nm (Ex/Em), a combination of peak A and peak M (Table S1). In this study, the two peaks were dominant in the EEM of terrestrially derived organic matter (i.e., LLOM), indicating that this component may relate to microbial transformation products of terrestriallyderived organic matter. The primary and the secondary fluorescence maxima of C4 were located at Ex/Em of 265/490 nm and 370/490 nm, respectively. The component appears to be a combination of peak A and peak C (Table S1), but the maxima were shifted to longer emission wavelengths as compared to C2 and C3. This component was previously defined as terrestrial humic-like component in much of the literature (Murphy et al., 2008; Yamashita et al., 2008; Kowalczuk et al., 2009). A recent study has shown that the shift of the EEM peak maxima into longer emission wavelengths likely relates to a larger molecular size of a terrestiral humic acid (Lee et al., 2015). The relative distributions of the four components differed according to the sources of DOM. For example, LLOM displayed the

highest abundance of C1 (55.9e60.6%) followed by C3 (25.3e31.8%) and C4 (12.2e15.2%). PLFA had a higher abundance of C2 (73.2%) than C4 (26.8%) with the absence of C1 and C3, while C4 occupied greater abundance in ESHA (59.3%) than C2 (25.4%) and C1 (15.3%) (Table S2). The high abudance of C1 in LLOM may be associated with tannin-like structures contained in leave litter (Maie et al., 2007; Hur et al., 2011). The dominant presence of C2 in PLFA can be explained by the main source of microbially and algal production for the well characterized fulvic acid. 3.3.2. Photodegradation of PARAFAC components at different conditions Similar to DOC and UV254, all the PARAFAC components exhibited greater adsorption with a higher TiO2 dose and a lower pH (Supplementary Information, Figs. S1 and S2). In most cases with the exceptions of C2 of ESHA, C4 of LLOM at 6 and 9 lamps, and C4 of LLOM at pH 10, the initial sharp decreases with irradiation were consistently observed for all the components (Fig. 1; Supplementary Information, Figs. S1eS3), indicating that all the identified fluorophore groups were subject to direct photochemical degradation (Del Vecchio and Blough, 2002). The photocatalytic degradation rates of the PARAFAC components fit well the pseudo first-order model in the Fmax values (R2 ¼ 0.82e0.99). Our results suggest that all the individual components follows the general trend of a first-order exponential decay for their degradation behaviors, which is widely described for DOM photodegradation with time, thus making it feasible to directly compare the degradation behaviors of different components with the kinetic rates. The total removal ratios of the components were very high, ranging from 94.7 to 100.0% (Tables 2 and 3). Even in the absence of TiO2, UV-irradiation on LLOM notably reduced the Fmax values of C1, C3, and C4 by 17.8, 41.5, and 43.3%, respectively (Table 2), which contrasts with the previous observation of little to no changes in DOC and UV254 values. This result indicates that fluorescent compounds respond to the UV light much more sensitively than other non-fluorescent structures, strongly suggesting that the fluorescent components could be usefully

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Table 2 Extent of adsorption, photodegradation, and total removal of the individual PARAFAC components for different UVA irradiation systems. No.

Experimental conditions TiO2 dose (g/L)

Leaf litter 1 0.0 2 0.1 3 0.3 4 0.5 5 0.7 6 0.5 7 0.5 8 0.5 9 0.5 Pony Lake 10 0.5 Elliott soil 11 0.5

Adsorption (%)

pH

Photodegradation (%)

Total removal (%)

Number of lamps

C1

C2

C3

C4

C1

C2

C3

C4

C1

C2

C3

C4

7

12

e

e

e

e

17.8

e

41.5

43.3

20.7

e

43.8

45.4

7

12

7.8

e

4.9

15.5

89.9

e

93.5

83.2

97.8

e

98.4

98.7

7

12

9.5

e

9.4

39.3

87.6

e

89.4

59.2

97.1

e

98.8

98.5

7

12

16.3

e

17.7

44.6

83.5

e

81.8

54.2

99.7

e

99.5

98.8

7

12

18.4

e

22.5

52.9

79.2

e

77.5

47.1

97.5

e

99.9

100.0

4

12

22.0

e

24.7

65.4

75.6

e

71.6

34.6

97.7

e

96.4

100.0

10

12

11.9

e

7.0

34.0

83.7

e

92.3

65.5

95.7

e

99.3

99.5

7

6

17.4

e

16.9

42.9

79.4

e

82.2

55.4

96.8

e

99.1

98.3

7

9

17.0

e

20.8

48.4

80.8

e

78.3

50.9

97.8

e

99.1

99.3

7

12

e

17.8

e

32.5

e

82.0

e

67.4

e

99.8

e

100.0

7

12

50.9

25.9

e

67.7

47.8

68.8

e

31.7

98.7

94.7

e

99.3

Table 3 First-order photodegradation rates of the individual PARAFAC components for different UVA irradiation systems. No.

Leaf litter 1 2 3 4 5 6 7 8 9 Pony Lake 10 Elliott soil 11

Photodegradation rates (min1)

Experimental conditions TiO2 dose (g/L)

pH

Number of lamps

C1

C2

C3

0.0 0.1 0.3 0.5 0.7 0.5 0.5 0.5 0.5

7 7 7 7 7 4 10 7 7

12 12 12 12 12 12 12 6 9

0.0009 0.0236 0.0351 0.0485 0.0348 0.0258 0.0286 0.0192 0.0246

e e e e e e e e e

0.0029 0.0155 0.0178 0.0289 0.0223 0.0217 0.0184 0.0119 0.0125

0.5

7

12

e

0.0296 ± 0.0012

e

0.0334 ± 0.0007

0.5

7

12

0.0280 ± 0.0044

0.0073 ± 0.0011

e

0.0197 ± 0.0014

applied to track the subtle changes in DOM composition upon UVirradiation. It was interesting to observe that the total removal rates in the PARAFAC components of LLOM were consistently greater than those of DOC, and that the degradation rates were higher than those of DOC and UV254 (t-test, p < 0.001). The faster degradation of fluorescence components versus UV-absorbing moieties (i.e., UV254) may be explained by the fluorescence arising from the p* - p transitions in DOM molecules and its rapid extinction under UV irradiation (Cho and Choi, 2002). The observed differences agreed well with other prior studies on photobleaching of natural water (Moran et al., 2000; Mostofa et al., 2007) and photocatalytic oxidation of DOM (Valencia et al., 2013a). 3.3.3. Behaviors of the individual PARAFAC components Regardless of the DOM sources and the experimental conditions, the extent of the adsorption onto TiO2 was consistently higher for C4 than for the other components (Paired t-test, p < 0.001). The component of C4 with its peaks at longer wavelengths (i.e., redshifting) may be associated with the structural condensation and

± ± ± ± ± ± ± ± ±

0.0001 0.0011 0.0019 0.0020 0.0021 0.0018 0.0022 0.0007 0.0011

C4 ± ± ± ± ± ± ± ± ±

0.0002 0.0007 0.0013 0.0011 0.0025 0.0013 0.0011 0.0018 0.0009

0.0035 0.0144 0.0163 0.0229 0.0193 0.0100 0.0125 0.0097 0.0116

± ± ± ± ± ± ± ± ±

0.0004 0.0015 0.0025 0.0025 0.0019 0.0011 0.0023 0.0018 0.0017

polymerization of DOM (Chen et al., 2003). Wu et al. (2003) and Hur and Kim (2009) also reported more pronounced fluorescence features at longer emission wavelengths for the EEMs of larger sized and/or more hydrophobic DOM fractions. In this context, our results are comparable with previous studies showing preferential adsorption of more hydrophobic and larger molecular sized DOM onto minerals and/or nanoparticles including TiO2 (Valencia et al., 2013b; Mwaanga et al., 2014; Lee et al., 2015). The most striking finding of this study was that the photodegradation behavior of each PARAFAC component was not the same for the systems with and without TiO2. In the absence of TiO2, the reductions of C1, C3, and C4 upon the photolysis of LLOM reached 17.8, 41.5, and 43.3%, and the rates corresponded to 0.0009 ± 0.001, 0.0029 ± 0.0002, and 0.0035 ± 0.0004 min1, respectively (Table 3), which followed the order of C4 > C3 > C1. Our results were consistent with the prior photobleaching studies using aquatic DOM samples (Moran et al., 2000; Mostofa et al., 2007; Hur et al., 2011), in which protein-like fluorescence peaks were more resistant to photodegradation compared with the other

D.D. Phong, J. Hur / Water Research 87 (2015) 119e126

peaks (e.g., peaks A, M, and C) and the fluorescence peaks (i.e., C4 for this study) with the excitation wavelength falling into the UVA band (i.e., 315e400 nm) were diminished to a greater extent (Hur et al., 2011; Ishii and Boyer, 2012). the peak with C3 would be degraded by UVA light to a lesser extent than C4 because the excitation peak wavelength (i.e., 310 nm) featured in C3 does not fully fall into the UVA band (i.e., 315e400 nm) (Hur et al., 2011; Ishii and Boyer, 2012). Very interestingly, however, the TiO2-assisted photodegradation presented the opposite trend in the degradation rates, following the order of C1 > C3 > C4 (p < 0.05) (Table 3). The different behaviors of the fluorescence components are possibly explained by the TiO2-associated reaction pathway previously proposed (Nguyen et al., 2003; Liu et al., 2008). In other words, electron and hole pairs are generated upon illumination as follows: hv

 TiO2 !hþ VB þ eCB:

The holes participate in the oxidation reaction of DOM. Alternatively, the holes may oxidize water or hydroxyl ions to give hydroxyl radicals, which, in turn, oxidize the DOM present in the solution. Under situations in which TiO2 exerts pronounced effects on photodegradation, the DOM component adsorbed on the TiO2 surface to a lesser extent (i.e., C1) is likely to be more preferably degraded by hydroxyl radicals in the solution, leading to the highest degradation rate among the identified fluorescence components. Meanwhile, such oxidization can be a limiting step for the other components with the higher adsorption affinity (e.g., C4), resulting in the lowest degradation rate. Furthermore, the reactive holes are likely involved in oxidizing the adsorbed DOM components with large size (e.g., C4) into smaller fragments, which could delay the overall degradation process. Upon the initial irradiation on ESHA, C2 showed a slight increase by 7.4% in the Fmax value until the irradiation of 45 min, followed by a steady decrease afterwards. The elevation in C2 and the slower rate of C2 (0.0073 min1) versus C1 (0.028 min1) and C4 (0.0197 min1) may result from the photo-produced effect previously reported, which is a gradual shift in Ex/Em of peak C in irradiated DOM samples into shorter wavelengths (i.e., blueshifting of the peak position) (Mostofa et al., 2007; Coble et al., 2014).

125

feasible via fluorescence sensors. For example, Shutova et al. (2014) suggested four pairs of Ex/Em for on-line tracking of DOM changes based on the behaviors of PARAFAC components in five drinking water treatment plants. Another possible but challenging option is to scan EEM in-situ rapidly followed by on-line PARAFAC modeling (Carstea et al., 2010). More sensitive responses of fluorescence components versus UV254 and DOC to UV irradiation with TiO2 heighten the probability of a successful application of on-line monitoring based on EEM-PARAFAC for photocatalytic systems. 4. Conclusions The general responses of DOM in photocatalytic degradation to varying TiO2 dose and light intensities were the same for DOC, UV254, and fluorescence: a faster degradation in accordance to higher TiO2 doses and stronger UV light. The photocatalytic changes in DOM with solution pH was dependent on TiO2 surface modification by DOM adsorption, the aggregation of TiO2, and the generation of OH for DOM oxidation. The photodegradation rates on the basis of DOC were lower than those of UV254 or fluorescence, suggesting the preferential removal of aromatic moieties within DOM upon UVA exposure. The changes in UV254 and fluorescence were fitted well by the pseudo first-order kinetic model, whereas those of DOC did not follow the model as much due to the release of DOC from the TiO2 surface. EEM-PARAFAC allowed one to estimate the degradation rates of the individual fluorescence components and to track the behaviors, thus providing new insights into the underlying mechanisms during the photodegradation. In the absence of TiO2, humic-like components with the featured excitation wavelengths corresponding to the irradiated light exhibited the fastest degradation rates. In the TiO2/UVA system, by contrast, protein-like component was preferentially removed by photocatalytic degradation probably due to its direct and preferable oxidation by hydroxyl radicals in solution. Acknowledgments This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No.2014R1A2A2A09049496). Appendix A. Supplementary data

3.4. Environmental implications and further applications In water treatment processes, EEM-PARAFAC has proven to be a promising monitoring tool for easily tracking the evolution of the individual organic components with different chemical compositions (Yang et al., 2015). Different fluorescent components have their own unique structures and characteristics. A recent review article has provided much evidence that humic-like components (i.e., C2, C3, and C4) could be used as the surrogates for disinfection by-products, and the protein-like component (e.g., C1) could be used for microbial pollution and membrane foulants (Yang et al., 2015). Our study provided insights into different responses and the behaviors of each fluorescent component of DOM upon photocatalytic degradation. This is of great benefit in suggesting the mechanisms underlying the processes and determining the subsequent treatment options. For example, the highest degradation rate of C1 shown in this study suggests that the photocatalytic treatment with TiO2 could effectively reduce the potentials of membrane fouling and bacterial re-growth in distribution pipes. In contrast, the treatment efficiency of trihalomethane precursors may not be as pronounced as much, as revealed by the relatively low degradation rates of humic-like components. Online monitoring of drinking water treatment plants is now

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