Effect of UV-LED wavelengths on direct photolytic and TiO2 photocatalytic degradation of emerging contaminants in water

Chemical Engineering Journal 300 (2016) 414–422

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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

Effect of UV-LED wavelengths on direct photolytic and TiO2 photocatalytic degradation of emerging contaminants in water Mohammad Reza Eskandarian a,b, Hyeok Choi b,⇑,1, Mostafa Fazli a, Mohammad Hossein Rasoulifard c,⇑,1 a

Department of Applied Chemistry, Faculty of Chemistry, Semnan University, Semnan 363-35196, Iran Department of Civil Engineering, The University of Texas at Arlington, Arlington, TX 76019-0308, United States c Water & Wastewater Treatment Research Laboratory, Department of Chemistry, University of Zanjan, Zanjan 313-45195, Iran b

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Light emitting diode (LED) as an

alternative UV source to mercury lamps was tested.  Direct photolytic and TiO2 photocatalytic decomposition of PPCPs was studied.  Sulfamethoxazole > diclofenac > ibuprofen > acetaminophen were successfully decomposed.  UV-LED was also effective to decompose a biological toxin, microcystin-LR.  Shorter wavelength LEDs in order of UVC > UVB > UVA were more effective.

a r t i c l e

i n f o

Article history: Received 1 February 2016 Received in revised form 19 April 2016 Accepted 11 May 2016 Available online 20 May 2016 Keywords: Light-emitting diode UV wavelength Photolysis TiO2 photocatalysis Emerging contaminants

a b s t r a c t Ultraviolet (UV) irradiation is known to be effective for chemical oxidation and disinfection of water and wastewater. Recently, light emitting diode (LED) has been recognized as a cost-effective, environmentally friendly, and sustainable source of UV to replace conventional mercury lamps, so-called UV-LED. Less is known about the effectiveness of UV-LEDs, i.e., UVA, UVB, and UVC in comparison, for the decomposition of recalcitrant organic chemicals. In this study, direct photolytic decomposition and TiO2 photocatalytic decomposition of model pharmaceuticals, including acetaminophen (ACT), diclofenac (DCF), ibuprofen (IBP), and sulfamethoxazole (SMX), was tested, and the effects of different UV-LED wavelengths on their decomposition were investigated. UV wavelength was found to be a more important parameter for the decomposition than light intensity. Shorter wavelength UV in order of UVC > UVB > UVA was more effective for the decomposition of the pharmaceuticals. Photocatalytic decomposition was much more significant than photolytic decomposition. Decomposition kinetics of the pharmaceuticals followed SMX > DCF > IBP > ACT, reflecting their molecular structures. Results on the mineralization of the pharmaceuticals also supported the observed trends for their disappearance. The investigation was resumed with microcystin-LR (MC-LR), a known lethal biological toxin often found in water resource, and the similar result to the pharmaceuticals was also observed for MC-LR. This study and the consequent results would facilitate applications of UV-LEDs for the treatment of water contaminated with recalcitrant toxic chemicals. Ó 2016 Elsevier B.V. All rights reserved.

⇑ Corresponding authors. 1

E-mail addresses: [email protected] (H. Choi), [email protected] (M.H. Rasoulifard). These authors contributed equally.

http://dx.doi.org/10.1016/j.cej.2016.05.049 1385-8947/Ó 2016 Elsevier B.V. All rights reserved.

M.R. Eskandarian et al. / Chemical Engineering Journal 300 (2016) 414–422

1. Introduction Pharmaceuticals and personal care products (PPCPs), as emerging contaminants often found in water, have drawn significant attention due to their potential environmental and health impact. PPCPs include a diverse group of organic compounds, including medical drugs and hormones, suncreams, and sanitizers [1–3]. They have been widely used in high quantities around the world [4,5]. In addition to the anthropogenic chemicals, there are naturally occurring toxic chemicals in water. Cyanotoxins produced and released from blue-green algae are of great concern. In particular, as one of the most commonly occurring cyanotoxins, microcystins (MCs) such as MC-LR are extremely toxic and show significant health risk [6]. The removal efficiency of organic contaminants through water and wastewater treatment facilities is various, from negligible to almost complete removal [7,8]. However, most of such pharmaceuticals are, in general, recalcitrant to oxidation during conventional chemical processes and toxic to microorganisms used for biological processes. This means they are not easily decomposed and thus prevalent in the environment. In recent years, ultraviolet (UV) irradiation has been proposed to be effective for chemical oxidation and disinfection of water and wastewater [9]. Direct photolytic decomposition and transformation of organic chemicals has been observed [10–14]. UV irradiation is now a routinely-used technique in many water treatment facilities. Even UV technology can be combined with other chemical processes such as UV/H2O2, UV/O3, and UV/TiO2 in order to produce highly strong oxidizing radical species such as hydroxyl radicals (OH, E0 = 2.8 eV). In particular, UV/TiO2, so-called TiO2 photocatalytic process, leverages both direct photolysis by providing photon energy given by UV and photocatalysis by producing hydroxyl radicals generated from TiO2 under UV radiation. Organic chemicals can be significantly decomposed by the two routes and thus UV/TiO2 has high potential to decompose recalcitrant chemicals such as PPCPs and MCs in water. In UV technologies for water treatment and disinfection, germicidal UVC (260 nm) has been overwhelmingly utilized over other UV regions such as UVA (365 nm) and UVB (310 nm) [15,16]. For TiO2 photocatalysis, UVA has been exclusively used. Research studies demonstrated successful decomposition of organic chemicals by UVC while less is known about the photolytic efficiency of UVA and UVB [17–21]. Meanwhile, the most common artificial illumination practiced up to recent studies is use of low and medium pressure UV lamps [22,23]. Conventional mercury UV lamps are characterized with short bulb lifetime, low energy efficiency, and mercury pollution potential [24]. Recently, light emitting diode (LED) has been recognized as a more cost-effective, environmentally friendly, and sustainable source of UV to replace mercury lamps, so-called UV-LED [25–27]. LED provides several advantages, including low warm-up time, no mercury disposal problem, stability and easy handling, operation at low voltage, and low power requirement [28]. The potential lifetime of LEDs has been reported to be 35,000–50,000 h while that of low pressure UV lamps is estimated to be only 9000– 12,000 h [29–31]. UV-LED has found various applications in the degradation of organic compounds such as phenolic compounds, various dyes, and natural organic materials [32–37]. Lowintensity UVC-LED has been effectively applied for water treatment while high-intensity UVC-LED has been recently developed [38– 41]. Fabrication and utilization of functionalized LEDs is still under development. Less is known about the effectiveness of UV-LEDs, i.e., UVA, UVB, and UVC in comparison, for the decomposition of recalcitrant organic chemicals. The overall objective of this present study is to evaluate the efficiency of UV-LEDs for the decomposition of emerging contaminants in water. Direct photolytic decomposition and TiO2

415

photocatalytic decomposition of some model PPCPs and MC-LR was tested and the effects of different UV-LED wavelengths on their decomposition were also investigated. Different reaction kinetics were interpreted with the molecular structures of the chemicals and the characteristics of the UV-LED wavelengths. 2. Experimental 2.1. Materials Among many PPCPs, acetaminophen (ACT), diclofenac (DCF), ibuprofen (IBP), and sulfamethoxazole (SMX) were chosen as target chemicals to decompose, considering their environmental significance (e.g., prevalence, recalcitrance, toxicity, public attention). MC-LR was also chosen as a biological toxin. They were purchased from Sigma–Aldrich, USA. The detailed information on the chemicals is presented in Table 1. For chromatographic analysis, acetonitrile, phosphoric acid, and trifluoroacetic acid were supplied from Sigma–Aldrich, USA. High purity water from a MilliQ-Water System (Millipore, Bedford, MA, USA) was utilized for the preparation of all solutions, suspensions, and chromatographic mobile phases. As a photocatalyst, P-25 TiO2 particles (Degussa, Germany) were used as received without further treatment. P-25 is nanocrystalline in a mixture of anatase and rutile (7:3) and characterized with surface area of 50 ± 15 m2/g and average primary particle size of 30 nm. 2.2. UV-LEDs and photoreactors UV lamps called UVCLEAN (TO39 UVLED) were equipped with multi-chip arrays of UV-LEDs with different wavelengths at 365 ± 10 nm, 365 ± 5 nm, 300 ± 5 nm, and 260 ± 10 nm for UVA, high-intensity UVA (UVAH), UVB, and UVC, respectively. UV-LEDs were purchased from International Light Technologies, Inc. (Peabody, MA, USA) for UVA and UVAH, Crystal IS Co. (Green Island, NY, USA) for UVB, and Sensor Electronic Technology Inc. (Columbia, SC, USA) for UVC. The specification of UV-LEDs and LED reactor is given in Table 2. Spectra of the emission wavelengths of the UVLEDs were measured by an ultrafast fiber optic spectrometer (AvaSpec-128, Avantes, Netherlands). LEDs were packaged in a standardized transistor with an internal heat sink for heat dissipation. The multi-chip array of LEDs enabled to obtain higher UV intensities compared to conventional array configuration. A 140-mm crystallizer was used as a photoreactor and was placed under the UV-LED lamps. The reactor was continuously mixed with a magnetic stirrer. Fig. 1 illustrates the schematic diagram of the circuit connection of UV-LED lamps. Three LEDs were soldered to a circular-shaped programmable circuit board (Nex Logic Co., CA, USA). An aluminum heat sink was used to dissipate heat from UV-LEDs. LEDs were powered by a constant-current power supply (KORAD KA3005D, Walpole, MA, USA). Distance between the crystallizer and the LEDs was 5 cm. 2.3. Photolytic and photocatalytic experiments and chemical analysis A batch experiment was setup for photolytic and photocatalytic experiments. Each of the target PPCPs was individually tested at 20 mg/L while MC-LR was tested at much lower concentrations of 1 mg/L and 5 mg/L due to the cost of MC-LR and its disposal issue after use. The concentrations of PPCPs and MC-LR used in this study are much higher than those typically found in water at several-tens lg/L. Such a higher concentration can make it possible to measure concentration of organic carbon accurately for calculation of total mineralization later and to measure concentration of the target chemicals reliably without significant analytical effort.

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Table 1 Characteristics and molecular structures of emerging organic contaminants tested in this study. Compound

Molecular weight, g/mol

Class

Column/ Detection

Acetaminophen (ACT) C8H9NO2

151.16

Analgesic drug

C18 column UV at 243 nm

Diclofenac (DCF) C14H11Cl2NO2

296.15

Anti-inflammatory drug

C18 column UV at 275 nm

Ibuprofen (IBP) C13H18O2

206.29

Anti-inflammatory drug

C18 Column UV at 214 nm

Sulfamethoxazole (SMX) C10H11N3O3S

253.28

Antibiotic drug

C18 column UV at 240 nm

Microcystin-LR (MC-LR) C49H74N10O12

995.17

Cyanobacterial toxin

C18 column UV at 238 nm

Table 2 Specification of UV-LED photoreactor.

a

Type

kMax (nm)

Wave lengths (nm)

Output power (mW)

Voltage (V)

Current (mA)

UVA UVAHa UVB UVC

365 365 300 250

355–375 360–370 295–305 250–280

10 3000 10 10

11.2 11 11.5 30

300 1000 280 300

UVA with high intensity.

(2)

5cm

UV-LEDs

Molecular structure

water and acetonitrile at 75:25, 30:70, and 50:50 (v/v) for ACT, DCF, and SMX, respectively, was used [42,43]. A mixture of phosphoric acid buffer and acetonitrile at 50:50 (v/v) was utilized for IBP [44]. The wavelengths for UV detection of ACT, DCF, IBP, and SMX were predetermined by using a UV–visible spectrophotometer (UV 2550; Shimadzu, Japan) at 243 nm, 275 nm, 214 nm, and 240 nm, respectively. Detailed analytical conditions for each PPCP could be found in literatures [42–44]. The similar analytical method above was also applied for MC-LR [45,46]. A mixture of 0.05% trifluoroacetic acid in water and 0.05% trifluoroacetic acid in acetonitrile at 60:40 (v/v) was utilized as a mobile phase. MCLR was detected at 238 nm. He et al. reported detailed analytical conditions for MC-LR [46]. Total organic carbon (TOC) was monitored by a TOC analyzer (TOC-VCSH/CSN, Shimadzu, Japan) at the end of the experiments to determine degree of mineralization of the chemicals.

(3) On Off

(1)

(4)

3. Results and discussion 3.1. PPCPs

Circuit

(6)

(5) Fig. 1. Schematic of a UV-LED photoreactor; (1) UV-LED modules, (2) aluminum heat sink, (3) crystallizer, (4) magnetic stirrer, (5) electrical circuit, and (6) DC power supply.

In addition, reactions of the chemicals at very low concentrations are too fast to measure their decomposition kinetics, which makes it hard to compare the effectiveness of the various systems tested in this study. Reaction volume was fixed at 150 mL. Voltage and current applied for the different UV-LEDs is given in Table 2. For photocatalytic experiments, P-25 TiO2 particles at 0.5 g/L were added to the reactor after they were completely ground and predispersed in water by sonication (VCX750 Sonicator, Sonics & Materials, Inc., CT, USA). In every 15 min, 0.5 mL of suspension sample was taken and filtered with a PTFE syringe (0.45 lm, Sigma–Aldrich, USA). High performance liquid chromatography with a UV detector (1200 series, Agilent Technologies, Santa Clara, CA, USA) was used to quantify ACT, DCF, IBP, SMX, and MC-LR. The concentrations of the chemicals were determined in a reversed-phase configuration with C18 column (2.1  150 mm, 5 lm particle size, Agilent Technologies, Santa Clara, CA, USA). As a mobile phase, a mixture of

3.1.1. Direct photolysis Direct photolytic decomposition of PPCPs by different UV-LEDs was studied as shown in Fig. 2. Result on dark degradation test indicated no noticeable decomposition of PPCPs in the absence of UV. Fig. 2(a) shows negligible photolytic decomposition of PPCPs by UVA (365 nm, 10 mW). SMX was decomposed at 6% for 3 h, following DCF, IBP and ACT at 5%, 4%, and 3%, respectively. Since the longest wavelength UVA did not work, UVA with much higher intensity, UVAH (365 nm, 3000 mW) was tested, as shown in Fig. 2(b). Compared to UVA, UVAH improved the decomposition of PPCPs for 3 h at 9%, 7%, 5% and 4% for SMX, DCF, IBP, and ACT, respectively. However, the improvement was not significant considering 300 times higher intensity. The order of PPCP decomposition was very similar for both UVA and UVAH. UVAH showed slightly more immediate decomposition of PPCPs than UVA. A more powerful UV light source, UVAH in this case, can provide more photons for definite irradiation time but it does not supply higher energy that could be more effective for photolysis (and photocatalysis later). Both of UVA and UVAH have the same photon energy while power output from UVAH is higher than that from UVA. UVAH provides more photons per area (high power density). From the observed results, it is clear that higher flux in UVAH increased the speed of the degradation slightly, but generally it

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1.0

1.0 1.00

Acetaminophen Diclofenac Ibuprofen Sulfamethoxazole

0.8 0.99 0.98

0.4

1.00

Acetaminophen Diclofenac Ibuprofen Sulfamethoxazole

0.98

0.6

0.97

0.96

C/C0

C/C0

0.6

0.8

0.96

0.4 0.94

0.95

0.2 0.94 0.93 0

0.0 0

0.2

(a) 20

20

40

40

60

60

80

80

100

100

120

140

120

140

160

160

0.92

(b)

0.90

180

0

0.0

180

0

20

20

40

40

60

60

Time (min)

80

80

100

100

120

140

120

140

160

160

180

180

Time (min) 1.0

0.8

0.8

0.6

0.6

C/C0

C/C0

1.0

0.4

0.4

Acetaminophen Diclofenac Ibuprofen Sulfamethoxazole

0.2

(c)

0.0 0

20

40

60

80

Acetaminophen Diclofenac Ibuprofen Sulfamethoxazole

0.2

100

120

140

160

(d)

0.0

180

Time (min)

0

20

40

60

80

100

120

140

160

180

Time (min)

Fig. 2. Direct photolytic decomposition of target PPCPs by different UV-LED sources: (a) UVA, (b) UVAH, (c) UVB, and (d) UVC (T = 25 °C, natural pH, and [ACT]0 = [DCF]0 = [IBP]0 = [SMX]0 = 20 mg/L, independently).

did not improve the overall efficiency of the degradation process significantly. Previous studies also reported that higher intensity provides more photons within a given time, accelerates photolytic reaction from the beginning, and thus makes it possible to achieve maximum degradation yield earlier, while not significantly improving overall degradation efficiency [45–47]. Since UVA and UVAH did not work well for photolytic decomposition of PPCPs, shorter wavelength UVB (300 nm, 10 mW) was tested, as shown in Fig. 2(c). UVB was much more effective for the photolytic decomposition of PPCPs than UVA and UVAH. Decomposition of SMX, DCF, IBP, and ACT for 3 h was at 37%, 29%, 21%, and 13%, respectively. Recent studies also revealed effectiveness of UVB illumination on photolytic decomposition of various PPCPs [48–50]. Lastly, the shortest wavelength LED, UVC (265 nm, 10 mW) was introduced for the photolytic decomposition of PPCPs, as shown in Fig. 2(d). Decomposition of PPCPs by UVC was fastest. Decomposition of SMX, DCF, IBP, and ACT for 3 h was at 56%, 50%, 36%, and 22%, respectively. 3.1.2. TiO2 photocatalysis In order to accelerate the decomposition of PPCPs, small amount of TiO2 photocatalysts at 0.5 g/L was added, as shown in Fig. 3. Result on dark degradation test indicated no noticeable removal of PPCPs, implying no significant adsorption of PPCPs onto TiO2. This makes it easy to interpret data on TiO2 photocatalysis. It should also be noted that the decreases in the concentration of the chemicals in Fig. 3 are ascribed to their photocatalytic

decomposition combined with photolytic decomposition (just simply photocatalysis). TiO2 was first irradiated with UVA, as shown in Fig. 3(a). Considering that the band gaps of TiO2 in anatase phase and rutile phase are at 3.20 eV and 3.03 eV, respectively, the band gap of P-25 used in this study (a mixture of anatase and rutile at 7:3 ratio) was estimated at around 3.15 eV, equivalent to 393 nm. As a result, UVA could successfully activate TiO2. The combination of TiO2 with UVA is considered as a common practice for TiO2 Photocatalysis. Photocatalytic decomposition of PPCPs for 3 h was significant at 58%, 50%, 35%, and 26% for SMX, DCF, IBP, and ACT, respectively, compared to photolytic decomposition shown in Fig. 2(a) at only 6%, 5%, 4%, and 3%. Similarly, photocatalytic decomposition of PPCPs under UVAH shown in Fig. 3(b) was much more significant than their photolytic decomposition under UVAH shown in Fig. 2 (b). There was no significant difference in photocatalytic decomposition between UVA and UVAH, which is in agreement with the results on photolysis by UVA in comparison to UVAH. Fig. 3(c) and (d) show photocatalytic decomposition of PPCPs under UVB and UVC, respectively. In both cases, photocatalytic decomposition was much more significant than photolytic decomposition. Overall, TiO2/UVC was more effective than TiO2/UVB. In particular, decomposition of SMX and DCF was significantly improved by TiO2/UVC. Almost complete decomposition of SMX was observed within 1 h by TiO2/UVC. In TiO2 photocatalysis, decomposition of SMX and DCF highly depended on UV wavelengths, compared to IBP and ACT.

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1.0

0.8

0.8

0.6

0.6

C/C0

C/C0

1.0

0.4

0.4

Acetaminophen Diclofenac Ibuprofen Sulfamethoxazole

0.2

0.0

0

20

40

60

80

Acetaminophen Diclofenac Ibuprofen Sulfamethoxazole

0.2

(a) 100

120

140

160

0.0

180

0

20

40

60

Time (min)

80

(b) 100

120

140

160

180

Time (min) 1.0

0.8

0.8

0.6

0.6

Acetaminophen Diclofenac Ibuprofen Sulfamethoxazole

C/C0

C/C0

1.0

0.4

0.4

Acetaminophen Diclofenac Ibuprofen Sulfamethoxazole

0.2

0.0 0

20

40

60

80

0.2

(c) 100

120

140

160

(d)

0.0 180

Time (min)

0

20

40

60

80

100

120

140

160

180

Time (min)

Fig. 3. TiO2 photocatalytic decomposition of target PPCPs by different UV-LED sources: (a) UVA, (b) UVAH, (c) UVB, and (d) UVC (TiO2 = 0.5 g/L, T = 25 °C, natural pH, and [ACT]0 = [DCF]0 = [IBP]0 = [SMX]0 = 20 mg/L, independently).

3.1.3. Photolysis vs. photocatalysis First, in all cases, the order of the effectiveness of UV-LEDs was UVC > UVB > UVAH > UVA. Shorter wavelength UV-LED was more effective for photolytic decomposition of PPCPs. In photolysis, PPCPs are broken down by absorbing photon energy. Any photons with sufficient energy provided by shorter wavelengths can alter the chemical bonds in PPCP chemicals. Considering much enhanced decomposition of PPCPs by UVC, compared to UVAH with 300 times higher intensity, UV wavelength was a more important parameter for the decomposition of PPCPs than light intensity. Second, photocatalytic decomposition of PPCPs was much more significant than their photolytic decomposition in all cases. In addition to direct photolytic decomposition of PPCPs by absorbing UV energy, hydroxyl radicals generated by the TiO2/UV system could significantly contribute to the decomposition of PPCPs. It is wellknown that hydroxyl radicals non-selectively and readily attack organic chemicals. Their oxidation capability is strong enough to decompose almost all organic chemicals in water and complete mineralization of many organic chemicals has been reported [51]. Lastly, the order of the decomposition of PPCPs was SMX > DCF > IBP > ACT in all cases, regardless of UV wavelengths (UVA, UVAH, UVB, UVC) and decomposition pathways (photolysis and photocatalysis). SMX was most vulnerable to chemical attack due to the presence of a NH group (note Table 1). Many previous studies demonstrated that the NH group of SMX is one of the desirable dissociation sites because of the electronegativity of nitrogen atom [52–54]. The structure is more subject to rearrangement by

simple electron transfer reaction [55,56]. SMX has been reported to easily fractionize to two fragments; hexagonal ring with sulfur dioxide group (Para-amino benzene sulfonic acid) and pentagonal ring with –NH– group which makes it possible to have unsaturated conjugated p bond [57]. Conjugated p bond provides the possibility of unsaturated electron motion between ring and nitrogen nonbonding electrons [57,58]. Other studies proposed the enhanced cleavage of S N bond [59–61]. Decomposition of DCF is expected to be very similar to that of SMX due to the similarity in their chemical structure, i.e., NH group between two benzene rings. The similar electron rearrangement and unsaturated conjugated p bond are possible around the NH group in DCF. The NH group placed between two aromatic rings is prone to cleavage. In addition, two Cl atoms attached to one of the aromatic rings can be easily replaced [62]. COONa in DCF not only provides the feasibility of electron transfer for conjugated system but also makes it easy to decompose mainly because of dissociation of the COO group [62–64]. IBP and ACT have been reported to be more stable and thus more recalcitrant [65–71]. Considering the molecular structure of IBP, its decomposition proceeds via rearrangement of acidic group and formation of hydroxyl group, followed by decarboxylation reaction and dehydrogenation [65,66]. IBP also benefits from electron transfer reaction but not as significant as SMX and DCF, resulting in less flexibility in degradation sites [67–69]. ACT was most recalcitrant. Removal of amid group (CH3CONH) in ACT is the first step in its decomposition. Then, as direct hole oxidation, phenolic radical losses a proton and then results in the formation of

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M.R. Eskandarian et al. / Chemical Engineering Journal 300 (2016) 414–422 Table 3 Total organic carbon removal efficiencies (%) of the photolytic and photocatalytic processes for PPCPs.a Contaminants

Direct photolysis

Acetaminophen Diclofenac Ibuprofen Sulfamethoxazole a

TiO2 photocatalysis

UVA

UVAH

UVB

UVC

UVA

UVAH

UVB

UVC

1.7 5.5 2.3 6.32

1.9 5.6 3.25 7.0

2.08 6.7 4.4 10.3

5.1 15.3 9.2 19.9

20.3 31.2 16.6 35.1

21.6 34.4 18.7 37.3

21.1 37.9 25.8 40.5

31.4 46.1 36.6 59.9

TOC removal after 3 h.

1.0

1.0

0.8

0.8

0.6

0.6

1.00

C/C0

C/C0

0.95 0.90

0.4

0.4

0.2

200

400

600

800

1000

1200

0.75 0.70

(a)

0.0 0

UVA UVAH UVB UVC

0.80

UVA UVAH UVB UVC

0.2

0.85

0.0 0

1400

0

50 200

100

400

600

1.0

1.0 UVA UVAH UVB UVC

C/C0

0.6 0.4

0.8

0.6

20

40

60

80

(c) 200

400

600

800

1000

0.0

0

20

40

1200

1400

Time (min)

60

Time (min)

0.2

0.0 0

0.2

100

Time (min)

0.2

1400

0.4

0.4

0.0 0

1200

UVA UVAH UVB UVC

0.8

0.6

0.2

0.4

1000

1.0

C/C0

C/C0

0.6

800

C/C0

0.8

0.8

(b)

200

Time (min)

Time (min) 1.0

150

80

100

(d)

0.0 0

200

400

600

800

1000

1200

1400

Time (min)

Fig. 4. Decomposition of microcystin-LR by direct photolysis and TiO2 photocatalysis under different UV-LED sources: (a) direct photolysis of MC-LR at 1 mg/L, (b) direct photolysis of MC-LR at 5 mg/L, (c) TiO2 photocatalysis of MC-LR at 1 mg/L by using 0.5 g/L TiO2, and (d) TiO2 photocatalysis of MC-LR at 5 mg/L by using 0.5 g/L TiO2. T = 25 °C and natural pH.

phenoxy radical that reacts with superoxide radical to surrender quinoneimine followed by 1,4-benzoquinone [70]. Continuous degradation of ACT and its intermediates is not as easy as the other PPCPs described [70,71]. There are electron acceptor groups (i.e., carbonyl group) which tend to have more electrons. In this study, hydroxyl radical generated from TiO2/UV was most effective for SMX among the investigated PPCPs (ACT, DCF, IBP, and SMX). Meanwhile, Nfodzo et al. reported that sulfate radical generated by a Fenton-like system was much less effective for SMX than ACT [43]. The organic attack mechanisms of hydroxyl radical and sulfate radical are different [72]. Hydroxyl radical reacts via electron transfer, hydrogen abstraction, and/or hydrogen addition, while sulfate radical reacts more selectively via electron transfer. Hydroxyl radical normally reacts with saturated organics

via hydrogen abstraction while they react with unsaturated organics primarily via addition reaction. Sulfate radical is known to be superior to hydroxyl radical for the decomposition of some chemicals [73]. It is also notable that photolytic and photocatalytic decomposition of PPCPs under UVA seemed steady-state after initial significant decomposition while that under UVB and UVC continued to progress over time. 3.1.4. Mineralization Mineralization efficiency of the photolytic and photocatalytic processes was measured in terms of TOC removal after 3 h, as summarized in Table 3. Compared to Figs. 2 and 3 reporting just disappearance of target PPCPs (i.e., its transformation to reaction intermediate molecules), mineralization might be more important

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Table 4 Total organic carbon removal efficiencies (%) of the photolytic and photocatalytic processes for MC-LR.a Contaminants

MC-LR at 1 mg/L MC-LR at 5 mg/L a

Direct photolysis

TiO2 photocatalysis

UVA

UVAH

UVB

UVC

UVA

UVAH

UVB

UVC

5.8 3.5

7.6 4.4

11.2 6.6

32.4 25.5

32.8 21.1

36.8 26.1

49.1 33.2

60.1 44.9

TOC removal after 24 h.

to evaluate the performance of the processes for further continuing decomposition of even reaction intermediates. All of the trends in PPCP disappearance observed previously were also found in PPCP mineralization, including effectiveness of UV wavelengths (UVC > UVB > UVAH > UVA), decomposition order of PPCPs (SMX > DCF > IBP > ACT), and more effective photocatalysis than photolysis. As expected, PPCP mineralization slightly retarded compared to PPCP disappearance. Significant mineralization of SMX at around 60% was observed in TiO2 photocatalysis under UVC after 3 h.

position of model PPCPs and MC-LR. UV wavelength was a more important parameter for their decomposition than light intensity. Shorter wavelength UV in order of UVC > UVB > UVA was more effective for photolytic and photocatalytic decomposition of PPCPs and MC-LR. Photocatalytic decomposition was much more significant than photolytic decomposition in all UV wavelengths. Decomposition kinetics of PPCPs followed SMX > DCF > IBP > ACT, reflecting their molecular structure. All of the observed trends in the disappearance of PPCPs and MC-LR were also found in their mineralization. Even though significant mineralization of the chemicals was observed, the mineralization slightly retarded in particular for MC-LR. As a result, UV-LEDs as an alternative source of UV to conventional mercury lamps were proven to be effective for the photolytic and photocatalytic decomposition of emerging chemicals found in water. More detailed studies on decomposition of the chemicals at more environmentally relevant concentrations (lg/L), simultaneous and competitive decomposition of the chemicals in a mixture, elucidation of reaction pathways and mechanisms (e.g., identification of reaction intermediates in all cases), and the effects of operating parameters (e.g., pH, UV intensity, impact of natural organic matter) should be followed in near future.

3.2. Decomposition of microcystin-LR In addition to PPCPs, an emerging chemical of concern MC-LR, one of the most toxic and prevalent natural biological toxins, was tested to evaluate the effect of different UV-LED wavelengths on its direct photolytic and TiO2 photocatalytic decomposition. The same experiments for PPCPs were revisited with MC-LR. Result on dark degradation test indicated no noticeable decomposition of MC-LR in the absence of light and no significant adsorption of MC-LR onto TiO2, as also observed for PPCPs. Fig. 4(a) and (b) show photolytic decomposition of MC-LR at 1 mg/L and 5 mg/L, respectively. Since the decomposition kinetics for 3 h was slow, the experiment was extended up to 24 h. MCLR was decomposed fairly well by UV in order of UVC > UVB > UVAH > UVA, which is in agreement with the results for PPCPs. As expected, higher concentration of MC-LR at 5 mg/L resulted in slower decomposition kinetics. Meanwhile, photocatalytic decomposition of MC-LR at 1 mg/L and 5 mg/L is shown in Fig. 4 (c) and (d), respectively. Photocatalytic decomposition of MC-LR at 1 mg/L was almost immediate within 30 min under all UV wavelengths mainly because its initial concentration tested in this study was very low compared to 20 mg/L used for PPCPs. Even at higher concentration of MC-LR at 5 mg/L, MC-LR was decomposed significantly fast. Similarly to the results on photolytic decomposition, UVC > UVB > UVAH > UVA in order was effective. Table 4 summarizes the mineralization of MC-LR after 24 h. The similar trends observed for PPCPs are also found in the mineralization of MC-LR, including effectiveness of UV wavelengths (UVC > UVB > UVAH > UVA) and retarded mineralization of MC-LR than its disappearance. Although almost complete decomposition of MC-LR by photocatalysis under UVC was observed within 30 min and 240 min in cases of initial concentrations of MC-LR at 1 mg/L and 5 mg/L, respectively (note Fig. 4(c) and (d)), its mineralization was relatively slow at 60.1% and 44.9% after 24 h. The mineralization of MC-LR seemed significant slower compared to PPCPs most probably because the chemical structure of MC-LR, a huge macromolecule, is much more complex than PPCPs and thus its complete mineralization takes huge time and effort (note Table 1). 4. Conclusions This study investigated the effect of LEDs with different UV wavelengths on direct photolytic and TiO2 photocatalytic decom-

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