VUV-Photocatalytic Degradation of Bezafibrate by Hydrothermally

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VUV- Photocatalytic Degradation of Bezafibrate by Hydrothermally Synthesized Enhanced {001} Facets TiO/Ti Film 2

Murtaza Sayed, Pingfeng Fu, Luqman Ali Shah, Hasan M.Khan, Jan Nisar, Muhammad Ismail, and Pengyi Zhang J. Phys. Chem. A, Just Accepted Manuscript • Publication Date (Web): 16 Dec 2015 Downloaded from http://pubs.acs.org on December 16, 2015

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The Journal of Physical Chemistry

VUV- Photocatalytic Degradation of Bezafibrate by Hydrothermally Synthesized Enhanced {001} Facets TIO2 / Ti Film Murtaza Sayed*¶, §, £, Pingfeng Fu¥, Luqman Ali Shah§, Hasan M.Khan§, Jan Nisar§, M. Ismail§, and Pengyi Zhang¶* ¶

State Key Joint Laboratory of Environment Simulation and Pollution Control, School of

Environment, Tsinghua University, Beijing 100084, China. §

National Centre of Excellence in Physical Chemistry, University of Peshawar, 25120,

Peshawar, Pakistan. £

Department of Chemistry, COMSATS Institute of Information Technology, Abbottabad, 22060,

Pakistan. ¥

School of Civil and Environment Engineering, University of Science and Technology Beijing,

Beijing 100083, China.

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Abstract

In the present study, a novel TiO2 / Ti film with enhanced {001} facets was synthesized by hydrothermal technique followed by calcination for studying the removal of bezafibrate (BZF), from an aqueous environment. The synthesized photocatalyst was characterized by FESEM, XRD, HR-TEM, and PL-technique. The second order rate constant of •OH with BZF was found out to be 5.66 × 109 M-1s-1. The steady state [•OH] was measured as 1.16 × 10-11 M, based on oxidation of terephthalic acid. The photocatalytic degradation of BZF followed pseudo-first order kinetics according to Langmuir-Hinshelwood model (k1 = 2.617 mgL-1min-1 and k2 = 0.0796 (mg L-1)-1). The effects of concentration and nature of various additives including inorganic anions (NO3‒, NO2‒, HCO3‒, CO32‒, Cl‒) and organic specie (fulvic acid) at various pHs (2, 4, 6, 9) on photocatalytic degradation of BZF were investigated. It was found that the nature and concentration of studied additives significantly affected the photocatalytic degradation of BZF. The efficiency of photocatalytic degradation process in terms of electrical energy per order was estimated. Degradation schemes were proposed based on the identified degradation byproducts by Ultra - performance liquid chromatography.


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1. Introduction

The existence of pharmaceuticals in the aquatic environment is always considered as a critical environmental concern since last few decades. The earlier reported work indicates the presence of pharmaceuticals in sewage treatment effluents, surface water, seawater, ground water and even in drinking water.1,2 Bezafibrate, that is, 2-(4-{2-[(4-chlorobenzoyl)amino]ethyl} phenoxy)-2-methylpropanoic acid, is a extensively used lipid regulator drug throughout the world, and consequently it has been frequently detected in the aqueous environment.3 Among various pharmaceuticals excreted into the sewage system, bezafibrate (BZF) is the most commonly detected pollutant and is classified as persistent organic compound.4 In drinking water its concentration has been detected in the range of 27 ng / L,

5

in rivers in the concentration

range of 0.1 ‒ 0.15 µg/L, 5 in small streams in the range of 0.5 – 1.9 µg / L, 5 in surface waters in the range of 3.1 µg/ L, 6 and up to 4.6 µg / L level in sewage treatment plant effluents. 6 Due to its high use and persistence nature, the removal of BZF from aqueous media has emerged as a hot research topic. Degradation kinetics and the quantification of identifying degradation products are also of great concern. Keeping in view all these issues , the present study was aimed to investigate the photodegradation process of BZF and its ultimate removal from aquatic environment using hydrothermally synthesized enhanced {001} facets TiO2/Ti film. There are many advanced treatment options for the removal of BZF from aqueous media, such as nanofilteration techniques, ultraviolet (UV) radiation and advanced oxidation processes (AOPs) 7 and these have been reviewed. AOPs as compared to other treatment techniques are more reliable as the pollutants of interest are converted to more stable, harmless inorganic species such as carbon dioxide, water and mineral salts. AOPs are classified as ozonation (O3), H2O2, O3/H2O2/photocatalysis, and O3/H2O2/UV photocatalysis.7,

8

TiO2 photocatalysis is 3

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considered as more promising technique among semiconductor photocatalysis.

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9-11

TiO2 has

shown a great potential in many applications, including water splitting to generate O2 and H2, 12, 13

water and wastewater treatment, 14, 15 gas phase treatment, 16-17 as well as in solar cells. 18 The

photocatalytic activity of TiO2 anatase crystal with long distance in {001} direction is reported to be superior than {101} facets. 3 However, mostly anatase TiO2 (both natural as well as synthetic) is composed of less reactive {101} facets because of its more thermodynamic stability as compared to {001} facets. The exposure of these high energy {001} facets can diminish rapidly during crystal growth due to minimization of surface energy; therefore the synthesis of anatase TiO2 particles with good exposure of {001} facets is a challenge. Although, a lot of efforts have been made to synthesize anatase TiO2 particles with increased percentage of exposed {001} facets

3, 19-30

however, to synthesize TiO2 films with

dominant {001} facets and with high photocatalytic performance is still a challenge. The film photocatalyst system has several advantages over powder system; (i) the catalyst filtration step after photocatalytic procedure is not necessary and (ii) the photocatalyst can be used for long term applications. 31 This research work has been focused on the synthesis of extraordinary efficient TiO2 film dominated by high energy {001} facets on Ti – substrate by hydrothermal technique followed by calcination. The synthesized TiO2 film with dominant {001} facets exhibited higher photocatalytic activity for decomposition of bezafibrate than commercially available P25. Though in synthetic wastewater, the efficiency decreased due to scavenging ability of additives, however, it can be almost completely rectified by using 185 nm UV to replace 254 nm UV irradiation.


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2. Experimental section 2.1.

Materials

Ti plates (99.5% in purity, Baoji Shengrong Titanium Corporation, Shuanxi Province, China), with a size of 180 mm × 120 mm × 0.15 mm was used as supporting material. BZF was provided by Sigma-Aldrich. The molecular mass, structure and other characteristics of BZF are shown in table 1. Methanol (HPLC grade) with 99.93% purity was purchased from Sigma Aldrich. HCl (trace metal grade), sodium hydroxide, sodium nitrate, sodium chloride, sodium carbonate, sodium bicarbonate, sodium nitrite, fulvic acid and phenol (99% +) were provided by fisher scientific, UK and used as received. All the solutions including standards used in the experiments were prepared with deionized water made with Milli-Q water purification system (Millipore, USA). 2.2.

Catalyst preparation

TiO2 / Ti thin film with dominant {001} facets was prepared by a simple hydrothermal technique followed by calcination. Ti plates used in the study were washed by heating in oxalic acid solution for 40 mins at 100 ºC prior to hydrothermal treatment to remove adsorbed oxides from the surface. In a typical synthesis procedure, Ti plate (180 mm×120 mm) was dipped in a mixture of chemical solution consisting of 110 mL Milli-Q water, 60 mL iso-propanol and hydrogen fluoride with a concentration of 0.03M at pH 2.62. The mixture was then transferred to 180 mL Teflon – lined stainless steel autoclave and maintained at 180 ˚C for hydrothermal time of 3 hrs. After the hydrothermal treatment, {001} facets dominated TiO2/Ti film was taken out from the autoclave, cooled down and washed with Milli-Q water. Finally, it was calcinated at 600 ˚C for 90 minutes to remove the surface adsorbed fluoride ions.



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2.3.

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Characterization of {001} facet TiO2/Ti film

X-ray diffraction of resulting photocatalyst was carried out with a Rigaku D/max-RB using Cu Kߙ radiation (ߣ = 0.15418nm), operated at 40 kV and 100 mA. The surface morphologies of {001} facet TiO2/Ti film were observed using field emission scanning electron microscope (FE-SEM, 5500, Hitachi). The high resolution transmission electron microscopy (HR-TEM) analysis was conducted using a JEM-2011F electron microscope (JEOL, Japan). The quantitative analysis of •OH-radical produced photocatalytically was carried out by using Hitachi F-7000 fluorescence spectrophotometer. 2.4.

Photocatalytic experiments

Photocatalytic experiments were conducted in a 120 mL cylindrical vessel with inner diameter of 30 mm and length of about 300 mm (Fig. S1). During our experiments, two low – pressure mercury lamps of Cnlight Co Ltd. were used to provide UV and VUV-irradiation. A UV lamp emitted at 254 nm (15 W, hereafter referred as UV) and VUV–lamp emitted at both 254 nm and 185 nm (15 W, hereafter referred as VUV). The lamp was introduced in the middle of photoreactor with quartz envelope, the inner diameter of which was 18 mm. The UV intensity (254 nm) calculated by iodide/iodate actinometery32 was found to be 1.65 mW/cm2 and the VUV intensity (185 nm) was 0.24 mW/cm2, measured by the oxygen actinometery.33 The photocatalytic reactor was maintained at room temperature (25 ˚C) throughout the experiment by continuously circulating cold water around it. Oxygen gas was supplied from the bottom of the reactor at the flow rate of 30 mL /min. Before irradiation experiments, the bezafibrate solution was kept in the dark to ensure adsorption equilibrium of BZF on catalyst. The initial pH of BZF solution was 6.5. Approximately, 4 mL of BZF solution was taken out after every 15 mins of irradiation. In order to check the reproducibility of the experimental results, the experiments


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were performed in triplicate. The BZF (10 mg/L) test solutions were prepared in Milli-Q water (pH = 7.6) and synthetic wastewater. The synthetic wastewater was prepared by alternative addition of NO3‒ (2 – 15 mM), CO3-2 (2 -15 mM), HCO3‒ (2-15 mM), NO2‒ (2 -15 mM), Cl‒ (2 -15mM), fulvic acid (2-15 mM) and at various pHs (2, 5, 7, 11). 2.5.

Calculation of •OH-radical concentration

The amount of •OH-radials formed during photocatalysis of BZF aqueous solution was measured by irradiating the aqueous solution of 6 × 10-4 M terephthalic acid and then monitoring the concentration of its byproduct, 2-hydroxy terephthalic acid by photoluminescence technique (PL). 3 The production of 2-hydroxy terephthalic acid is shown below;

O

O

OH

HO .

HO

HO

OH O

2.6.

OH

k = 4.4 × 10 9 M -1 s -1

O

2 -h yd ro xy te re p h th a lic a cid

Analytical procedures

To check the variation in the BZF concentration during photocatalytic process, the aliquots were analyzed on a high performance liquid chromatography (HPLC, Shimadzu, LC10AD) with a UV detector (SPD-10AV) at 230 nm and a Kromcil C18 column (250 × 4.6 mm) for separation. The mobile phase was a mixture of methanol and water (70:30, v/v) at a flow rate of 1 mL min-1. The qualitative analysis of BZF degradation products was done on ultra-performance liquid chromatography – tandem mass spectrometry (UPLC-MS/MS). The optimized mass parameters were as follows: capillary voltage, 2.1 kV; source temperature, 120 ˚C; desolvation



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temperature, 280 ˚C; and desolvation gas flow rate, 650 L / h. The parent ion, daughter ion, cone voltage (V), and collision energy (eV) for BZF detection were 360.22 > 274.12, 22, and 15. In addition, the full scan mode was used to detect parent ions for acquiring more information about intermediates. Samples were directly injected into the mass spectrometer ion source by pure methanol with the cone voltage of 10 V. 3. Results and Discussion 3.1.

Characterization of {001} facets TiO2/Ti films

Fig. 1 (a) and (b) show FE - SEM micrograph of {001} facets exposed TiO2/Ti film and particle size distribution respectively. It can be seen that TiO2 crystals are fairly uniform in distribution and have sheet-like morphology. From this image the average {001} facets size was determined to be 0.35 µm by measuring the size of 50 particles through “Nano Measure 1.2” software. The fine structure of synthesized {001} facets TiO2/Ti film was further characterized by HR-TEM, and the results are displayed in Fig. 2. This figure shows TEM image of an individual nanostructure TiO2 crystal with exposed {001} facets. At the bottom of the TiO2 crystal, the high-resolution TEM (HRTEM) image was taken, the inset shows the selected area electron diffraction (SAED) pattern. The continuous atomic planes with a lattice fringe spacing of 0.22 nm, correspond to the (0 0 1) crystal planes of anatase TiO2. Both the SAED and the HRTEM confirm that the synthesized TiO2/Ti with exposed {001} facets is single crystalline anatase TiO2 and thus favors the migration of photoengraved electron-hole pairs.34, 35 Our previous study indicated that calcination treatment at 600˚C effectively removed F‒ ions.3 This observed phenomena supports our present observations. The XRD patterns of TiO2 samples before and after calcination treatment are shown in Fig. 3. It is clear that the diffraction peaks at 2θ = 25.22º, 36.82º, 37.67º, 38.54º, 47.94º, 53.79º, 55.02º,


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62.64º, 68.72º and 70.51º can be attributed to (101), (103), (004), (112), (200), (105), (211), (213), (204) and (110) of the standard card JCPDS 21-1272 corresponding to anatase, suggesting a high phase purity for the achieved titania. It is also clear from our XRD patterns before and after calcination treatment that there was no change in crystal structure after heat treatment at 600˚C. The percentage of rutile and anatase character was determined by using the following equation; 36, 37 Xa = [1+1.26(Ir/Ia)]-1 and

Xr = 1-Xa

so,

% anatase = Xa × 100 % rutile = Xr × 100

Where Xa represents the rate of anatase in mixture and Xr is the rate of rutile in the same mixture. Ia corresponds to the integrated intensity (101) reflection of anatase, and Ir represents the integrated intensity of the (110) reflection of rutile. The synthesized TiO2 / Ti film consist of ca. 83% anatase and 17 % rutile. Besides TiO2, there are other diffraction peaks at 2θ = 35.96º, 39.95º, 52.92º which correspond to titanium (Ti) sheet (JCPDS No. 05-0628). There is also a diffraction peak at 2θ = 53.82º for sample prepared with addition of isopropanol which belongs to TiOx. The appearance of Ti diffraction peaks in XRD of the TiO2 films grown on Ti-substrate was also reported.

38

The rutile character may be

due to the Ti-substrate to which the TiO2 film was directly attached. 3.2.

Measurement of reaction rate constant of • OH with BZF

Absolute bimolecular rate constant of •OH with BZF was calculated using parachlorobenzoic acid (p-CBA) as probe molecule by competition kinetics equation established by Pereira and coworkers39 as depicted in equation (1).



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k • OH ( s ) =

ks (UV / H2O2 ) − ks (UV ) × k • OH ( ref ) kref (UV / H2O2 ) − kref

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(1)

Where, k•OH, k (UV/H2O2) and k (UV) represent the second order rate constant of hydroxyl radical, UV/H2O2,fluence based rate constant of UV/ hydrogen peroxide process and UV, fluence based rate constant of direct photolysis, respectively. The notations “s” and “ref” represents the substrate and reference compounds, which in our case is BZF and p-CBA, respectively. For the determination of •OH rate constant with BZF, two sets of experiments were performed. In one set of experiments, the solution containing 27.63 µM of BZF, 27.63 µM of p-CBA and 1 mM of H2O2 was exposed to UV irradiation, while another set of experiments was free of H2O2 to calculate kUV. The concentration of H2O2 was kept higher for ensuring production of efficient •

OH with UV-photocatalysis. Figure 4 shows degradation curves for BZF and p-CBA, both BZF and p-CBA were found

to follow pseudo-first order degradation kinetics. The second order rate constant of •OH with p CBA is 5.0 × 109 M-1s-1. 40 By substituting pseudo-first order degradation constants (kUV/H2O2 and kUV) values in equation 7, the bimolecular rate constant of • OH with BZF was calculated and found out to be 5.66 × 109 M-1s-1. 3.3.

Measurement of • OH-radical concentrations

The steady - state concentration of • OH-radical in BZF solution due to photocatalysis using {001} facets TiO2/Ti film was measured based on oxidation of terephthalic acid, as illustrated in equation (2). 41

d [T A − O H ] = k [T A ] [ • O H ] dt

(2)

Where “k” is the bimolecular rate constant of •OH-radical with terephthalic acid, [TA] is the


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concentration

of

terphthalic

acid,

[TA-OH]

is

the

concentration

of

produced

2-

hydroxyterephthalic and [• OH] is the concentration of hydroxyl radical. From equation (2), the concentration of

•

OH was calculated as 1.16 × 10-11 M. Which is nearly 105 times higher than

the concentration of • OH found in natural waters. 42-44 3.4.

Photocatalytic performance of {001} facets TiO2/ Ti film towards degradation of BZF.

During dark controlled photocatalytic experiments,

no obvious loss of bezafibrate

molecule was detected, indicating negligible degaradtion by hydrolysis or thermal degaradtion . Fig. 5 shows the comparsion on the photocatalytic performance of {001} facets TiO2/Ti film towards degaradtion of BZF under UV-254 nm, UV-254 nm + {001} facets TiO2/Ti, VUV- 185 nm, VUV- 185 nm + {001} facets TiO2/Ti and in the presence of TiO2/Ti (P25). It can be seen that the appearant rate constant (kapp) for the degradation of BZF was decreased in the order of VUV + {001} facets TiO2/Ti > VUV + P25 > VUV alone > UV + {001} facets TiO2/Ti > UV + P25> UV alone. VUV (185 nm) – radiations are strongly absorbed by water with absorption coefficient of α = 1.8 cm-1. 45 As a result of VUV absorption by water, homolysis and photochemical ionization of water takes place as shown by equations (3) and (4), respectively. 46 •

hν (185 nm ) H 2 O  → OH + • H

Φ (+• OH) = 0.33

•

hν (185 nm ) H 2 O  → OH + + H + e-aq Φ (+e-aq ) = 0.045

(3) (4)

In oxygen saturated medium, •H atoms and eaq‒ are converted to HO•2 and O2•‒ respectively as shown by equations (5) and (6). 47 O 2 + H • → HO 2•

k = 1 × 1010 M-1s-1

(5)

O 2 + e-aq → O 2•-

k = 2 × 1010 M-1s-1

(6) 11

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Because of the direct formation of

•

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OH, VUV irradiation can be regarded as an advanced

oxidation process (AOP). Besides • OH – radicals, eaq

‒

and •H are also generated which can

initiate many reactions in aqueous media with the help of molecular oxygen and oxygenated species.

48

Furthermore, VUV – 185 nm light can easily be obtained from commonly available

low-pressure mercury lamps with low prices and thus further makes it a promising technique for practical applications. Effect of Initial BZF concentration

3.5.

It is important to investigate the effect of contaminant concentration on the photocatalytic degradation process for future large scale practical applications. The initial concentration of BZF was varied in the range of 9.57 – 35.47 mg/L. As shown in Fig. 6, the initial BZF concentration decreases with increase in the irradiation time. However, at 30 min of irradiation time the concentration of remaining BZF increases with increase in the initial concentration of BZF. The reasons for this behavior can be explained follows; firstly, as the concentration of BZF molecule is increased more active sites of {001} facets TiO2/Ti films are adsorbed by BZF molecules which in turn effects the photocatalytic activity. Secondly, with increase in the BZF concentration the photons are get adsorbed by BZF molecules rather than catalyst surface and hence lowers the production of •OH radicals. Langmuir- Hinshelwood (L-H) pseudo–first order kinetics was used to study the dependence of initial concentration of BZF on the photocatalytic degradation rate as follows. 49 r=

k1k2 [ BZF ] = kapp [ BZF ] 1 + k2 [ BZF ]0

(7)

Which also gives,

1 kapp

=

[ BZF ]0 1 + k1k2 k1

(8)


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Where [BZF]0 is the initial concentration of BZF, k1 (mg L-1 min-1) is the kinetic rate constant of surface reaction, k2 (mgL-1)-1 is taken to be Langmuir adsorption constant. This L-H kinetic model has been applied by several many authors for heterogonous photocatalytic reactions.

50-53

Fig. 7 shows a linear representation of L-H model by plotting 1/kapp Vs [BZF]0.

The values obtained for “k1” and “k2” were 2.617 mg L-1min-1 and 0.0796 (mg L-1)-1, respectively. 3.6.

Effect of organic/inorganic additives and initial solution pH

The effect of various anions and fulvic acid on the photocatalytic degradation of BZF was investigated using immobilized {001} facets TiO2/Ti films at different concentrations and the results are shown in Fig. 8 (a). It can be seen that all the anions have significantly decreased the degradation rate of BZF. The negative effect of inorganic anions was found in the order of NO2‒ > CO32‒ > NO3‒ > HCO3‒ > Cl‒. Since the common cation of all the added anions is Na+, which is inert, hence only anions are involved in the inhibition process. Moreover, the degradation rate of BZF decreases with increase in the concentration of these anions. The inhibitory effect of these inorganic anions can be explained in the following ways; (i)

There is competitive adsorption of anions and BZF molecules onto the surface of

TiO2 / Ti film, and thus resulting in the blockage of the active sites on the film surface which causes the decrease in photocatalytic degradation of BZF molecule by oxidative species.

54, 55

There is also a possibility that these negatively charged anions reacted with positive holes (h+) and hydroxyl radicals (•OH) to form less reactive species and thus these anions act as scavenger for h+ and •OH resulting in a decrease in the photocatalytic degradation efficiency. 56 Chloride ions act as scavenger of •OH and h+ as shown by reactions (9) and (10), respectively. The two Cl• will then recombine to terminate the reaction as shown by reaction



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(11). 57 Cl‒ + •OH → Cl• + OH‒

k = 3 × 109 M-1s-1

(9)

Cl‒ + h+ → Cl

(10)

Cl• + Cl• → Cl2

(11)

HCO3‒ and CO32‒ scavenge •OH to form CO3‒• (reactions (12) and (13)) which is less reactive than •OH54 and thus resulting in the decrease of photocatalytic degradation of BZF. HCO3‒ + •OH → CO3‒• + H2O

k = 1 × 107 M-1s-1

(12)

CO32‒ + •OH → CO3‒• + OH‒

k = 4 × 108 M-1s-1

(13)

Nitrite ions (NO2‒) are believed to be the strongest scavengers of •OH radicals as shown by reaction (14). 40 This is the reason why nitrite ions showed greatest decrease in photocatalytic degradation rate. NO2‒ + •OH → OH‒ + NO2•

k = 1.4 × 1010 M-1s-1

(14)

Nitrate ions (NO3‒) themselves are not the scavengers of • OH radicals, however, by the absorption of photons they can give NO2‒ ions as byproduct (reaction (19)),

47

which in turn

compete with BZF molecule for • OH (reaction (14)) and thus a decrease in the photocatalytic process is observed. NO3‒ + hν → NO2‒ + O

(15)

NO3‒ + hν → O•‒ + NO2•

(16)

Surprisingly, when fulvic acid (FA) was added as natural organic matter, The VUVphotocatalytic degradation rate of BZF using {001} facets TiO2/Ti film was increased. FA has strong adsorption capability on the semiconductor particles, with an apparent association constant of 12000 ± 500 M-l. This increase in the degradation rate when FA is added could be attributed to sensitization mechanism of FA.58 The excited FA injects electrons into the


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conduction band (CB) of TiO2 which can be represented by the following elementary steps; kad FA +TiO 2 ←→ FA − TiO2

hν  → FAaq* FA aq ← hν '

hν FA-TiO 2  → FAaq* - TiO 2

FAaq* + TiO2 → FAox + TiO2 (e‒cb) As a result, these injected conduction band electrons may enhance the VUV photocatalytic oxidation of BZF molecule using {001} facets TiO2/Ti film. The initial pH of the solution is another parameter that can affect the degradation rate of BZF using {001} facets TiO2/Ti film. Due to a direct relationship between the pH and the surface property of the catalyst, the initial solution pH influences the adsorption property of the target pollutant and sometimes even the degradation pathway.

59

It can be seen from Fig. 8 (b) that the

degradation rate of BZF is higher under basic media (pH =9) and decreases as the solution pH goes to acidic media (pH = 2). In basic media, excess of hydroxyl (OH‒) ions are available around the surface of the catalyst, which then reacts with photogenerated holes and thus higher concentration of hydroxyl radical are formed as compared to acidic media. As the photocatalytic degradation of BZF is associated with the presence of •OH radicals in the solution, therefore, efficient removal of BZF is achieved in alkaline media. TiO2 has an isoelectric point of 6.5, and the surface of {001} facets TiO2/Ti film has negatively charged at pH higher than 6.5 and positively charged at lower pH values. Besides this, the pKa-value of BZF is 3.62, where the main part of BZF carries an overall positive charge. The decrease in photocatalytic degradation of BZF at pH=2 might be due to repulsive forces between TiO2 surface and BZF molecule. 3.7.

Electrical energy consumption for VUV-photocatalytic degradation of BZF Electrical energy per order (EEO), provides best approach to investigate the energy 15 ACS Paragon Plus Environment


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efficiency of photocatalytic process. In the case of low pollutant concentration (which applies in our case) the electrical energy per order (EEO) may be defined as “the amount of kilowatt hour of electrical energy needed to decrease the concentration of a pollutant by 1 order of magnitude (90 %) in a unit volume of contaminated water”. For a batch type reactor the EEO (kWh/m3/order) can be calculated as follows. 60

EEO =

P × t × 1000 V × 60 × log(Ci /Cf )

EEO =

38.4 × P V × kapp

(18)

(19)

Where “P” is the lamp power (kW), “t” is the irradiation time (min),“V” is the treated volume (L), “Ci” and “Cf” are the initial and final concentrations of pollutant, and “kapp” is the pseudofirst order degradation rate constant (min-1). The factor 38.4 in Eq. (2) is the conversion factor 1000 × ln (10)/60. The calculated EEO values for photocatalytic degradation of BZF at initial concentration of 10, 20, 30 and 40 mg/L are given in Table 2. It is more appropriate to apply EEO values found in this study for treatment costs. For example, if the electricity cost in China is $ 0.107 per kWh, then the treatment cost for removal of 40 mg/L of BZF from aqueous solution will be $ 9.183 per m3. In addition, there will be small cost factors for the chemicals used and for the VUV-lamp replacement. 3.8.

BZF degradation pathways

Based on the structures of identified intermediates, two schemes are presented for the degradation of bezafibrate. As shown in scheme 1, the bezafibrate molecule undergoes single (m/z = 275.73072), and double (m/z 291.7286) hydroxylation. Since •OH radical had a strong electrophilic character so it will preferably attack the carbon atom having more electron density, 16 ACS Paragon Plus Environment

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i.e. the phenoxy ring. The degradation intermediate (DI) with m/z of 154.382, identified as 4chlorobenze amide was resulted after the successive, oxidation-decarboxylation cycles. 4chlorobenzoic acid with m/z of 156.5660, generated via the hydrolytic cleavage of amide bond of BZF, has also been identified as major hydrolysis product by microbial degradation.

61

4-

chlorobenzoic acid may then undergo decarboxylation reaction resulting into the formation of 4chlorophenol (m/z = 128.5568). Since the Cl group of 4- chlorophenol is ortho- para- directing group, so it will direct the incoming OH group at ortho and para positions on the ring giving DI with m/z = 144.5561 (4-chlorocatechol or 4- chlorocresorcinol). OH OH H N CH2 Cl

C

O Bezafiberate

O

C O C

H3 C

CH3

.OH -C4H8O2

OH Cl

HN CH2 C O

.OH

OH

H N CH2 Cl

C

.OH, NH2 Cl

-CO2

O

O

m/z = 154.382

m/z = 291.728

m/z = 275.730

C

.OH

-NH2

OH

.OH Cl

CO2+H2O

Cl

OH

OH HO

Cl

-CO2

C

m/z = 128.556

m/z = 144.556

Scheme 1

The possible degradation pathway, when the reactive hydroxyl radical attack the 4chlorobezoyl ring side is presented in scheme 2. Initially, •OH-radical replaces Cl- atom from BZF molecule resulting into the formation of degradation product with m/z = 343.372. Degradation product with m/z = 257.284 may be obtained as a result of hydroxylation of degradation product with m/z = 343.373. 4 - hydroxy benzoic acid (m/z = 138.121) is the


O

m/z = 156.566


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outcome of hydrolytic cleavage of amide bond of degradation product with m/z = 257.284. 4hydroxy benzoic acid (m/z = 138.121) after net loss of CO2 molecule is converted to hydroquinone (m/z = 110.114). The degradation product with m/z = 126.110, identified as hydroxy hydroquinone was produced from hydroxylation of hydroquinone, which is in agreement with the previous study conducted for photocatalytic degradation of chlorobenzoic isomer.

62

The detection of low molecular weight organic acids like α – hydroxyiso-butyric acid

(m/z = 104.0991) and lactic acid (m/z = 90.0779) are expected to be final products before mineralization.

OH

OH O

H N CH2 Cl

C

C O

.OH

CH3

-Cl

H N CH2

C H3C

O Bezafiberate

HO

C

O

O

C O

.

OH

H N CH2

C H3 C

CH3

-C4H8O2

HO

C

m/z = 343.372

OH

O

m/z = 257.284 OH

OH O

OH C

HO

O

OH C

CH CH3

m/z = 90.077

HO

C

HO

.OH

HO

OH

OH

CH3 CH3

HO

m/z = 126.110

-CO2

C

m/z = 138.121

m/z = 110.111

m/z = 104.104

Scheme 2 Conclusions TiO2 - thin films with enhanced {001} facets was successfully synthesized by a simple hydrothermal technique followed by calcination at 6000C. It was then characterized by different various techniques and was found out as a promising photocatalyst for the destruction of organic pollutants from aqueous media. The reaction rate constant of •OH with BZF was calculated and found out to be 5.66 × 109 M-1s-1, by competition kinetics method. •OH concentration was


O

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calculated as 1.16 × 10-11 M by VUV photocatalysis in the presence of {001} faceted TiO2/Ti film. Langmuir-Hinshelwood pseudo-first order kinetics was successfully applied at initial concentration in the range of 9.57 – 35.47 mg/L. It was found that inorganic additives such as Cl‒, NO3‒, CO32‒, HCO3‒ and NO2‒ restrained the degradation of BZF and caused a negative effect on the photocatalytic processes. However, fulvic acid as organic additive enhanced the photocatalytic degradation of BZF. The initial solution pH values also affected the photocatalytic degradation of BZF from aqueous solution. It was noticed that the basic condition (pH= 9) was more favorable for the photocatalytic degradation of BZF using {001} facets exposed TiO2/Ti film. Two possible degradation schemes were proposed based on identified degradation products.

SUPPORTING INFORMATION Experimental setup for evaluating the photocatalytic activity. This material is available free of charge via the internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *

Murtaza Sayed. Email addresses:[email protected] number:-+92-3326811276

*

Pengyi Zhang. Email address:- [email protected]. Phone number:- +86-13801107587

ACKNOWLEDGEMENTS This work was funded by the National Basic Research Program of China (2013CB632403), Science Fund for Creative Research Groups (21221004), and the Collaborative Innovation Center for Regional Environmental Quality. Murtaza Sayed is thankful to Higher Education Commission (HEC) of Pakistan for partial funding of this work.



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Tables

Table 1 Characteristics of bezafibrate Chemical structure Molecular formula

Molecular mass

pka - value

Cl

HN

C19H20ClNO4

361.819 g mol-1

O

3.61

O O

OH


Page 27 of 38

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Table 2 Apparent rate constants (kapp), half-lives (t1/2) and electrical energy per order (EEO) for photocatalytic degradation of BZF at different initial consternations.

[BZF]0 mg/L 10

kapp (min-1)

t1/2 (min)

EEO (kWh m-3)

0.1134

6.112

39.506

20

0.0844

8.212

53.080

30

0.0675

10.268

66.370

40

0.0522

13.278

85.823



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Figures

(a)

(a)


Page 29 of 38

(b) 70

60

50

Frequency %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40

30

20

10

0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Mean/µm

Fig.1 FESEM image of {001} facet TiO2 film on Ti-substrate (a), particle size distribution (b). Other conditions are 110 mL water solution, 60 mL iso – propanol, HF= 0.03M, hydrothermal temperature of 180˚C, pH = 2.8 (natural pH of hydrothermal solution) and hydrothermal time of 3hrs.



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Fig. 2 TEM image of single crystal anatase TiO2 scratched off from Ti-substrate, the corresponding HR-TEM image of anatase TiO2 single crystal recorded along {001} axis and SAED pattern.


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A: Anatse R: Rutile Ti: Titanium

A

A Ti Ti

A

A

20

25

30

35

40

A

A

A

45

50

55

A

60

65

70

75

80

2 theta / degree Fig.3 XRD pattern of the TiO2/Ti film with exposed {001} facets



1.0

0.8

BZF pCBA pCBA + H2O2

2.5

BZF + H2O2

0.6

C/C0

y = 10.1E-03x 2.0

ln C0/C

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 38

0.4

y = 8.8E-03x 1.5

1.0

0.5

y = 3.67E-04x

0.2

y = 2.02E-04x

0.0

0

100

200 300 2 UV fluence (mJ/cm )

400

500

0.0 0

100

200

300

400

500

2

UV fluence (mj/cm ) Fig. 4 Determination of bimolecular rate constant of BZF with • OH; inset shows the degradation kinetics of BZF alone, pCBA alone, BZF+ H2O2, pCBA + H2O2 exposed to UV-irradiation.


Page 33 of 38

UV VUV UV+P25 VUV+P25 UV+{001} facets TiO2/Ti

0.14

0.12

VUV+{001} facets TiO2/Ti 0.10

-1

kapp (min )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.08

0.06

0.04

0.02

0.00

Fig. 5 Comparative photocatalytic degradation of BZF by UV-alone, VUV-alone, UV+P25, VUV+P25, UV + {001} facets TiO2/Ti and VUV + {001} facets TiO2/Ti.



40 35

9.5 mg/L 18.89 mg/L 28.75 mg/L 35.47 mg/L

30 25

Conc. (mg/L)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 38

20 15 10 5 0 0

5

10

15

20

25

30

Irradiation Time (mints.)

Fig. 6 Effect of initial BZF concentration on photocatalytic efficiency of {001} facets TiO2 / Ti thin film.


Page 35 of 38

25

y = 0.382x + 4.798 2 R = 0.983 20

1/kapp

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


15

10

5

10

15

20

25

30

35

40

[BZF]0 Fig. 7 Representation of Langmuir Hinshelwood for photocatalytic degradation of BZF by {001} facets TiO2/Ti film.



(a) Absence of additive − Cl − HCO3

0.14 0.12

fulvic acid − NO3

0.10

k o b s (m in − 1 )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 38

2−

CO3

0.08

−

NO2 0.06 0.04 0.02 0.00

0

2

5

10

15

mM


Page 37 of 38

(b) 1.0

pH = 2 pH = 4 pH= 6 pH = 9

0.8

0.6

C/C0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.4

0.2

0.0 0

5

10

15

20

25

30

Time (min)

Fig. 8 (a) Effect of inorganic anions and fulvic acid concentration on the photocatalytic degradation of BZF using {001} facets TiO2 / Ti film. (b) The influence of pH on the photocatalytic degradation of BZF by {001} facets TiO2/Ti.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 38

TOC graphic for manuscript


VUV-Photocatalytic Degradation of Bezafibrate by Hydrothermally

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