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peroxymonosulfate process: Kinetics and mechanistic investigations
Accepted Manuscript Title: Efficient degradation of lindane by visible and simulated solar light-assisted S-TiO2 /peroxymonosulfate process: Kinetics and mechanistic investigations Author: Sanaullah Khan Changseok Han Hasan M. Khan Dominic L. Boccelli Dionysios D. Dionysiou PII: DOI: Reference:
S1381-1169(16)30512-X http://dx.doi.org/doi:10.1016/j.molcata.2016.11.035 MOLCAA 10132
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date: Revised date: Accepted date:
27-9-2016 23-11-2016 24-11-2016
Please cite this article as: Sanaullah Khan, Changseok Han, Hasan M.Khan, Dominic L.Boccelli, Dionysios D.Dionysiou, Efficient degradation of lindane by visible and simulated solar light-assisted S-TiO2/peroxymonosulfate process: Kinetics and mechanistic investigations, Journal of Molecular Catalysis A: Chemical http://dx.doi.org/10.1016/j.molcata.2016.11.035 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Efficient degradation of lindane by visible and simulated solar lightassisted S-TiO2/peroxymonosulfate process: Kinetics and mechanistic investigations
Sanaullah Khan1,2,3,†, Changseok Han3,†, Hasan M. Khan2 , Dominic L. Boccelli3 , Dionysios D. Dionysiou3,4 *
1
Departmen of Chemistry, Shaheed Benazir Bhutto University, Sheringal Dir (Upper), Khyber Pakhtunkhwa, Pakistan 2
Radiation Chemistry Laboratory, National Centre of Excellence in Physical Chemistry, University of Peshawar, Peshawar 25120, Pakistan
3
Environmental Engineering and Science Program, University of Cincinnati, Cincinnati, Ohio 45221-0012, USA 4
Nireas-International Water Research Centre, University of Cyprus, Nicosia 1678, Cyprus *
Corresponding author Email: [email protected] Tel: +1-513-556-0724; Fax: +1-513-556-2599
†Contributed equally to this work.
Lindane removal (%) after 6 h
Graphical abstract
100
80
60
40
20
0
n
o iti nd
S-
O Ti
o kc r a /D 2
T S-
ht ht ht lig lig lig r r e l la la ib so so - /vis d d e e 5 at at ul SO ul m m /2 H i - /Si /S O 2 Ti O 5 i O S T S S/H 2 O Ti St
h lig e bl isi /V
iO 2
Highlights
Removal of lindane was studied by visible and simulated solar light-assisted S-TiO 2 .
Efficiency of S-TiO2 photocatalysis dramatically increased in the presence of HSO 5 −.
Operational parameters such as pH, concentration of lindane and HSO 5 − were optimized.
Degradation mechanism was proposed based on identified intermediates and final products.
S-TiO 2 photocatalysis is an effective AOP to treat lindane contaminated waters.
Abstract Organochlorine pesticides (OCPs) are toxic and are among the most potent endocrine disrupting chemicals in the environment. Most OCPs are resistant towards oxidation by •OH due to presence of electron-withdrawing chlorine group in their molecular structures. Here, we investigated
a
visible
and
simulated
solar
light-assisted
sulfur
doped
TiO 2
(S-
TiO2 )/peroxymonosulfate (HSO 5 −) process to eliminate a selected OCP, lindane. Initially, visible and simulated solar light-assisted S-TiO 2 photocatalysis resulted in 31.0 and 63.4% removal of lindane (C0 = 1.0 µM), respectively in 6 h. The photocatalytic activity of S-TiO 2 was dramatically increased in the presence of 0.2 mM HSO 5 −, leading to 68.2 and 99.9% lindane removal under visible and simulated solar light illumination, respectively in 6 h. The observed pseudo first-order rate constant for simulated solar light-assisted S-TiO2 /HSO5 − decreased with increasing initial concentration of lindane, corresponding to 8.98 × 10−1 , 6.58 × 10−1 and 3.84 × 10−1 h−1 at [lindane]0 of 0.5, 1.0 and 2.0 µM, respectively. The degradation kinetics were significantly affected by solution pH, leading to 88.2, 99.9 and 71.4% removal of lindane in 6 h at pH 4.0, 5.8 and 8.0, respectively. S-TiO2 film exhibited a high mechanical strength with only 6.4% loss of efficiency after four repeated cycles. Based on the detected reaction intermediates, a possible reaction mechanism was proposed, suggesting dechlorination, dehydrogenation, and hydroxylation via •OH, SO4 •− and O2 •− attack. The results suggest that visible and simulated solar light-assisted S-TiO2 /HSO 5 − is a promising alternative for treatment of water contaminated with most OCPs.
Keywords:
Lindane;
S-TiO2
photocatalysis;
Visible and
Peroxymonosulfate; Reaction mechanism; Water treatment.
simulated
solar light activity;
1. Introduction Organochlorine pesticides (OCPs) represent a major source of emerging water pollutants in
recent
decades
[1].
Among
organochlorine
compounds,
the
gamma
isomer
of
hexachlorocyclohexane (γ-HCH), lindane, has been widely used as a broad spectrum insecticide in agriculture since the 1940s [2]. It is also a commonly found ingredient in some lotions, creams and shampoos due to its antiparasitic property [3]. Therefore, its residues have been detected worldwide in many environmental segments [2], ultimately entering into human body through food chains [4]. In particular, lindane poses potential health risks to humans and other living organisms due to its high toxicity [5]. For example, acute exposure to lindane can cause mild skin irritation, dizziness, headaches, diarrhea, nausea, vomiting, convulsions, and death. Chronic exposure can adversely affect the liver and nervous systems of animals. Lindane was reported to be a potential carcinogen and teratogen [6]. Although the use of lindane as an insecticide has been restricted in many European countries as well as the United States (US) since 1975, it is still being used in many other countries [7]. Lindane was used in the United States and Canada for seed treatment, till recent years [8]. Therefore, decontamination of lindane contaminated water has become of great importance in recent years. Advanced oxidation processes (AOPs) have attracted much attention for decomposing a wide range of recalcitrant organic pollutants in water among the currently available technologies due to the in-situ generation of highly reactive oxygen species (ROS) at ambient pressure and temperature conditions [9]. During the last few decades, extensive research studies have been carried out on TiO 2 based AOPs for abating environmental pollution, because of low cost, easy availability, wide versatility, and good chemical and thermal stability of TiO 2 [10-14]. Upon UV light (λ < 388 nm) absorption, TiO 2 produces conduction band electrons (eCB−) and valence band
holes (hVB+) (reaction (1)) [11] which despite having recombining and/or trapping affinity in the TiO2 lattice, migrate to the catalyst surface and initiate various chemical redox processes (2)-(5)) [11, 15]. TiO2 + hν → hVB+ + eCB−
(1)
hVB+ + eCB− → Energy
(2)
hVB+ + H2 O → •OH
(3)
hVB+ + HO − → •OH
(4)
eCB− + O2 → O2 •−
(5)
In recent decades, there have been increasing research studies into the development of more efficient non-metal doped TiO 2 materials which can utilize low energy (visible light) photon for excitation of valence band electrons [16-20], thus making TiO 2 photocatalysis a sustainable alternative employing solar energy. Among non-metals (carbon, nitrogen, fluorine, etc.), sulfur is one of the most widely used candidates for synthesizing visible light-active TiO 2 photocatalyst in recent years [20-23]. Although significant advances have been made, visible and solar light-active non-metal doped TiO2 photocatalysts exhibit slow degradation rate or low quantum yield on degradation of highly bio-refractory organic compounds [24]. One more effective strategy was the coupling of various inorganic oxidants such as hydrogen peroxide (H2 O2 ), peroxymonosulfate (HSO 5 −) or persulfate (S2 O 8 2−) with TiO2 photocatalysts, which enhances the degradation kinetics by ways of (i) electron trapping thus limiting the rate of electron-hole recombination and (ii) the generation of •
OH and/or SO 4 •− radicals [19, 24-27]. Both •OH and SO 4 •− are strong oxidizing agents with
standard oxidation–reduction potentials of 2.4-2.7 and 2.5–3.1 V, respectively [28], and capable of decomposing most organic pollutants in water. Different from •OH, which tends to react
rapidly with organic molecules through hydrogen abstraction or addition reactions, SO 4 •− usually participates in electron transfer reaction [29]. Consequently, SO4 •− is more selective for oxidation reactions and with many organic compounds SO 4 •− reacts as a more efficient oxidant than •OH [29]. Peroxymonosulfate (HSO 5 −) was chosen as an oxidant due to its strong oxidation properties [30, 31] as well as its ability to generate both •OH and SO 4 •−. In this study, photocatalytic activity of a nanostructured sulfur doped TiO 2 (S-TiO2 ) photocatalyst film synthesized by a sol-gel method was investigated for degradation of lindane under visible and simulated solar light irradiation. In particular, the enhancing effect of HSO 5 − on the efficiency of S-TiO2 photocatalysis for lindane degradation was studied. The effect of some crucial parameters of practical applications such as initial concentration of lindane, initial concentration of HSO 5 − and solution pH was investigated. The transformation intermediates of lindane were detected using GC-MS and a potential degradation mechanism was subsequently proposed. Lastly, multi-cycle tests were conducted to investigate the mechanical stability and reusability of the photocatalyst. 2. Materials and methods 2.1 Materials Lindane (C 6 H6 Cl6 , 97%), Oxone® (2KHSO 5 ·KHSO4 ·K 2 SO 4 ), polyoxyethylene (80) sorbitan monooleate (Tween 80), and titanium (IV) isopropoxide (TTIP, 97%) were purchased from Sigma Aldrich. Sulfuric acid (H2 SO 4 , 95–98%) and isopropyl alcohol (i-PrOH, 99.8%) were obtained from Pharmco. All solutions were prepared using MilliQ grade water (resistivity of 18.2 MΩ cm). All chemicals in this study were used as received. 2.2 Synthesis of S-TiO2 film
S-TiO 2 photocatalyst was prepared by a sol–gel method using a self-assembly technique. H2 SO4 as a precursor for S and TTIP as a Ti source were used while Tween 80 was used as a pore directing agent. The details of the S-TiO2 sol-gel synthesis and film preparation are described in our previous publication [21]. Briefly, Tween 80 was dissolved in i-PrOH followed by the addition of TTIP. Then, H2 SO4 was added in the solution. The solution was stirred for 24 h at room temperature. The resulting solution was transparent and stable with light yellowish color. The solution pH and viscosity were around 3.0 and 6.48 ± 0.12 cP, respectively. The molar ratio of the ingredients was Tween 80:i-PrOH:TTIP:H2 SO 4 = 1:45:1:1. In this way, S-TiO2 photocatalyst films containing five coating layers were prepared by a dip-coating method. In the preparation of reference TiO 2 (ref-TiO2 ), Tween 80 was excluded and H2 SO 4 was replaced with acetic acid, using the same molar ratio of ingredients. The entire characterization of the synthesized S-TiO2 is given in the previous publication [21]. 2.3 Photocatalytic experiments The photocatalytic experiments were carried out in a borosilicate glass Petri dish (dia. 10 cm) with a quartz cover. In a typical experiment, 20 mL of aqueous lindane solution (1.0 µM) containing a film of S-TiO2 was irradiated in the photoreactor. The visible light experiments were performed under two 15 W fluorescent lamps (Cole-Parmer) with a UV block filter (UV420, Opticology). The light irradiance (Ee) was found to be 4.05 × 10−4 W cm−2 . In order to ensure adsorption equilibrium of lindane on S-TiO2 film, the solution was stirred for 10 min in dark, prior to irradiation. The simulated solar light experiments were carried out under a Xenon lamp (OF 300 W 67, 005, Newport, Oriel Instrument) with light irradiance (Ee) of 4.71 × 10−2 W cm−2 . Experiments were performed in triplicate, and the error bars on the figures represent the standard error of the mean.
2.4 Analytical methods A gas chromatograph (GC, Agilent 6890) with a mass selective detector (Agilent 5975, Wilmington, DE, USA) was used to analyze lindane and its reaction intermediates, employing the method described in our previous study [32]. Briefly, extraction of the sample was performed by a solid phase micro extraction (SPME) technique. Separation of the analyte was achieved on an HP-5 (5% phenyl methylsiloxane) capillary column (Hewlett-Packard, 30 m, i.d. 0.25 µm). Mass spectra were obtained in an electron impact ionization mode (EI+) at 70 eV. The mass spectra in the scan mode were recorded in the scan range of m/z 50-550. The samples were analyzed using mass spectral search program (NIST, USA) installed in the GC-mass spectrometer and spectra of the samples were compared with those of the standards in the NIST library. Measurement of total organic carbon (TOC), as non-purgeable organic carbon, was performed using a Shimadzu VCSH-ASI TOC analyzer. The chloride ion (Cl−) released was identified by a Dionex DX 500 ion chromatograph (IC) equipped with a CD 25 conductivity detector, a GP 50 gradient pump, and an IonPac AS 14 analytical column (4 mm × 250 mm). Both TOC and Cl− analyses have been qualitatively performed, mainly to ensure the endproducts of the photocatalysis of lindane. 3. Results and discussion 3.1 Photocatalytic activity of S-TiO2 under visible light irradiation Figure 1 shows the visible light activity of S-TiO2 for degradation of lindane in aqueous solution. The results of control experiments, including (i) visible light only, (ii) ref-TiO 2 /dark, (iii) S-TiO 2 /dark, and (iv) ref-TiO 2 /visible light, revealed that neither activation of ref-TiO2 (band gap energy, EG = 3.18 eV) [21], nor direct photodegradation of lindane with visible light (λ
> 420 nm) was effective in this study. The results; however, showed that significant degradation of lindane occurred in visible light-assisted S-TiO2 photocatalysis (S-TiO 2 /visible), leading to 31.0% lindane removal in 6 h. The visible light activity of S-TiO2 was associated with its reduced band gap energy (i.e., 2.94 eV), induced by substitutional doping of S 2− in the TiO 2 lattice [21]. Consequently, the absorption edge of S-TiO2 was shifted to lower energy region, thereby capable of absorbing visible light photon for promotion of electrons to the conduction band [21, 22].
The resulting photogenerated electrons (e −) can reduce the surface adsorbed
oxygen (O 2 ) to yield superoxide radical anion (O 2 •−) as reactive oxidizing species (reaction (5)) [33, 34]. Contrary to UV/TiO 2 , the photogenerated hole (h+) from visible light-assisted dopedTiO2 process cannot oxidize H2 O or HO − because of thermodynamics constrains, thus avoiding the formation of •OH [33, 34]. However, formation of •OH in visible light-assisted S-TiO2 photocatalysis can take place via O 2 •− reaction pathways involving production and consumption of H2 O2 molecule (reactions (6)-(10)) [33, 35]. The various reactive oxygen species formed in visible light-assisted S-TiO2 (i.e., O 2 •− and •OH) were most likely responsible for the degradation of lindane in the above system. eCB− + O2 •− + 2H+ → H2 O2
(6)
O 2 •− + O2 •− + 2H+ → H2 O2 + O2
(7)
O 2 −• + H+ → HO2 •
(8)
2HO 2 • + 2H+ → 2H2 O2 + O2
(9)
eCB− + H2 O2 + H+ → H2 O + •OH
(10)
3.2 Photocatalytic activity of S-TiO2 under simulated solar light irradiation Photocatalytic degradation of lindane using S-TiO2 was investigated under simulated solar light irradiation and the results are shown in Figure 2. As seen in Figure 2, direct
photodegradation of lindane by simulated solar light irradiation was negligible within 6 h. However, the photocatalytic efficiency of TiO 2 based photocatalysts was significantly enhanced under simulated solar light irradiation, leading to 36.7 and 63.4% lindane removal using ref-TiO2 and S-TiO 2 films, respectively, for 6 h (Table 1). Solar light consists of about 5% UV light radiation, which has energy greater than band gap energy of ref-TiO2 , thus promoting the electrons from valence band to conduction band and generating an electron-hole pair (eCB− + hVB+) according to reaction (1). Subsequently, these electron-hole pair can generate various ROS such as O2 •− and •OH, as discussed in the previous sections. Compared to ref-TiO2 /simulated solar, S-TiO2 /simulated solar light process yielded far better degradation results mainly because of a fairly strong potential of S-TiO 2 for absorbing visible light radiation, besides, the increased BET surface area and high porosity [21]. This is in accordance with the findings of Fotiou et al. [36] and Triantis et al. [37], who reported that solar light-assisted doped-TiO 2 photocatalyst showed higher performance than the corresponding reference TiO 2 for the degradation of pollutants. The observed pseudo first-order rate constants for simulated solar light-assisted refTiO2 and S-TiO 2 photocatalysis were found to be 7.31 × 10−2 and 1.63 × 10−1 h−1 , respectively. 3.3 Influence of peroxymonosulfate (HSO5 −) on visible and simulated solar light-assisted S-TiO2 photocatalysis of lindane Figure 3 shows the effect of HSO 5 − on TiO 2 photocatalysis of lindane under visible light irradiation. The results of control experiments showed that only 4.1% lindane was removed by HSO 5 − direct oxidation under visible light irradiation in 6 h, indicating that HSO 5 − cannot be effectively activated under visible light irradiation. The ref-TiO 2 /HSO 5 −/visible process showed 7.0% lindane removal in 6 h, demonstrating that addition of HSO 5 − had no significant effect on the
efficiency
of visible
light-assisted
ref-TiO2
photocatalysis
of lindane.
Interestingly,
photocatalytic activity of S-TiO2 /vis was significantly increased in the presence of 0.2 mM HSO 5 −, leading to 68.2% lindane removal in 6 h. The enhanced removal efficiency was potentially due to the dual role of HSO 5 − as: (i) an electron acceptor, thereby reducing the rate of electron-hole recombination, i.e., opposing reaction (2) [11], and (ii) an efficient source of SO 4 •− and •OH, according to reactions (11) and (12), respectively [25]. In our previous study it was shown that lindane could be readily oxidized by both •OH and SO 4 •− radicals [32, 39]. It was also revealed that the latter had a higher reactivity than the former [39], which could successfully explain the obtained results. HSO 5 − + eCB− → •OH + SO 4 2−
(11)
HSO 5 − + eCB− → OH− + SO 4 •−
(12)
The
percent
removal efficiency
(%)
of lindane
by
visible light-assisted
TiO 2
photocatalysis in presence of HSO 5 − changed in the following order: ref-TiO 2 /visible < refTiO2 /HSO 5 −/visible < S-TiO2 /visible < S-TiO2 /HSO 5 −/visible, corresponding to 4.2, 7.3, 31.0 and 68.2% removal, respectively, in 6 h (Table 2). The observed pseudo first-order rate constant (k obs) and calculated half-life (t 1/2 ) for the processes studied herein were compared in Table 2, showing a significant enhancing effect by the addition of HSO 5 −. The influence of HSO 5 − on simulated solar light-assisted TiO 2 photocatalysis of lindane was investigated and the results are shown in Figure 4. HSO 5 − direct oxidation under simulated solar light irradiation led to 15.0% removal of lindane in 6 h, attributable to its activation by UV light (a portion of solar light radiation), hence generating SO 4 •− and •OH following reaction (13) [40]. The photocatalytic activity of simulated solar light-assisted TiO 2 photocatalysis was significantly enhanced in the presence of HSO 5 −, with 85.4 and 99.9% lindane removal in 6 h by ref-TiO2 /HSO 5 −/solar and S-TiO2 /HSO 5 −/solar processes, respectively. A plausible reason could
be reduced rate of electron-hole recombination and generation of SO 4 •− and/or •OH as discussed above on the effect of HSO 5 − on visible light-assisted TiO 2 photocatalysis of lindane. The value of k obs was found to be 3.10 × 10−1 and 6.58 × 10−1 h−1 by ref-TiO2 /HSO 5 −/simulate solar and STiO2 /HSO 5 −/simulated solar processes, respectively (Table 1). HSO 5 − + hν → SO 4 •− + •OH
(13)
A comparison of various simulated solar light-assisted TiO 2 photocatalytic processes for lindane degradation followed the order: ref-TiO 2 /simulated solar < S-TiO2 /simulated solar < refTiO2 /HSO 5 −/simulated solar < S-TiO 2 /HSO 5 −/simulated solar, corresponding to 36.7, 63.4, 85.4 and 99.9% lindane removal in 6 h, respectively (Table 1). Thus the addition of 0.2 mM HSO 5 − showed a strong enhancing effect on k obs, with a corresponding large decrease in the calculated half-life (t 1/2 ) of the reactions, as shown in Table 1. This study suggested that the addition of HSO 5 − was very beneficial to TiO 2 -based photocatalysis by way of reducing size of the photocatalytic reactor. Consequently, high lindane removal can be achieved in a considerably less reaction time, thus making visible and simulated solar light-assisted S-TiO2 photocatalysis a suitable alternative for application purposes. Simulated solar light-assisted S-TiO2 /HSO 5 − was the most effective for degradation of lindane among the studied processes. A further study was performed to investigate effect of critical operation parameters such as initial concentration of lindane, initial concentration of HSO 5 −, and solution pH on the degradation of lindane by the simulated solar light-assisted STiO2 /HSO 5 − process. 3.4 Factors affecting the efficiency of simulated solar light-assisted S-TiO2 /HSO5 − process 3.4.1
Influence of initial concentrations of lindane
Degradation of lindane by simulated solar light-assisted S-TiO2 /HSO 5 − process followed pseudo first-order kinetics, as shown by following equation (14): C -ln kobs t C0
(14)
where C0 and C represent the initial and final concentration of lindane before and after irradiation; t is the radiation time; and k obs is the observed pseudo first-order rate constant. Figure 5 depicts the influence of initial concentration of lindane on the observed pseudo first-order rate constant, k obs, using simulated solar light-assisted S-TiO2 /HSO 5 − process. The k obs was found to decrease with increasing the initial concentration of lindane. Under the experimental condition in this study, the value of k obs was 8.98 × 10−1 , 6.58 × 10−1 and 3.84 × 10−1 h−1 for 0.5, 1.0 and 2.0 µM of initial concentration of lindane, respectively. These findings were consistent with the previous studies regarding the effect of solute concentration on the photocatalytic degradation of organic pollutants [41-44]. The most plausible reason for the decreasing k obs could be the formation of reaction by-products which competed with lindane for various ROS including SO4 •−, at higher initial concentrations of lindane [45, 46]. For a given concentration of reactive radicals produced under a specific condition, the high concentration of pollutant will require more time to attain the same extent of degradation, thus explaining the reduced removal efficiency of lindane. The initial degradation rate of lindane (calculated by the change in concentration with time at an initial reaction time of 1 h) was also determined at three different initial concentrations (i.e., 0.5, 1.0 and 2.0 µM) and the results are shown in Figure 5. As seen in Figure 5, the initial degradation rate of lindane increased since the number of lindane molecules exposed to more ROS at higher initial concentrations of lindane [47]. Under the experimental conditions used in
this study, the degradation rate of lindane was 0.20, 0.31 and 0.52 µM h−1 at using 0.5, 1.0 and 2.0 µM of initial concentration of lindane, respectively. 3.4.2
Influence of initial concentrations of HSO5 − The effect of initial concentration of HSO 5 − on the efficiency of simulated solar light-
assisted S-TiO 2 /HSO 5 − process was investigated, and the results are shown in Figure 6. As can be seen, the value of k obs increased with increasing the initial concentration of HSO 5 −. A plausible explanation for this behaviour could be the increased production of SO 4 •− and •OH (reactions (11) and (12)), at using higher initial concentration of HSO 5 − [32]. As stated earlier, the electron-hole recombination was inhibited by HSO 5 −, so, an increase in the concentration of HSO 5 − may cause an enhanced inhibition of electron-hole recombination, thereby increasing the removal efficiency of lindane as well, and hence k obs was increased. The calculated k obs was found to be 3.84 × 10−1 , 5.81 × 10−1 , 8.01 × 10−1 , and 9.97 × 10−1 h−1 , when initial concentration of HSO 5 − was 0.1, 0.2, 0.5 and 1.0 mM, respectively. An initial HSO 5 − concentration of 0.2 mM was used in all subsequent experiments, corresponding to its minimum amount needed to achieve complete (99.9%) lindane removal in 6 h. 3.4.3
Influence of solution pH A solution pH may influence the extent of photocatalytic degradation of organic
pollutants by changing the surface charge of the photocatalyst, state of ionization of the pollutant, and concentration of the reactive radicals produced [10, 26, 48, 49]. The degradation of lindane by simulated solar light-assisted S-TiO 2 /HSO 5 − process was studied at three different pH values of 4.0, 5.8 and 8.0, and the results are shown in Figure 7. The degradation kinetics varied significantly with solution pHs and the highest lindane removal was achieved at pH 5.8, corresponding to 99.9% removal in 6 h. In contrast, the removal efficiency decreased in stronger
acidic as well as basic conditions, indicating 88.2 and 71.4% removal at pH 4.0 and 8.0, respectively, after 6 h of simulated solar light irradiation. The lindane degradation followed pseudo first-order kinetics model in the studied pH range with k obs of 3.56 × 10−1 , 6.58 × 10−1 and 2.09 × 10−1 h−1 for pH 4.0, 5.8, and 8.0, respectively. The obtained results are in good agreement with the literature reports,
dealing with photocatalytic degradation of other
insecticides such as atrazine and dimethoate [16, 49]. Since the electrostatic force of attraction or repulsion between the photocatalyst surface and the pollutant is affected by the solution pH [49, 50], the degradation efficiency of S-TiO 2 photocatalysis was found to change with the varying pH. At lower pH, the concentration of O 2 −• was reduced by reacting with H+ ions (reaction (8)) [11], and so the removal efficiency of lindane was reduced. The removal efficiency of lindane was also reduced at pH 8.0, which can be attributed to the surface charge change of photocatalysts, corresponding to surface electrostatic repulsion with the pollutant. Literature studies showed that TiO 2 photocatalysis of atrazine resulted in a lower removal efficiency at the increasing solution pH, i.e., at pH 7 or 10 than at pH 4 or 5 [51]. The change in the removal efficiency may also be due to varying behaviour of HSO 5 − under simulated solar light (i.e., UV component), at different pH values [52]. The scavenging of SO4 •− and •OH by HO − following reactions (15) and (16), respectively, may probably lower the removal efficiency at higher solution pH [29, 53]. Since the oxidation potential of •OH decreased with the increasing pH; it could also partially explain the lower efficiency of lindane at high pH [54]. Above all, our results clearly indicated that the highest photocatalytic activity was observed at pH 5.8. SO 4 •− + HO− → SO 4 2− + •OH
(k = 8.3 × 107 M−1 s−1 , alkaline pH)
(15)
•
(k = 61.2 × 1010 M−1 s−1 , alkaline pH)
(16)
OH + HO − → O•− + H2 O
3.5 Identification of reaction intermediates and degradation mechanism GC-MS analysis revealed six reaction intermediates, i.e., hexachlorobenzene (HCB), pentachlorocyclohexene (PCCH),
tetrachlorocyclohexene (TeCCH), trichlorobenzene (TCB),
dichlorobenzene (DCB), and trichlorophenol (TCP) during simulated solar light-assisted STiO2 /HSO 5 − photocatalysis of lindane. CO 2 and Cl− were found to be the end-products of lindane, as revealed by TOC and IC analyses. Based on the results of identification of intermediates and end-products, a plausible degradation mechanism was proposed, as shown in Figure 8. The simulated solar light-assisted S-TiO 2 /HSO 5 − photocatalytic degradation of lindane was expected to have initiated by attack of •OH, SO4 •− and/or O 2 •− species, generated via reactions (3)-(5), (10), (11), (12) and (13). For example, the formation of PCCH was presumably resulted from the dehalogenation of lindane by O 2 •−, as described earlier by Antonaraki et al., using polyoxometalates (POMs) photocatalysis [55]. This mechanism was similar to the dehalogenation of aliphatic halocarbons by O 2 •−, whereby the reacting molecule was converted into corresponding alkenes [56]. PCCH was also previously reported as an intermediate product of lindane during TiO 2 photocatalysis [57]. The PCCH on successive dechlorination may lead to the formation of lesser chlorinated by-products such as TeCCH, TCB and DCB. For instance, TCB was reported during dehalogenation of PCCH in subcritical water [58]. Both •OH and SO4 •− may easily react with aliphatic compounds following hydrogen abstraction pathway (i.e., H abstraction) [29, 53]. Thus, lindane was likely to be dehydrogenated and converted into HCB by a reaction of •OH and/or SO4 •− via H abstraction [39]. The formation of a stable HCB by-product may allow the reaction to be kinetically feasible [59]. Antonaraki et al. [55], Guo et al. [59] and Nitoi et al. [60] also reported HCB as intermediate of lindane using
POMs photocatalysis, POM/SiO 2 photocatalysis, and photo-Fenton process, respectively, attributing its formation to the reaction of •OH radical. TCP could be formed by hydroxylation of TCB mediated by •OH and/or SO4 •− radical mechanisms [39]. The •OH radical abstracts hydrogen atom from aromatic hydrocarbons, and thereby forming a carbon centered radical. The resulting carbon centered radical on termination with another •OH radical may give rise to a phenolic compound [61], e.g., TCP in this case. Antonaraki et al. [55] have reported TCP as an intermediate product during POMs photocatalysis of lindane, attributing its formation to •OH radical attack. The various intermediates detected in this study on subsequent reactions with different reactive species might have undergone ring cleavage, as indicated from the CO 2 evolved. CO2 was also reported as end-product of lindane using POMs photocatalysis [55] and photo-Fenton process [60]. 3.6 Test of reusability of S-TiO2 photocatalyst films Mechanical stability and
reusability are among the most important features of a
photocatalyst, which were investigated in this study by conducting four repeated cycle experiments using a single film of S-TiO 2 , and the results are shown in Figure 9. As seen in Figure 9, comparable degradation results were achieved in the four successive cycles, corresponding to 99.9, 98.7, 97.4 and 96.6% lindane removal, respectively, after 6 h of simulated solar light irradiation. This result showed that the photocatalytic activity of S-TiO2 film was maintained during four cycles, indicating high mechanical stability and recycling capacity for STiO2 film under the studied experimental conditions. 4. Conclusions Nanocrystalline S-TiO2 film, synthesized by a sol–gel method, showed a considerable efficiency on the degradation of lindane under visible and simulated solar light irradiation. The
photocatalytic activity of visible and simulated solar light-assisted S-TiO 2 was dramatically improved in the presence of 0.2 mM HSO 5 −, leading to 68.2 and 99.9% lindane removal, respectively in 6 h. Thus with addition of HSO 5 −, sizes of photocatalytic reactors can be considerably reduced in practical applications. The photocatalytic efficiency of S-TiO2 /HSO 5 − process decreased with increasing initial concentration of lindane. The solution pH had a significant effect on the efficiency of S-TiO2 /HSO 5 − process. The highest removal efficiency was achieved at near neutral pH (5.8). The photocatalytic activity of S-TiO2 film was maintained after four repeated runs, confirming mechanical stability for S-TiO2 film. The results of intermediate
and
end-products
analyses
led
to
the
conclusion
that
dechlorination,
dehydrogenation, hydroxylation, and ring cleavage, via •OH, SO4 •− and O2 •− attack were the main reaction steps followed during S-TiO 2 /HSO 5 − photocatalysis of lindane. The results indicated that visible and
simulated solar light-assisted S-TiO2 /HSO 5 − is very effective for the
detoxification of water contaminated with chlorinated pesticides such as lindane. Acknowledgments The Higher Education Commission (HEC), Islamabad, Pakistan is highly acknowledged for funding this research project through an International Research Support Initiative Program (IRSIP). This work was also partially funded by the Cyprus Research Promotion Foundation through Desmi 2009-2010 which is co-funded by the Republic of Cyprus and the European Regional
Development
IPODOMI/STRATH/0308/09.
Fund
of
the
EU
under
contract
number NEA
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Figure captions
Figure 1. Degradation of lindane by visible light-assisted S-TiO2 photocatalysis. [lindane]0 = 1.0 µM, [S-TiO2 ]0 = 0.23 g/L, pH = 5.8. Figure 2. Degradation of lindane by simulated solar light-assisted S-TiO2 photocatalysis. [lindane]0 = 1.0 µM, [S-TiO 2 ]0 = 0.23 g/L, pH = 5.8. Figure 3. Effect of 0.2 mM HSO 5 − on S-TiO2 photocatalysis of lindane under visible light irradiation. [lindane]0 = 1.0 µM, [S-TiO 2 ]0 = 0.23 g/L, pH = 5.8. Figure 4. Effect of 0.2 mM HSO 5 − on S-TiO2 photocatalysis of lindane under simulated solar light irradiation. [lindane]0 = 1.0 µM, [S-TiO2 ]0 = 0.23 g/L, pH = 5.8. Figure 5. Variation of rate constant (k obs) and initial degradation rate with different initial concentration of lindane by simulated solar light-assisted S-TiO2 /HSO 5 − process. Initial degradation rate corresponds to the first hour of decay. [S-TiO2 ]0 = 0.23 g/L, pH = 5.8. Figure 6. Variation of rate constant (k obs) with different initial concentration of HSO 5 − by simulated solar light-assisted S-TiO2 /HSO 5 − process. [lindane]0 = 1.0 µM, [S-TiO 2 ]0 = 0.23 g/L, pH = 5.8. Figure 7. Effect of initial solution pH on the efficiency of simulated solar light-assisted STiO 2 /HSO 5 − process. [lindane]0 = 1.0 µM, [S-TiO2 ]0 = 0.23 g/L, [HSO 5 −]0 = 0.2 mM. Figure 8. Proposed degradation pathway of lindane by simulated solar light-assisted STiO 2 /HSO 5 − process. [lindane]0 = 10.0 μM, [S-TiO 2 ]0 = 0.23 g/L, [HSO 5 −]0 = 0.2 mM, pH = 5.8. Figure 9. Multi-cycle tests for S-TiO 2 photocatalyst in degrading lindane under simulated solar light for 6 hr. lindane]0 = 1.0 µM, [S-TiO 2 ]0 = 0.23 g/L, pH = 5.8.
Table Captions Table 1. Pseudo first-order rate constant (k obs), removal efficiency (%) and half-life (t 1/2 ) of simulated solar light-assisted S-TiO2 photocatalytic processes for lindane degradation, in the presence of 0.2 mM HSO 5 −. [lindane]0 = 1.0 µM, [S-TiO2 ]0 = 0.23 g/L, pH = 5.8. Table 2. Pseudo first-order rate constant (k obs), removal efficiency (%) and half-life (t 1/2 ) of visible light-assisted S-TiO 2 photocatalytic processes for lindane degradation, in the presence of 0.2 mM HSO 5 −. lindane]0 = 1.0 µM, [S-TiO 2 ]0 = 0.23 g/L, pH = 5.8.
Figure 1.
1.0
C/C0
0.9
0.8 Visible light alone Ref-TiO2/dark 0.7
S-TiO2/dark Ref-TiO2/visible light S-TiO2/visible light
0.6
0
1
2
3
4
Irradiation time (h)
5
6
Figure 2.
1.0
C/C0
0.8
0.6
Simulated solar light alone Ref-TiO2/Simulated solar light
0.4
S-TiO2/Simulated solar light
0
1
2
3
4
Irradiation time (h)
5
6
Figure 3.
1.0
C/C0
0.8
0.6
0.4
HSO5-/Visible light Ref-TiO2/HSO5-/Visible light
0.2
S-TiO2/Visible light S-TiO2/HSO5-/Visible light
0.0 0
1
2
3
4
Irradiation time (h)
5
6
Figure 4.
1.0
C/C0
0.8
0.6
0.4 HSO5-/Simulated solar light
0.2
Ref-TiO2/HSO5-/Simulated solar light S-TiO2/HSO5-/Simulated solar light
0.0
0
1
2
3
4
Irradiation time (h)
5
6
Figure 5.
Rate constant, kobs 0.5
-1
Degradation rate
0.8
0.4
0.6
0.3
0.4
0.2
0.5
1.0
1.5
[Lindane]0 (M)
2.0
Degradation rate (M.h-1)
Rate constant, kobs (h )
1.0
Figure 6.
-1
Rate constant, kobs, (h )
1.2
1.0
0.8
0.6
0.4
0.0
0.2
0.4
0.6
[HSO5-] (mM)
0.8
1.0
Figure 7.
1.0
C/C0
0.8
0.6
0.4
0.2
pH = 8.0 pH = 4.0 pH = 5.8
0.0 0
1
2
3
4
Irradiation time (hr)
5
6
Figure 8.
Figure 9.
Lindane degradation (%)
100
80
60
40
20
0 1
2
3
Number of cycles
4
Table 1. Simulated solar light-assisted AOPs
k obs (h−1 )
Removal efficiency (%)
t1/2 (h)
Ref-TiO2 /Simulated solar light
7.31 × 10−2
36.7
9.5
S-TiO2 /Simulated solar light
1.63 × 10−1
63.4
4.3
Ref-TiO2 /HSO5 −/Simulated solar light
3.10 × 10−1
85.4
2.2
S-TiO2 /HSO5 −/Simulated solar light
6.58 × 10−1
99.9
1.2
Table 2. Visible light-assisted AOPs
k obs (h−1 )
Removal efficiency (%)
t1/2 (h)
Ref-TiO2 /visible light
6.61 × 10−3
4.2
105.0
Ref-TiO2 /HSO5 −/visible light
1.19 × 10−2
7.3
58.2
S-TiO2 /visible light
5.93 × 10−2
31.0
11.7
S-TiO2 /HSO5 −/visible light
1.87 × 10−1
68.2
3.7
S1381-1169(16)30512-X http://dx.doi.org/doi:10.1016/j.molcata.2016.11.035 MOLCAA 10132
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date: Revised date: Accepted date:
27-9-2016 23-11-2016 24-11-2016
Please cite this article as: Sanaullah Khan, Changseok Han, Hasan M.Khan, Dominic L.Boccelli, Dionysios D.Dionysiou, Efficient degradation of lindane by visible and simulated solar light-assisted S-TiO2/peroxymonosulfate process: Kinetics and mechanistic investigations, Journal of Molecular Catalysis A: Chemical http://dx.doi.org/10.1016/j.molcata.2016.11.035 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Efficient degradation of lindane by visible and simulated solar lightassisted S-TiO2/peroxymonosulfate process: Kinetics and mechanistic investigations
Sanaullah Khan1,2,3,†, Changseok Han3,†, Hasan M. Khan2 , Dominic L. Boccelli3 , Dionysios D. Dionysiou3,4 *
1
Departmen of Chemistry, Shaheed Benazir Bhutto University, Sheringal Dir (Upper), Khyber Pakhtunkhwa, Pakistan 2
Radiation Chemistry Laboratory, National Centre of Excellence in Physical Chemistry, University of Peshawar, Peshawar 25120, Pakistan
3
Environmental Engineering and Science Program, University of Cincinnati, Cincinnati, Ohio 45221-0012, USA 4
Nireas-International Water Research Centre, University of Cyprus, Nicosia 1678, Cyprus *
Corresponding author Email: [email protected] Tel: +1-513-556-0724; Fax: +1-513-556-2599
†Contributed equally to this work.
Lindane removal (%) after 6 h
Graphical abstract
100
80
60
40
20
0
n
o iti nd
S-
O Ti
o kc r a /D 2
T S-
ht ht ht lig lig lig r r e l la la ib so so - /vis d d e e 5 at at ul SO ul m m /2 H i - /Si /S O 2 Ti O 5 i O S T S S/H 2 O Ti St
h lig e bl isi /V
iO 2
Highlights
Removal of lindane was studied by visible and simulated solar light-assisted S-TiO 2 .
Efficiency of S-TiO2 photocatalysis dramatically increased in the presence of HSO 5 −.
Operational parameters such as pH, concentration of lindane and HSO 5 − were optimized.
Degradation mechanism was proposed based on identified intermediates and final products.
S-TiO 2 photocatalysis is an effective AOP to treat lindane contaminated waters.
Abstract Organochlorine pesticides (OCPs) are toxic and are among the most potent endocrine disrupting chemicals in the environment. Most OCPs are resistant towards oxidation by •OH due to presence of electron-withdrawing chlorine group in their molecular structures. Here, we investigated
a
visible
and
simulated
solar
light-assisted
sulfur
doped
TiO 2
(S-
TiO2 )/peroxymonosulfate (HSO 5 −) process to eliminate a selected OCP, lindane. Initially, visible and simulated solar light-assisted S-TiO 2 photocatalysis resulted in 31.0 and 63.4% removal of lindane (C0 = 1.0 µM), respectively in 6 h. The photocatalytic activity of S-TiO 2 was dramatically increased in the presence of 0.2 mM HSO 5 −, leading to 68.2 and 99.9% lindane removal under visible and simulated solar light illumination, respectively in 6 h. The observed pseudo first-order rate constant for simulated solar light-assisted S-TiO2 /HSO5 − decreased with increasing initial concentration of lindane, corresponding to 8.98 × 10−1 , 6.58 × 10−1 and 3.84 × 10−1 h−1 at [lindane]0 of 0.5, 1.0 and 2.0 µM, respectively. The degradation kinetics were significantly affected by solution pH, leading to 88.2, 99.9 and 71.4% removal of lindane in 6 h at pH 4.0, 5.8 and 8.0, respectively. S-TiO2 film exhibited a high mechanical strength with only 6.4% loss of efficiency after four repeated cycles. Based on the detected reaction intermediates, a possible reaction mechanism was proposed, suggesting dechlorination, dehydrogenation, and hydroxylation via •OH, SO4 •− and O2 •− attack. The results suggest that visible and simulated solar light-assisted S-TiO2 /HSO 5 − is a promising alternative for treatment of water contaminated with most OCPs.
Keywords:
Lindane;
S-TiO2
photocatalysis;
Visible and
Peroxymonosulfate; Reaction mechanism; Water treatment.
simulated
solar light activity;
1. Introduction Organochlorine pesticides (OCPs) represent a major source of emerging water pollutants in
recent
decades
[1].
Among
organochlorine
compounds,
the
gamma
isomer
of
hexachlorocyclohexane (γ-HCH), lindane, has been widely used as a broad spectrum insecticide in agriculture since the 1940s [2]. It is also a commonly found ingredient in some lotions, creams and shampoos due to its antiparasitic property [3]. Therefore, its residues have been detected worldwide in many environmental segments [2], ultimately entering into human body through food chains [4]. In particular, lindane poses potential health risks to humans and other living organisms due to its high toxicity [5]. For example, acute exposure to lindane can cause mild skin irritation, dizziness, headaches, diarrhea, nausea, vomiting, convulsions, and death. Chronic exposure can adversely affect the liver and nervous systems of animals. Lindane was reported to be a potential carcinogen and teratogen [6]. Although the use of lindane as an insecticide has been restricted in many European countries as well as the United States (US) since 1975, it is still being used in many other countries [7]. Lindane was used in the United States and Canada for seed treatment, till recent years [8]. Therefore, decontamination of lindane contaminated water has become of great importance in recent years. Advanced oxidation processes (AOPs) have attracted much attention for decomposing a wide range of recalcitrant organic pollutants in water among the currently available technologies due to the in-situ generation of highly reactive oxygen species (ROS) at ambient pressure and temperature conditions [9]. During the last few decades, extensive research studies have been carried out on TiO 2 based AOPs for abating environmental pollution, because of low cost, easy availability, wide versatility, and good chemical and thermal stability of TiO 2 [10-14]. Upon UV light (λ < 388 nm) absorption, TiO 2 produces conduction band electrons (eCB−) and valence band
holes (hVB+) (reaction (1)) [11] which despite having recombining and/or trapping affinity in the TiO2 lattice, migrate to the catalyst surface and initiate various chemical redox processes (2)-(5)) [11, 15]. TiO2 + hν → hVB+ + eCB−
(1)
hVB+ + eCB− → Energy
(2)
hVB+ + H2 O → •OH
(3)
hVB+ + HO − → •OH
(4)
eCB− + O2 → O2 •−
(5)
In recent decades, there have been increasing research studies into the development of more efficient non-metal doped TiO 2 materials which can utilize low energy (visible light) photon for excitation of valence band electrons [16-20], thus making TiO 2 photocatalysis a sustainable alternative employing solar energy. Among non-metals (carbon, nitrogen, fluorine, etc.), sulfur is one of the most widely used candidates for synthesizing visible light-active TiO 2 photocatalyst in recent years [20-23]. Although significant advances have been made, visible and solar light-active non-metal doped TiO2 photocatalysts exhibit slow degradation rate or low quantum yield on degradation of highly bio-refractory organic compounds [24]. One more effective strategy was the coupling of various inorganic oxidants such as hydrogen peroxide (H2 O2 ), peroxymonosulfate (HSO 5 −) or persulfate (S2 O 8 2−) with TiO2 photocatalysts, which enhances the degradation kinetics by ways of (i) electron trapping thus limiting the rate of electron-hole recombination and (ii) the generation of •
OH and/or SO 4 •− radicals [19, 24-27]. Both •OH and SO 4 •− are strong oxidizing agents with
standard oxidation–reduction potentials of 2.4-2.7 and 2.5–3.1 V, respectively [28], and capable of decomposing most organic pollutants in water. Different from •OH, which tends to react
rapidly with organic molecules through hydrogen abstraction or addition reactions, SO 4 •− usually participates in electron transfer reaction [29]. Consequently, SO4 •− is more selective for oxidation reactions and with many organic compounds SO 4 •− reacts as a more efficient oxidant than •OH [29]. Peroxymonosulfate (HSO 5 −) was chosen as an oxidant due to its strong oxidation properties [30, 31] as well as its ability to generate both •OH and SO 4 •−. In this study, photocatalytic activity of a nanostructured sulfur doped TiO 2 (S-TiO2 ) photocatalyst film synthesized by a sol-gel method was investigated for degradation of lindane under visible and simulated solar light irradiation. In particular, the enhancing effect of HSO 5 − on the efficiency of S-TiO2 photocatalysis for lindane degradation was studied. The effect of some crucial parameters of practical applications such as initial concentration of lindane, initial concentration of HSO 5 − and solution pH was investigated. The transformation intermediates of lindane were detected using GC-MS and a potential degradation mechanism was subsequently proposed. Lastly, multi-cycle tests were conducted to investigate the mechanical stability and reusability of the photocatalyst. 2. Materials and methods 2.1 Materials Lindane (C 6 H6 Cl6 , 97%), Oxone® (2KHSO 5 ·KHSO4 ·K 2 SO 4 ), polyoxyethylene (80) sorbitan monooleate (Tween 80), and titanium (IV) isopropoxide (TTIP, 97%) were purchased from Sigma Aldrich. Sulfuric acid (H2 SO 4 , 95–98%) and isopropyl alcohol (i-PrOH, 99.8%) were obtained from Pharmco. All solutions were prepared using MilliQ grade water (resistivity of 18.2 MΩ cm). All chemicals in this study were used as received. 2.2 Synthesis of S-TiO2 film
S-TiO 2 photocatalyst was prepared by a sol–gel method using a self-assembly technique. H2 SO4 as a precursor for S and TTIP as a Ti source were used while Tween 80 was used as a pore directing agent. The details of the S-TiO2 sol-gel synthesis and film preparation are described in our previous publication [21]. Briefly, Tween 80 was dissolved in i-PrOH followed by the addition of TTIP. Then, H2 SO4 was added in the solution. The solution was stirred for 24 h at room temperature. The resulting solution was transparent and stable with light yellowish color. The solution pH and viscosity were around 3.0 and 6.48 ± 0.12 cP, respectively. The molar ratio of the ingredients was Tween 80:i-PrOH:TTIP:H2 SO 4 = 1:45:1:1. In this way, S-TiO2 photocatalyst films containing five coating layers were prepared by a dip-coating method. In the preparation of reference TiO 2 (ref-TiO2 ), Tween 80 was excluded and H2 SO 4 was replaced with acetic acid, using the same molar ratio of ingredients. The entire characterization of the synthesized S-TiO2 is given in the previous publication [21]. 2.3 Photocatalytic experiments The photocatalytic experiments were carried out in a borosilicate glass Petri dish (dia. 10 cm) with a quartz cover. In a typical experiment, 20 mL of aqueous lindane solution (1.0 µM) containing a film of S-TiO2 was irradiated in the photoreactor. The visible light experiments were performed under two 15 W fluorescent lamps (Cole-Parmer) with a UV block filter (UV420, Opticology). The light irradiance (Ee) was found to be 4.05 × 10−4 W cm−2 . In order to ensure adsorption equilibrium of lindane on S-TiO2 film, the solution was stirred for 10 min in dark, prior to irradiation. The simulated solar light experiments were carried out under a Xenon lamp (OF 300 W 67, 005, Newport, Oriel Instrument) with light irradiance (Ee) of 4.71 × 10−2 W cm−2 . Experiments were performed in triplicate, and the error bars on the figures represent the standard error of the mean.
2.4 Analytical methods A gas chromatograph (GC, Agilent 6890) with a mass selective detector (Agilent 5975, Wilmington, DE, USA) was used to analyze lindane and its reaction intermediates, employing the method described in our previous study [32]. Briefly, extraction of the sample was performed by a solid phase micro extraction (SPME) technique. Separation of the analyte was achieved on an HP-5 (5% phenyl methylsiloxane) capillary column (Hewlett-Packard, 30 m, i.d. 0.25 µm). Mass spectra were obtained in an electron impact ionization mode (EI+) at 70 eV. The mass spectra in the scan mode were recorded in the scan range of m/z 50-550. The samples were analyzed using mass spectral search program (NIST, USA) installed in the GC-mass spectrometer and spectra of the samples were compared with those of the standards in the NIST library. Measurement of total organic carbon (TOC), as non-purgeable organic carbon, was performed using a Shimadzu VCSH-ASI TOC analyzer. The chloride ion (Cl−) released was identified by a Dionex DX 500 ion chromatograph (IC) equipped with a CD 25 conductivity detector, a GP 50 gradient pump, and an IonPac AS 14 analytical column (4 mm × 250 mm). Both TOC and Cl− analyses have been qualitatively performed, mainly to ensure the endproducts of the photocatalysis of lindane. 3. Results and discussion 3.1 Photocatalytic activity of S-TiO2 under visible light irradiation Figure 1 shows the visible light activity of S-TiO2 for degradation of lindane in aqueous solution. The results of control experiments, including (i) visible light only, (ii) ref-TiO 2 /dark, (iii) S-TiO 2 /dark, and (iv) ref-TiO 2 /visible light, revealed that neither activation of ref-TiO2 (band gap energy, EG = 3.18 eV) [21], nor direct photodegradation of lindane with visible light (λ
> 420 nm) was effective in this study. The results; however, showed that significant degradation of lindane occurred in visible light-assisted S-TiO2 photocatalysis (S-TiO 2 /visible), leading to 31.0% lindane removal in 6 h. The visible light activity of S-TiO2 was associated with its reduced band gap energy (i.e., 2.94 eV), induced by substitutional doping of S 2− in the TiO 2 lattice [21]. Consequently, the absorption edge of S-TiO2 was shifted to lower energy region, thereby capable of absorbing visible light photon for promotion of electrons to the conduction band [21, 22].
The resulting photogenerated electrons (e −) can reduce the surface adsorbed
oxygen (O 2 ) to yield superoxide radical anion (O 2 •−) as reactive oxidizing species (reaction (5)) [33, 34]. Contrary to UV/TiO 2 , the photogenerated hole (h+) from visible light-assisted dopedTiO2 process cannot oxidize H2 O or HO − because of thermodynamics constrains, thus avoiding the formation of •OH [33, 34]. However, formation of •OH in visible light-assisted S-TiO2 photocatalysis can take place via O 2 •− reaction pathways involving production and consumption of H2 O2 molecule (reactions (6)-(10)) [33, 35]. The various reactive oxygen species formed in visible light-assisted S-TiO2 (i.e., O 2 •− and •OH) were most likely responsible for the degradation of lindane in the above system. eCB− + O2 •− + 2H+ → H2 O2
(6)
O 2 •− + O2 •− + 2H+ → H2 O2 + O2
(7)
O 2 −• + H+ → HO2 •
(8)
2HO 2 • + 2H+ → 2H2 O2 + O2
(9)
eCB− + H2 O2 + H+ → H2 O + •OH
(10)
3.2 Photocatalytic activity of S-TiO2 under simulated solar light irradiation Photocatalytic degradation of lindane using S-TiO2 was investigated under simulated solar light irradiation and the results are shown in Figure 2. As seen in Figure 2, direct
photodegradation of lindane by simulated solar light irradiation was negligible within 6 h. However, the photocatalytic efficiency of TiO 2 based photocatalysts was significantly enhanced under simulated solar light irradiation, leading to 36.7 and 63.4% lindane removal using ref-TiO2 and S-TiO 2 films, respectively, for 6 h (Table 1). Solar light consists of about 5% UV light radiation, which has energy greater than band gap energy of ref-TiO2 , thus promoting the electrons from valence band to conduction band and generating an electron-hole pair (eCB− + hVB+) according to reaction (1). Subsequently, these electron-hole pair can generate various ROS such as O2 •− and •OH, as discussed in the previous sections. Compared to ref-TiO2 /simulated solar, S-TiO2 /simulated solar light process yielded far better degradation results mainly because of a fairly strong potential of S-TiO 2 for absorbing visible light radiation, besides, the increased BET surface area and high porosity [21]. This is in accordance with the findings of Fotiou et al. [36] and Triantis et al. [37], who reported that solar light-assisted doped-TiO 2 photocatalyst showed higher performance than the corresponding reference TiO 2 for the degradation of pollutants. The observed pseudo first-order rate constants for simulated solar light-assisted refTiO2 and S-TiO 2 photocatalysis were found to be 7.31 × 10−2 and 1.63 × 10−1 h−1 , respectively. 3.3 Influence of peroxymonosulfate (HSO5 −) on visible and simulated solar light-assisted S-TiO2 photocatalysis of lindane Figure 3 shows the effect of HSO 5 − on TiO 2 photocatalysis of lindane under visible light irradiation. The results of control experiments showed that only 4.1% lindane was removed by HSO 5 − direct oxidation under visible light irradiation in 6 h, indicating that HSO 5 − cannot be effectively activated under visible light irradiation. The ref-TiO 2 /HSO 5 −/visible process showed 7.0% lindane removal in 6 h, demonstrating that addition of HSO 5 − had no significant effect on the
efficiency
of visible
light-assisted
ref-TiO2
photocatalysis
of lindane.
Interestingly,
photocatalytic activity of S-TiO2 /vis was significantly increased in the presence of 0.2 mM HSO 5 −, leading to 68.2% lindane removal in 6 h. The enhanced removal efficiency was potentially due to the dual role of HSO 5 − as: (i) an electron acceptor, thereby reducing the rate of electron-hole recombination, i.e., opposing reaction (2) [11], and (ii) an efficient source of SO 4 •− and •OH, according to reactions (11) and (12), respectively [25]. In our previous study it was shown that lindane could be readily oxidized by both •OH and SO 4 •− radicals [32, 39]. It was also revealed that the latter had a higher reactivity than the former [39], which could successfully explain the obtained results. HSO 5 − + eCB− → •OH + SO 4 2−
(11)
HSO 5 − + eCB− → OH− + SO 4 •−
(12)
The
percent
removal efficiency
(%)
of lindane
by
visible light-assisted
TiO 2
photocatalysis in presence of HSO 5 − changed in the following order: ref-TiO 2 /visible < refTiO2 /HSO 5 −/visible < S-TiO2 /visible < S-TiO2 /HSO 5 −/visible, corresponding to 4.2, 7.3, 31.0 and 68.2% removal, respectively, in 6 h (Table 2). The observed pseudo first-order rate constant (k obs) and calculated half-life (t 1/2 ) for the processes studied herein were compared in Table 2, showing a significant enhancing effect by the addition of HSO 5 −. The influence of HSO 5 − on simulated solar light-assisted TiO 2 photocatalysis of lindane was investigated and the results are shown in Figure 4. HSO 5 − direct oxidation under simulated solar light irradiation led to 15.0% removal of lindane in 6 h, attributable to its activation by UV light (a portion of solar light radiation), hence generating SO 4 •− and •OH following reaction (13) [40]. The photocatalytic activity of simulated solar light-assisted TiO 2 photocatalysis was significantly enhanced in the presence of HSO 5 −, with 85.4 and 99.9% lindane removal in 6 h by ref-TiO2 /HSO 5 −/solar and S-TiO2 /HSO 5 −/solar processes, respectively. A plausible reason could
be reduced rate of electron-hole recombination and generation of SO 4 •− and/or •OH as discussed above on the effect of HSO 5 − on visible light-assisted TiO 2 photocatalysis of lindane. The value of k obs was found to be 3.10 × 10−1 and 6.58 × 10−1 h−1 by ref-TiO2 /HSO 5 −/simulate solar and STiO2 /HSO 5 −/simulated solar processes, respectively (Table 1). HSO 5 − + hν → SO 4 •− + •OH
(13)
A comparison of various simulated solar light-assisted TiO 2 photocatalytic processes for lindane degradation followed the order: ref-TiO 2 /simulated solar < S-TiO2 /simulated solar < refTiO2 /HSO 5 −/simulated solar < S-TiO 2 /HSO 5 −/simulated solar, corresponding to 36.7, 63.4, 85.4 and 99.9% lindane removal in 6 h, respectively (Table 1). Thus the addition of 0.2 mM HSO 5 − showed a strong enhancing effect on k obs, with a corresponding large decrease in the calculated half-life (t 1/2 ) of the reactions, as shown in Table 1. This study suggested that the addition of HSO 5 − was very beneficial to TiO 2 -based photocatalysis by way of reducing size of the photocatalytic reactor. Consequently, high lindane removal can be achieved in a considerably less reaction time, thus making visible and simulated solar light-assisted S-TiO2 photocatalysis a suitable alternative for application purposes. Simulated solar light-assisted S-TiO2 /HSO 5 − was the most effective for degradation of lindane among the studied processes. A further study was performed to investigate effect of critical operation parameters such as initial concentration of lindane, initial concentration of HSO 5 −, and solution pH on the degradation of lindane by the simulated solar light-assisted STiO2 /HSO 5 − process. 3.4 Factors affecting the efficiency of simulated solar light-assisted S-TiO2 /HSO5 − process 3.4.1
Influence of initial concentrations of lindane
Degradation of lindane by simulated solar light-assisted S-TiO2 /HSO 5 − process followed pseudo first-order kinetics, as shown by following equation (14): C -ln kobs t C0
(14)
where C0 and C represent the initial and final concentration of lindane before and after irradiation; t is the radiation time; and k obs is the observed pseudo first-order rate constant. Figure 5 depicts the influence of initial concentration of lindane on the observed pseudo first-order rate constant, k obs, using simulated solar light-assisted S-TiO2 /HSO 5 − process. The k obs was found to decrease with increasing the initial concentration of lindane. Under the experimental condition in this study, the value of k obs was 8.98 × 10−1 , 6.58 × 10−1 and 3.84 × 10−1 h−1 for 0.5, 1.0 and 2.0 µM of initial concentration of lindane, respectively. These findings were consistent with the previous studies regarding the effect of solute concentration on the photocatalytic degradation of organic pollutants [41-44]. The most plausible reason for the decreasing k obs could be the formation of reaction by-products which competed with lindane for various ROS including SO4 •−, at higher initial concentrations of lindane [45, 46]. For a given concentration of reactive radicals produced under a specific condition, the high concentration of pollutant will require more time to attain the same extent of degradation, thus explaining the reduced removal efficiency of lindane. The initial degradation rate of lindane (calculated by the change in concentration with time at an initial reaction time of 1 h) was also determined at three different initial concentrations (i.e., 0.5, 1.0 and 2.0 µM) and the results are shown in Figure 5. As seen in Figure 5, the initial degradation rate of lindane increased since the number of lindane molecules exposed to more ROS at higher initial concentrations of lindane [47]. Under the experimental conditions used in
this study, the degradation rate of lindane was 0.20, 0.31 and 0.52 µM h−1 at using 0.5, 1.0 and 2.0 µM of initial concentration of lindane, respectively. 3.4.2
Influence of initial concentrations of HSO5 − The effect of initial concentration of HSO 5 − on the efficiency of simulated solar light-
assisted S-TiO 2 /HSO 5 − process was investigated, and the results are shown in Figure 6. As can be seen, the value of k obs increased with increasing the initial concentration of HSO 5 −. A plausible explanation for this behaviour could be the increased production of SO 4 •− and •OH (reactions (11) and (12)), at using higher initial concentration of HSO 5 − [32]. As stated earlier, the electron-hole recombination was inhibited by HSO 5 −, so, an increase in the concentration of HSO 5 − may cause an enhanced inhibition of electron-hole recombination, thereby increasing the removal efficiency of lindane as well, and hence k obs was increased. The calculated k obs was found to be 3.84 × 10−1 , 5.81 × 10−1 , 8.01 × 10−1 , and 9.97 × 10−1 h−1 , when initial concentration of HSO 5 − was 0.1, 0.2, 0.5 and 1.0 mM, respectively. An initial HSO 5 − concentration of 0.2 mM was used in all subsequent experiments, corresponding to its minimum amount needed to achieve complete (99.9%) lindane removal in 6 h. 3.4.3
Influence of solution pH A solution pH may influence the extent of photocatalytic degradation of organic
pollutants by changing the surface charge of the photocatalyst, state of ionization of the pollutant, and concentration of the reactive radicals produced [10, 26, 48, 49]. The degradation of lindane by simulated solar light-assisted S-TiO 2 /HSO 5 − process was studied at three different pH values of 4.0, 5.8 and 8.0, and the results are shown in Figure 7. The degradation kinetics varied significantly with solution pHs and the highest lindane removal was achieved at pH 5.8, corresponding to 99.9% removal in 6 h. In contrast, the removal efficiency decreased in stronger
acidic as well as basic conditions, indicating 88.2 and 71.4% removal at pH 4.0 and 8.0, respectively, after 6 h of simulated solar light irradiation. The lindane degradation followed pseudo first-order kinetics model in the studied pH range with k obs of 3.56 × 10−1 , 6.58 × 10−1 and 2.09 × 10−1 h−1 for pH 4.0, 5.8, and 8.0, respectively. The obtained results are in good agreement with the literature reports,
dealing with photocatalytic degradation of other
insecticides such as atrazine and dimethoate [16, 49]. Since the electrostatic force of attraction or repulsion between the photocatalyst surface and the pollutant is affected by the solution pH [49, 50], the degradation efficiency of S-TiO 2 photocatalysis was found to change with the varying pH. At lower pH, the concentration of O 2 −• was reduced by reacting with H+ ions (reaction (8)) [11], and so the removal efficiency of lindane was reduced. The removal efficiency of lindane was also reduced at pH 8.0, which can be attributed to the surface charge change of photocatalysts, corresponding to surface electrostatic repulsion with the pollutant. Literature studies showed that TiO 2 photocatalysis of atrazine resulted in a lower removal efficiency at the increasing solution pH, i.e., at pH 7 or 10 than at pH 4 or 5 [51]. The change in the removal efficiency may also be due to varying behaviour of HSO 5 − under simulated solar light (i.e., UV component), at different pH values [52]. The scavenging of SO4 •− and •OH by HO − following reactions (15) and (16), respectively, may probably lower the removal efficiency at higher solution pH [29, 53]. Since the oxidation potential of •OH decreased with the increasing pH; it could also partially explain the lower efficiency of lindane at high pH [54]. Above all, our results clearly indicated that the highest photocatalytic activity was observed at pH 5.8. SO 4 •− + HO− → SO 4 2− + •OH
(k = 8.3 × 107 M−1 s−1 , alkaline pH)
(15)
•
(k = 61.2 × 1010 M−1 s−1 , alkaline pH)
(16)
OH + HO − → O•− + H2 O
3.5 Identification of reaction intermediates and degradation mechanism GC-MS analysis revealed six reaction intermediates, i.e., hexachlorobenzene (HCB), pentachlorocyclohexene (PCCH),
tetrachlorocyclohexene (TeCCH), trichlorobenzene (TCB),
dichlorobenzene (DCB), and trichlorophenol (TCP) during simulated solar light-assisted STiO2 /HSO 5 − photocatalysis of lindane. CO 2 and Cl− were found to be the end-products of lindane, as revealed by TOC and IC analyses. Based on the results of identification of intermediates and end-products, a plausible degradation mechanism was proposed, as shown in Figure 8. The simulated solar light-assisted S-TiO 2 /HSO 5 − photocatalytic degradation of lindane was expected to have initiated by attack of •OH, SO4 •− and/or O 2 •− species, generated via reactions (3)-(5), (10), (11), (12) and (13). For example, the formation of PCCH was presumably resulted from the dehalogenation of lindane by O 2 •−, as described earlier by Antonaraki et al., using polyoxometalates (POMs) photocatalysis [55]. This mechanism was similar to the dehalogenation of aliphatic halocarbons by O 2 •−, whereby the reacting molecule was converted into corresponding alkenes [56]. PCCH was also previously reported as an intermediate product of lindane during TiO 2 photocatalysis [57]. The PCCH on successive dechlorination may lead to the formation of lesser chlorinated by-products such as TeCCH, TCB and DCB. For instance, TCB was reported during dehalogenation of PCCH in subcritical water [58]. Both •OH and SO4 •− may easily react with aliphatic compounds following hydrogen abstraction pathway (i.e., H abstraction) [29, 53]. Thus, lindane was likely to be dehydrogenated and converted into HCB by a reaction of •OH and/or SO4 •− via H abstraction [39]. The formation of a stable HCB by-product may allow the reaction to be kinetically feasible [59]. Antonaraki et al. [55], Guo et al. [59] and Nitoi et al. [60] also reported HCB as intermediate of lindane using
POMs photocatalysis, POM/SiO 2 photocatalysis, and photo-Fenton process, respectively, attributing its formation to the reaction of •OH radical. TCP could be formed by hydroxylation of TCB mediated by •OH and/or SO4 •− radical mechanisms [39]. The •OH radical abstracts hydrogen atom from aromatic hydrocarbons, and thereby forming a carbon centered radical. The resulting carbon centered radical on termination with another •OH radical may give rise to a phenolic compound [61], e.g., TCP in this case. Antonaraki et al. [55] have reported TCP as an intermediate product during POMs photocatalysis of lindane, attributing its formation to •OH radical attack. The various intermediates detected in this study on subsequent reactions with different reactive species might have undergone ring cleavage, as indicated from the CO 2 evolved. CO2 was also reported as end-product of lindane using POMs photocatalysis [55] and photo-Fenton process [60]. 3.6 Test of reusability of S-TiO2 photocatalyst films Mechanical stability and
reusability are among the most important features of a
photocatalyst, which were investigated in this study by conducting four repeated cycle experiments using a single film of S-TiO 2 , and the results are shown in Figure 9. As seen in Figure 9, comparable degradation results were achieved in the four successive cycles, corresponding to 99.9, 98.7, 97.4 and 96.6% lindane removal, respectively, after 6 h of simulated solar light irradiation. This result showed that the photocatalytic activity of S-TiO2 film was maintained during four cycles, indicating high mechanical stability and recycling capacity for STiO2 film under the studied experimental conditions. 4. Conclusions Nanocrystalline S-TiO2 film, synthesized by a sol–gel method, showed a considerable efficiency on the degradation of lindane under visible and simulated solar light irradiation. The
photocatalytic activity of visible and simulated solar light-assisted S-TiO 2 was dramatically improved in the presence of 0.2 mM HSO 5 −, leading to 68.2 and 99.9% lindane removal, respectively in 6 h. Thus with addition of HSO 5 −, sizes of photocatalytic reactors can be considerably reduced in practical applications. The photocatalytic efficiency of S-TiO2 /HSO 5 − process decreased with increasing initial concentration of lindane. The solution pH had a significant effect on the efficiency of S-TiO2 /HSO 5 − process. The highest removal efficiency was achieved at near neutral pH (5.8). The photocatalytic activity of S-TiO2 film was maintained after four repeated runs, confirming mechanical stability for S-TiO2 film. The results of intermediate
and
end-products
analyses
led
to
the
conclusion
that
dechlorination,
dehydrogenation, hydroxylation, and ring cleavage, via •OH, SO4 •− and O2 •− attack were the main reaction steps followed during S-TiO 2 /HSO 5 − photocatalysis of lindane. The results indicated that visible and
simulated solar light-assisted S-TiO2 /HSO 5 − is very effective for the
detoxification of water contaminated with chlorinated pesticides such as lindane. Acknowledgments The Higher Education Commission (HEC), Islamabad, Pakistan is highly acknowledged for funding this research project through an International Research Support Initiative Program (IRSIP). This work was also partially funded by the Cyprus Research Promotion Foundation through Desmi 2009-2010 which is co-funded by the Republic of Cyprus and the European Regional
Development
IPODOMI/STRATH/0308/09.
Fund
of
the
EU
under
contract
number NEA
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Figure captions
Figure 1. Degradation of lindane by visible light-assisted S-TiO2 photocatalysis. [lindane]0 = 1.0 µM, [S-TiO2 ]0 = 0.23 g/L, pH = 5.8. Figure 2. Degradation of lindane by simulated solar light-assisted S-TiO2 photocatalysis. [lindane]0 = 1.0 µM, [S-TiO 2 ]0 = 0.23 g/L, pH = 5.8. Figure 3. Effect of 0.2 mM HSO 5 − on S-TiO2 photocatalysis of lindane under visible light irradiation. [lindane]0 = 1.0 µM, [S-TiO 2 ]0 = 0.23 g/L, pH = 5.8. Figure 4. Effect of 0.2 mM HSO 5 − on S-TiO2 photocatalysis of lindane under simulated solar light irradiation. [lindane]0 = 1.0 µM, [S-TiO2 ]0 = 0.23 g/L, pH = 5.8. Figure 5. Variation of rate constant (k obs) and initial degradation rate with different initial concentration of lindane by simulated solar light-assisted S-TiO2 /HSO 5 − process. Initial degradation rate corresponds to the first hour of decay. [S-TiO2 ]0 = 0.23 g/L, pH = 5.8. Figure 6. Variation of rate constant (k obs) with different initial concentration of HSO 5 − by simulated solar light-assisted S-TiO2 /HSO 5 − process. [lindane]0 = 1.0 µM, [S-TiO 2 ]0 = 0.23 g/L, pH = 5.8. Figure 7. Effect of initial solution pH on the efficiency of simulated solar light-assisted STiO 2 /HSO 5 − process. [lindane]0 = 1.0 µM, [S-TiO2 ]0 = 0.23 g/L, [HSO 5 −]0 = 0.2 mM. Figure 8. Proposed degradation pathway of lindane by simulated solar light-assisted STiO 2 /HSO 5 − process. [lindane]0 = 10.0 μM, [S-TiO 2 ]0 = 0.23 g/L, [HSO 5 −]0 = 0.2 mM, pH = 5.8. Figure 9. Multi-cycle tests for S-TiO 2 photocatalyst in degrading lindane under simulated solar light for 6 hr. lindane]0 = 1.0 µM, [S-TiO 2 ]0 = 0.23 g/L, pH = 5.8.
Table Captions Table 1. Pseudo first-order rate constant (k obs), removal efficiency (%) and half-life (t 1/2 ) of simulated solar light-assisted S-TiO2 photocatalytic processes for lindane degradation, in the presence of 0.2 mM HSO 5 −. [lindane]0 = 1.0 µM, [S-TiO2 ]0 = 0.23 g/L, pH = 5.8. Table 2. Pseudo first-order rate constant (k obs), removal efficiency (%) and half-life (t 1/2 ) of visible light-assisted S-TiO 2 photocatalytic processes for lindane degradation, in the presence of 0.2 mM HSO 5 −. lindane]0 = 1.0 µM, [S-TiO 2 ]0 = 0.23 g/L, pH = 5.8.
Figure 1.
1.0
C/C0
0.9
0.8 Visible light alone Ref-TiO2/dark 0.7
S-TiO2/dark Ref-TiO2/visible light S-TiO2/visible light
0.6
0
1
2
3
4
Irradiation time (h)
5
6
Figure 2.
1.0
C/C0
0.8
0.6
Simulated solar light alone Ref-TiO2/Simulated solar light
0.4
S-TiO2/Simulated solar light
0
1
2
3
4
Irradiation time (h)
5
6
Figure 3.
1.0
C/C0
0.8
0.6
0.4
HSO5-/Visible light Ref-TiO2/HSO5-/Visible light
0.2
S-TiO2/Visible light S-TiO2/HSO5-/Visible light
0.0 0
1
2
3
4
Irradiation time (h)
5
6
Figure 4.
1.0
C/C0
0.8
0.6
0.4 HSO5-/Simulated solar light
0.2
Ref-TiO2/HSO5-/Simulated solar light S-TiO2/HSO5-/Simulated solar light
0.0
0
1
2
3
4
Irradiation time (h)
5
6
Figure 5.
Rate constant, kobs 0.5
-1
Degradation rate
0.8
0.4
0.6
0.3
0.4
0.2
0.5
1.0
1.5
[Lindane]0 (M)
2.0
Degradation rate (M.h-1)
Rate constant, kobs (h )
1.0
Figure 6.
-1
Rate constant, kobs, (h )
1.2
1.0
0.8
0.6
0.4
0.0
0.2
0.4
0.6
[HSO5-] (mM)
0.8
1.0
Figure 7.
1.0
C/C0
0.8
0.6
0.4
0.2
pH = 8.0 pH = 4.0 pH = 5.8
0.0 0
1
2
3
4
Irradiation time (hr)
5
6
Figure 8.
Figure 9.
Lindane degradation (%)
100
80
60
40
20
0 1
2
3
Number of cycles
4
Table 1. Simulated solar light-assisted AOPs
k obs (h−1 )
Removal efficiency (%)
t1/2 (h)
Ref-TiO2 /Simulated solar light
7.31 × 10−2
36.7
9.5
S-TiO2 /Simulated solar light
1.63 × 10−1
63.4
4.3
Ref-TiO2 /HSO5 −/Simulated solar light
3.10 × 10−1
85.4
2.2
S-TiO2 /HSO5 −/Simulated solar light
6.58 × 10−1
99.9
1.2
Table 2. Visible light-assisted AOPs
k obs (h−1 )
Removal efficiency (%)
t1/2 (h)
Ref-TiO2 /visible light
6.61 × 10−3
4.2
105.0
Ref-TiO2 /HSO5 −/visible light
1.19 × 10−2
7.3
58.2
S-TiO2 /visible light
5.93 × 10−2
31.0
11.7
S-TiO2 /HSO5 −/visible light
1.87 × 10−1
68.2
3.7