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Article

Exhaustive Photocatalytic Lindane Degradation by Combined Simulated Solar Light-Activated Nanocrystalline TiO2 and Inorganic Oxidants

by
Sanaullah Khan
1,2,3,†,
Changseok Han
4,†,
Murtaza Sayed
2,
Mohammad Sohail
5,
Safeer Jan
2,6,
Sabiha Sultana
7,
Hasan M. Khan
2 and
Dionysios D. Dionysiou
3,8,*
1
Department of Chemistry, Women University Swabi, Swabi 23430, Pakistan
2
Radiation and Environmental Chemistry Laboratory, National Centre of Excellence in Physical Chemistry, University of Peshawar, Peshawar 25120, Pakistan
3
Environmental Engineering and Science Program, Department of Chemical and Environmental Engineering (ChEE), University of Cincinnati, Cincinnati, OH 45221-0012, USA
4
Department of Environmental Engineering, INHA University, Incheon 22212, Korea
5
Institute of Chemical Sciences, University of Swat, Swat 19130, Pakistan
6
Hubei Key Laboratory of Electrochemical Power Sources, College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China
7
Department of Chemistry, Islamia College University Peshawar, Peshawar 25120, Pakistan
8
Nireas-International Water Research Centre, University of Cyprus, 1678 Nicosia, Cyprus
*
Author to whom correspondence should be addressed.
†
Contributed equally to this work.
Catalysts 2019, 9(5), 425; https://doi.org/10.3390/catal9050425
Submission received: 25 February 2019 / Revised: 4 April 2019 / Accepted: 8 April 2019 / Published: 7 May 2019
(This article belongs to the Special Issue Nanostructured Materials for Photocatalysis)
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Abstract

:
Organochlorine compounds (OCs) are very toxic, highly persistent, and ubiquitous contaminants in the environment. Degradation of lindane, a selected OC, by simulated solar light-activated TiO2 (SSLA-TiO2) photocatalysis was investigated. The film types of the TiO2 photocatalyst were prepared using a dip-coating method. The physical properties of the films were investigated using X-ray diffraction, transmission electron microscopy, and environmental scanning electron microscopy. The SSLA-TiO2 photocatalysis led to a lindane removal of 23% in 6 h, with 0.042 h−1 of an observed pseudo first-order rate constant (kobs). The SSLA-TiO2 photocatalysis efficiency was greatly enhanced by adding hydrogen peroxide (H2O2), persulfate (S2O82−), or both combined, corresponding to a 64%, 89%, and 99% lindane removal in the presence of 200 µM of H2O2, S2O82−, or equimolar H2O2-S2O82−, respectively. The hydroxyl and sulfate radicals mainly participated in lindane degradation, proven by the results of a radical scavenger study. The degradation kinetics were hindered in the presence of the water constituents, indicated by a 61%, 35%, 50%, 70%, 88%, and 91% degradation of lindane in 6 h, using a SSLA-TiO2/S2O82−/H2O2 photocatalysis system containing 1.0 mg L−1 humic acid (HA), or 1 mM of CO32−, HCO3−, NO3−, SO42−, and Cl−, respectively. The TiO2 film demonstrated high reusability during four runs of lindane decomposition experiments. The SSLA-TiO2/S2O82−/H2O2 photocatalysis is very effective for the elimination of a persistent OC, lindane, from a water environment.

Graphical Abstract

1. Introduction

Many organochlorine compounds (OCs), such as chlorinated alkanes, alkenes, benzenes, phenols, and biphenyls, are often introduced into the environment in the form of solvents, disinfectants, soil fumigants, pesticides, and as dye precursors [1]. A considerable number of OCs also enter the environment as by-products from waste incineration, the chlorination of drinking water and wastewater, and bleaching of pulp with chlorine [2]. OCs are generally considered as very toxic, highly bioaccumulative, and strongly resistant towards biodegradation [2].
Hexachlorocyclohexanes (HCHs) constitute one of the largest classes of chlorinated alkanes that have extensively been used as organochlorine pesticides during the last several decades [3]. The HCHs consist of eight isomers, namely β, γ, δ, θ, ε, η, and two α-enantiomers. The insecticidal property of HCHs is solely due to the γ-isomer, commonly known as lindane (i.e., 99% γ-HCH) [4]. The low cost and high efficiency of lindane led to its excessive usage in a wide application range, such as an insecticide and a seed treatment agent in the agricultural and forest industry, a vector control in public health, and as anti-mice and anti-lice agents for livestock and domestic purposes [5,6]. Because of the high chemical stability and mobility of lindane, it ubiquitously disperses in the environment, including biota via the food chain [7]. Owing to the presence of large number of chlorine atoms in the molecule, lindane is a highly toxic compound and is recognized as an endocrine disruptor in the environment [8]. The development of more efficient methods for removing lindane from water is of fundamental importance for environmental cleanup.
Advanced oxidation processes (AOPs) are promising alternatives to the conventional water and wastewater treatment processes, owing to their wide versatility and high efficiency [9]. Among AOPs, TiO2 photocatalysis is considered as one of the most promising technologies because of the low cost, easy availability, and environmentally benign nature of TiO2 [10]. The solar light-induced TiO2 photocatalysis has recently gained much attention in water decontamination and disinfection, owing to the availability and sustainability of sunlight radiation [11,12]. The photocatalytic performance of the catalysts is improved significantly with an increased crystallinity, large surface area, and tailor-designed morphology [13,14,15]. The efficiency of solar light-activated TiO2 photocatalytic processes is usually low, because only a small portion of the sunlight (i.e., 5%, namely UV radiation) is involved in the activation of TiO2 (Reaction (1)) [11].
TiO2 + hν → hVB+ (valence band hole) + eCB− (conduction band electron)
Hydrogen peroxide (H2O2), persulfate (S2O82−), and, more recently, peroxymonosulfate (HSO5−) are emerging inorganic oxidants employed or explored in water and/or wastewater treatment processes, owing to the generation of the strongly reactive oxidants of the hydroxyl radical (•OH) and sulfate radical (SO4•−) [16,17,18]. Several inorganic anions (e.g., CO32−, HCO3−, NO3−, SO42−, and Cl−) and organic acids (e.g., humic acid (HA)) are frequently found in water sources, owing to the natural abundances and man-made activities [19]. Depending on the mode of the reaction, these inorganic and organic constituents in water may differently affect the removal efficiency of pollutants using different AOPs [20]. Only limited information is available about the effect of these constituents on the efficiency of combined photocatalytic–photochemical processes, so far.
In this study, the efficiency of a simulated solar light-activated nanocrystalline TiO2 photocatalyst for decomposing a selected OC, lindane, in an aqueous solution was investigated. The synthesized TiO2 photocatalyst films were characterized using X-ray diffraction (XRD), environmental scanning electron microscopy (ESEM), and transmission electron microscopy (TEM), for determining its surface morphology and structural properties. The effect of H2O2 and S2O82− on the activity of the simulated solar light-activated TiO2 (SSLA-TiO2) photocatalysis for the removal of lindane was investigated. Radical scavenging experiments were conducted so as to examine the relative importance of the reactive species towards the degradation of lindane. The effect of natural water constituents (i.e., HA and inorganic ions) on the efficiency of the SSLA-TiO2/S2O82−/H2O2 process was investigated, considering practical applications. Finally, the performance sustainability of the synthesized TiO2 photocatalyst was evaluated using four runs of lindane decomposition experiments. The obtained results could provide useful data on the application of SSLA-TiO2 photocatalysis for removing persistent OCs, such as lindane, from the water environment.

2. Results and Discussion

2.1. Characteristics of Synthesized TiO2 Films

Figure 1a shows the ESEM image for the film of the TiO2 photocatalyst. As the film was dried under infrared illumination and then slowly heated up to 350 °C, no cracks in the TiO2 film were observed at a magnification of 2000×. The peaks of the XRD analysis of the synthesized TiO2 corresponding to the anatase phase of TiO2 were observed (Figure 1b). This indicates that the anatase phase of TiO2 dominated in the films with the synthesis method, based on the observed peaks at 2θ (degree) = 24.8, 37.3, 47.6, 53.5, 55.1, 62.2, 68.8, 70.1, and 74.9, corresponding to the (101), (004), (200), (105), (211), (204), (116), (220), and (215) planes, respectively, of anatase TiO2 (JCPDS card no. 21-1272). Figure 1c,d shows the HR-TEM images of the sample. The average crystal size of the TiO2 photocatalyst was 20 ± 5.7 nm. The measured BET surface area was 13.1 ± 0.04 m2 g−1 (Figure 1e,f). The spacing between the lattice fringes was calculated as 0.351 nm, similar to the TiO2 anatase lattice spacing of the (101) plane (i.e., 0.352 nm) [21]. The calculated band-gap energy of the synthesized TiO2 film by the Kubelka–Munk transformation was 3.1 eV (Figure S1). These results confirmed the formation of the anatase crystalline phase, which has a better photocatalytic activity compared to rutile [22]. The small particle size with the corresponding high surface area, as well as the large number of active sites, could lead to the high photocatalytic activity of the TiO2 anatase crystalline phase [23]. Lin et al. [24] also reported that the band gap of the TiO2 nanoparticles is a function of primary particle size, so that when the TiO2 particle size decreased (i.e., 29 to 17nm), the band gap decreased as well.
The valence band electrons of the synthesized TiO2 film are excited to a conduction band when a solar light of sufficient energy illuminates the film. The electrons in the conduction band (eCB−) interact with oxygen to generate the highly reducing superoxide radical anion (O2•−) (i.e., Reaction (2)), while the valence band holes (hVB+) react with H2O or OH– producing the strongly oxidizing •OH, following Reactions (3) and (4), respectively [10].
eCB− + O2 → O2•−
hVB+ + H2O → •OH + H+
hVB+ + OH– → •OH

2.2. SSLA-TiO2 Photocatalysis of Lindane

Figure 2 shows the degradation of lindane by SSLA-TiO2 photocatalysis, indicating that 23% lindane was removed in 6 h, corresponding to an observed rate constant (kobs) of 0.042 h−1. Many papers were published on the SSLA-TiO2 photocatalysis of organic compounds [25,26,27].
The •OH produced in Reactions (3) and (4) was mainly responsible for the decomposition of organic contaminants [28,29]. The O2•− formed in Reaction (2) might also participate in pollutant degradation, and may likely react with a water molecule to produce additional •OH [30]. Khan et al. [31] previously reported an efficient lindane degradation by •OH, besides a minor lindane removal by O2•−, employing simulated solar light-activated sulfur-doped TiO2 (S-TiO2) photocatalysis. In the current case too, it was seen that •OH mainly participated in lindane degradation, although O2•− also contributed slightly in the degradation process, as will be discussed latter in Section 2.4. Literature studies show that the degradation efficiency of some other organic compounds by solar TiO2 photocatalysis was rather high [32,33,34]. The reaction rate constants for the SSLA-TiO2 photocatalysis of some other pesticides, such as λ-cyhalothrin, chlorpyrifos, and diazinon (C0 = 0.1, 0.57, and 0.36 mM, respectively) were 0.48, 0.44, and 0.25 h−1, respectively [25]. Adishkumar and Kanmani [26] found that the reaction rate constant for SSLA-TiO2 photocatalysis of phenol (C0 = 1.06 mM) was 1.76 h−1. The apparent discrepancy in the degradation rate constants could be because of the effect of the light intensity [35,36,37], as well as the molecular structure differences, as previously reported regarding organic pollutants degradation using other AOPs [38]. Parra et al. [36] showed that the degradation efficiency of atrazine using solar simulator/TiO2 photocatalysis was significantly enhanced by increasing the solar light intensity from 50 to 90 mW/cm2. Khataee and Kasiri [39] reported that monoazo dyes have higher photocatalytic degradation rates as compared to the dyes with an antraquinone structure. The presence of a methyl or chloro group in the dye molecule showed a decreasing effect, while the nitrite group has an increasing effect on the degradation efficiency. The photocatalytic efficiency of dye molecules decreases by a sulfonic substituent, while the hydroxyl group has an opposite effect [39].
Zaleska et al. [40] reported a 77% lindane degradation ([lindane]0= 0.137 mM; [TiO2]0 = 0.5 g/L) after 2.5 h irradiation in the anatase TiO2/UV photocatalytic process. The discrepancy between our results and those reported by Zaleska et al. [40] is in agreement with the findings by Parra et al. [36], showing a higher degradation efficiency of atrazine using UV/TiO2 than solar simulator/TiO2 photocatalysis, obviously due to an efficient activation of TiO2 for •OH radical generation by UV than simulated solar light.
Despite the significant lindane degradation achieved in 6 h, the efficiency of the SSLA-TiO2 process in the current case may be regarded as low, particularly considering the practical applications. In an attempt to achieve a higher degradation efficiency by SSLA-TiO2 photocatalysis, inorganic oxidants, such as S2O82− and H2O2, were employed as additives in the subsequent experiments, as discussed below.

2.3. Effect of S2O82− and H2O2 on Lindane Degradation by SSLA-TiO2 Photocatalysis

Figure 3 shows the effect of S2O82− and H2O2 on the removal efficiency of lindane using SSLA-TiO2 photocatalysis. The removal efficiency of lindane was remarkably enhanced by adding 200 µM of S2O82− or H2O2, indicated by an 89% and 64% lindane removal in 6 h, corresponding to an observed pseudo first-order rate constant (kobs) of 0.369 and 0.098 h−1, respectively. Both H2O2 and S2O82− are strong electron acceptors, capable of scavenging the photogenerated eCB−, according to Reactions (5) and (6), respectively, thus increasing the concentration of hVB+, with a subsequent higher •OH concentration on the TiO2 surface [41]. The photolysis of H2O2 and S2O82− can lead to the generation of additional •OH, as well as SO4•−, according to Reactions (7) and (8), respectively [42,43]. The above-mentioned phenomena, that is, the promotion of the charge separation followed by an increased production of •OH and the generation of additional •OH and SO4•−, could explain the enhancing effect of H2O2 and S2O82− [44]. The comparatively higher enhancing effects exerted by the S2O82− than H2O2 were probably due to an easier activation of S2O82− than H2O2 by simulated solar radiation containing UV light [45]. The quantum yield results show that the concentration of SO4•− generated by the simulated solar radiation (containing UV light) activation of S2O82− was high, compared to that of •OH that resulted from H2O2 [46]. Antonopoulou and Konstantinou [47], and Koltsakidou et al. [32] also reported that the addition of S2O82− showed a stronger enhancing effect than H2O2 on the decomposition of N,N-diethyl-m-toluamide and cytarabine, respectively, by simulated solar-light assisted TiO2 photocatalysis.
H2O2 + eCB− → •OH + HO−
S2O82− + eCB− → SO4•− + SO42−
H2O2 + hv → 2 •OH (λ = 253.7 nm, ϕ = 1.0)
where “ϕ” is the quantum yield, that is, the number of species formed per photon absorbed.
S2O82− + hv → 2SO4•− (λ = 253.7 nm, ϕ = 1.8)
•OH + •OH → H2O2 (k = 5.3 × 109 M−1 s−1)
SO4•− + SO4•− → S2O82− (k = 4 × 108 M−1 s−1)
Furthermore, an even stronger enhancing effect on the lindane degradation efficiency (Figure 3) was noticed, indicated by a 99% lindane removal in 6 h, corresponding to a kobs of 0.550 h−1 when each 100 µM of S2O82− and H2O2 was added into the system together. Table 1 shows the kinetics analyses of degradation of various pesticides using TiO2 photocatalytic processes under different conditions [25,48].
The degradation of organic compounds usually takes place in several steps, involving the generation and destruction of various reaction by-products [49]. The various reaction by-products of lindane identified in the SSLA-TiO2/S2O82−/H2O2 process included hexachlorobenzene, tetrachlorocyclohexene, and dichlorophenol, probably resulting via hydrogen abstraction and hydroxylation pathways by using SO4•− and •OH [20,31]. All of the generated by-products eventually disappeared after 6 h of irradiation. The reactivity of the various by-products generated during the degradation process could be different towards •OH and SO4•− [50]. For example, chlorobenzene, which is identified in this study as well as frequently reported as a lindane by-product elsewhere using TiO2 photocatalysis [31,51], typically showed a higher rate constant towards •OH than SO4•−, probably because of the high tendency of the former species for addition reactions due to the multiple bonds [50]. Literature studies show that the efficiency of UV/S2O82− and UV/H2O2 systems that are capable of generating SO4•− and •OH, respectively, varied when applied to different types of chemical compounds [52,53]. This might explain the larger enhancing effect while using both S2O82− and H2O2 combined, because it could provide an opportunity for the different by-products generated to be efficiently degraded under the conditions favorable to it, that is, either SO4•− or •OH, or both combined.
Furthermore, the residual oxidant analysis revealed that out of the 100 µM of H2O2 and S2O82− each employed in the SSLA-TiO2/S2O82−/H2O2 process, 7 and 23 µM were left as a residue after 6 h of treatment, respectively.
Figure 4 shows the effect of the initial concentration of S2O82− and H2O2 on the observed pseudo first-order rate constant (kobs) of lindane using the SSLA-TiO2 photocatalysis. The value of kobs increased at a higher initial S2O82− and H2O2 concentration, attributable to an increased promotion of the charge separation, as well as the higher concentrations of SO4•− and •OH. However, the increase in kobs was less in the case of H2O2 than S2O82−, probably due to the increased recombination rate in the former case, that is, the higher second-order rate constant of Reaction (9) than Reaction (10) [20]. Bekkouche et al. [54] determined an enhancing effect due to an increasing initial concentration of S2O82− on the solar-UV/TiO2/S2O82− photocatalytic degradation of Safranin O, attributed to an increased generation of reactive radicals. Saien et al. [45] reported the effects of the initial concentration of S2O82− and H2O2 using the UV/TiO2 photocatalysis of Triton X-100, showing an enhancing effect as a result of increasing the initial concentrations of the oxidants. Koltsakidou et al. [32] reported a decreasing effect as a result of increasing the initial concentrations of H2O2 on the SSLA-TiO2 photocatalytic degradation of cytarabine, attributed to the fast scavenging of •OH at comparatively higher H2O2 concentrations (i.e., 1–4 mM).

2.4. Radical Scavenger Studies and Role of the Reactive Species

The •OH, SO4•−, and O2•− are the main reactive species present in the SSLA-TiO2/S2O82−/H2O2 system, as discussed above. To identify the role of the individual reactive species in the lindane decomposition, appropriate radical scavenger experiments were performed.
Benzoquinone, tert-butanol, and iso-propanol were employed to scavenge O2•−, •OH, and both •OH and SO4•−, respectively, according to Reactions (11)–(14) [50,55]. In the presence of 50 mM benzoquinone, 89% lindane was decomposed by SSLA-TiO2/S2O82−/H2O2 in 6 h (Figure 5). In the absence of a radical scavenger, a 99% removal of lindane could be achieved by SSLA-TiO2/S2O82−/H2O2 in 6 h. The result showed a 10% decrease in the lindane removal efficiency as a result of the addition of benzoquinone, attributable to the reactions of O2•−. By adding 50 mM tert-butanol, a 58% removal of lindane was achieved in 6 h (Figure 5), indicating a 41% loss in the efficiency of the SSLA-TiO2/S2O82−/H2O2 system, attributable to the role of •OH. In the presence of 50 mM iso-propanol, only an 11% removal of lindane occurred in 6 h (Figure 5), indicating an 88% decrease in the removal efficiency of lindane by SSLA-TiO2/S2O82−/H2O2, attributable to the role of the combination of •OH and SO4•−. By subtracting, the role of •OH and SO4•− was found to be 41% and 47%, respectively. The relatively larger contribution of SO4•− than •OH could be due to the high concentration of the former species [45], as well as its high rate constant with lindane [20].
Benzoquinone + O2•− → Benzoquione•− + O2 k = 9.6 × 108 M−1 s−1
(CH3)3COH + •OH → (CH3)2•CH2COH + H2O  k= 5.2 × 108 M−1 s−1
(CH3)2CHOH + •OH → (CH3)2•COH + H2O k = 1.9 × 109 M−1 s−1
(CH3)2CHOH + SO4•− → (CH3)2•COH + HSO4−  k = 8.2 × 107 M−1 s−1

2.5. Influence of Natural Water Constituents

Inorganic ions are one of the most frequently found ingredients in water and wastewater, and are derived from various anthropogenic and non-anthropogenic sources [19]. The presence of inorganic ions can influence the photocatalytic and/or photochemical degradation of organic compounds [56,57,58,59]. CO32−, HCO3−, NO3−, SO42−, and Cl−, typical components of natural waters, were selected as inorganic ions in this study. As seen in Figure 6, the efficiency of the SSLA-TiO2/S2O82−/H2O2 system was reduced by 65%, 50%, 30%, 12%, and 9% in the presence of 1.0 mM of CO32−, HCO3−, NO3−, SO42−, and Cl−, respectively. The inorganic ions could be adsorbed onto the TiO2 catalyst, thereby decreasing the active site number on its surface [60]. The inhibiting effect may be due to the reduced generation of the reactive radicals owing to the blocking of the TiO2 active sites [61], as well as the scavenging of the reactive radicals by the ions [59,62]. Liang et al. [58] reported that a degradation rate of the TiO2 photocatalysis of 2,3-dichlorophenol decreased in the presence of NO3−, Cl−, SO42−, and HPO4−, attributed to the competitive adsorption on the surface of TiO2, besides the scavenging of the •OH by the ions. Muruganandham and Swaminathan determined that the UV/TiO2 photocatalytic oxidation of Reactive Yellow 14 was decreased by adding Cl− or CO32−, attributed to the •OH scavenging [63]. Our previous study [64] reported a 51%, 34%, 3%, and 1% decrease on the lindane removal efficiency by UV/HSO5− process in the presence of 1.0 mM CO32−, HCO3−, Cl−, or SO42−, respectively, which was attributed to the scavenging of SO4•− and •OH. Contrary to our previous study involving lindane degradation by an UV/HSO5− system [64], the comparatively larger inhibiting effect observed in the current case may indicate an additional TiO2 deactivation by the ions, besides the scavenging of the reactive radicals, according to Reactions (15)–(19) [50,65,66]. Despite the high affinity of Cl− for the scavenging of reactive radicals (Reactions (18) and (19)), a rather small inhibiting effect was observed on the degradation efficiency of lindane. A plausible reason could be the involvement of the reactive Cl• in the degradation process, as was previously described [20].
The efficiency of lindane degradation by the SSLA-TiO2/S2O82−/H2O2 process decreased by 39% in the presence of 1.0 mg L−1 HA (Figure 6). The result was consistent with the findings of Bekkouche et al. [54], showing that the solar-UV/TiO2/S2O82− photocatalytic degradation of Safranin O decreased in the presence of HA. Plausible reasons for the inhibitory effect could be (i) the scavenging of •OH and SO4•− by HA (Reactions (20) and (21)) [67,68], (ii) the blocking of active sites on the TiO2 surface attributable to HA adsorption [69], and (iii) the absorption of photons by HA [59]. Doorslaer et al. [70] reported that the UV/TiO2 photocatalytic degradation rate of moxifloxacin decreased by adding different types of dissolved organic matter (DOM), including HA and fulvic acid, attributed to the scavenging of reactive species (i.e., •OH) as well as the absorption of UV light by DOM. The results might suggest that the natural water constituents studied may adversely affect the efficiency of the TiO2/oxidant-based AOPs applied for the decontamination of field waters, which might need further attention.
HCO3− + •OH → H2O + CO3•− (k = 8.5 × 106 M−1 s−1) [50]
HCO3− + SO4•− → CO3•− + SO42− + H+ (k = 3.5 × 106 M−1 s−1) [65]
CO32− + SO4•− → CO3•− + SO42− (k = 4.1 × 106 M−1 s−1) [66]
Cl− + •OH → ClHO•−   (k = 4.3 × 109 M−1 s−1) [50]
Cl− + SO4•− → Cl• + SO42− (k = 2.6 × 108 M−1 s−1) [66]
•OH + NOM → Products (k = 2.23 × 108 L (mol C)−1 s−1) [67]
SO4•− + NOM → Products (k > 6 × 106 L (mol C)−1 s−1) [68]

2.6. Performance Sustainability of the Synthesized TiO2 Film

The stability and performance sustainability of the nanocrystalline photocatalyst was tested using the same TiO2 film for four repeated runs. The percent degradation results achieved by the SSLA-TiO2/S2O82−/H2O2 photocatalysis during four successive runs equaled a 99%, 96%, 95%, and 93% lindane removal in 6 h (Figure 7). The obtained results might indicate a high performance sustainability for the synthesized TiO2 film under the experimental condition in this study. Han et al. [56] and Pelaez et al. [71] reported a similar performance sustainability using nanocrystalline S-TiO2, and nitrogen and fluorine doped TiO2 (NF-TiO2) photocatalysts, respectively, synthesized via a similar method. Hung et al. [72] recently reported that the sol–gel synthesized doped-TiO2 exhibited a higher reusability than the film synthesized by a hydrothermal method, attributable to the high calcinations temperature in the former case. The high activity and reusability may recommend the synthesized TiO2 photocatalyst as a promising choice for application purposes.

3. Materials and Methods

3.1. Materials

Titanium (IV) isopropoxide (TTIP; 97%), lindane (C6H6Cl6; 97%), and persulfate (S2O82−) were obtained from Sigma-Aldrich (St. Louis, MO, USA). Acetic acid (CH3COOH, HAc), hydrogen peroxide (H2O2, 50%, v/v), sodium carbonate (Na2CO3), sodium chloride (NaCl), sodium nitrate (NaNO3), sodium bicarbonate (NaHCO3), isopropyl alcohol (i-PrOH; Certified ACS), and sodium sulfate (Na2SO4) were purchased from Fisher Scientific (Hampton, NH, United States). For a reference natural organic matter (NOM) in this study, standard Suwannee River humic acid (SRHA) was obtained from the International Humic Substances Society (IHSS, University of Minnesota, St. Paul, MN, USA). Benzoquinone (C6H4O2, BQ), iso-propanol ((CH3)2CHOH), and tert-butanol (CH3)3COH) were used as radical scavengers and were obtained from ACROS Organics (Morris, NJ, USA). Milli-Q grade water (resistivity: 18.2 MΩ cm) was used to prepare the aqueous solutions during the experiments. All of the chemicals were used without further treatment.

3.2. TiO2 Film Preparation

The preparation of film types of TiO2 photocatalyst was performed with a dip-coating technique using a TiO2 solution, and according to the procedure described in our previous paper [56]. In short, the TTIP was dissolved in i-PrOH, and then acetic acid was added in the solution. The solution was kept under vigorous stirring for 24 h at room temperature. As a result, a stable solution was obtained with a white color. The molar ratio of i-PrOH:TTIP:HAc was 45:1:1. The synthesized TiO2 material was immobilized on a glass substrate (Gold Seal® Micro Slides (75 × 25mm; 1mm thick), Portsmouth, NH, USA) in a five-layered nanocrystalline thin film employing a dip-coating method. After the coating process, the film was dried under infrared illumination for 20 min, and then calcined at 350 °C for 2 h in a Paragon HT-22-D furnace (Thermcraft Inc., Winston-Salem, NC, USA). The temperature of 350 °C was chosen so as to avoid the formation of cracks as a result of the high stress towards the coating during the calcination process. Also, at the relatively low calcination temperature, the phase transformation from anatase to rutile, as well as the collapse of porous structure of the film were inhibited during the film preparation. The thickness of the prepared film was 1.02 ± 0.02 µm and the total mass of the immobilized TiO2 was 4.51 ± 0.18 mg.

3.3. Characterization of Synthesized TiO2 Films

The characterization techniques used in this study were similar to those in our previously published paper [56]. Briefly, the surface morphology of the synthesized TiO2 photocatalyst film was characterized using an ESEM (Philips XL 30ESEM-FEG, Eindhoven, The Netherlands). The crystal size and crystal structure of the TiO2 photocatalyst were characterized using a JEM-2010F high resolution-TEM (HR-TEM, JEOL, Tokyo, Japan) with a field emission gun at 200 kV. An X-ray diffraction (XRD) analysis was performed using a X’Pert PRO XRD diffractometer (Philips, Almelo, The Netherlands) with Cu Ka radiation (λ = 1.5406 Å) so as to examine the crystal structures of TiO2 photocatalyst. The light absorption property of synthesized TiO2 film was investigated using a Shimadzu 2501 PC UV-visible spectrophotometer equipped with an ISR 1200 integrated sphere attachment. The reference material for the analysis was BaSO4. The Brunauer, Emmett, and Teller (BET) surface area of the sample was measured with a Micromeritics Tristar 3000 (Norcross, GA, USA). For the XRD, BET, UV-visible light absorption, and HR-TEM analyses, powders from the TiO2 coatings were collected using a blade, which were then used for analysis.

3.4. Photocatalytic Experiments

The photocatalytic experiments were performed in a batch mode photoreactor of a borosilicate glass Petri dish with a diameter of 10 cm. For the UV transmission, a quartz cover was used. A 20 mL aqueous solution of lindane (1 µM) at pH 5.8 and containing two thin film-coated TiO2 slides was irradiated with simulated solar light in the photoreactor. In the experiments concerning the effects of oxidants or natural water constituents on lindane degradation, desired amounts of the chemicals (i.e., 100 µM S2O82− or H2O2, 1 mM inorganic ions, and 1 mg/L HA) were added to the lindane solution at the start of the experiment. A 300 W Xenon lamp (Newport, Oriel Instrument, Irvine, CA, USA) emitted simulated solar light radiation. The wavelength of the solar light was mainly from 330 to 760 nm. A schematic diagram of the photocatalytic experimentation is shown in Figure 8. The measured light irradiance (Ee) using a Newport broadband radiant power meter was 4.71 × 10−2 W cm−2. The experiments were repeated at least three times.

3.5. Analytical Methods

The lindane concentration was monitored using an Agilent 6890 gas chromatograph (GC; Wilmington, DE, USA) with an Agilent 5975 mass spectrometer (MS; Wilmington, DE, USA), using the method previously reported in our study [64]. Briefly, a solid phase micro extraction (SPME) technique was used for the sample extraction. An HP-5MS (5% phenyl methylsiloxane) capillary column (length: 30 m and i.d.: 0.25 µm) was employed for the separation of the analyte. The mass spectra of the compounds in the samples were obtained in an electron impact ionization mode (EI+) at 70 eV, with an m/z ranging from 50 to 550. An online mass spectral search program (National Institute of Standards and Technology (NIST), Gaithersburg, MD, USA, https://www.nist.gov/) was used to interpret the obtained mass spectra using the GC-MS. For the residual analysis of H2O2 and S2O82−, two colorimetric methods, described by Liang et al. [73] and Allen et al. [74], were used.

4. Conclusions

The efficiency of a sustainable technology employing solar photocatalysis for the elimination of lindane, a persistent organochlorine compound, from an aqueous solution was evaluated using a sol-gel synthesized nanocrystalline TiO2 photocatalyst film. The results of the ESEM and TEM analyses showed a highly uniform and smooth surface morphology comprising anatase as the dominant crystalline phase in the TiO2 film. The synthesized TiO2 photocatalyst decomposed 23% lindane in 6 h, reflecting a limited lower removal efficiency in a practical application. Meanwhile, the efficiency of the SSLA-TiO2 photocatalysis was remarkably improved by the addition of environmentally friendly and economically attractive oxidants (i.e., S2O82− or H2O2), achieving an 89% and 64% removal of lindane, respectively, in 6 h. The removal efficiency of lindane further increased by 10% when using an equimolar mixture of both of the oxidants simultaneously (i.e., S2O82− and H2O2), which corresponds to a 99% removal within 6 h. The SO4•− and •OH were mostly involved in lindane degradation using an SSLA-TiO2/S2O82−/H2O2 system. The lindane removal efficiency for the SSLA-TiO2/S2O82−/H2O2 process decreased significantly in the presence of natural water constituents (i.e., HA, CO32−, HCO3−, NO3−, SO42−, and Cl−), requiring a high energy input in the contaminated field water treatment processes. The sustained catalytic activity of the TiO2 film during four runs manifested a high stability and reusability of the synthesized photocatalyst. The SSLA-TiO2/S2O82−/H2O2 is very effective for the elimination of lindane, and potentially other OCs, from a water environment. Importantly, as the remaining S2O82- in the process after the complete removal of OCs may result in secondary contamination [75,76], the S2O82- concentration must be kept as low as possible in order to treat the OCs in the process. It may be the limitations of the SSLA-TiO2/S2O82−/H2O2 system for practical applications.

Supplementary Materials

The following are available online at https://www.mdpi.com/2073-4344/9/5/425/s1. Figure S1: Tauc plot of Kubelka–Munk transformation of synthesized TiO2 films. Inserts shows the UV-visible absorption spectrum of the TiO2 film.

Author Contributions

Planning and designing of the research was made by S.K. and C.H. The experiments were performed by S.K. Materials were synthesized and characterized by S.K. and C.H. Drafting of manuscript was done by S.K., C.H., M.S., S.J. and S.S. Critical revision was performed by D.D.D. and H.M.K. Planning and supervision of the research was made by Dionysiou D.D.D.

Funding

This research was funded by Women University, Swabi, Pakistan. The APC was funded by the Cyprus Research Promotion Foundation through Desmi 2009–2010, which is co-funded by the Republic of Cyprus and ERDF under contract number NEA IPODOMI/STRATH/0308/09.

Acknowledgments

The authors are thankful to Dominic L. Boccelli of the University of Cincinnati (UC) for allowing us to use the GC-MS. We acknowledge the financial assistance from the Women University, Swabi, Pakistan. We also acknowledge the financial support from the Cyprus Research Promotion Foundation through Desmi 2009–2010, which is co-funded by the Republic of Cyprus and ERDF under contract number NEA IPODOMI/STRATH/0308/09. Also, D.D. Dionysiou acknowledges support from the UC through a UNESCO co-Chair Professor position on “Water Access and Sustainability”, and the Herman Schneider Professorship in the College of Engineering and Applied Sciences.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) Environmental scanning electron microscopy (ESEM) image of the TiO2 film, (b) X-ray diffraction (XRD) spectrum of the TiO2 film, (c,d) high-resolution transmission electron microscopy (HR-TEM) images of the TiO2 film, (e) pore size distribution, and (f) N2 adsorption–desorption isotherm of TiO2 film.
Figure 1. (a) Environmental scanning electron microscopy (ESEM) image of the TiO2 film, (b) X-ray diffraction (XRD) spectrum of the TiO2 film, (c,d) high-resolution transmission electron microscopy (HR-TEM) images of the TiO2 film, (e) pore size distribution, and (f) N2 adsorption–desorption isotherm of TiO2 film.
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Figure 2. Simulated solar light-activated TiO2 photocatalysis of lindane. [lindane]0 = 1.0 µM; mass of TiO2 film = 9.02 mg; thickness of TiO2 film = 1.02 µm; area of TiO2 film = 3750 mm2; pH = 5.8.
Figure 2. Simulated solar light-activated TiO2 photocatalysis of lindane. [lindane]0 = 1.0 µM; mass of TiO2 film = 9.02 mg; thickness of TiO2 film = 1.02 µm; area of TiO2 film = 3750 mm2; pH = 5.8.
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Figure 3. Effect of S2O82− and H2O2 additives on the simulated solar light-activated TiO2 photocatalysis of lindane. [lindane]0 = 1.0 µM; [S2O82−]0 = [H2O2]0 = 200 µM; mass of TiO2 film = 9.02 mg; thickness of TiO2 film = 1.02 µm; area of TiO2 film = 3750 mm2; pH = 5.8.
Figure 3. Effect of S2O82− and H2O2 additives on the simulated solar light-activated TiO2 photocatalysis of lindane. [lindane]0 = 1.0 µM; [S2O82−]0 = [H2O2]0 = 200 µM; mass of TiO2 film = 9.02 mg; thickness of TiO2 film = 1.02 µm; area of TiO2 film = 3750 mm2; pH = 5.8.
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Figure 4. Variation of kobs with increasing initial concentration of S2O82− and H2O2 on the simulated solar light-activated TiO2 photocatalysis of lindane. [lindane]0 = 1.0 µM; mass of TiO2 film = 9.02 mg; thickness of TiO2 film = 1.02 µm; area of TiO2 film = 3750 mm2; pH = 5.8.
Figure 4. Variation of kobs with increasing initial concentration of S2O82− and H2O2 on the simulated solar light-activated TiO2 photocatalysis of lindane. [lindane]0 = 1.0 µM; mass of TiO2 film = 9.02 mg; thickness of TiO2 film = 1.02 µm; area of TiO2 film = 3750 mm2; pH = 5.8.
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Figure 5. The role of •OH, SO4•−, and O2•− in the simulated solar light-activated TiO2/S2O82−/H2O2 photocatalysis of lindane. [lindane]0 = 1.0 µM; [iso-propanol]0 = 50 mM; [tert-butanol]0 = 50 mM; [benzoquinone]0 = 50 mM; mass of TiO2 film = 9.02 mg; thickness of TiO2 film = 1.02 µm; area of TiO2 film = 3750 mm2; pH = 5.8.
Figure 5. The role of •OH, SO4•−, and O2•− in the simulated solar light-activated TiO2/S2O82−/H2O2 photocatalysis of lindane. [lindane]0 = 1.0 µM; [iso-propanol]0 = 50 mM; [tert-butanol]0 = 50 mM; [benzoquinone]0 = 50 mM; mass of TiO2 film = 9.02 mg; thickness of TiO2 film = 1.02 µm; area of TiO2 film = 3750 mm2; pH = 5.8.
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Figure 6. Effect of humic acid and inorganic anions (CO32−, HCO3−, NO3−, SO42−, and Cl−) on the SSLA-TiO2/S2O82−/H2O2 photocatalysis of lindane. [lindane]0 = 1.0 µM; mass of TiO2 film = 9.02 mg; thickness of TiO2 film = 1.02 µm; area of TiO2 film = 3750 mm2; [humic acid]0 = 1 mg/L; [inorganic ions]0 = 1 mM.
Figure 6. Effect of humic acid and inorganic anions (CO32−, HCO3−, NO3−, SO42−, and Cl−) on the SSLA-TiO2/S2O82−/H2O2 photocatalysis of lindane. [lindane]0 = 1.0 µM; mass of TiO2 film = 9.02 mg; thickness of TiO2 film = 1.02 µm; area of TiO2 film = 3750 mm2; [humic acid]0 = 1 mg/L; [inorganic ions]0 = 1 mM.
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Figure 7. Test of reusability for the simulated solar light-activated TiO2/S2O82−/H2O2 photocatalyst of lindane. [lindane]0 = 1.0 µM; mass of TiO2 film = 9.02 mg; thickness of TiO2 film = 1.02 µm; area of TiO2 film = 3750 mm2; pH = 5.8.
Figure 7. Test of reusability for the simulated solar light-activated TiO2/S2O82−/H2O2 photocatalyst of lindane. [lindane]0 = 1.0 µM; mass of TiO2 film = 9.02 mg; thickness of TiO2 film = 1.02 µm; area of TiO2 film = 3750 mm2; pH = 5.8.
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Figure 8. A schematic diagram of the photocatalytic experimental procedure.
Figure 8. A schematic diagram of the photocatalytic experimental procedure.
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Table
Table 1. Kinetics analyses of degradation of pesticides using different photocatalytic processes.
Table 1. Kinetics analyses of degradation of pesticides using different photocatalytic processes.
Pesticideskobs(h−1)Reaction TypeReference
Vinclozoline0.408a[48]
Quinalphos0.426a[48]
Malathion4.266a[48]
Fenarimol0.168a[48]
Fenitrothion0.300a[48]
Dimethoate0.666a[48]
Lambda-Cyhalothrin0.504b[25]
Chlorpyrifos0.498b[25]
Diazinon0.270b[25]
Lindane0.550cthis study
a Natural sunlight/TiO2/S2O82−; [S2O82−]0 = 250 mg/L; TiO2 (P25) = 200 mg/L. b Natural sunlight/TiO2/H2O2; [H2O2]0 = 1000 mg/L; TiO2 = 2.0 g/L. c Simulated solar light/TiO2/S2O82−/H2O2; [S2O82−]0 = [H2O2]0 = 100 µM. Mass of TiO2 film = 9.02 mg; thickness of TiO2 film = 1.02 µm; area of TiO2 film = 3750 mm2.

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Khan, S.; Han, C.; Sayed, M.; Sohail, M.; Jan, S.; Sultana, S.; Khan, H.M.; Dionysiou, D.D. Exhaustive Photocatalytic Lindane Degradation by Combined Simulated Solar Light-Activated Nanocrystalline TiO2 and Inorganic Oxidants. Catalysts 2019, 9, 425. https://doi.org/10.3390/catal9050425

AMA Style

Khan S, Han C, Sayed M, Sohail M, Jan S, Sultana S, Khan HM, Dionysiou DD. Exhaustive Photocatalytic Lindane Degradation by Combined Simulated Solar Light-Activated Nanocrystalline TiO2 and Inorganic Oxidants. Catalysts. 2019; 9(5):425. https://doi.org/10.3390/catal9050425

Chicago/Turabian Style

Khan, Sanaullah, Changseok Han, Murtaza Sayed, Mohammad Sohail, Safeer Jan, Sabiha Sultana, Hasan M. Khan, and Dionysios D. Dionysiou. 2019. "Exhaustive Photocatalytic Lindane Degradation by Combined Simulated Solar Light-Activated Nanocrystalline TiO2 and Inorganic Oxidants" Catalysts 9, no. 5: 425. https://doi.org/10.3390/catal9050425

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