Natural solar light driven degradation of refractory chlorophenolic pollutant using

Journal of Environmental Chemical Engineering 2 (2014) 1804–1812

Contents lists available at ScienceDirect

Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece

Natural solar light driven degradation of refractory chlorophenolic pollutant using Carbon doped and carbon–iron co-doped electrospun titania fibers Jeevitha Raji Rathinavelu, Kandasamy Palanivelu * Centre for Environmental Studies, Anna University Chennai, Chennai 600 025, India

A R T I C L E I N F O

A B S T R A C T

Article history: Received 29 April 2014 Accepted 20 July 2014

The present investigation focused on a clean and green destruction of 2,4-dichlorophenol (2,4-DCP) under natural sunlight using pristine, carbon doped and carbon–iron co-doped electrospun TiO2 fibers. The synthesized fibers were characterized by XRD, HR-SEM, EDX, UV–vis spectroscopy and BET specific surface area analysis. From XRD, the co-doped samples were found to contain beneficial fraction of 73% anatase and 27% rutile phases. From HR-SEM, the fibrous structure was found to be retained after the doping and co-doping processes. The effect of operating variables like pH, catalyst dosage, oxidant concentration, solar intensity and initial pollutant concentration was also investigated. Solar photoFenton’s degradation of 2,4-DCP was favorable at pH 3 than at basic pH and an oxidant concentration of 9.9 mM/L was sufficient to effectively mineralize 2,4-DCP. The rate of degradation was remarkably high when the intensity of sunlight was >85 kiloLUX and decreased with decrease in solar intensity, however, complete degradation of 2,4-DCP was possible till very low solar intensity of 20 klx. ã 2014 Elsevier Ltd. All rights reserved.

Keywords: 2,4-Dichlorophenol Titania fibers Solar photo-Fenton’s treatment Dechlorination Sun intensity

Introduction In 1976, the European Union (EU) categorized 132 hazardous pollutants based on their toxicity, stability and bioaccumulation [1]. Among them, dichlorophenols (DCPs) were recognized as the most dangerous and recalcitrant class of pollutants in wastewater [2]. The existence of DCPs in wastewater stems principally from industries during chlorination of phenolic effluents. Their high toxicity is attributed to the number of chlorine atoms on the benzene ring and there are two mechanisms such as (i) uncoupling of oxidative phosphorylation and (ii) narcotic action, by which DCPs affect biological systems [3,4]. Thus, dichlorophenols attract significant focus of environmental remediation by a reliable technology like advanced oxidation process (AOPs) [5]. Among the various AOPs, TiO2 semiconductor photocatalysis has received a prime attention in destroying a wide variety of pollutants. TiO2 photocatalyst is usually applied in the form of nanoparticles with high surface area, but unfortunately, the nanometric size limits the recovery of catalyst after treatment. Most recently, electrospun fibers have attracted a vast interest due to their ultrathin diameter and one dimensional structure which

* Corresponding author. Tel.: +91 44 2235 9014; fax: +91 44 2235 4717. E-mail address: [email protected] (K. Palanivelu). http://dx.doi.org/10.1016/j.jece.2014.07.022 2213-3437/ ã 2014 Elsevier Ltd. All rights reserved.

makes them suitable candidates for better efficiency and recovery after treatment [6]. Though TiO2 photocatalysis is an efficient technology, the wide band gap energy (3.2 eV) associated with anatase TiO2 limits its application under natural sunlight. To circumvent this limitation, attention is being paid to metal/non-metal doping of TiO2 which is believed to alter both photophysical behavior and photochemical properties, and turns TiO2 photoactive under sunlight [7]. Co-doped titania with metal–non-metal elements has gained more interest since the detrimental recombination effect associated with monodoping could be passivated by co-doping [8–10]. On the other hand, heterogeneous photo-Fenton’s system combining TiO2 photocatalysis and Fenton’s activity is also found to remarkably degrade refractory pollutants without generating sludge as witnessed in conventional homogeneous Fenton’s system [11–14]. Thus, the present study aimed to (i) prepare TiO2 in the form of fibers to facilitate the recovery and reuse of catalyst (ii) tailor the catalytic properties by doping with carbon and co-doping with carbon–iron and (iii) evaluate the potential of pristine and doped fibers in the destruction of 2,4-DCP by solar photocatalytic and photo-Fenton’s process under natural sunlight. In our earlier works, carbon doped nanoparticles and iron doped fibers [15,16] were prepared and their photocatalytic performance was investigated by the degradation of colored pollutant like dyes. Hence, emphasis was shown in this study to prepare carbon

J. Raji Rathinavelu, K. Palanivelu / Journal of Environmental Chemical Engineering 2 (2014) 1804–1812

doped and carbon–iron co-doped electrospun fibers to evaluate its potential in the degradation of colorless and refractory pollutant like 2,4-DCP. Materials and methods Materials All chemicals used in the present study were of analytical grade. Titanium(IV) propoxide (Sigma–Aldrich), isopropyl alcohol (Samchun Chemical), coconut oil (Parachute), ferrous sulphate heptahydrate (Samchun Chemical) and 2,4-dichlorophenol (SRL) were used as received from the suppliers without further purification. All reagents and solutions were prepared using de-ionized water. Synthesis of electrospinning gel The electrospinning of pristine TiO2 fibers was accomplished without the assistance of polymers and the electrospinning protocol is given in Fig. 1. The calcined electrospun fibers were designated as “Pure-TiO2-F”. Carbon doping of titania fibers Pristine titania fibers (Pure-TiO2-F) were subjected to carbon doping using coconut oil as carbon precursor [15]. In the carbon doping process, the materials Pure-TiO2-F, coconut oil and acetone was mixed in 1:0.1:0.9 w/w ratio, so that the ratio of carbon precursor was 10 wt% with respect to Pure-TiO2-F. The acetone solvent was evaporated by heating and the mixture was kept undisturbed for 24 h. The fibers were then subjected to closed thermal treatment at a temperature of 250–300
C for 70 min and allowed to attain the room temperature. The thermally treated catalyst was then subjected to microwave irradiation for 1 h in medium-high mode using microwave oven (model: IFB 17PMMEPI). The resulting carbon doped fibers were designated as “C–TiO2F”.

Mixing Titanium tetra propoxide, isopropanol and HCl in 1:1:1 molar ratio

Hydrolysis and polycondenstaion reaction for 8 h at room temperature (31 C) and relative humidity (R. H. = 70%) under constant stirring Electrospinning of the hydrolyzed titania gel at 12 kV in the flow rate of 6 – 8 mL/h from 30 mL syringe equipped with a stainless steel 18-guage needle

1805

Carbon–iron co-doping of titania fibers In the C–Fe co-doping process, Pure-TiO2-F (calcined at 400
C) was first subjected to carbon doping process where the carbon precursor to titania ratio was varied as 2 wt% and 4 wt%. The carbon doped materials were subjected to co-doping using ferrous sulphate as Fe2+ precursor [16] in which the proportion of iron was varied as 1 wt% and 2 wt%. The resulting C–Fe co-doped catalysts were designated as C2.0Fe1.0TiO2F, C2.0Fe2.0TiO2F, C4.0Fe1.0TiO2F and C4.0Fe2.0TiO2F respectively. Catalyst characterization In order to determine the crystal phase and crystalline nature of the pure and doped titania fibers, powder X-ray diffraction pattern of the catalysts was taken using ET 816 X-ray diffractometer (2u = 10.0–70.0; Cu Ka radiation l = 1.5405 Å) with scintillation counter as detector. The surface morphology of the catalysts was characterized by field emission scanning electron microscopy (FESEM) using JSM – 6300, JEOL. In order to estimate the quantity of carbon/iron doped into TiO2 fibers, EDX analysis was performed using energy dispersive X-ray spectrometers from Hitachi (S3400N). For inspecting visible light absorption characteristics of the fibers, optical absorption spectra of the samples were recorded using UV–vis spectrophotomer (Cary 5E). The specific surface area of the catalysts was determined by BET surface area analysis (Micrometritics ASAP2020) with N2 physisorption at liquid nitrogen temperature. Solar photocatalytic and photo-Fenton’s experiments The photochemical experiments were carried out in 600 mL capacity glass cylindrical cells with 200 mL of working solution containing 0.1 g of catalyst and 100 ppm of 2,4-DCP. The diameter and length of the glass reactor was 10 cm and 12.5 cm; and the length of the reactant suspension was 3.5 cm respectively. The pH adjustments of the dichlorophenol solutions were carried out by the addition of diluted NaOH/H2SO4. All the reactant suspensions were first subjected to dark adsorption for 30 min and then subjected to daylight solar irradiation between 10:00 a.m. and 2:00 p.m., when the intensity of sunlight was between 90 klx and 30 klx, measured using LUX meter (Model: TES 1332). All the photocatalytic studies with pure and carbon doped TiO2 fibers were accomplished under natural sunlight in open atmospheric air with and without hydrogen peroxide (29.7 mM/L). In all the photo Fenton’s experiments with pure and C–Fe co-doped fibers, hydrogen peroxide was added before subjecting the suspensions to solar irradiation. All the degradation studies were carried out in duplicates. During the adsorption and irradiation period, aliquots were collected and the chlorophenol concentration (as phenol) was estimated spectrophotometrically by 4- amino antipyrine method (Method No. 5530D) in Systronics Visiscan 167 spectrophotometer [17]. Chemical oxygen demand (COD) was determined by open reflux, dichromate titrimetric method as described in standard methods [17]. Instrumental analysis

Collection of electrospun fibers on a cylindrical aluminum target at a tip to collector distance of 9 cm and calcination of the fibers at 500 C / 400 C for 4 h Fig. 1. Electrospinning protocol.

Dechlorination of 2,4-DCP was studied by analyzing the liberation of chloride ion using ion chromatograph (model: Dionex DX-120) equipped with a conductivity detector (Dionex DX-120, Germany) and Dionex IonPac AS14 analytical column (4 mm  250 mm). Mineralization studies were accomplished by analyzing total organic carbon (TOC) with the help of TOC analyzer (Analytik Jena, Germany multi N/C 2100S).

1806

J. Raji Rathinavelu, K. Palanivelu / Journal of Environmental Chemical Engineering 2 (2014) 1804–1812

Results and discussion Crystal characterization X-ray diffraction patterns of pristine TiO2 fibers (calcined at 500
C), carbon doped and carbon–iron co-doped fibers (calcined at 400
C) are shown in Fig. 2. The XRD pattern of pure and C–TiO2 fibers corresponded to both anatase and rutile forms of titania with the latter as the predominating one. The formation of rutile phase might be attributed to the high temperature calcination (500
C for 4 h). The mass fraction of anatase and rutile phase in the calcined titania samples was calculated by Spurr formula (Eq. (1)). XR ¼

1 1 þ 0:8ðIA =IR Þ

(1)

where, IA and IR = Integrated peak intensities of anatase and rutile peaks respectively [18]. From the Spurr formula, the fraction of anatase and rutile in pristine fibers considering the peaks at 2u = 25 and 55
(anatase) and 27
(rutile) was found to be 27% and 73% respectively. The average domain size of the crystalline phases was calculated with the help of Scherrer formula as given in Eq. (2) [13,19,20]. DScherrer ¼

kl bcosu

(2)

where: k = 0.9 a shape factor for spherical particles. l = the wavelength of the incident radiation (l = 0.154 nm). u = half of the diffraction angle (deg). b = B  b the line broadening (rad). B is the full width at half maximum (FWHM) of the measured diffraction peak and b is the instrumental broadening. From Scherrer’s formula, the average domain size of pure titania fibers was found to be in the range of 48–54 nm and that of the C–TiO2 fibers was 36–40 nm showing a minimal reduction in the particle size after carbon doping process. The XRD patterns of C–Fe TiO2 fibers revealed that the co-doped fibers chiefly composed of

anatase phase without losing the desired crystallinity. This superior property in the co-doped samples was attributed to the suitable calcination temperature (400
C for 4 h) opted in the synthesis procedure rather than calcination at higher temperature. From the Spurr formula, all the C–Fe catalysts were calculated to be uniformly composed of 73% anatase and 27% rutile phases considering the peaks at 2u = 25 and 28
respectively. From Scherrer’s formula, the average domain size of C–Fe–TiO2 catalyst fibers was found to be in the range of 13–20 nm. Morphological characterization The HR-SEM images of all the samples (Fig. 3) represent a large number of thin identical fibers. From the high and low magnification images, it is clear that the TiO2 fiber morphology was retained even after the doping and co-doping processes. The average diameter of the carbon doped fibers lain in the range of 500 nm and the diameter of all the co-doped catalyst fibers lain in the range from 200 nm. In the SEM micrograph of the catalyst, ‘C4.0Fe2.0TiO2F’ a bushy and thin fibrous patch was noticed, which might indicate the shrinkage in the diameter of the catalyst after carbon and iron doping process without causing significant damage to the fiber morphology. Compositional analysis The EDX spectra of the carbon doped and carbon–iron co-doped titania fibers are shown in Fig. 4. As shown in the EDX spectra, the presence of carbon was evident both in the carbon doped and carbon–iron co-doped catalysts. In the case of co-doped fibers, the amount of iron added in the reaction mixture during the catalyst preparation was 1.0 wt% and 2.0 wt% in the catalysts C2.0Fe1.0TiO2F, C2.0Fe2.0TiO2F, C4.0Fe1.0TiO2F and C4.0Fe2.0TiO2F (as indicated in the catalyst code) and the amount of iron estimated by EDX analysis was 0.39 wt%, 0.40 wt%, 0.36 wt% and 0.43 wt% respectively. In our previous study, the amount of iron incorporation was found to be appreciable (up to 1.0 wt%) in the case of iron doping of fibers [16]. Whereas, in this work, the limited amount of iron incorporation during co-doping might be ascribed to the reason that the carbon doping prior to iron doping might have majorly covered the surface of titania fibers and limited the incorporation of iron during iron doping process. Visible light characterization Fig. 5 shows the UV–vis absorbance spectra of pure titania fibers (calcined at 500
C and 400
C), C–TiO2 fibers and C–Fe codoped fibers. From the figure, it could be observed that the pure fibers (calcined at 500
C) and C–TiO2 fibers exhibited a strong absorption in the wavelengths around UV region but the absorbance gradually decreased and became stable in the visible region. This might be due to the presence of large fraction of rutile phase in the fibers calcined at 500
C [21]. The pure titania fibers calcined at 400
C was found to possess characteristic absorption in UV region and no absorption in visible region, whereas, all the co-doped samples were found to possess absorption in visible region. The co-doped fiber sample C2.0Fe2.0TiO2F was found to possess superior absorption properties on comparing the other co-doped samples. The onset of absorption in the sample took place at the wavelength of 430 nm, which corresponded to a band gap of 2.88 eV. Surface area analysis

Fig. 2. X-ray diffraction patterns of (a) Pure-TiO2F, (b) C–TiO2F, (c) C2.0Fe1.0TiO2F, (d) C2.0Fe2.0TiO2F, (e) C4.0Fe1.0TiO2F, and (f) C4.0Fe2.0TiO2F.

The surface area of pure fibers calcined at 500
C for 4 h was low and found to be around 17 m2/g, whereas, the surface area of the

J. Raji Rathinavelu, K. Palanivelu / Journal of Environmental Chemical Engineering 2 (2014) 1804–1812

1807

Fig. 3. FE-SEM images of (a) Pure-TiO2F, (b) C2.0Fe1.0TiO2F, (c) C2.0Fe2.0TiO2F, (d) C4.0Fe1.0TiO2F, and (e) C4.0Fe2.0TiO2F.

pure fibers calcined at 400
C for 4 h was high and found to be around 51 m2/g. After carbon–iron co-doping process it was found to be around 46 m2/g. Though the surface area of the fibers was low on comparing the nanoparticles (180 m2/g) which we synthesized in our previous work [15], the fibers calcined at 400
C possess a surface area which is fairly closer to that of the commercially renowned Degussa P25 catalyst (50 m2/g). Solar photocatalytic degradation of 2,4-DCP In the solar photochemical degradation of 2,4-DCP, a detailed investigation on the role of initial pH of DCP solution was carried out between pH 3 and pH 9. Studies were not conducted below pH 3 due to the fact that under strong acidic conditions, the catalyst lattice collapses and consequently loses its catalytic activity [22]. The general photodegradation reaction of 2,4-DCP could be expressed as in Eq. (3). hn

2C6 H4 OCl2 þ 12O2 ! 12CO2 þ 2H2 O þ 4HCl

(3)

In the adsorption study, the adsorption of 2,4-DCP onto pure and carbon doped electrospun fibers varied from 7–10% to 14–17% from basic to acidic range respectively. In the case of C–Fe

co-doped catalysts, the adsorption of 2,4-DCP was lower than that over C–TiO2 catalysts around 11–13% in basic to acidic range with the maximum adsorption at acidic pH 3. Fig. 6a and b depicts the degradation of 2,4-DCP with pure and C–TiO2 fibers in the presence and absence of hydrogen peroxide respectively. The results revealed that when compared to the undoped titania, the C–TiO2 fibers performed better in the degradation of 2,4-DCP. The enhanced performance of carbon containing catalyst might be due to increased adsorption of the pollutant and its subsequent degradation on the active sites. It could be seen from Fig. 6a and b that the degradation of 2,4-DCP was effective at acidic pH 3 than at basic pH (60% at pH 3 and 45% at pH 9 after 180 min of solar irradiation) when the experiments were conducted in the absence of H2O2. This could be due to the catalyst-pollutant interaction as follows: for TiO2, the point of zero charge (pzc) is around 5.8–6.0. Hence, at acidic pH below 5.8, the aqueous medium can supply more protons which make the particle surface possess large number of positive charge, whereas, at pH above 5.8, the aqueous medium provides hydroxyl ions and the particle surface carries negative charge. Since 2,4-DCP is an anionic pollutant, it is assumed to be largely in its neutral form at acidic pH and anionic form at basic pH as shown in Eqs. (4) and (5). Therefore at acidic pH, the electrostatic repulsion between

1808

J. Raji Rathinavelu, K. Palanivelu / Journal of Environmental Chemical Engineering 2 (2014) 1804–1812

Fig. 4. EDX spectra of (a) C–TiO2F, (b) C2.0Fe1.0TiO2F, (c) C2.0Fe2.0TiO2F, (d) C4.0Fe1.0TiO2F, and (e) C4.0Fe2.0TiO2F.

the positively charged catalyst surface and the neutral 2,4-DCP is low on comparing the repulsion between the negatively charged catalyst surface and negatively charged 2,4-DCP at basic pH. at acidic conditions

       ! C6 H3 Cl2 OH ðNeutral compoundÞ C6 H3 Cl2 O þHþ  pKa of 2;4DCP¼7:8

(4) at basic conditions

      ! Exists in the same anionic nature C6 H3 Cl2 O þOH  (5)

Fig. 5. UV–vis absorption spectra of pure, doped and co-doped fibers.

On the other hand, in the presence of H2O2, the degradation was effective at basic pH 9 (70% at pH 3 and 90% at pH 9). In spite of the repulsion between catalyst and the pollutant at basic pH, the degradation of 2,4-DCP was more pronounced, which might be due to the following reasons: (i) At pH 9, more HO ions are introduced into the reactant solution and come into contact with the photogenerated holes under solar irradiation which in turn lead to the generation of more HO radicals and subsequent oxidation of

J. Raji Rathinavelu, K. Palanivelu / Journal of Environmental Chemical Engineering 2 (2014) 1804–1812

1809

Fig. 6. Solar photocatalytic degradation of 2,4-DCP with pure and C–TiO2 fibers at varied pH (a) without H2O2, (b) with H2O2 (initial solar intensity 75 kiloLUX; catalyst dosage = 0.5 g/L; H2O2 concentration = 29.7 mM/L).

pollutant (ii) In addition the H2O2 added into the solution might have also induced the generation of HO radicals from HO ions [23]. Another reason for this enhanced degradation at pH 9 could be that in the case of 2,4-DCP, it has two Cl atoms which are electron attracting groups. Thus chlorine element makes the aromatic ring electron deficient and hinders the electrophilic attack by HO radicals. At basic pH, when there are more HO ions, the electron donating HO ions normally increase the electron density of aromatic ring and the subsequent electrophilic attack by HO radicals [24]. Solar photo-Fenton’s degradation of 2,4-DCP In the solar photo-Fenton’s degradation of 2,4-DCP, the degradation of the pollutant was prominent in acidic pH than in basic pH. The enhanced degradation in acidic pH was due to the well known classic Fenton’s process which is generally efficient at pH around 3.0 [25]. The solar photo-Fenton’s degradation profile of 2,4-DCP with pure and carbon–iron co-doped titania fibers from acidic to basic pH is illustrated in Fig. 7a–d respectively. From the figures, it is clear that the degradation of 2,4-DCP was significantly higher at pH 3 with the catalyst, ‘C2.0Fe2.0TiO2F’ containing lower carbon and higher iron loading. The efficiency of Fenton’s oxidation was decreased at pH greater than 5, which could be due to the formation of ferric hydroxide and decrease in the oxidation rate. Moreover, at pH greater than 5, the complexation of Fe2+ as [Fe(II)(H2O)6]2+ was favored, which reacted more slowly with hydrogen peroxide deteriorating the hydroxyl radical generation and affected the oxidation rate [26]. A significant degradation (>92%) of the phenolic pollutant was observed within 45 min of solar irradiation with the co-doped catalysts C2.0Fe1.0TiO2F, C2.0Fe2.0TiO2F and C4.0Fe2.0TiO2F with C2.0Fe2.0TiO2F being more efficient at all the pH studied. Over all, on comparing the efficiency of the pure and carbon doped titania catalysts, the carbon–iron co-doped catalyst was found to be efficient in degrading DCP. The results are in accordance with Xiaoping et al. [27] where the synergistic effects of Fe and C co-doping into TiO2 resulted in improved photocatalytic activities for degradation of bisphenol A and clofibric acid as compared to C–TiO2, Fe–TiO2 and P25 under visible light and simulated solar light irradiation [27]. Several other research works also concluded that co-doping/multidoping of titania significantly enhances the visible light absorption property and photoefficiency of the catalyst on comparing the pristine or single dopant containing titania [28–31].

The performance of C–Fe co-doped titania in the photodecomposition of the pollutant must be attributed to (i) the crystallinity of anatase phase (ii) the adsorption of pollutant onto carbon layer and (iii) enhanced Fenton’s effect due to the presence of iron. On observing the degradation of 2,4-DCP with all the catalysts, the degradation was found to be efficient with the catalyst, “C2.0Fe2.0TiO2F”. Hence, further degradation studies in 2,4-DCP was conducted with the co-doped catalyst “C2.0Fe2.0TiO2F” at the optimized solution pH of 3.0. To study the effect of catalyst dosage in the solar photo-Fenton’s degradation of 2,4-DCP, four different catalyst dosages such as 0.25, 0.5, 0.75 and 1.0 g/L were studied. It was observed that the degradation of 2,4-DCP was effective when the amount of catalyst was increased from 0.25 to 1.0 g/L. This might be due to the enhanced catalyst-pollutant interaction and higher Fenton’s activity at higher catalyst loading. Hence, further optimization studies were conducted with 1 g/L of catalyst as optimum dosage. Effect of hydrogen peroxide concentration In presence of the efficient catalyst (C2.0Fe2.0TiO2F), The effect of oxidant concentration in the photo-Fenton’s degradation of 2,4-DCP in terms of mineralization and dechlorination was studied by varying the concentration of H2O2 from 9.9 mM/L to 39.6 mM/L. The experimental results are illustrated in Fig. 8. It could be observed that a concentration as low as 9.9 mM/L was sufficient to effectively mineralize 2,4-DCP at an irradiation duration of 60 min. The extent of mineralization of 2,4-DCP was detrimentally affected by high concentration of H2O2 from 29.7 mM/L to 39.6 mM/L. The retarded performance at higher oxidant concentration might be due to the fact that at higher H2O2 concentrations, hydrogen peroxide itself can act as a scavenger of hydroxyl radicals and render them unavailable for Fenton’s reaction [32,33]. On the other hand, on observing the dechlorination pattern of 2,4-DCP at varied oxidant concentration, almost complete dechlorination was observed over the entire range of hydrogen peroxide concentration studied. As observed in the mineralization profile, the dechlorination profile also suggests that the degradation of 2,4-DCP could be significantly achieved at a minimal oxidant dosage and does not depend on higher oxidant consumption. Effect of initial pollutant concentration The effect of initial concentration of 2,4-DCP in its degradation, dechlorination and mineralization is provided in Table 1. The experiments were conducted at other optimum conditions varying

1810

J. Raji Rathinavelu, K. Palanivelu / Journal of Environmental Chemical Engineering 2 (2014) 1804–1812

Fig. 7. Solar photo-Fenton’s degradation of 2,4-DCP with pure and carbon–iron co-doped fibers at a) pH 3, b) pH 5, c) pH 7, d) pH 9 (initial solar intensity 50 kiloLUX; catalyst dosage = 0.5 g/L; H2O2 concentration = 29.7 mM/L).

only the initial concentration of 2,4-DCP from 200 to 500 ppm. The concentration of 2,4-DCP was varied from 200 to 500 ppm to simulate the phenol concentrations present in petrochemical and herbicide wastewater [34]. When observing the results in the table, it could be noticed that the destruction of 2,4-DCP followed a characteristic

pattern in the order of degradation, dechlorination and mineralization irrespective of the initial concentration as follows: 2;4DCPremoval > CODremoval > Dechlorination > Mineralization It is to be noticed from the table that H2O2 concentration as low as 9.9 mM/L was sufficient to degrade even higher concentrations of 2,4-DCP. Similar kind of efficiency at lower H2O2 concentrations was also observed by Tamimi et al. [32] in which the degradation of 0.123 mM/L of methomyl pesticide was observed at a faster rate at pH 3 when the H2O2 concentration was 1 mM/L. Above 1 mM/L, the increase in the rate of degradation was affected due to the hydroxyl radical scavenging effect at higher oxidant concentrations. Effect of solar intensity

Fig. 8. Effect of hydrogen peroxide concentration in the mineralization and dechlorination of 2,4-DCP (initial solar intensity 50 kiloLUX; catalyst dosage = 1.0 g/L).

In this work, since all the degradation experiments were conducted under natural sunlight, the intensity of the solar radiation played a major role in the rate of degradation of 2,4-DCP. The solar photodegradation studies were conducted from the month of April–October except during the precipitation days. It was observed that complete degradation of 2,4-DCP could be achieved during all the months in which the photochemical studies were conducted,

J. Raji Rathinavelu, K. Palanivelu / Journal of Environmental Chemical Engineering 2 (2014) 1804–1812

1811

Table 1 Effect of initial pollutant concentration in the solar photo-Fenton’s degradation, dehalogenation and mineralization of 2,4-DCP. Si. no.

Initial concentration of 2,4-DCP (ppm)

Phenol removal (%)

COD removal (%)

Dechlorination (%)

TOC removal (%)

1 2 3 4

200 300 400 500

99.9 99.1 98.5 95.5

94.3 89.3 78.8 73.2

92.1 83.5 70.7 69.4

91.8 82.3 70 62.1

except that, the rate of degradation of 2,4-DCP was comparably slower during post-summer than the rate during peak summer. This might be attributed to the decrease in insolation and minimal solar intensity during post-summer seasons [35,36]. The measured solar intensity during the course of study and the observed rate of degradation of 2,4-DCP are illustrated in Fig. 9. As observed from Fig. 9, the rate of degradation of 2,4-DCP was remarkably high when conducting the experiments during peak summer when the solar intensity was >85 kiloLUX. As the intensity of summer decreased, the rate of degradation of 2,4-DCP decreased gradually and the lowest rate was observed in the month of October when the solar intensity reached around 20 kiloLUX. It might be due to the well known reason that at higher solar intensities, more photons will be provided by the solar source which come into contact with the photocatalyst and lead to enhanced degradation and vice versa as the intensity decrease. The degradation of 2,4-DCP became insignificant when the intensity of sunlight diminished below 20 kiloLUX.

The reaction kinetics fitted well for the first order rate equation with a regression co-efficient of 0.99 and disobeys the second order kinetics with a regression co-efficient of 0.68. The kinetics clearly revealed that though hydroxyl radical attack (by the addition of H2O2) and Fenton’s activity (active Fe sites on catalyst) played major roles in the degradation process, they only have a pseudo effect and the rate of reaction was chiefly dependent on the concentration of the pollutant. Hence, it could be stated that the reaction followed a pseudo first order kinetics. This is in accord with the results obtained by various researchers in which the heterogeneous photo-Fenton’s degradation of pollutants was found to follow pseudo first order kinetics [37,38,39]. In this study, the pseudo first order rate constants for the TOC removal of 200–500 ppm of 2,4-DCP at the optimized conditions of catalyst dosage (1.0 g/L) and pH (pH 3.0) with 9.9 mM/L of H2O2 was found to be 0.115, 0.078, 0.07 and 0.052 min1 respectively. The corresponding half lives were 6.02, 8.88, 9.76 and 13.3 min respectively.

Kinetics

Leaching and reusablity studies

The kinetics of the heterogeneous solar photo-Fenton’s degradation of 2,4-DCP was studied applying first and second order rate equations (Eqs. (6) and (7) respectively) in the presence of the efficient catalyst, “C2.0Fe2.0TiO2F” at optimized conditions.

Leachability experiments were conducted to examine the leaching of iron species and organic carbon from the most efficient catalyst C2.0Fe2.0TiO2F. The initial concentration of 2,4-DCP was 100 ppm, the catalyst dosage was 1.0 g/L and H2O2 concentration was 9.9 mM/L at pH 3. It was observed that the leaching of iron was negligible, releasing a maximum of 0.1 ppm of iron into the reaction medium after the commencement of photochemical experiments. Since the catalyst preparation procedure involved the use of coconut oil as carbon precursor, TOC analysis was also carried out in blank samples to ascertain that no residual organic carbon was present and leached into the reaction medium after 180 min of solar irradiation. It was found that no organic carbon was released into the reaction medium. In order to evaluate the practical application potential of the electrospun fibers, reusability studies were conducted with the efficient fiber catalyst, “C2.0Fe2.0TiO2F”. The fibers were recovered after each run, washed, annealed at 400
C for 1 h and reused for the next cycle of study. It was possible to reuse the catalyst with no significant loss in the catalytic activity for three cycles, after the 3rd cycle the electrospun fibers were found to disintegrate and became flake like structures.

1 CP0 k1 ¼ ln t CPt k2 ¼

  1 CP0 t CP0  CPt

(6)

(7)

where: k1 = first order rate constant, min1. k2 = second order rate constant, L/mg min1. t = time, min. CP0 = concentration of pollutant at t = 0, mg/L. CPt = concentration of pollutant at any time “t”, mg/L.

Conclusions

Fig. 9. Effect of solar intensity in the degradation of 2,4-DCP (initial DCP concentration = 100 mg/L; initial solution pH 3; H2O2 concentration = 9.9 mM/L; catalyst dosage = 1.0 g/L).

In summary, pristine TiO2 fibers were prepared by sol–gel derived electrospinning process and subjected to carbon doping and carbon–iron co-doping. Solar photo-Fenton’s degradation of 2,4-DCP with the co-doped catalysts was found to be efficient than solar photocatalytic process. Under optimized conditions, H2O2 concentration as low as 9.9 mM was found to be sufficient for almost complete mineralization of 2,4-DCP. The destruction pattern of 2,4-DCP followed the order as follows: 2,4-DCP removal > COD removal > Dechlorination > TOC removal. From the t1/2 values, it is clear that the rate of degradation obeyed pseudo first order kinetics. On the whole, the current research reveals that the most hazardous and recalcitrant

1812

J. Raji Rathinavelu, K. Palanivelu / Journal of Environmental Chemical Engineering 2 (2014) 1804–1812

2,4-dichlorophenol could be effectively destructed using the synthesized C–Fe co-doped fibers even under low solar intensity, presenting an energy-efficient alternative to the energy intensive treatment technologies for pollutant eradication. Acknowledgements Authors are thankful to Nanotechnology Group, Chungnam National University (South Korea) for providing the electrospinning set-up and their valuable assistance in the electrospinning process. Mrs. R. Jeevitha Raji is thankful to the University Grants Commission (UGC), New Delhi, India for Senior Research Fellowship in Science for Meritorious students. References [1] O.E. Igbinosa, E.E. Odjadjare, N.V. Chigor, H.I. Igbinosa, O.A. Emoghene, O.F. Ekhaise, O.N. Igiehon, G.O. Idemudia, Toxicological profile of chlorophenols and their derivatives in the environment: the public health perspective, Sci. World J. (2013) 1–11 Article ID460215. [2] IARC-International Agency for Research on Cancer, Monographs on the evaluation of carcinogenic risk of chemicals to humans – Occupational exposures to Chlorophenols. 41 (1986) pp. 319–356. [3] R. Eisler, Pentachlorophenol Hazards to Fish, Wildlife and Invertebrates: A Synoptic Review, U.S. Fish and Wildlife Service, Patixent Wildlife Research Center, Laurel, MD, 1989. [4] S. Ramamoorthy, S. Ramamoorthy, Chlorinated Organic Compounds in the Environment: Regulatory and Monitoring Assessment, CRC Press, Taylor Francis group, 1997. [5] D. Avisar, I. Horovitz, L. Lozzi, F. Ruggieri, M. Baker, M.L. Abel, H. Mamane, Impact of water quality on the removal of carbamazepine in natural waters by N-doped TiO2 photo-catalytic thin film surfaces, J. Hazard. Mater. 244–245 (2013) 463–471, doi:http://dx.doi.org/10.1016/j.jhazmat.2012.09.058. 23141378. [6] S. Kedem, J. Schmidt, Y. Paz, Y. Cohen, Composite polymer nanofibers with carbon nanotubes and titanium dioxide particles, Langmuir 21 (2005) 5600– 5605, doi:http://dx.doi.org/10.1021/la0502443. 15924496. [7] C.B. Murray, C.R. Kagan, M.G. Bawendi, Synthesis and characterization of monodisperse nano crystals and close-packed nanocrystal assemblies, Ann. Rev. Mater. Sci. 30 (2000) 545–550. [8] C. Li, Z. Jiang, Z. Yao, Fabrication and characterization of multi-metal co-doped titania films for a water-splitting reaction, Dalton Trans. 39 (2010) 10692– 10696, doi:http://dx.doi.org/10.1039/c0dt00448k. 20938538. [9] S.T. Hussain, A. Siddiqa, Iron and chromium doped titanium dioxide nanotubes for the degradation of environmental and industrial pollutants, Int. J. Environ. Sci. Technol. 8 (2011) 351–362. [10] K. Pathakoti, S. Morrow, C. Han, M. Pelaez, X. He, D. Dionysiou, H.-M. Hwang, Photoinactivation of Escherichia coli by sulfur-doped and nitrogen-fluorinecodoped TiO2 nanoparticles under solar simulated light and visible light irradiation, Environ. Sci. Technol. 47 (2013) 9988–9996, doi:http://dx.doi.org/ 10.1021/es401010g. 23906338. [11] L.C.A. Oliveria, M. Goncalves, M.C. Guerreiro, T.C. Ramalho, J.D. Fabris, M.C. Pereira, K. Sapag, A new catalyst material based on Niobia/Iron oxide composite on the oxidation of organic contaminants in water via heterogeneous Fenton mechanisms, Appl. Catal., Part A 316 (2007) 117–124. [12] S. Navalon, M. Alvaro, H. Gracia, Heterogeneous Fenton catalysts based on clays, silicas and zeolites, Appl. Catal., Part B 99 (2010) 1–26. [13] B. Zhao, G. Mele, I. Pio, J. Li, L. Palmisano, G. Vasapollo, Degradation of 4nitrophenol (4-NP) using Fe–TiO2 as a heterogeneous photo-Fenton catalyst, J. Hazard. Mater. 176 (2010) 569–574, doi:http://dx.doi.org/10.1016/j.jhazmat.2009.11.066. 20004519. [14] M. Hamadanian, A. Reisi-Vanani, M. Behpour, A.S. Esmaeily, Synthesis and characterization of Fe, S-codoped TiO2 nanoparticles: application in degradation of organic water pollutants, Desalination 281 (2011) 319–324. [15] R. Jeevitha Raji, K. Palanivelu, Sunlight-induced photocatalytic degradation of organic pollutants by carbon-modified nanotitania with vegetable oil as precursor, Ind. Eng. Chem. Res. 50 (2011) 3130–3138.

[16] J. Raji Rathinavelu, M.-J. Jung, J.-S. Im, Y. Seak-Lee, K. Palanivelu, Electrospinning of polymer unaided TiO2 fibers and iron impregnation for sunlightinduced photo-Fenton’s degradation of dyes, Environ. Eng. Sci. 30 (2013) 653– 662. [17] APHA-AWWA-WPCF, Standard Methods for the Examination of Water and Wastewater, American Public Health Association, Washington, D.C, 2005. [18] R.A. Spurr, H. Meyers, Quantitative analysis of anatase–rutile mixtures with an X-ray diffractometer, Anal. Chem. 29 (1957) 760–762. [19] B. Jongsomjit, T. Wongsalee, P. Praserthdam, Study of cobalt dispersion on titania consisting various rutile:anatase ratios, Mater. Chem. Phy. 92 (2005) 572–579. [20] J. Gamage McEvoy, W. Cui, Z. Zhang, Degradative and disinfective properties of carbon-doped anatase–rutile TiO2 mixtures under visible light irradiation, Catal. Today 207 (2013) 191–199. [21] B. Sun, P.G. Smirniotis, Interaction of anatase and rutile TiO2 particles in aqueous photooxidation, Catal. Today 88 (2003) 49. [22] J. Massam, D.R. Brown, Acid-treated montmorillonite supports for copper(II) nitrate oxidation reagents, Appl. Catal., Part A 172 (1998) 259–266. [23] R. Chen, J.J. Pignatello, Role of quinone intermediates as electron shuttles in Fenton and photoassisted Fenton oxidations of aromatic compounds, Environ. Sci. Technol. 31 (1997) 2399–2406. [24] R.P. Juliya Dorathi, K. Palanivelu, Sonochemical degradation of p-chlorophenol in aqueous solution using hypervalent iron, Indian J. Chem. Technol. 17 (2010) 111–119. [25] X.K. Zhao, G.P. Yang, Y.J. Wang, X.C. Gao, Photochemical degradation of dimethyl phthalate by Fenton reagent, J. Photochem. Photobiol. A: Chem. 161 (2004) 215–220. [26] H. Liu, A. Imanishi, Y. Nakato, Mechanisms for photooxidation reactions of water and organic compounds on carbon-doped titanium dioxide as studied by photocurrent Masurements, J. Phy. Chem. C 111 (2007) 8603–8610. [27] W. Xiaoping, T. Yuxin, L. Ming-Yian, L. Teik-Thye, Solvothermal synthesis of Fe– C doped TiO2 nanoparticles for visible-light photocatalytic removal of emerging organic contaminants in water, Appl. Catal., Part A 409–410 (2011) 257–266. [28] P. Wang, P.-S. Yap, T.-T. Lim, C-N-S tridoped TiO2 for photocatalytic degradation of tetracycline under visible-light irradiation, Appl. Catal., Part A 399 (2011) 252–261. [29] T.-H. Kim, V. Rodríguez-González, G. Gyawali, S.-H. Cho, T. Sekino, S.-W. Lee, Synthesis of solar light responsive Fe, N co-doped TiO2 photocatalyst by sonochemical method, Catal. Today 212 (2013) 75–80. [30] Y. Zhang, K. Cheng, F. Lv, H. Huang, B. Fei, Y. He, Z. Ye, B. Shen, Photocatalytic treatment of 2,4,6-trinitotoluene in red water by multi-doped TiO2with enhanced visible light photocatalytic activity, Colloid Surf., Part A 452 (2014) 103–108. [31] I. Morjan, R. Alexandrescu, C.T. Fleaca, A. Badoi, E. Dutua, A.M. Niculescu, C. Luculescu, E. Vasile, J. Wang, S. Bouhadoun, N. Herlin-Boimec, Enhancing the visible light absorption of titania nanoparticles by S and C doping in a singlestep process, Appl. Surf. Sci. 302 (2014) 11–18. [32] M. Tamimi, S. Qourzal, N. Barka, A. Assabbane, Y. Ait-Ichou, Methomyl degradation in aqueous solutions by Fenton’s reagent and the photo-Fenton system, Sep. Purif. Technol. 61 (2008) 103–108. [33] S. Karthikeyan, A. Titus, G. Gnanamani, A.B. Mandal, G. Sekaran, Treatment of textile wastewater by homogeneous and heterogeneous Fenton oxidation processes, Desalination 281 (2011) 438–445. [34] R.A. Conway, R.D. Ross, Handbook of Industrial Waste Disposal, Litton Educational Publishing Inc., New York, 1980. [35] B. Alfanoa, A.E. Cassano, R. Dillert, R. Goslich, Photocatalysis in water environments using artificial and solar light, Catal. Today 58 (2000) 199–230. [36] C. Adan, J. Carbajo, A. Bahamande, I. Oller, S. Malato, A. Martinez, A. Arias, Solar light assisted photodegradation of phenol with hydrogen peroxide over iron doped titania catalysts: role of iron leached/readsorbed species, Appl. Catal., Part B 109 (2011) 168–176. [37] Y.J. Shih, C.H. Ho, Y.H. Huang, Photo-Fenton oxidation of azo dye reactive Black B using an immobilized iron oxide as heterogeneous catalyst, Water Environ. Res. 85 (2013) 340–345. 23697238. [38] Q. Chena, P. Wu, Z. Danga, N. Zhua, P. Lia, J. Wua, X. Wanga, Iron pillared vermiculite as a heterogeneous photo-Fenton catalyst for photocatalytic degradation of azo dye reactive brilliant orange X-GN, Sep. Purif. Technol. 71 (2010) 315–323. [39] J.Y. Feng, X.J. Hu, P.L. Yue, Novel bentonite clay-based Fe-nanocomposite as a heterogeneous catalyst for photo-Fenton discoloration and mineralization of Orange II, Environ. Sci. Technol. 38 (2004) 269–275. 14740746.