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Photocatalytic degradation of malachite green dye using zero valent iron doped polypyrrole

The zero valent iron doped polypyrrole (Ppy/Fe) was synthesized via chemical oxidative polymerization method in diethylene glycol medium for the first time in this study. The characterization of synthesized Ppy and Ppy/Fe composites was realized by using fourier transform infrared spectrometer (FTIR), UV-visible diffuse reflectance spectrometer (DRS), scanning electron microscope (SEM), elemental dispersion X-ray analysis (EDX), four point probe electrical conductivity X-Ray diffraction spectroscopy (XRD) and pH at point of zero charge (pHpzc) methods. These characterizations show that there is an interaction between zero valent iron (Fe) and Ppy polymer. The photocatalytic activity of synthesized Ppy and Ppy/Fe composite were investigated by degradation of malachite green (MG) dye under UV light irradiation. Ppy/Fe composite was achieved complete degradation within 60 min, which is 5.92 times faster than pure Ppy. The effect of irradiation time, ratio of Ppy and Fe in the synthesized composites, photocatalyst amount to photocatalytic efficiency of Ppy/Fe is studied. Also, the photocatalytic stability of Ppy/Fe is investigated under UV light illumination for degradation MG dye.


Introduction
In many developing countries, textile industries are essential for economic development. Different materials like cotton, fibers, and chemicals, including dyes, have been used in textile industries [1]. These synthetic dyes are harmful, detrimental to mammals, and aquatic environments. Due to the infertile dyeing process used in textile industries, huge synthetic dyes are charged into the environment and about 17-20% of industrial wastewater is produced from the textile dyeing and finishing process [2]. Although these industries ensure critical economic advantages, they also pollute the environment to generate toxic wastewater. An effective treatment method should be applied before it is dumped into the environment to reduce the negative effects of textile wastewater. Textile wastewater has been treated using various technologies like adsorption, membrane filtration, coagulation, ion exchange, biological treatment, photocatalysis, etc. Among these treatment methods, photocatalysis has attracted a great deal in recent years [3,4]. In a photocatalytic reaction, the photon energy, hν, is equal or exceeds the band gap of the semiconductor; a photoexcited electron (e-) is jumped from the valance band (VB) of a photocatalyst to the conduction band (CB) [5][6][7]. So, generated e-/h+ couples transferred to the semiconductor's surface and then react with the other reactants on the photocatalyst surface [8]. An ideal photocatalyst has a suitable band gap, large surface area, and low cost [9]. In recent years, conductive polymers like polypyrrole (Ppy), polyaniline, and polythiophene are getting attention due to its easy synthesis procedure, chemical stability, low cost and good conductivity photocatalysis applications. Ppy is a highly preferred conductive polymer due to conjugated structure; Ppy has good electron transport properties with excellent photoinduced charge separation [10]. Ppy has been synthesized easily using chemical [11,12] and electrochemical [13] methods. In the literature, Ppy composite polymer photocatalysts have been used for the photocatalytically organic dye degradation process [14][15][16]. Zero valent iron (Fe 0 ) due to its small particle size and reduction properties used as a catalyst in wastewater treatment. Fe 0 NPs have superparamagnetic properties, non-toxic, low cost, and easy synthesis procedure. Although these unique properties, magnetic Fe0 tends to agglomeration due to high surface energy and superparamagnetic properties in waste-water medium [17]. To overcome these drawbacks, magnetic Fe 0 NPs combined with various materials like metal oxide [18][19][20], conductive polymers [21][22][23].
There are various studies about Ppy/Fe composite in the literature. However, up to our knowledge, no published study described the use of Ppy/Fe composite in the degradation of MG dyes in the wastewater. This study aims to synthesize Ppy/Fe composite with a facile chemical oxidative polymerization method for degradation MG dye. The characterization of Ppy/Fe was completed via FTIR, DRS, SEM, EDX, four probe electrical conductivity, XRD and pHpzc methods. The photocatalytic efficiency of Ppy/Fe composite was investigated under UV light irradiation as a function of irradiation time, the ratio of Ppy and Fe 0 , photocatalyst amount, and photocatalytic stability.

Materials
Pyrolle (py) monomer was purchased from Merck Chemical and distilled under atmosphere pressure before use. Ammonium peroxidisulphat (APS), diethylene glycol (DEG), Fe(NO3) 3 .9H2O, malachite green (MG) were purchased from Merck Chemicals. All reagents were used without purification. Deionized water was used in experiments.

Synthesis of Ppy/Fe Photocatalyst
Ppy polymer was synthesized in aqueous DEG using APS as an oxidant using chemical oxidation polymerization described elsewhere [24].

Characterization of Ppy/Fe Photocatalyst
The characterization of Ppy/Fe was completed by using optic, spectroscopic and morphological methods. The optical property of Ppy/Fe was examined using a UV-visible diffuse reflectance spectrometer (DRS) with HITACHI U3900 instrument. Fourier transform infrared spectra FTIR were recorded at 4,000-500 cm -1 wavenumber range by Perkin Elmer 1725. The scanning electron microscope SEM images were recorded with ZEISS Evo LS 10 SEM. The absorption spectra of dye were recorded using the Ocean Optics HR4000 UV-visible spectrophotometer at photocatalytic activity experiments. The pHpzc of Ppy/Fe was determined by pH drift method. The crystallinity of Ppy and Ppy/Fe were investigated via Shimadzu XRD-6000 X-ray diffractometer (Cu Kα radiation (λ = 0.15418 nm; 20-70°and 2°/min).

Investigation of Photocatalytic Activity of Ppy/Fe
The photocatalytic efficiency of Ppy/Fe composite was searched by degradation MG dye under UV light illumination in a quartz tube. Photocatalytic activity experiments were carried in a Luzchem 4V (Luzhem Research Inc. Canada) photo reactor with 8 UVC lamps (emission at 254 nm). Photocatalytic experiment details were in our previous research [26]. The photocatalysts (x mg/mL; x = 0.2, 0.4, 0.8, 1.2 and 1.6) and 1.0*10 -5 M MG dye solution (3 mL) added a quartz tube. The adsorption/desorption equilibrium of dye on the Ppy/Fe was stored in dark conditions for 60 min. After, UV light switched on for initiate photocatalytic reaction. For 15 minintervals time, the absorption spectra of MG were recorded with UV-vis absorption spectrophotometer. The degradation efficiency of MG dye was calculated from Eq. (1): Where C 0 is the initial concentration of dye and C t is the concentration of dye at time t.

Characterization of Ppy/Fe Nanocomposite
The FTIR spectra investigated to analyze the chemical bond of Ppy and Ppy/Fe composites (Fig. 1). The characteristic FTIR peaks of Ppy were observed in Fig. 1(a). The peak at centered at 1,458 cm -1 is assigned to C-N stretching vibration in the pyrrole ring [27]. C-C stretching vibration is observed at 1,544 cm -1 correspond to [28]. The peaks at 1,281 and 754 cm -1 are assigned to C-H. These peaks confirm the polymerization of pyrrole monomer. The peak at around 565 cm -1 in Fig. 1(b)-(e) can be attributed Fe-O stretches in Ppy/Fe composite [29]. The FTIR peaks of Ppy mention above partially shifted to higher wavenumber after addition Fe 0 particles to polymerization medium due to interaction between Ppy and Fe 0 .
For investigate the optical properties of Ppy and Ppy/Fe composite, the UV-vis diffuse reflectance spectrum were recorded as given Fig. 2(a). Three possible states of Ppy which are neutral, poloron (radical cation) and bipoloron (dication) can be observed all at the same time. These three characteristic absorption bands of Ppy are seen at 250-270 nm, 310-360 nm and 600-650 nm [30]. The bands at visible range are attributed to the doping level and introduction of polaron and bipolaron lattices, which represent the protonation stages of Ppy chain [30]. As can be seen from Fig.  3(a) Ppy shows an absorption band at 250 nm and a shoulder at around 310 nm. The absorption peak at 210 nm which is character-istic π-π* transition of neutral Ppy [31]. With the addition Fe 0 particles to polymerization medium, the intensities of absorption band of Ppy/Fe composite are increased. Also, the synthesized with highest Fe amount; Ppy/Fe (1:2) has third characteristic absorption peak of Ppy around 600 nm at visible range.
The optical band gaps (Eg) of synthesized Ppy and Ppy/Fe composites were calculated with the help UV-vis absorption spectra using Tauc plot [32,33]. The Eg values of the Ppy and Ppy/Fe at UV and visible range have been computed by extrapolation of the plot of (αhν) 2 vs hν in Fig. 2(b), and the results are shown in Fig.  2 of the Ppy increased in the UV region while they decreased in the visible region. The smallest Eg value belongs to Ppy/Fe (1:1) in the visible range. The SEM images of Ppy and Ppy/Fe composites are given in Fig. 3(a)-(e). The morphology is one of the key factor for photocatalysis because of effected the photocatalytic efficiency directly [34]. All SEM images are recorded same magnification given in Fig. 3. As can be seen from SEM images, the morphology are roughly spherical in shape with sizes Fig. 3(a) and with the increase in adding Fe 0 particles in the polymerization medium grain size decreases. Fe amount in Ppy/Fe composite were determined via EDX methods and the results were given in Fig. 3(f). As shown in Fig. 3(f), as the amount or Fe 0 particles increased in polymerization medium, the percentage of Fe (wt%) increases in the polymer composite.
The electrical conductivity of Ppy and Ppy/Fe composite were given in Fig. 4. The electrical conductivity of pure Ppy is 1.7x10 -5 S/cm. When Ppy combined with the Fe 0 particles, the electrical conductivity is increased. The conductivity increases with increase in Fe 0 contents in Ppy composite. The electrical conductivity of Ppy/Fe (1:1) is ten times higher than pure polymer. The conductivity reached its maximum value for Ppy/Fe (1:1), but it partially decreased when the amount of Fe 0 in the composite was increased further. The reason for the partial decrease in conductivity at high Fe 0 amount is the disruption of the homogeneous distribution of Fe 0 nanoparticles in the composite and the formation of agglomeration. The conductivity of a photocatalyst depends of the availability of electrons in the conduction band. In conductive polymer, the band gap small enough to excite the valence band electrons to conduction band by absorbance of light. So if the band gap of a photocatalyst is small, the conductivity will increase. As electrical conductivity increased, the charge transfer ability rise [34]. The change in conductivity with the addition of Fe 0 particles to the polymerization environment is another confirmation of composite formation and polymer-Fe interactions. Fig. 5 indicates XRD patterns of the Ppy and Ppy/Fe composite. As shown Fig. 5, Ppy is amorphous in nature and the characteristic peak of amorphous Ppy has been obtained at 2= 24.6 [35]. For the composite, the Fe nanoparticles could not influence the crystalline structure of Ppy due. However, a very weak Fe(110) diffraction peak has been detected at a diffraction angle of about 47.16° [36].  The pHpzc of Ppy/Fe was determined by pH drift method. For this purpose 0.01 M NaCl solution was placed in a beaker and N 2 was bubbled through the solution to stabilize the pH by preventing the dissolution of CO 2 . The pH was then adjusted to initial values 2, 4, 6, 8 and 10 by adding either 0.1 M HCl or 0.1 M NaOH and the 0.2 g Ppy/Fe was added to the solution. After 48 h, the final pH of Ppy/Fe was measured and plotted the initial pH (Fig. S2). The pH at which the curve crosses the line pH (final) = pH (initial) is taken as the pHpzc of the adsorbents. The pHpzc of Ppy/Fe was determined to be 2.96. The natural pH value of MG solution was found to be 5.7. At the MG solution pH > pHpzc, the Ppy/Fe surface negatively charged and promote degradation of cationic dyes due to increased electrostatic force of attraction. At pH < pHpzc, the surface becomes positively charged, concentrations of H + were high and they compete with positively charged MG cations for vacant adsorption sites causing a decrease in dye degradation. So, MG which is cationic dye degradation by using Ppy/Fe is favored at pH higher than pHpzc.

Photocatalytic Activities of Ppy and Ppy/Fe Nanocomposite
The photocatalytic activity of the synthesized Ppy and Ppy/Fe composites was investigated by the degradation of MG dye under UV light illumination. Fig. 6(a) indicates the effect of Ppy:Fe ratio on photocatalytic activity of the Ppy/Fe composite materials for 60 min UV light irradiation. As shown in Fig. 6, Ppy/Fe (1:1) has higher photocatalytic activity than Ppy and other Ppy/Fe composites under UV light irradiation. According to the Fig. 6(a), optimum photocatalyst for degradation of MB dye under UV light illumination was selected Ppy/Fe (1:1). The apparent rate constant of MG photodegradation was calculated by relation following Eq. (2). ln(C t /C 0 ) = k app t ( 2 ) where k app is the apparent rate constant, C 0 is the initial concentration and C t is the concentration of dyes at the given time following pseudo-first order kinetics for different Fe amount in the polymer-ization medium ( Fig. 6(b)). The k aap value of produced Ppy and Ppy/Fe composite were given inset Fig. 6(b). As can be seen from inset Fig. 6(b), the highest k app value, 0.0936 min -1 belongs to Ppy/Fe (1:1) polymer composite. This can be attributed of its smallest Eg value and highest electrical conductivity. When the Ppy/Fe (1:1) is exposure with UV light, an electron is jumped from valence band to conduction band. This photogenerated charge has the high transfer ability due to its high electrical conductivity. So, the Ppy/Fe (1:1) has the higher photocatalytic activity than pure Ppy and other Ppy/Fe composites. The photocatalytic activity of the catalyst can usually be improved by doping or by creating nanocomposites [9]. The high photocatalytic activity of the polymer/metal or polymer/metal oxide nanocomposites depends on the homogeneous distribution of the dopant in the polymer. With the increase in the amount of Fe 0 in the Ppy/Fe (1:2) composite, the homogeneous distribution of Fe 0 is distorted and Fe 0 agglomerations can be occur in the composite. As a result, a decrease in photocatalytic activity is observed. Fig. S3 shows MG degradation in the presence of Ppy and Ppy/Fe composite under dark and UV light conditions. The MG dye stored under dark for 60 min for understand adsorption mechanism. The decolorization efficiency of MG dye is 14 and 8.9% with adsorption process in the presence of Ppy and Ppy/Fe composite, respectively. After 60 min UV light irradiation, the degradation efficiency of MG reached up 95% by using Ppy/Fe as photocatalysts. At the same time, pure Ppy is degraded the MG dye only 58.7% under UV light irradiation. According to Fig. S3, Fe 0 particles addition to polymerization medium increased the photocatalytic activity.
The effect of photocatalysts amount on the decolorization of MG was researched under UV light irradiation a fixed 60 min irradiation time (Fig. S4). The experimental results were given in Table S1. The photocatalytic degradation rate of MG dye is increased with the increase of Ppy/Fe amount. The photocatalysts amount threshold was evaluated as 1.2 mg/mL, but the photocatalytic degradation rate of MG dye for the higher photocatalyst amount loadings is related to due to irregular distribution of active sites and, presumably, agglomeration. Generally, because of the increment in active sites, hydroxyl radical generated from illuminated photocatalyst and leads to an improvement in the rate of photocatalytic degradation. Beyond the optimum photocatalyst amount, the degradation rate is declined due to rise in the opacity of the suspension, and thus increasing the light scattering and also the infiltration depth of the photons is reduced and fewer photocatalysts could be activated. Also, the agglomeration of nanoparticles at high concentrations leads to a decline in the number of surface active sites. Fig. 7 shows the change in absorption spectra of MG dye under UV light for different periods by using Ppy/Fe as photocatalyst. The reduction of the characteristic bands intensities of MG dye observed at 425 and 615 nm shows that MG has been degraded by Ppy/Fe composite under UV light irradiation in 60 min. Also, the degradation efficiency of MB dye is around 61% in the presence a b   (Fig. 7). The inset images in Fig. 7 demonstrated the decolorization of MG dye during 60 min under UV light irradiation.
To investigate the photocatalytic stability of Ppy/Fe composite, the photocatalytic decolorization experiment was recurrence up to five times. Each photocatalytic experimental results were shown the average of three repetitions ± standard deviations and the results are given in Fig. 8. For each photocatalytic cycle, Ppy/Fe is gathered via centrifugation, and then washed with deionized water several times. After first photocatalytic usage, the photocatalytic efficiency of Ppy/Fe was decreased. MG dye degraded in 120 min under UV light irradiation in the second photocatalytic usage. As shown in Fig. 8, no apparent deactivation of Ppy/Fe was observed after second usage under UV light irradiation for degradation of MG dye.
FTIR analyzes were carried out to confirm the photocatalytic stability of Ppy/Fe composite after fifth photocatalytic experiment. As shown, Fig. S5, the similar spectrums have obtained before and after photocatalytic degradation of MG dyes via Ppy/Fe composite under UV light irradiation. The structure of Ppy/Fe composite was not affected during photocatalytic process.
In recent studies, Yuan and co-workers [37] synthesized the Ppy n hexagonal meso phases (used as soft templates) by chemical polymerization (Ppy-NS-c). The photocatalytic activity of Ppy-NS-c was investigated under UV and visible light irradiation by the degradation of phenol. Ppy-NS-c presents the highest activity for photodegradation of phenol under UV light with degradation rate of phenol about 100% after 4.5 h. In the other work; the apparent rate constant of methyl orange photodegradation is 0.0130 min -1 by Ppy/TiO 2 under UV light illumination [38]. The photocatalytic activity of Ag/TiO2@PPy heterostructure was studied by using methylene blue (MB) degradation under UV light irradiation. The MB dye completely degraded after around 100 min UV light irradiation [39]. In this study, Ppy/ Fe synthesized with a facile synthesized method and it has been performed high photocatalytic performance under UV light irradiation for the MG dye degradation.

Conclusions
In this study, Ppy/Fe composite have been synthesized via facile chemical oxidative polymerization methods in the diethylene glycol medium for the first time. The characterization of Ppy and Ppy/Fe composite were completed by using FTIR, UV-vis diffuse reflectance, SEM, EDX four probe electrical conductivity, XRD and pHpzc methods. The electrical conductivity of Ppy is increased the 10 times when Fe 0 particles added to polymerization medium. The interaction between Fe 0 particles and Ppy demonstrated these characterization methods. The degradation of MG dye was studied for 60 min to evaluate Ppy/Fe photocatalytic performance under UV light irradiation. The photocatalytic efficiency of Ppy/Fe composite were investigated under UV light irradiation as a function of irradiation time, ratio of Ppy and Fe 0 , photocatalyst amount and photocatalytic stability. MG dye completely degraded after 60 min later under UV light irradiation. The k app value of Ppy/Fe composite is 0.0936 min -1 for degradation of MG dye under UV light irradiation. As the results were evaluated, Ppy/Fe composite is a promising photocatalyst for degradation the textile industries wastewater by using photocatalytic wastewater treatment.