Solid-phase photocatalytic degradation of polyethylene–goethite composite film under UV-light irradiation

Journal of Hazardous Materials 172 (2009) 1424–1429

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Solid-phase photocatalytic degradation of polyethylene–goethite composite film under UV-light irradiation G.L. Liu, D.W. Zhu, S.J. Liao ∗ , L.Y. Ren, J.Z. Cui, W.B. Zhou Laboratory of Plant Nutrition and Ecological Environment Research, Centre for Microelement Research of Huazhong Agricultural University, Key Laboratory of Subtropical Agriculture and Environment, Ministry of Agriculture, Wuhan 430070, China

a r t i c l e

i n f o

Article history: Received 17 February 2009 Received in revised form 3 August 2009 Accepted 4 August 2009 Available online 8 August 2009 Keywords: Polyethylene Goethite Solid-phase Photocatalytic degradation

a b s t r a c t A novel photodegradable polyethylene–goethite (PE–goethite) composite film was prepared by embedding the goethite into the commercial polyethylene. The degradation of PE–goethite composite films was investigated under ultraviolet light irradiation. The photodegradation activity of the PE plastic was determined by monitoring its weight loss, scanning electron microscopic (SEM) analysis and FT-IR spectroscopy. The weight of PE–goethite (1 wt%) sample steadily decreased and led to the total 16% reduction in 300 h under UV-light intensity for 1 mW/cm2 . Through SEM observation there were some cavities around the goethite powder in the composite films, but there were few changes except some surface chalking phenomenon in pure PE film. The degradation rate could be controlled by changing the concentration of goethite particles in PE plastic. The degradation of composite plastic initiated on PE–goethite interface and then extended into polymer matrix induced by the diffusion of the reactive oxygen species generated on goethite particle surface. The photocatalytic degradation mechanism of the composite films was briefly discussed. © 2009 Elsevier B.V. All rights reserved.

1. Introduction The major component of plastic agriculture film is polyethylene (PE) used widely for the protection and cultivation of some vegetables and crops in some regions in China. The increase in the use of plastics in agriculture has enabled farmers to increase their crop production and to alleviate the dependence of their production on the climatic conditions. Today, the use of plastics in agriculture results in the increment of yield, earlier harvest for crops and vegetables, less reliance on herbicides and pesticides, better protection of food products and more efficient water conservation. However, the popular use of plastics in agriculture has always been accompanied by a serious negative effect which is that thousands tons of the plastic wastes are disposed of. A large portion of plastic films was left on the field or burnt uncontrollably by the farmers, emitting harmful substances with the associated negative consequences to the environment [1–3]. In order to reduce cost, the thickness of application agriculture films is less than 5 ␮m in some regions in China, which result in difficult to recycle. And because the process of recycling is expensive and time-consuming, only a small percentage of the agricultural plastic waste is currently recycled at the end of cultivation in China [4]. Biodegradable plastics are regarded as an ideal solution to this kind of problem [5,6].

∗ Corresponding author. Tel.: +86 27 87287184; fax: +86 27 87397735. E-mail addresses: [email protected], [email protected] (S.J. Liao). 0304-3894/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2009.08.008

However, the biodegradable plastics till now cannot completely solve the problem due to its chemical stability, and the farmers usually could not afford the cost [7–9]. Consequently, investigating the progress of PE degradation in the environment to develop efficient and economical degradation technologies for PE is of significance for sustainable agricultural development and the protection of the natural environment, and benefits farmers as well. Heterogeneous photocatalytic oxidation occurring at moderate conditions has been widely used to deal with aquatic or air pollutants [10]. Soild-phase photocatalytic degradation of polymers such as PVC or even chlorine-less polymer such as polyethylene (PE) has attracted intense interest in recent years [11–16]. A few recent reports [11–14] describe the use of TiO2 as the photocatalyst for oxidative degradation of PE. However, poor solar efficiency has hindered the commercialization of this technology [17,18]. Iron oxide is one of the most abundant minerals in soil and a main constituent of atmospheric particulate and plays a critical role in many chemical and biological processes [19]. Major iron oxides include goethite (␣-FeOOH), hematite (␣-Fe2 O3 ), lepidocrocite (␥-FeOOH), and maghemite (␥-Fe2 O3 ). Iron oxides can act as natural photocatalysts to catalyze degradation of organic pollutants in environment [20,21]. Recently, the use of goethite has been found to effectively oxidize organic compounds due to the catalysis on goethite surface and ferrous ion generation [22–26]. To our best knowledge, little has been done on the solid-phase photodegradation of plastic over sensitized goethite catalyst. Therefore, the present study focused on solid-phase photocatalytic degradation of polyethylene plastic

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with goethite as photocatalyst. PE–goethite composite films were prepared and their photocatalytic degradation under ultraviolet light in the ambient air was studied. Photodegradation intermediate products were speculated and the mechanism of the solid-phase photocatalytic reaction was probed. 2. Materials and methods 2.1. Chemicals PE (grade-LD103) was purchased from Yanshan Petrochemical Company Ltd. The average molecular weight (Mw) was about 100,000. Fe(NO3 )3 (AR grade), KOH (AR grade), cyclohexane (AR grade) were supplied from Guoyao Chemical Co. (Shanghai, China). All chemicals were used without further purification and deionized water was used in all the experiments. 2.2. Preparation goethite photocatalysts The original goethite was prepared according to the method of Liao et al. [27]. It was prepared in a flask by hydrolysis of a stirred 0.150 mol L−1 Fe(NO3 )3 solution with 2.5 mol L−1 KOH, and the alkali was introduced by titration at a constant rate of 5 ml min−1 up to a solution pH of 11.9. The suspension was then equilibrated in a covered polyethylene bottle at 60 ◦ C for 48 h. Then the suspensions were centrifuged, filtered, washed repeatedly with deionized water and dried at 60 ◦ C. The dried samples were ground in an agate mortar, passed through a 0.16 mm sieve and stored in a desiccator. The properties of oxide have been previously investigated using N2 adsorption method (BET), infrared spectrometer, X-ray diffraction and transmission electron microscope and reported by Liao et al. [27].

Fig. 1. Schematic diagram of photoelectrocatalytic reactor. 1: lamp-housing box (50 cm × 40 cm × 30 cm); 2: ultraviolet lamp (20 W); 3: air and water inlet; 4: sample (5 cm × 5 cm).

multibounce HATR accessory with ZnSe crystal at 45◦ . The surface morphologies of all samples after irradiation for 300 h were observed by scanning electron microscope (SEM, JSM 6390LV). 3. Result and discussion 3.1. Weight loss Fig. 2 shows the photoinduced weight loss of pure PE film and PE–goethite composite film samples in air under UV irra-

2.3. Preparation of PE–goethite composite films PE–goethite composite films were cast as follows [14]. The polymer stock solution was prepared by dissolving 1.0 g of PE in 50 ml cyclohexane at 70 ◦ C under stirring for 60 min. Then, goethite powder was suspended uniformly in the above 50 ml solution to give 0.4 wt% and 1.0 wt% goethite contents with respect to the total mass of PE. An aliquot of 20 ml PE–goethite solution was spread on a glass plate (R = 4.5 cm) and first dried for 20 min at 70 ◦ C, then dried for 48 h at room temperature. The thickness of the resulting PE–goethite composite film sample measured by SEM was 20–45 ␮m. Its weight was ca. 0.4 g. Fig. 2. Effect of goethite loading on the photocatalytic degradation of PE film.

2.4. Photodegradation and characterization of PE and PE–goethite composite films In order to reveal the photocatalytic degradation behavior and mechanism of PE–goethite plastic, the photodegradation reaction was conducted under ambient air at around 25 ◦ C in a lamphousing box (50 cm × 40 cm × 30 cm) reactor with ultraviolet lamp, as shown in Fig. 1. The pure films and composite films were irradiated under 20 W ultraviolet lamp (ZSZ-D, Changsha Guangming Co. Ltd.). The typical surface area of the film samples was around 25 cm2 . The samples were placed 15 cm away from the lamp, where the light intensity was measured using a UV intensity meter (UV-I, Beijing Shida Ltd.) at primary wavelength (254 nm). The degradation of the films was evaluated directly by their weight loss. FT-IR (Nicolet Avatr330) spectrophotometer was used to study the spectrum character of these films before and after the irradiation. Measurement range was 4000–500 cm−1 , with a 4 cm−1 resolution, 0.475 cm−1 /s scan speed and 32 scans. The technique applied was attenuated total reflectance (ATR) with an Avantar

Fig. 3. Effect of intensity of UV radiation on the photocatalytic degradation of PE–goethite (1.0 wt%) composite film.

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Table 1 Regression analysis on film weight loss treated with different goethite concentrations (n = 8). Goethite concentration

Equation

k

a

R2

0 wt% 0.4 wt% 1.0 wt%

Qi = kt + a

0.0084 0.0288 0.0493

−0.2100 −0.5733 −0.7217

0.9673*** 0.9903*** 0.9958***

Qi stands for weight loss, t stands for irradiation hours, respectively. *** Stands for significant difference at P < 0.001, according to the SAS system.

diation. The weight loss rate was much greater for PE–goethite samples than for the pure PE. The degradation rate of PE–goethite composite films increased while goethite concentration increased. The weight of PE–goethite (1.0 wt%) sample decreased to 16% in 300 h under UV-light intensity for 1 mW/cm2 . While PE sample showed only 2% weight loss under the same experimental conditions. The above weight loss data indicates that the photocatalytic reaction of PE–goethite composite films has occurred and might produce a mass of volatile products. This result was similar to that described in the works [14]. The volatile products might be carbon dioxide, methane, ethene, ethane, propane, acetaldehyde,

formaldehyde and acetone. As shown in Fig. 2, at goethite concentrations employed in the study, the photocatalytic degradation of films obeyed zero order kinetics in 300 h under UV-light irradiation. The rate equation from the irradiation time and weight loss at different concentrations of goethite loading was given in Table 1. The degradation rate is proportional to the concentrations of goethite loading and the photocatalytic degradation of composite films could be controlled by changing the concentration of goethite particles in PE plastic. The influence of the UV irradiation is shown in Fig. 3. The results indicated that the photocatalytic degradation of the com-

Fig. 4. SEM images of different samples before and after irradiation with 1 mW/cm2 UV-light intensity for 300 h: (a) PE sample before (i) and after (ii) irradiation; (b) PE–goethite (0.4 wt%) sample before (i) and after (ii) UV irradiation; (c) PE–goethite (1.0 wt%) sample before (i) and after (ii) irradiation.

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posite films increased as the light irradiation was intensified. The weight loss of PE–goethite (1.0 wt%) sample reached 24% in 300 h under UV-light intensity for 2 mW/cm2 , which was higher in light intensity for 1 mW/cm2 . 3.2. The surface morphology of the films after photodegradation As shown in Fig. 4, scanning electron microscope was carried out to observe the surface morphology of the films with photodegradation. The surface of PE and the PE–goethite composite films were smooth before and after irradiation (Fig. 4). Fig. 4b(ii) and c(ii) shows the texture of PE–goethite (0.4 wt%) and the texture of PE–goethite (1.0 wt%) composite films irradiated for 300 h under UV-light intensity for 1 mW/cm2 in air, respectively. After 300 h irradiation, there were some cavities around the goethite powder in the composite films which were also observed in the identical experiments. The formation of these cavities might be induced by

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the escape of volatile products from PE matrix [14]. After 300 h irradiation, there were more cavities on the surface of PE–goethite (1.0 wt%) composite films. Fig. 4a(ii) shows the texture of pure PE film that was irradiated for 300 h under UV-light intensity for 1 mW/cm2 in air. There were few changes except some surface chalking phenomenon in pure PE film. This result was in accordance with the weight loss data shown in Fig. 2. SEM images suggested that the degradation of PE matrix started from PE–goethite interface and led to the formation of cavities around goethite particle aggregates. It implied that the active oxygen species generated on goethite surface diffused and etched the polymer matrix. 3.3. Spectroscopic characterization The photocatalytic degradation of PE films was examined by FT-IR spectroscopy. Fig. 5 shows the FT-IR spectra of the composite films before and after irradiation for 300 h. The spectrum

Fig. 5. FT-IR spectra of the films before and after irradiation with 1 mW/cm2 UV-light intensity: (a) PE sample before (i) and after (ii) irradiation; (b) PE–goethite (0.4 wt%) sample before (i) and after (ii) UV irradiation; (c) PE–goethite (1.0 wt%) sample before (i) and after (ii) irradiation.

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of the original PE–goethite composite films shows the characteristic absorption of long alkyl chain in the region of 2923 cm−1 , 2850 cm−1 , 1472 cm−1 and 719 cm−1 , which were the same as that in pure PE film. It shows that the IR spectra character of polymeric matrix is not affected by embedding goethite particles. Fig. 5a(ii), b(ii) and c(ii) shows the FT-IR spectra of the different films after 300 h irradiation. There was a new absorption peak for PE–goethite (1.0 wt%) composite film in the region of 1713 cm−1 and 1177 cm−1 after irradiation, which could be assigned to C O and C–O stretching vibrations, respectively [11]. It shows that the intensity of C O and C–O groups in PE–goethite (1.0 wt%) film is the strongest in the films. However, the intensity of alkyl characteristic peaks in PE–goethite (1.0 wt%) was much weaker than those in the PE–goethite (0.4 wt%) and pure PE film. And those in the PE–goethite film were weaker than that in pure PE film, in which alkyl characteristic peaks had almost no change. These all indicate that the goethite can make the photodegradation of PE film and more goethite has a higher catalytic activity. 3.4. Photocatalytic degradation mechanism of PE–goethite The photocatalytic degradation of pure PE has been extensively studied [28,29]. The reaction of PE under ultraviolet irradiation occurred via direct absorption of photons by the PE macromolecule to create exited states, and then undergo chain scission, branching cross-linking and oxidation reaction [30]. For composite samples, the photocatalytic degradation is the main reaction, which is quite different from the photolytic degradation of the pure PE sample. For PE–goethite composite film, the photodegradation of PE mainly happened on the film surface where electrons or holes combined with adsorbed oxygen molecules or hydroxyl ion to produce • O2 − or • OH, two very important reactive oxygen species for the degradation of PE. The photocatalytic reaction mechanism of PE–goethite could be written as follows. Goethite + h → e− + h+

(1)

H2 O + h+ → H+ + • OH

(2)

−

O2 + e → • O2 •O − 2

−

+ H+ → HOO•

2HOO•

→ H2 O2 + O2

(3) (4) (5)

H2 O2 + e− → OH− + • OH

(6)

–(CH2 CH2 )– + • OH → Degradationproducts

(7)

When irradiated by UV-light, electrons receive energy from the photons and are thus excited from valence band to conduction band as in Eq. (1) [19,20]. After formation of electron–hole pair, the hydroxyl radicals (• OH) with higher redox potential can be generated by a series of reactions as Eqs. (2)–(6). And then polymer could be oxidized by • OH. The degradation process spatially extends into the polymer matrix through the diffusion of the reactive oxygen species. 4. Conclusions In summary, the photocatalytic degradation process of PE–goethite composite films has been studied under the influence of UV radiation at 254 nm. The degradation of composite film was much faster and more complete than the simple photolysis of pure PE films with more goethite loading and higher light intensity. The degradation rate of composite films could be controlled by changing the concentration of goethite particles in PE plastic. The present paper intends to study goethite as photocatalytst for degradating

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