Photocatalytic activity and biodegradation of polyhydroxybutyrate films containing titanium dioxide

Polymer Degradation and Stability 91 (2006) 1800e1807 www.elsevier.com/locate/polydegstab

Photocatalytic activity and biodegradation of polyhydroxybutyrate films containing titanium dioxide Saw-Peng Yew, Hui-Ying Tang, Kumar Sudesh* School of Biological Sciences, Universiti Sains Malaysia, 11800 Penang, Malaysia Received 17 October 2005; received in revised form 14 November 2005; accepted 23 November 2005 Available online 18 January 2006

Abstract This study aims to evaluate the photocatalytic activity and biodegradation of polyhydroxybutyrate (PHB) films containing titanium dioxide (TiO2). Nanosized TiO2 photocatalysts were immobilized onto PHB film to overcome the difficulty of the recovery process. PHB is a suitable base material as it is naturally biodegradable and is produced from renewable resources. The photocatalytic degradation of organic compounds, photocatalytic sterilization activity and biodegradation rate in garden soil of PHBeTiO2 composite films were investigated. After an hour under solar illumination, 96% of methylene blue solution was decolorized. The antibacterial activity against Escherichia coli (E. coli) using PHBe TiO2 composite film exhibited enhanced photocatalytic sterilization activity over time. As for the ability to biodegrade, PHBeTiO2 composite films placed on soil surface with no direct solar illumination showed slower degradation rate compared to those receiving direct solar illumination. Interestingly, the latter composite films showed faster degradation rates compared to pure PHB films indicating that the degradation is mainly due to photocatalytic activity. PHBeTiO2 composite films buried in soil generally showed slower degradation rates compared to pure PHB films and were dependent on the soil microbial activity. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: PHBeTiO2 composite film; Biodegradation; Photocatalytic degradation; Photocatalytic sterilization; Microbial degradation

1. Introduction This study aims to evaluate the photocatalytic activity and biodegradation of polyhydroxybutyrate (PHB) films containing titanium dioxide (TiO2). Photocatalysis had been demonstrated to be an inexpensive and effective method for treating a wide range of pollutants in both water and air. Titanium dioxide is close to being an ideal photocatalyst in environmental photocatalysis. It is also relatively inexpensive and highly stable chemically. When a photocatalyst TiO2 captures ultraviolet light (UV), either from the sun or fluorescent light, it forms activated oxygen from water or oxygen in the air [1]. The activated oxygen is strong enough to oxidize and decompose organic materials or toxic gas, and to kill bacteria. The very strong oxidizing power of TiO2 can destroy the bacterial

* Corresponding author. Tel.: þ604 6533888x4367; fax: þ604 6565125. E-mail address: [email protected] (K. Sudesh). 0141-3910/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2005.11.011

cell membrane, causing leakage of the cytoplasm which in turn inhibits bacterial activity. Ultimately, this results in the death and decomposition of the bacteria [2,3]. The mechanism of TiO2 oxidation is as illustrated in Fig. 1 (modified from [4]). TiO2 had been approved by the food testing laboratory of the United States Food and Drug Administration (FDA) and it is considered safe and harmless to human. It is commonly used in paint, printing ink, plastics, paper, synthetic fibers, rubber, condensers, electronic components, food and cosmetics [5]. Organic dyes, when present in considerable amounts, are one of the major pollutants in wastewaters produced from textile and other industrial processes [6]. In the treatment of dyecontaining wastewaters, complete destruction of dye pollutants is difficult when conventional methods such as flocculation, carbon adsorption, reverse osmosis and activated sludge process are being employed [7]. TiO2 is the most preferable material among the various photocatalysts available, and it is proven to be able to degrade organic carbon into CO2 effectively [8,9]. The combination of photocatalysis and solar technologies

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Ultraviolet radiation (UV)

conduction band

electron

O2 reduction •O2-

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manufactured by R&M Chemical, Canada; and DIFCO Laboratories, United Kingdom, respectively. All of the other chemicals were of analytical grade quality and were used without further purification. The pH of the solution was adjusted with either diluted HCl or NaOH. 2.2. Bacterial strain and growth media

UV

•OH valence band

oxidation hole

H2O

Fig. 1. Simple scheme showing the reaction mechanism of TiO2 photocatalysis modified from [4].

provides an alternative means for water purification and it is a new process for preserving the environment [10]. However, TiO2 suspensions in photocatalysis require separation and recycling of the photocatalyst after usage. In the case of ultrafine TiO2 photocatalysis, the development of the recovery technology is more time consuming and expensive. As such the powder slurry of TiO2 is not suitable for large-scale applications, such as purification of water and/or of air. For practical photocatalytic application, it is more advantageous to immobilize TiO2 in the form of a coating. At present, heterogeneous immobilized photocatalysts such as TiO2 are commonly produced by solegel processes and spraying techniques [11]. In this study, a multifunctional PHBeTiO2 composite film was developed by coating the TiO2 photocatalyst (Degussa’s P-25 formulation) on polyhydroxybutyrate (PHB) film. PHB is the most common polymer among the polyhydroxyalkanoates (PHAs). PHA is a bio-based thermoplastic synthesized by various microorganisms and it can be decomposed and assimilated by many microbial species (biodegradable) [12,13]. The entire cycle of the PHA production and disposal is a sustainable process where PHA becomes part of the biological carbon cycle [14]. To examine the decolorizing efficiency of this PHBeTiO2 composite film, photocatalytic decolorization of the organic dyes methylene blue and rose bengale was studied. The durability of disinfection via photocatalytic sterilization was also evaluated and compared using Escherichia coli (E. coli) as the biological indicator for disinfection efficiency. Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analyses were used to characterize the PHBeTiO2 composite films. Biodegradation study of PHBe38 wt% TiO2 and PHBe57 wt% TiO2 composite films was conducted at two sampling sites that had different soil microbial activity. 2. Materials and methods

The bacterial strain of E. coli JM109 was inoculated into LuriaeBertani broth and was grown overnight at 37  C under constant agitation in aerobic condition. Aliquots of the overnight culture were inoculated into fresh medium and incubated aerobically at 37  C until it reached mid-exponential phase. Bacterial growth was monitored by optical density at 600 nm. In the exponential growth phase (OD600 nm w 0.6), the bacterial cells were collected by centrifugation at 4000 g for 10 min. The bacterial pellet was then washed twice with sterile distilled water. The cell suspension was diluted with sterile distilled water to the required cell density with a final optical density of 0.025. The cell suspension (20 ml) was further aliquoted into petri dishes containing PHBe57 wt% TiO2 composite films and the same method was also applied for slurry test using powder TiO2. The petri dishes were continuously agitated on a see-saw shaker. Samples were taken periodically under three conditions: firstly under fluorescent light illumination (1020 lux); secondly under ultraviolet A (UVA, 320e400 nm) provided by two black light tubes each of 40 W and placed at a distance of 20 cm; and thirdly in the dark. The intensity of light was determined by using Lux Meter ISO-TECH ILM350. Serial dilutions were prepared, when necessary, in sterile distilled water and the samples were then plated onto LuriaeBertani agar plates. The plates were incubated at 37  C for 24 h before counting the resulting colonies. All of these experiments were being repeated three times. Control experiment was performed for each series of the experiments. The control experiment was carried out using pure PHB film without TiO2 under fluorescent light, UVA illumination and in the dark. 2.3. Preparation of PHBeTiO2 composite films The PHB used in this study was kindly provided by PHB Industrial S/A, Brazil (600,000 Da). The PHB was further purified by dissolving it in hot chloroform and precipitating in cold methanol. All films were prepared by conventional solvent-cast technique. The ratio of PHB to TiO2 for PHBe 57 wt% TiO2 composite film was 1.0:1.3 (w/w) while that for PHBe38 wt% TiO2 was 1.0:0.6 (w/w). These mixtures in chloroform were poured into glass petri dishes and the solvent was allowed to evaporate at ambient conditions. The resulting films appeared homogeneous and were rinsed with distilled water prior to use.

2.1. Chemicals 2.4. Photocatalytic decolorization experiments TiO2 (P-25, ca. 80% anatase, 20% rutile; BET area, ca. 50 m2 g
1) was kindly provided by JJ Degussa Co., Malaysia. The organic dyes, Methylene Blue and Rose Bengale were

The methylene blue solution (1 mM) was prepared in distilled water and the pH was further adjusted to 4.0. For

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comparison purposes, 1 mM rose bengale was used. The freshly prepared solution of 20 ml was distributed into petri dishes containing PHBe57 wt% TiO2 composite film. A similar experiment with pure PHB film was also performed under the same conditions. Samples (2 mL) that were withdrawn periodically for the determination of photocatalytic degradation of methylene blue were being monitored using spectrophotometer at a wavelength of 580 nm. 2.5. Morphological analysis of nanosized TiO2 TiO2 powder was suspended in distilled water and the suspension was mounted onto copper grids. After a minute, the suspension was observed using the Philip CM-12 transmission electron microscope (TEM).

the soil bacteria colonizing the surfaces of the polymer films. The colonies of the soil bacteria present on the agar plates were then counted and expressed in terms of log10 colony forming unit per gram (log10 CFU/g). 2.9. Scanning electron microscopy (SEM) micrographs and energy dispersive X-ray (EDX) analysis SEM micrographs, with a magnification of 1000e80,000 times, of the various polymer films before and after the degradation were recorded on Leo Supra 50 VP Field Emission SEM equipped with Oxford INCA 400 EDX Microanalysis System. EDX analysis was used to quantify the elemental compositions of these films before and after degradation. 3. Results and discussion

2.6. Sampling sites and experiment design Two sampling sites in a garden, designated as Location A and Location B were chosen through the determination of their respective soil pH, microbial activity and light intensity. Location A was under the shade of a tree with sunlight intensity ranging from 2530 to 11,800 lux. The soil had a microbial activity of approximately 27,000 CFU/g with pH 4.6. In contrast, Location B was exposed to direct sunlight with intensity ranging from 62,400 to 115,300 lux. The microbial activity recorded was about 13,000 CFU/g and the soil pH was 4.4. 2.7. Biodegradation tests Films of PHB, PHBe38 wt% TiO2 and PHBe57 wt% TiO2 were allowed to age for a week before exposing them to the garden soils. Biodegradation tests were carried out by placing 20 pieces of PHBe38 wt% TiO2 and 20 pieces of PHBe57 wt% TiO2 composite films on the soil surface; while another batch of 20 pieces of each type were buried in the soil. The pure PHB films were also being experimented under the same conditions for comparison purposes. The duplicate test pieces were retrieved from the soils, washed in sterile distilled water, dried for one day at room temperature, and weighed. The degradation of these films was evaluated conventionally in terms of percentage weight loss using to the following formula:   weight of initial film
weight of film after degradation weight of initial film  100% 2.8. Enumeration and isolation of soil bacteria Soil samples were collected from the topsoil (0e3 cm in depth) at the two sampling sites in the garden. Soil suspension was prepared by adding 1.0 g of the soil sample to 10 ml of sterile distilled water. The supernatant of the soil suspension was then serially diluted and the dilutions were inoculated onto Nutrient Agar plates. Similarly, the washings from the films that had been incubated in the soil were used to isolate

3.1. Characteristics of TiO2 photocatalyst used in this study Fig. 2 shows the morphology of TiO2 nanoparticles (JJ Degussa P-25) under transmission electron microscopy (TEM). The TEM revealed that the photocatalyst was of uniform crystal size with a mean of 23.45 nm. Slurry test was performed to determine the photocatalytic efficiency of the TiO2 photocatalyst. The survival ratio of E. coli (Table 1) indicated that the TiO2 (P-25, Degussa) is a photocatalytically active compound. Generally, anatase is considered to be the photoactive form, while rutile is considered to have a low photocatalytic activity. However, for reasons that are not yet understood, mixtures of anatase and rutile (Degussa P-25, which consists of about 80% anatase and 20% rutile) have better photocatalytic activity than either phase [15]. Experiment was carried out under light and UVA illumination with different concentrations of TiO2. Table 1 shows a strong dependence of the E. coli inactivation efficiency on the TiO2 concentration under fluorescent light illumination. The bacteria showed complete inactivation under UVA illumination with low concentration of TiO2 (0.1 mg/mL). 3.2. Photocatalytic degradation of organic dye Photocatalytic degradation of organic dyes is well documented [16e18] and can be used to determine the efficiency of the photocatalyst. In this study, PHBe57 wt% TiO2 composite film was used to determine the photocatalytic degradation of methylene blue (1 mM) when illuminated by solar light, UVA and fluorescent light. The results (Fig. 3) illustrate that solar light exhibits the highest efficiency in photocatalytic degradation of methylene blue in comparison to UVA light or fluorescent light with 96% dye decolorization after an hour of illumination. Interestingly, solar light illumination gave better results compared to UVA light illumination. This is most probably due to an increase in temperature of the dye solution when exposed to solar light illumination [19]. One of the most important mechanisms of TiO2 is that photocatalytic activity can be induced even with a few photons of

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Fig. 2. Morphology of nano-TiO2 (JJ Degussa P-25) observed under transmission electron microscopy.

3.3. Photocatalytic sterilization of microorganisms Recently, there is a growing interest in the use of TiO2 mediated photocatalysis for water disinfection. To avoid the use of TiO2 powder which entails later separation from the water, various researchers began to work on the ways of immobilizing TiO2 particles, for example in thin films. One of the first reports on the preparation of TiO2 films was that of Matthews [20]. This idea has also been tested by Anderson [21], Heller [22] and Fujishima groups [23e28]. One of the major advantages of the material used in this study is its biodegradability. PHA is polyester, produced by a range of microbes, and it does not cause toxic effects on the host (biocompatible) [29]. In nature, a vast consortium of microorganisms is able to degrade PHA. The degradation rate of PHA is typically in the order of a few months to years depending on the microbial activity and physical factors. Here we have tested for the first time the photocatalytic sterilization activity of TiO2 immobilized onto the most common PHA, i.e., PHB. For this, PHBe57 wt% TiO2 composite films were exposed to either fluorescent or UVA light and E. coli was used as the biological indicator for disinfection efficiency. Table 1 Effects of TiO2 (P-25 formulation; Degussa) concentrations on the killing of E. coli TiO2 concentrations (mg/mL) 0 0.10 0.25 0.50 1.00

Survival ratio (%)a Dark 98 98 94 95 93

Light illumination

UVA illumination

99 80 55 25 10

98 0 0 0 0

The data shown are representative of two independent experiments. a Ratio of the cell concentration after 30 min in the dark, light or UVA illumination to the corresponding cell concentration in the dark.

Bacterial inactivation by fluorescent light and UVA light in the presence of PHBe57 wt% TiO2 is shown in Fig. 4. During the experiment under UVA illumination, the number of surviving bacteria decreased exponentially before reaching nondetectable level. After 3 h of illumination under UVA, 72% of the E. coli JM109 was inactivated in contrast to the E. coli being still viable in dark condition. Prolonged illumination up to 6 h resulted in more than 99% inactivation. On the other hand, when UVA illumination was replaced by fluorescent light, the E. coli inactivation efficiency decreased as expected (Fig. 4). After 3 h of illumination, only 6.7% of cells were inactivated. Prolonged illumination up to 6 h increased inactivation efficiency whereby 56.9% of cells were inactivated. The results indicated that disinfection with TiO2 in the dark does not occur. It is known that TiO2 particles show weak or no toxicity either in vitro or in vivo. The results showed that the TiO2 immobilized onto PHB retains its photocatalytic activity and can achieve significant sterilization effect even under fluorescent light illumination. 3.4. Microbial degradation of films at sampling sites The data indicating the percentage of initial weight recovered for pure PHB films and PHBeTiO2 composite films at Location A (high microbial activity, low light intensity) and 1

100 90

Color removal (%)

energy [1]. This means that ordinary fluorescent lighting is sufficient to induce photocatalytic activity in the presence of TiO2 photocatalysts. It was observed that after an hour under fluorescent light illumination, 16% dye removal was achieved. When methylene blue was replaced with rose bengale, similar trends were observed (results not shown).

80

2

70 60 50 40 30

3

20 10

4

0 1

2

3

4

5

6

7

Time (min) Fig. 3. Discoloration of 1 mM methylene blue under different experiment conditions using PHBe 57 wt% TiO2 as photocatalyst: (1) solar light; (2) UVA light; (3) fluorescent light; and (4) dark condition (each half of error bar represents the standard deviation for triplicate experiments).

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Survival ratio (%)

1804 100 90 80 70 60 50 40 30 20 10 0

% of initial weight recovered 1

A 100 95

2

90

3 0

3

6

9

85 12

Time (hours) Fig. 4. Performance of E. coli JM109 sterilization with PHBeTiO2 composite films under: (1) dark, (2) fluorescent light, and (3) UVA illumination (each half of error bar represents the standard deviation for triplicate experiments).

B

100 90 80

Location B (low microbial activity, high light intensity) are shown in Fig. 5AeD. In both locations, films that were placed on the soil surface showed slower degradation in comparison to the films buried in the soil. This trend is expected given the high numbers of microorganisms present in the soil (Fig. 6AeD). The degradation of buried films is therefore dependent on the density of the microbial population. Fig. 5D shows that the order of degradation of all the films buried in the soil in Location B is similar to that of Location A, but the overall degradation is much slower. The results of CFU counting revealed that the number of microorganisms in Location B was lower than that of Location A (Fig. 6). Since Location B was exposed to full sunlight, it is possible that the soil may be dry causing the soil microorganisms to become less active. With the exception of the PHBeTiO2 composite films placed on the soil surface at Location B, the obtained results revealed the significance of the TiO2 incorporated into the PHB films. In comparison to the pure PHB films, both the PHBe38 wt% TiO2 and the PHBe57 wt% TiO2 composite films experimented on the soil surface and also buried in the soil, displayed slower rate of degradation. Hence, indicating that the presence of TiO2 photocatalyst actually slows down the progress of degradation by the soil microorganisms. The oxidizing power of TiO2 affects single cell organisms that include all bacteria and fungus. Upon exposure to mild UVA, TiO2 produces highly reactive hydroxide (OH) radicals in the presence of water and oxygen [30]. These radicals cause rupture and leakage of the bacterial cytoplasm leading to the killing of the cells. Difficulty of the soil microorganisms to grow on the surface of the PHBeTiO2 composite films had contributed to the slow degradation of these films. Also, this is further supported by the fact that the microbes capable of growth on the PHBe57 wt% TiO2 composite films were about 1e2 orders of magnitude less prevalent than the pure PHB films as shown in Fig. 6. Theoretically, the films containing TiO2 would be expected to show slower degradation. Interestingly, the PHBeTiO2 composite films that were placed on the soil surface at Location B exhibited much higher weight loss than the pure PHB films. The fast degradation rates of the PHBeTiO2 composite films are most probably attributed to the higher light intensity

70 60 50 40

C

100 95 90 85 80

D

100

90

PHB PHB-38 wt% TiO2

80

PHB-57 wt% TiO2 0 0

7

12

16

20

24

28

32

37

40

43

Day Fig. 5. Degradation after 43 days for: A, pure PHB films and PHBeTiO2 composite films on soil surface at Location A; B, pure PHB films and PHBeTiO2 composite films buried in soil at Location A; C, Pure PHB films and PHBe TiO2 composite films on soil surface at Location B; D, pure PHB films and PHBeTiO2 composite films buried in soil at Location B (each half of error bar represents the standard deviation).

in this area rather than to the activity by the soil microorganisms. It is known that the increment of photocatalytic activity due to the exposure to intense light eventually leads to the attack of the base material. This offers an explanation to the high weight loss of the PHBeTiO2 composite films despite the low densities of the microbial populations found growing on these films.

S.-P. Yew et al. / Polymer Degradation and Stability 91 (2006) 1800e1807

PHBe57 wt% TiO2 composite film contained more TiO2 particles. Since the microbial degradation occurred by surface erosion, only the TiO2 particles adhering to the surface of base material were removed. The remaining TiO2 particles in the interior region could have contributed to the higher weight recovered for the PHBe57 wt% TiO2 composite film.

log10 CFU/g

A

6

5

4

3.5. Microbial enumeration Fig. 6AeD show the average log10 CFU/g for the duplicate experiments of each film taken from both sampling sites, and experimented on the soil surface as well as buried in the soil. The error bars represent the standard deviation of log10 CFU/g for the duplicate experiments. The data revealed several important points. Firstly, the overall densities of the microbial populations on all the films exhibited a relationship whereby Location A > Location B. This is in good agreement with the faster degradation of the buried films but not so for the films placed on the soil surface at Location A. Despite the higher microbial density on the films at Location A, the degradation of the PHBeTiO2 composite films on the soil surface was much slower than that of the films at Location B. Therefore, the degradation of the PHBeTiO2 composite films on the soil surface is not directly proportional to the microbial activity. Secondly, microbes tend to favor growth on the pure PHB films rather than the PHBe57 wt% TiO2 composite films in all conditions at both the sampling sites. Microbes capable of growth on the PHBe57 wt% TiO2 composite films were about 1e2 orders of magnitude less prevalent than the pure PHB films.

3

B

1805

8

7

6

5

4

C 5

4

3

3.6. SEM micrographs and energy dispersive X-ray (EDX) analysis

D 6

5

4

PHB

3

PHB-38 wt% TiO2 PHB-57 wt% TiO2

0 7

12

16

20

24

28

32

37

40

43

Day Fig. 6. Average of bacterial populations growing on: A, pure PHB films and PHBeTiO2 composite films on soil surface at Location A; B, pure PHB films and PHBeTiO2 composite films buried in soil at Location A; C, pure PHB films and PHBeTiO2 composite films on soil surface at Location B; D, pure PHB films and PHBeTiO2 composite films buried in soil at Location B (each half of error bar represents the standard deviation for duplicate experiments).

Based on the data, there is an apparent higher percentage of initial weight recovered for the PHBe57 wt% TiO2 composite films in comparison to the PHBe38 wt% TiO2 composite films. This is possibly due to the higher amount of TiO2 in the former film. The surface and interior region of the

The SEM micrographs (Fig. 7AeF) show the transition of topologies present on the films. With time, the surface of the buried films became progressively dented because of surface erosion. Different traces due to microbial degradation can be observed on the surfaces of the experimented film sheets. These traces revealed the morphology of the microorganisms that had acted upon the surfaces of the films [31]. Colonization of bacteria on the films caused the formation of hemispherical cavities (arrow a in Fig. 7D) while growth of fungal hyphae caused the formation of streaky furrows (arrow b in Fig. 7E). Table 2 shows the energy dispersive spectrometer analysis of the pure PHB and the PHBe57 wt% TiO2 films before and after 32 days of incubation in the soil. The EDX analysis was carried out on the surface of the film sheets using an accelerating voltage of 10 kV to demonstrate the elemental compositions. Assuming that the weight percentage of the Titanium (Ti) was not lost during degradation and by comparing the weight percentage of Carbon (C) relative to the weight percentage of Ti, the PHBe57 wt% TiO2 composite film contained lesser weight percentage of C after degradation. The obvious decrease in weight percentage of C indicated the degradation of this film. The microorganisms colonizing the surface of these films secreted enzymes, which degraded the PHB into its monomeric units. These units are assimilated by the microorganisms as

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B

A

PHB-38 wt% TiO2

PHB

D

C

a

PHB-57 wt% TiO2

E

PHB

F

b PHB

PHB-57 wt% TiO2

Fig. 7. Scanning electron microscopy of pure PHB, PHBe38 wt% TiO2 and PHBe57 wt% TiO2 composite films before and after degradation in soil. A, surface of pure PHB film before incubation in soil; B and C, surface of PHBe38 wt% TiO2 and PHBe57 wt% TiO2 composite films before incubation in soil; D, dented surface of pure PHB film placed on soil surface after 32 days; E, dented surface of buried pure PHB film in soil and fungal hyphae; and F, dented PHBe 57 wt% TiO2 composite films with soil particles after 32 days of soil incubation. In A, formation of hemispherical cavities due to bacterial degradation (a) and streaky furrows caused by fungal degradation (b) are shown.

a carbon source for biomass growth [32]. The EDX data of the pure PHB film (Table 2) after degradation, gave an interesting insight into the cleavage of the PHB molecules during degradation. Before degradation, the ratio of C to O was roughly 1:1 but after degradation, the ratio increased to approximately 2:1. The relative increment of C could be attributed to the cleavage of ester bonds which resulted in the removal of O. The presence of two new elements, Aluminium (Al) and Silica (Si), was also detected in the pure PHB and the PHBe57 wt% TiO2 films incubated in the soil (data not shown). The Si and Al were from soil particles trapped in the cavities formed on the deteriorated surfaces of the films.

4. Conclusion The PHBeTiO2 composite film designed in this study was found to be effective in the sterilization and decolorization of methylene blue and rose bengale. The use of the immobilized photocatalysts has important operational advantages as they do not require the separation process after photocatalytic treatment. In addition to this, the PHBeTiO2 composite film is also biodegradable and environmental-friendly. Its production is essentially neutral with respect to the carbon dioxide balance, and it is also biologically renewable. Therefore the usage of the PHBeTiO2 composite film may be developed as a novel

S.-P. Yew et al. / Polymer Degradation and Stability 91 (2006) 1800e1807 Table 2 Elemental composition by EDX of pure PHB film and PHBe57 wt% TiO2 composite film Element

Weight % of elements PHB film

C O Al Si Ti

[10]

PHBe57 wt% TiO2 composite film

Before degradation

After degradationa

Before degradation

After degradationa

58.0 42.0 n.d. n.d. n.d.

51.4 29.2 7.6 11.8 n.d.

22.0 44.9 n.d. n.d. 33.1

7.4 41.1 0.7 0.8 50.0

[11]

[12] [13] [14]

[15]

n.d.: Not detected. a Data are obtained from representative points on each film after 32 days of degradation in garden soil. [16]

method for the removal of organic dyes and the disinfection of treated wastewater. Acknowledgement We would like to thank PHB Industrial S/A of Brazil for kindly providing the pure PHB resin used in this work. We also thank the staffs of the Electron Microscopy Unit, School of Biological Sciences for their technical assistance during the TEM, SEM and EDX analyses. This study had been funded in parts by research grants from the Malaysian Government through IRPA RM8 and from YORK International Corporation. S.-P. Yew is a recipient of the National Science Foundation Fellowship. References [1] Fujishima A, Honda K. Electrochemical photocatalysis of water at semiconductor electrode. Nature 1972;238:27e8. [2] Wei C, Lin WY, Zainal Z, Williams NE, Zhu K, Kruzic AP, et al. Bactericidal activity of TiO2 photocatalyst in aqueous media: toward a solar-assisted water disinfection system. Environ Sci Technol 1994;28:934e8. [3] Pham HN, McDowell T, Wikins E. Photocatalytically-mediated disinfection of water using TiO2 as a catalyst and spore-forming Bacillus pumilus as a model. J Environ Sci Health A 1995;30:627e36. [4] Cho M, Chung H, Choi W, Yoon J. Different inactivation behaviors of MS-2 phage and Escherichia coli in TiO2 photocatalytic disinfection. Appl Environ Microbiol 2005;71:270e5. [5] Fujishima A, Rao TN, Tryk DA. Titanium dioxide photocatalysis. J Photochem Photobiol C Photochem Rev 2000;1:1e21. [6] Zollinger H. Color chemistry: synthesis, properties and application of organic dyes and pigments. VCH; 1991. [7] Konstantinou IK, Albanis TA. TiO2-assisted photocatalytic degradation of azo dyes in aqueous solution: kinetic and mechanistic investigations: a review. Appl Catal B Environ 2004;49:1e14. [8] Chen JS, Liu MC, Zhang JD, Ying XY, Jin LT. Photocatalytic degradation of organic wastes by electrochemically assisted TiO2 photocatalytic system. J Environ Manage 2004;70:43e7. [9] Lee SH, Kang M, Cho SM, Han GY, Kim BW, Yoon KJ, et al. Synthesis of TiO2 photocatalyst thin film by solvothermal method with a small

[17]

[18]

[19] [20] [21] [22] [23]

[24]

[25]

[26]

[27]

[28]

[29] [30]

[31]

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